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Horsepower - Please help - Confused!

  1. Feb 13, 2009 #1
    Let me preface this by saying I understand the basics of work, power, and energy. I know how horsepower is calculated from torque. I'm confused over how engines are designed for horsepower. I've read many mechanic books, many car magazines, etc. but never come across how to specifically design an engine for specific horsepower. I've heard all of the general answers - no replacement for displacement - and all that other stuff, but can someone please tell me the relationship between airflow, cams, etc. to produce specific torques at speeds?

    Essentially, I'm trying to figure out if you want x amount of horsepower at x speed, how do you know which parts to pick? Thank you.
  2. jcsd
  3. Feb 14, 2009 #2
  4. Feb 14, 2009 #3


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    Get a copy of "Internal Combustion Engine Fundamentals" by John Heywood. Read chapters 1, 2, and 8-10 and you'll have a pretty good understanding.
  5. Mar 13, 2009 #4
    I know you have probably read countless articles on how horsepower is work and engines are pumps and it's not clearing anything up for you so here goes.

    Torque is a twisting force upon an axis. If you have ever used a torque wrench then you are well aware that you can exert as much as 90ft/lb or torque on a bolt. If you haven't then take my word for it, you can.
    So for all practical purposes, a human can exert as much torque as a Toyota Yaris. The problem is speed. You see, no matter how much torque you can produce, you can't get a 2200LB car up to 70mph!
    Here's another example. If a man weighs 180lbs and he is on a bicycle. the arms on the bicycle pedals are 12 inches or 1 foot. He stands on the pedal to accelerate. 180ft/lbs of torque is being exerted on the pedal. He has just exerted as much torque as a V6 Pontiac GrandAm! However the car will still win the race in spite of the fact it weighs 20 times as much as the man on the bike. The best you can pedal is about 0.8 push per second or .4 revolutions per second or 24 rpm. So we have the formula (24*180)/5252=0.8 hp. In practicality you only have about 0.2 Hp but on a bicycle you have 0.8 which is why you can ride at about 80% of the speed of a running horse.

    So to sumarise. Torque is potential energy( See potential energy and kinetic energy) and horsepower is torque at speed. The faster you can create torque the more power you can create.
  6. Mar 13, 2009 #5
    Here is how you make more power. you need 2 things. 1 is a vacuum guage and 2 is a pressure guage. So you install the vacuum guage in the intake as close to the cumbustion chamber as possible. install the pressure guage (heat resistant) as close to the heads as possible in the exhaust. at wide open throttle there should be little or no vacuum in the intake at your Hp peak. if there is excessive vacuum then you need to open the intake. Port, polish, bigger throttle body, low restrict air cleaner etc. just open up the intake.

    on the other side or the chamber if there is excessive pressure (more than 2-3psi) at Hp peak then you will need to open up the eshaust. free flow cat, larger exhaust tubing, low resistance muffler, etc. Once you have accomplished these 2 feats then you can get a cam with more duration. only go about 5-10 degrees more. Get larger valves. Increase in diamer if about 5% will make a huge difference! These 2 things will make the engine breath better at higher rmps. torque at high rpm=more horsepower. then go back to the beginning. You can also increase compression to get more efficent fuel burn.
  7. Mar 14, 2009 #6

    Ranger Mike

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    Whooaa there Grasshopper!!
    I'll be happy to tell ya but we gotta figure out all yer questions..

    1. how to specifically design an engine for specific horsepower
    2. the relationship between airflow, cams, etc. to produce specific torques at speeds?
    3. if you want x amount of horsepower at x speed, how do you know which parts to pick?

    i suggest we talk about 2. the relationship between airflow, cams, etc. to produce specific torques at speeds? first...

    then go into how to specifically design an engine for specific horsepower....
    this will naturally assist in answering your 3rd question which, btw, is the most often asked. We have to use the production line engine as a startiong point and get real selective on the parts we use to amp up the H.P. ..unless your daddy is named Cosworth or Offenhouser or the like...
    first. the ref; i use

    Elements of Internal-Combustion Engines by A.R. Rogowski, S.M.
    published by McGraw-Hill

    i got grab a beer so hold that thought!!
  8. Mar 14, 2009 #7

    Ranger Mike

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    In my opinion, the Internal Combustion Engine (IC) is a big air pump. It uses a fuel/Air mixture which is converted to mechanical energy through the combustion process. for this discussion I will keep it simple. the IC we discuss is normally aspirated ( not turbocharger or supercharged) and uses gasoline as fuel..

