Peak Torque vs Boost?

  1. Herro guys, I'm trying to build a motor for shop class (from scratch really, new manifolds, ect :eek:), I'm only 15 so please take it easy on me :redface:

    Anyways, I've been trying to calculate my ideal runner length/diameter and stufflets for the intake manifold design. I've got a question about that, however. The motor is a 2.0 liter Camry motor (lol), twin T3/T4 turbos (I'm adamant about that). There will be obviously a rather large amount of turbo lag (which I like), but to calculate intake manifold diameters, you need to know where you'd like your "peak torque". Ideally, I'd like my peak torque about where I peak out my boost (~25psi, all out on these two smaller turbos), but I've noticed in some cases peak torque is reached later after peak boost... any idea why?

    I assumed that the T3/T4 may lag up until ~5k RPM and possibly hit full boost in 5.5-7k RPM... (maybe? :redface:), and because at max boost generally where I'd get most volumetric efficiency... that's where I'd get peak torque? (assuming the rest of my engine is build for high(er) RPM powa)

    One thing just struck me though, seeing that ~25psi I'd be absolutely stringing out the T3/T4's, in THEORY if I didn't have a intercooler large enough to cool it, my actual peak torque COULD go down in the rev range because of higher intake temperatures, and thus according to the ideal gas formula modified to get lbs/min of air... I could be getting less pounds o' air at higher revs if I can't keep it cool, thus actually "downing" of peak torque o.o

    And now another question xD, assuming that all that is done ^, I need to calculate the VE to find out the diameter of my runner length (according to Vizard's formula from this one book :P), but because it IS turbocharged, VE would be over 100% (or I hope it is, lol). But how exactly can one calculate that?

    Assuming I took off a few psi for turbulence and blockage o' flow from the intercooler at max boost, would the VE increase in porportion to the amount of pressure over atmospheric pressure there is in the intake manifold? Or am I dreaming? xD
  2. jcsd
  3. nobodeh :(?
  4. Danger

    Danger 9,663
    Gold Member

    Sorry, dude, but I'm an old-time driver. There's no substitute for cubic inches (or, as newbies to the scene say, "there's no replacement for displacement".)
    There's only so much that you can squeeze out of a squirrel on a treadmill, no matter how much oxygen you feed him.
    I can't think that an engine of the type that you propose could show anything like performance below 15,000 rpm's. That is not even vaguely applicable to street driving.
    By the bye, your entire engine is slightly larger than one cylinder of my 8-cylinder Roadrunner (446 ci) or El Camino (455 ci).
    My serious advice is to turf the lawnmower engine and throw in something that is suitable for a car.
    Last edited: Oct 14, 2010
  5. Totally irrelevent in the modern world, where we demand performance from an efficient package. Large engines are going the way of the dinosaur for road use for a reason. Turbocharged engines are the way forward, it provides power when you need it, but theefficiency of a small engine when you don't need power.

    Time moves on, we do things better now, it's called progress.

    Anyone who claims they need the pwer output of a large displacement engine for road is talking bollocks.

    Just to do a bit of geekmaths.
    Depending on the year of the engine made, the output from the 7.5L V8 will vary wdely. I've seen power figures as low as 250BHP from one. There isnt any point in comparing emissions figures becuase the modern engine will win hands down on emissions per km driven (and probably enven emissions per HP produced).

    But lets be generous. 400BHP at 5000rpm.

    BMEP: Pn/VN
    (298000 * 2 )/ 0.0075 * 83)
    =957 429 Pa = 9.5 bar

    Comapre to an engine over 1/5th the size.
    Take the VW 1.4 TSI, twin charged outputting 170BHP@ about 6000rpm.

    BMEP: Pn/VN
    (126000 * 2 )/ 0.0014 * 100)
    =1 800 000 Pa = 18 bar

    So as we can see, the 1.4TSI puts out less emissions and had a BMEP almost twice that of the larger dispacement engine. We could also compare efficiency figures, but it wouldnt change the outcome. The new smaller displacement turbo is simply a better engine.