    Air - what we breathe under 14.7 PSI pressure
    Fuel - gasoline, Av. gas if you can get it.note; gas is rated in octane..there are two methods used to rate octane but we won't go into detail here. The lame stuff we have to buy is low octane like 85..it used to be you could get 13 octane pump gas ..had lots of lead that prevented detonation. now illegal...av. gas comes close. The IC compression ration dictates the octane..a 8:1 engine can use 85 octane but if you run 12:1 compression you need av. gas 110 or higher octane. or you detonate...your engine pings, bucks and may even grenade.

    The IC takes in the fuel/air mixture

    ECONOMY.......... BEST ALL- AROUND .............. POWER

    Gasoline 17.1 16.0 15.1 14.7 14.7 14.7 14.7 14.0 13.2 12.1
    Alcohol 7.6 7.1 6.7 6.5 6.5 6.5 6.5 6.1 5.8 5.3
    Propane 17.9 16.8 15.9 15.6 15.6 15.6 15.6 15.0 14.0 13.0

    the fuel/air mixture depends on the design. I will discuss carburation as it is simplest to understand. google it for more details
    For this discussion it is a device that mixes air with fuel the mixture is determined by the carb jets used. bigger jets mean more fuel/air mixture..less means higher economy. Carbs are rated in CFM (cubic feet per minute) you can by um up to 1100 CFM
    most small block IC are happy with 650 cfm.

    Airflow requirement = CID / 2 x RPM/1728 x Volumetric Efficiency

    so a total prepared race 350 cube engine at 8000 RPM needs 811 CFM ( race engine is as close to 100 % Vol. Eff. as your are gonna get! ) and this is a requirement at Wide pen Throttle..which is NOT going to be for long as cranking a mill at 8 grand makes the parts guy at the race engine facility real happy
  9. Mar 14, 2009 #8

    Ranger Mike

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    the cam shaft controls entry and exit of the fuel /air mixture (fresh and spent) at points that are timed to piston position. It is also linked to the ignition in such a way that ignition of the compressed mixture is properly timed with valve / piston movement. exactly when valves open and close and when ignition occurs is dependant on what you desire from the engine. More on this later.
    The valve train of a typical overhead valve design V8 IC consists of the timing chain that connects the crankshaft to the camshaft, camshaft, lifters ( mechanical part that transforms rotary motion into linear motion, push rods, these connect the lifter to the rocker arm ( lever that actuated the valve) valve spring and valve it self along with valve retainer and keepers that secure the valve spring combination .
    there is a lot of mass in this valve train, the next step is to look at the overhead cam valve train - a timing chain or belt connects the crank to the cam. again we have a cam shaft, a rocker is directly actuated by the cam lobe and bumps the valve ,the valve spring, retainer valve, keepers).

    Volumes have been written on cam design..too much for this discussion. Generally, the cam has LIFT, Duration ( degrees of crankshaft rotation that the valves are of their seats and has a separate intake and exhaust profile. If the intake and exhaust are off their seats at the same time ..it is called overlap. Overlap helps draw spent mixture out of the cylinder and fresh mixture in.
    as a general rule, the greater the lift with out increase in duration usually improves IC low, mid and even some upper range ( 5000 rpm) engine performance . long timed cams ( long duration) normally move the performance bracket, in terms of RPM, higher in the engines operating range.
    Last edited: Mar 14, 2009
  10. Mar 14, 2009 #9

    Ranger Mike

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    ok ..we have this ideal engine we are building. we have a good fuel air mixture coming in through a manifold to the engine. we have a means of controlling it via cam shaft and valve train. and we have a means of igniting it. and a means of evacuation the spent mixture out of the combustion chamber. (exhaust manifold..or better yet, headers)...time to look at various design characteristics of IC

    The IC has been around for over 100 years and just about every wacky idea has been tried to improve on it. I will endeavor to review some of them..in my limited capacity on Saturday morning, over another beer!

    Ford flat head- this was an in line 4 cylinder IC. had 4 pistons in a row, firing order was 1342. Intake and ex valves where in the engine block. the cylinder head was FLAT ..millions were produced..easy to work on, repair in the early 1910..1930s.

    more power was needed so the next step was an in line 6 cylinder..same as above with two more slugs..
    the even made an inline 8 cylinder..I think Cadillac and Buick had straight 8s..main draw back was cost to produce, weight and space..a straight 8 was LONGGGG..took up a lot of hood to cover.. andf it was HEAVY

    the V8 came about to get more cubes, and reduce the engine package.more compact, shorter... plus .. it was SMOOTH running. having a piston sparking every 45 degrees is a lot smoother than having the ignition at 90 degrees ( 4 cylinder) or 60 degrees ( 6 cyl). Less stressful on crankshaft and drive train components.
    the head was still a flat head. this is the classic flat head V8 you still see in Hot Rod magazine...ran pretty good.

    the next step was to go to the overhead valve configuration. this opened up tow areas of performance improvement. Now you could design a cylinder head that was Wedge shaped, semi hemispherical or a true Hemispherical head).

    the most effective design to handle pressure in all directions is the Hemispherical head. it features a round combustion chamber and makes maximum use of the fuel air mixture and transmits the power to the top of the piston as equally as yer gonna get in the real world. The wedge-shaped combustion chamber is good at what it does AND CHEAP TO MANUFACTURE compared to the other designs..which is why it is so popular..as are all things in Detroit..it is all about economy of scale and cost to produce...the Hemi and the Semi were low volume and high production cost engines..
    today, i think everyone is using the hemi head wit up to 5 valves in some engines I have worked on...these are all overhead cam ICs and some have dual overhead cams..one operating the intake valves and on for the exhaust..further reduces valve train weight.

    Yes they make a V10 but very low volume and high dollars to biuld , buy , race and maintain...unless your daddy is Penske!
  11. Mar 14, 2009 #10

    Ranger Mike

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    I recommend you pick up a copy of Engine Blueprinting by Rick Voegelin
    also the BIBLE for me

    POWER SECRETS by Smokey Yunick..one absolute genius..he was THE MAN..way out front..decades ahead of his time...
    this is after you have rebuilt at least one or two engines..4 cyl, 6 or eight..ya gotta master the basics before doing any hot rodding..
    ok nuff of the theoretical..lets talk about working with what you have available..like the tried and true small block Chevy V8 or better yet...a 4 cylinder which, sadly today is about the most popular.

    lets say we have a race series with very restrictive rules regarding cubic inch displacement, cam shaft modifications etc...
    most every crew chief out there will tell you..if the rules don't say you can't do it..it is legal!
    cheating??? only if you get caught..this forum is not to debate ethics so I am not going to try to be a barracks lawyer..I will tell you that anyone wit half a brain is doing it..
    I discuss these points to get you thinking

    We raced a 4 cylinder one time..got a set of forged Chevy connecting rods (1.9 ratio) and had a custom piston made. we ran LONGER than stock connecting rods which did several things, required a custom piston that had the pin boss moved a lot closer to the piston rings
    hence, was a lot lighter, had a lot better rod angle, less stress on the crank shaft and cylinder walls, permitted us to use a much smaller wrist pin..less weight and less mech. friction than the stock configuration..we were able to take a whole lot of reciprocating weight out using this package....,ran like Hell, here is why..

    In general, most observations relate to engines used for some type of competition event and will in general produce peak power higher than 6000 RPM with good compression ring seal as defined by no more than 3 percent leak down per cylinder.

    Short Rod is slower at BDC range and faster at TDC range.

    Long Rod is faster at BDC range and slower at TDC range.


    A. Intake Stroke -- will draw harder on cyl head from 90-o ATDC to BDC.

    B. Compression Stroke -- Piston travels from BDC to 90-o BTDC faster than short rod. Goes slower from 90-o BTDC to TDC--may change ign timing requirement versus short rod as piston spends more time at top. However; if flame travel were too fast, detonation could occur. Is it possible the long rod could have more cyl pressure at ie. 30-o ATDC but less crankpin force at 70-o ATDC. Does a long rod produce more efficient combustion at high RPM--measure CO, CO2? Find out!!