    The only reason to own a colossal V8, is becuase they sound amazingly cool.

    EDIT: I'll read and give a responce to the OP in a little bit.
  6. brewnog

    brewnog 2,775
    Science Advisor
    Gold Member

    I was going to put a technical response together on this, but it'd take ages.

    OP, get yourself a copy of Heyward's masterpiece and do some reading, you'll get more from that than any forum.

    Danger, you're a legend, but with the utmost respect you're talking cobblers! :smile:
  7. Danger

    Danger 9,663
    Gold Member

    :uhh: I think that I know what that means. If I'm correct in that assumption, I respectfully disagree. You are my hero; if it weren't for you first greeting me, and then encouraging me, and occasionally prodding me, I wouldn't even be on PF.
    Still, I must say that if I made the same sort of mods on my engine that the OP is talking about, I'd be pushing 1,200 hp. (And by the bye, I don't like turbos. I know that they are inherently more efficient, but I really want to hear those straight-cut gears in a 6-71 whining under the pull of a 3" Gilmer belt. :biggrin:)
    Last edited: Oct 14, 2010
  8. brewnog

    brewnog 2,775
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    Gold Member

    EXACTLY! You'd be pushing 1,200hp! Which shows that they they DO do the job!

    I don't like turbocharged cars either but you can't beat progress, I'd love to give one of the new Alfa Multiair engine out.
  9. I acutally really like the way turbo cars drive. They are by far the easiest to modify cheaply, have bags of torque low down whilst still putting out top end power. This is especially true for restricted turbos, an almost flat torque curve makes it the ultimate lazy mans fast car.

    I do kind of miss the revs of a well tuned NA car though.

    To be on topic: OP do as Brewnog suggests and read Heywood.
  10. Danger

    Danger 9,663
    Gold Member

    But the point is that I can lump along at about 2,500 rpm, doing the speed limit of 100 km/h. Since she idles at 875 rpm, you can see that she's just lazing along at that speed, and ingesting very little fuel (25 mpg). If I then need to romp on it, I can downshift to 2nd, matt the sucker, and be at 200 km/h within about 5 seconds. My power band is from 4,500 to 6,500 rpm, and the top number of that is my redline. That, needless to say, sucks back a tad more juice (about 5 mpg), and shaves a bit of rubber off of the tires, but that's needed for only a few seconds at a time. A rice rocket would probably have to hit over 12,000 rpm to accomplish the same thing, if it could do it at all. Imagine what mine would do if I threw on the twin turbos and injectors and other high-tech junk.
    Sorry for the cross-referencing between measurement systems. Although we are officially metric in Canada, I've never quite tuned into that x litres/100 km mileage determination.
    Last edited: Oct 14, 2010
  11. o.o

    So any tips? Dx I have no idea what a Heyward is... :( (And Google doesn't come up with much anywho :\)

    How complicated is my question anyways? o_O
  12. From 7 litres?????? That's shocking. Hell, I can do that in my Golf 1.8T. AND I get 50 mpg (from a petrol) doing a steady 60mph. 60-120mph takes about 8 seconds ish on my last run (so a bit slower). Although a turbo remap (more boost) will bring that down to about 5. 60 mph in top gear is about 2500revs.

    I just don't get how Americans can manage to extract so little power from a road going engine. They are all the same. There is no doubt they CAN make a large engine but out huge power (NASCAR get about 800bhp from a pushrod carb V8 that revs to 9k), so why don't they do it for the road? Instead you are left with an obese, asthmatic, fuel hungry mongrel.
  13. brewnog

    brewnog 2,775
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    Gold Member

  14. I just meant his cars were american built.

    OP it's not that the question is really that complicated, there are just too many to deal with in a single thread. I've reread your post and there is probably 10 decent sized threads worth of discussion in there. It may be helpful to start a discussion on a single question, until you get a satisfactory answer and then take it from there.