    C. Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20 to 75 ATDC less force is exerted on the crank pin than a shorter rod. However, the piston will be higher in the bore for any given crank angle from 90-o BTDC to 90-o ATDC and thus cylinder pressure could be higher. Long rod will spend less time from 90-o ATDC to BDC--allows less time for exhaust to escape on power stroke and will force more exhaust out from BDC to 90-o BTDC. Could have more pumping loss! Could be if exhaust port is poor, a long rod will help peak power.

    D. Exhaust Stroke -- see above.

    II. Short Rod

    A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.

    B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.

    C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.

    D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.


    A. Rod Length Changes -- Appears a length change of 2-1/2% is necessary to perceive a change was made. For R & D purposes it appears a 5% change should be made. Perhaps any change should be 2 to 3%--ie. Ignition timing, header tube area, pipe length, cam shaft valve event area, cyl head flow change, etc.

    B. Short Rod in Power Stroke -- Piston is higher in the bore when Rod-Crank angle is at 90-o even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10o and ie. 90-o, the short rod will generate a greater force on the crank pin which will be in the 70-o to 75-o ATDC range for most engines we are concerned with.

    C. Stroke -- Trend of OEM engine mfgs to go to longer stroke and/or less over square (bore numerically higher than stroke) may be a function of L/R. Being that at slower engine speeds the effect of a short rod on Intake causes few problems. Compression/Power Stroke should produce different emissions than a long rod. Short rod Exhaust Stroke may create more reversion--EGR on a street engine.

    D. More exhaust lobe or a earlier exhaust opening may defeat a longer rod. I am saying that a shorter rod allows a earlier exhaust opening. A better exhaust port allows a earlier exhaust opening.

    E. Definition of poor exhaust port. Becomes turbulent at lower velocity than a better port. Flow curve will flatten out at a lower lift than a good port. A good exhaust port will tolerate more exhaust lobe and the engine will like it. Presuming the engine has adequate throttle area (so as not to cause more than 1" Hg depression below inlet throttle at peak power); then the better the exhaust port is, the greater the differential between optimum intake lobe duration and exhaust lobe duration will be--ie. exh 10-o or more longer than intake Carbon buildup will be minimal if cyl is dry.


    Short Rod -- Min Rod/Stroke Ratio -- 1.60 Max Rod/Stroke Ratio -- 1.80

    Long Rod -- Min Rod/Stroke Ratio -- 1.81 Max Rod/Stroke Ratio -- 2.00

    Any ratio's exceeding these boundaries are at this moment labeled "design screw-ups" and not worth considering until valid data supports it.

    A n engine relies on pressure above the piston to produce rotary power. Pressure above the piston times the area of the bore acts to create a force that acts through the connecting rod to rotate the crankshaft. If the crankshaft is looked at as a simple lever with which to gain mechanical advantage, the greatest advantage would occur when the force was applied at right angles to the crankshaft. If this analogy is carried to the connecting rod crankshaft interface, it would suggest that the most efficient mechanical use of the cylinder pressure would occur when the crank and the connecting rod are at right angles. Changing the connecting rod length relative to the stroke changes the time in crank angle degrees necessary to reach the right angle condition.

    A short connecting rod achieves this right angle condition sooner than a long rod. Therefore from a "time" perspective, a short rod would always be the choice for maximum torque. The shorter rod achieves the right angle position sooner and it does so with the piston slightly farther up in the bore. This means that the cyl pressure (or force on the piston) in the cylinder is slightly higher in the short rod engine compared to the long rod engine (relative to time).

    Another concern in selecting the rod length is the effects of mechanical stress imposed by increasing engine speed. Typically, the concept of mean piston speed is used to express the level of mechanical stress. However, the word "mean" refers to the average speed of the piston in going from the top of the bore to the bottom of the bore and back to the top of the bore. This distance is a linear distance and is a function of the engine stroke and engine speed, not rod length. Empirical experience; however, indicates that the mechanical stress is less with the longer rod length. There are two reasons for these results. Probably the primary reason for these results is that the profile of the instantaneous velocity of the piston changes with rod length. The longer rod allows the piston to come to a stop at the top of the bore and accelerate away much more slowly than a short rod engine. This slower motion translates into a lower instantaneous velocity and hence lower stresses on the piston. Another strong effect on mechanical stress levels is the angle of the connecting rod with the bore centerline during the engine cycle. The smaller the centerline angle, the less the side loading on the cylinder wall. The longer rod will have less centerline angle for the same crank angle than the shorter rod and therefore has lower side loadings.
    I am out of beer!!
    Last edited: Mar 14, 2009
  12. Mar 14, 2009 #11

    Ranger Mike

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    home work..read about bore to stroke ratio


    one of the hottest engines we ever ran was dreamed up by some unknown racer in the 1960s
    it was the Chevy 301 engine which was a Chevy 283 cid bored out to 4 inches so it had
    a 4 inch bore and a 3 inch stroke resulting in 1.33 ratio 301 CID was so successful that Chevy introduced the 302 CID engine in 1968 ( 67 they had a lot of mules running) the 3 inch 283 CID crank shaft was placed In a 327 CID Chevy block that had 4 inch bore.. was in the HOT Z28 Camaro...11 to 1 compression 780 cfm holley 4 barrel on aluminum manifold..one hot package..
    had 4 bolt main caps on the crankshaft to keep the bottom end together

    aother hot setup was the 426 Hemi by Chrysler
    had 4.25 bore and 3.75 stroke or 1.13 ratio
    the Pontiac GTO 389 cid had 1.08 ratio
    good base line numbers
    Bore to stroke ratio is one critical design factor when looking for the right engine package
  13. Mar 14, 2009 #12


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    Another very tough engine (and good performer) was the Buick 232 V-6. Nice little pushrod mill that got used in lots of applications, including with single and double turbos in the Grand National models and with a supercharger in the Park Avenue Ultra. Jeep used these engines in their 6-cyl CJs back in the '60s, too.
  14. Mar 15, 2009 #13

    Ranger Mike

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    exactly right Turbo !

    Buick V6 is one awesome package. I was on a super late model race car team in late 1990s where John Vallo was the driver. We ran a V6 and tore up the competition..was significantly lighter package and darn near same Hp as a V8..naturally..they changed the rules and mandated a V8 had to be used as opposed to any American engine..ifin you can't beat um..change the rules....sick
    the Buick V6 was introduced in 1961 during the first compact car wars..Buick needed smaller profile engine for the compact line of skylarks and specials..so they took the aluminum block V8, chopped off two cylinders and cast it in iron..took 6 months from concept to production..a Motown record..
    this is why we have a 90 degree V6..was 3.625 bore x 3.2 stoke ..ENGINEERING NOTE:
    when the engineers chopped of the two cylinders, they had to choose between two basic firing patterns..1,6,2,4,3,5 and 1,6,5,4,3,2,

    the first would produce 150º , 120º, 120º, 90º,120º, 120º crankshaft intervals between firing. needed dual plane manifold , would produce uneven exhaust pulses ..the latter firing order would give 150º, 90º, 150º, 90º,150º,90º, sequence..better overall torque fluctuation throughout the cycle, smoother with only two fluctuations versus three on the first , even intake spacing regarding manifold feeding, simpler single plane intake manifold design. that's why they chose the latter.
    the little 198 cid V6 was poked out to 225 cubes and eventually Buick sold the engine, patterns et all to Jeep in 1967..Jeep used it 'til 1971 when they consolidated on the inline 6. In 1975 Buick bought back the engine from Jeep ..they bored it another .050 inch so they could use the Buick 350 pistons. Buick decided to use the little V6 through the whole product line and recognized they had a vibration problem due to firing order. The ingenious solution was to divide each of the three crankshaft journals into two and stagger them 30 º with a narrow flange in between, to produce, in actuality. a six-throw crankshaft in the 90º block without changing much else in the mill. the combination allows the pistons to arrive at TDC in even intervals (120º crankshaft) thus producing an "even -Fire" 90º V-6. the split throw crank was not a new concept, Lancia and Caterpillar used it years before. The 231 CID was a popular mill and in 1978 Buick installed a factory turbo . in 1980 the V6 was popped up to 4.1 liters 252 CID,
    for more on this and other V6 get a copy of V-6 Performance by Pat Ganahl published by Cartech
  15. Mar 15, 2009 #14
    Ranger Mike,

    THANK YOU SOOO MUCH! Everything is making sense now. I really appreciate your (and everyone else's) contribution to my questions.
  16. Mar 15, 2009 #15
    Is the pentroof engine essentially a flattened hemi head?
  17. Mar 16, 2009 #16

    Ranger Mike

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    Like all things in racing it depends upon the application.
    Rragrding racing engine design, and of F1 engines at that, then it is correct that the pent roof is the ideal shape,
    as Keith Duckworth so cleverly demonstrated all those years ago with the Cosworth DFV. It is incorrect to deduce that this is because the shape is ideal, far from it.