    What makes it complicated is that when trying to answer two questions at once, you can get what seems like a contraditory answer as everything in an engine is very variable dependent.
  15. brewnog

    brewnog 2,775
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    Gold Member

    Agree, and I really stand by reading a copy of The Book.
  16. Ranger Mike

    Ranger Mike 1,675
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    how did i miss this debate???
    was at the road course with a 2000 cc car making 150 hp getting 8 mpg!
  17. brewnog

    brewnog 2,775
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    Haha, same figures as my everyday car round Mallory Park this year!
  18. Danger

    Danger 9,663
    Gold Member

    You are technically correct, and I know from the way that you stated it that you think the same way that I do on the subject. I'm American in that I live in North, Central or South America. Unfortunately, the term has very nasty connotations since the Yanks have tried to usurp it to their exclusive use. I prefer it be known that I'm a Canuk, residing in the good part of North America.
    Chris, two points here. One is that my car wasn't "American built", other than modified by the foregoing response to Brewski. The original owner, whose widow I bought it from, worked at the Chrysler plant in Windsor Ontario and built the entire thing himself from the frame up. It was initially a Yank design, to be sure, but not built to their standards. As a for-instance, the factory rating for the motor was 285hp, but the one that he put together was pushing 400. My subsequent modifications brought it up to about 650, and it will be a hell of a lot more once it has a decent cam.
    Secondly, I might have been a bit conservative in my 100-200 km/h time. I've never measured it. I do know, from the closest that the sweep second hand on my buddy's wristwatch could determine, is that 0-100 km/h takes 3.5 seconds. I have to shift to 2nd at 105, though, so subsequent times are twitchy.
    Anyhow, Brewski... my point is that if you build two engines exactly the same way, one being 2 litres and the other 7 litres, the big one is going to win every time.
    By the bye, my shift points are 2nd @ 105, 3rd @ 138, and 4th @ 185... and 4th is a .73:1 overdrive. Top end is 257.
    Keep in mind, as well, that this is a 1,750 kg vehicle without modern aerodynamic features.
    Last edited: Oct 25, 2010
  19. Unfortunately, this isn't about who can go fastest in a stright line (which is just about the only thing a large displacement engine can do well). It's about who gets the most power from the most efficient package. This is especially true when talking about road cars, why would you need to 'win'?

    I mean really, 650bhp? For a road car? Why? If this was a car designed for modified stock drag racing, then I can understand it.

    EDIT: I'm all for the right engine for the right job. For drag racing, yes I'd take the large displacement engine (almost) every time. For the road however, the amount of power they completely waste makes them obsolete.
  20. Danger

    Danger 9,663
    Gold Member

    I basically agree on general principle, but the fact that I can idle in first gear at the in-town speed limit, whereas rice-rockets are spooling their guts out in 2nd or 3rd gear at the same speed, indicates that there is something wrong. My engine simply doesn't have to work as hard to get the job done.
    As to "why"... one of the few pleasures that I've had in my life was constantly humiliating Porsches. I have nothing against the cars or the engineering behind them, but in my country the drivers are frequently rich jerks who severely need to be squished (especially in the presence of their trophy girlfriends).
    Leaving the Roadrunner aside for the moment, I'll compare two of my other cars—and go by imperial measurements, since that's still the way that I think. My Camaro with the 305 2-barrel and incredibly shallow differential gears got about 1/2 to 2/3 of the mileage of my 4-wheel-drive El Camino with the 455 4-barrel Olds motor.
    I case you hadn't noticed, I'm not particulary "eco-friendly". I continue to eat my canned tuna at least a couple of times a week, and think that dolphins are delicious.
    Last edited: Oct 25, 2010
  21. Why idle in first when you can idle in 5th and achieve the same thing? What you've just said there is your MINIMUM power output in lowest exceeds that needed for normal driving conditions.

    And bolded is the only legitimate reason for owning a powerful engine in a road car. I'd still rather go for a european turbo motor. The twin turbo V10 from the RS6 springs to mind.

    I'd love to have the RS6, fast in a stright line, AND goes round bends.

    I would never have guesed. :wink:
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