    It is important to remember that these engines have extreme bore/stroke ratios which seriously limits your options for combustion chamber shape, if high compression is desired, which it uniformly is for non turbo engines. The extreme bore/stroke ratio results from the simple geometry of total
    swept volume and maximizing piston area within that limit. A pent roof was essentially the only way to go, when Duckworth crunched the numbers.

    It is true that larger valves slow down the intake flow, but only through the valve. The speed of the intake flow in the inlet pipe goes up, which is more to the point. The cylinder filling resulting from intake effects is as much about resonance as it is about inlet tract to valve port angle or
    valve area or lift. Bigger valves breathe better, all other things being equal. More precisely, bigger total valve opening area breathes better AOTBE.

    A hemi head design still has advantages over pent roof for some applications. Not all engines can run at 18,000 rpm or head blowing compression ratios. The undersquare Alfa four is an example. The 164 engine also uses hemi heads in the two valve version even with a healthy oversquare engine. Bigger valves are possible with the hemi design, but multiple valves don't fit so well for reasons of actuation complexity. Pent
    roof accommodates multiple valves in an arrangement relatively easy to actuate with ordinary valve gear. Porcupine heads have been tried with some success in the past but they are complex, and who knows, maybe the valve
    actuation work being done now will result in a return to a semi hemi?i.e Chevy 396 and 427 big blocks
    The Pentroof design is a good compromise for high compression , in a street application and still giving good EPA emission numbers..sick....
    The quench zones are the flat areas of the cylinder head where the piston comes in close proximity to TDC. Pentroof DOHC cylinder heads typically have four quench zones at the ends of the combustion chamber. Quench zones promote more complete burning and reduce the likeliness of detonation by increasing turbulence of the fuel air mixture as the piston comes to TDC by squishing the fuel air mixture toward the sparkplug and away from the end zones of the combustion chamber. This reduces the amount of fuel-air mixture near the ends of the combustion chamber where it does not completely burn (thus being wasted) by pushing or squishing it toward the centrally located sparkplug where it can easily be ignited. When heads have additional quench area, they normally need less timing advance to make power, thus further preventing the detonation by rasiing the threshold, making the engine more reliable.

    The quench zones can be welded, milled and reshaped by hand to make them bigger, shaping the combustion chamber like a cloverleaf instead of the stock pentroof rectangle. This reduces the combustion chamber volume, increasing compression as well as making the quench zone more effective. This can also make the combustion chamber less likely to promote engine-damaging detonation because the turbulent air/fuel mixture squished by the bigger quench zones burns completely and smoothly.
  18. Mar 16, 2009 #17

    Ranger Mike

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    In a four-stroke naturally aspirated engine, the theoretical maximum volume of air that each cylinder can take in during the intake cycle is equal to the swept volume of that cylinder (0.7854 x bore x bore x stroke).

    Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine takes in compared to the theoretical maximum is called volumetric efficiency (VE). An engine operating at 100% VE is using its total displacement every two crankshaft revolutions.

    Its all about the air pimp as I said many times. Its how much air / fuel mixture you can stuff into the cylinder... That mass is directly proportional to (a) the air density and (b) the volumetric efficiency. There is a VERY close relationship between an engine's VE curve and its torque curve.

    In Old tech naturally-aspirated, two-valve-per-cylinder, pushrod engines a VE over 95% is excellent, and 100% is achievable, but extremely difficult to attain. Only full blown race prepped engine reach 110%, and that is by means of extremely expensive specialized intake manifolds, combustion chambers, exhaust header technology and super trick valve system components. The practical limit for normally-aspirated engines, typically DOHC layout with four or more valves per cylinder, is about 115%, which can only be achieved under the most highly-developed conditions, with precise intake and exhaust passage tuning.

    Generally, the RPM at peak VE is same as the RPM at the torque peak. Automotive engines rarely exceed 90% VE. There is a lot of good reasons for that performance, including the design requirements for automotive engines (good low-end torque, good throttle response, high mileage, low emissions, low noise, inexpensive production costs, restrictive form factors, etc.), as well as the allowable tolerances for components in high-volume production.

    For a known engine displacement and RPM, you can calculate the engine airflow at 100% VE, in sea-level-standard-day cubic feet per minute (scfm) as follows:

    100% VE AIRFLOW (scfm) = DISPLACEMENT (ci) x RPM / 3456

    Using that equation to evaluate a 540 cubic-inch engine operating 2700 RPM reveals that, at 100% VE, the engine will flow 422 SCFM.

    If we know how to calculate the fuel flow required for a given amount of power produced, we can calculate the mass airflow required for that amount of fuel, then by using that calculated airflow along with the engine displacement, the targeted operating RPM, and the achievable VE values, we can pretty much determine the resulting performance.

    Once we know the required fuel flow, we can determine the required airflow. Assumes 12.5 air fuel mixture.

    Using that best-power air-to-fuel ratio, you calculate required airflow:

    MASS AIRFLOW (pph) = 12.5 (Pounds-per-Pound) x FUEL FLOW (pph)

    But airflow is usually discussed in terms of volume flow (Standard Cubic Feet per Minute, SCFM). One cubic foot of air at standard atmospheric conditions (29.92 inches of HG absolute pressure, 59°F temperature) weighs 0.0765 pounds. So the volume airflow required is:

    AIRFLOW (scfm) = 12.5 (ppp) x FUEL FLOW (pph) / (60 min-per-hour x 0.0765 pounds per cubic foot)

    That equation reduces to:

    REQUIRED AIRFLOW (scfm) = 2.723 x FUEL FLOW (pph)

    OK, hang on. The really useful stuff is next.

    If I know my FUEL FLOW, I get:

    FUEL FLOW (pph) = HP x BSFC

    Replacing "FUEL FLOW" in with "HP x BSFC" :

    REQUIRED AIRFLOW (scfm) = 2.723 x HP x BSFC

    Now, we can estimate the airflow required for a given amount of horsepower, we can calculate the 100% VE airflow your engine can generate at a known RPM.

    Combining those equations will give us one equation which tells us how close we are to our performance goal by knowing Requited HP, RPM, CID and BSFC


    Here is an example of how useful that relationship can be. Suppose you decide that a certain 2.2 liter engine would make a great Mini Stock powerplant. You decide that 300 HP is a nice number, and 5200 RPM produces an acceptable mean piston speed

    The required VE for that engine will be:

    Required VE = (9411 x 300 x .45 ) / (134 x 5200 ) = 1.82 (182 %) no way..no how...

    Clearly that's not going to happen with a normally aspirated engine. Here's another example. Suppose you want 300 HP from a 540 cubic inch engine at 2700 RPM, and assume a BSFC of 0.45. Plugging the known values into the equation =

    Required VE = (9411 x 300 x .45 ) / (540 x 2700) = 0.87 (87 %)

    hope this clears things up a little
  19. Oct 8, 2009 #18
    How do you determine the fuel flow required for a given amount of horsepower?
  20. Oct 8, 2009 #19
    if you have a fuel injected engine then there is a very good chance you have a fuel pressure regulator. it keeps about 50 psi of fuel in the rail and returns the excess. if you tap into the return line and at full engine load+ WOT the fuel return stops then you need a more powerful fuel pump.

    My XJ-S gives me 4 miles/gal @ 130 mph. so i will be using 32 gals/hr. with this in mind you should shoot for a fuel system that can produce a minimum of 50gph@ 50psi if you want 400hp. from there you can extrapolate that 800hp will require somewhere in the neighbourhood of 100gph@ 50psi.

    One quick note. my XJ-s does NOT have 400 bhp. it is actually closer to 320.
  21. Oct 8, 2009 #20
    Yeah but I got the impression from Ranger Mike's post that when designing an engine, you first determine how much power you want to make, and work from the required fuel flow to make that much power. Is there a specific relationship between power and fuel flow that can be expressed as an equation?
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