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Camshaft base circle info

  1. Oct 10, 2015 #1
    Hi can anyone help me with info on the cause & effects of using a large 4" vs small 2" base circle camshaft. I want to know if I can use less rocker ratio and what software is used to look at lobe profiles- example a .5 inch lift lobe with a 2 inch base circle and a 1.75 rocker ratio (.875 lift) vs a .875 lobe lift and a 1 to 1 rocker using a 4 inch base circle. can these software programs show area under the curve? etc.
  2. jcsd
  3. Oct 11, 2015 #2
    I can help with part of your question, software is outside my purview.
    In the racing world the base circle of the cam and the lobe configuration is a closely guarded secret. This is due to the fact that valve opening and closing rates dramatically affect performance. The cams need to accelerate the valves off the seats as quickly as possible to move air. They also need to decelerate them and set them somewhat softly to prevent breaking the heads.

    The larger the base circle of the cam the more control and the more the variability one has. Also the more gentle the acceleration/deceleration rates. This directly affects the rpm limit and thus the peak horsepower. However, the bigger the base circle the bigger the block or heads ( depending on overhead cam or pushrod). Every builder has their own idea on how the best balance is achieved. The builders won't talk and the racers with their own shops wont say either. All I can say is that the current dominant motors like the LS7 have all gone to bigger base circle.
  4. Oct 12, 2015 #3

    Ranger Mike

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    scott, are you talking about old school V8 or newer over head cam design?
    Note 1 - all this is about a typical V engine with camshaft located inside the engine block ( not over head cam design).

    Advertised lift and duration are not the same as effective lift and duration. There is no real standard measurement although measurement of the lift at 0.050” lift is pretty much the established standard in hot rodding world.
    Base circle (BC) – not used too much and even less understood. The tappet spends more time on the cams BC than it does lifting the valve. Nose Height or maximum lift is understood, not so with BC. The BC is located 180° from the nose of the camshaft. The lobe lift is calculated as difference between the nose height and BC. Here is the main problem when dealing with camshaft specifications. Manufacturers use differing methods to measure things. Camshaft numbers are given in crankshaft degrees. Camshaft rotates at 1/2 the crankshaft speed. Lets say cam grinder A has a 300 ° cam. This really means the camshaft 300° rating is really 150° . Since the BC on our 300° is really only 210 ° and is should be noted that the BC is measured as a distance (height) from camshaft centerline.
    A typical V8 may have .750” distance from camshaft centerline to BC. add in a lobe lift of .333” and you have nose height of 1.083”.

    Here is the fun part. The nose of the cam must fit thru the camshaft bearings. Because of this we have a physical limitation on the biggest bump stick we can use in an engine. Cam grinders got past this limitation by using a smaller BC. This sounds bad but consider a max effort full race drag cam with .750” lift requires under cutting the BC by only .160”.
    What's left out of this discussion is the Ramps. All camshafts have ramps. This is a whole in depth subject in itself. How aggressive and wild do you want to get to open the valve? Ramps add considerably to the duration of the camshaft. Could run 20 to 30 ° per side of the lobe. AND... you may have symmetrical ramps ( same on bit sides ) or asymmetrical ramps ( fast opening slow to close). Add to the mix is lobe separation angle and that on characteristic can impact on the bump sticks functioning or flat out winning a race.

    Rocker arm ratio is another subject and im out of beer so enuff for now
    Last edited: Oct 12, 2015
  5. Oct 12, 2015 #4
    Thanks for looking at this. My question pertains to two different apps. 1, I am building a billet head with overhead camshafts. I want to look at using larger dia. camshafts so I can have lobes ground with more lift. 2, I am designing a 90 degree v twin engine based on a 900 c.i. Pro mod. I have the space to put in a very large(4-5 inch dia.) camshaft. I want a basic understanding of lobe design so I can decide how large to make the new camshafts. The camshaft people do not want to provide any info about software etc- thanks- scott
  6. Oct 12, 2015 #5


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    I realize that you have probably heard this many many times already; but, due all of the issues of balancing valve train inertia vs acceleration vs. maximum valve open duration during the intake and exhaust stroke vs maximum engine rpm before valve float; you are by far best served by reviewing all of the established high performance valve packages and designing around the one closest to the characteristics you need for your engine. Even then, there is still a chance that at some point you will want to change your cam selection; and, as a result, you will be best to start with a cam with the predominant bearing diameter available.
  7. Oct 12, 2015 #6
    Hi JBA, I guess I am not conveying my request correctly. I want to talk with someone who has some knowledge about this and what software is used to look at these issues. Here is an example- A 1 inch diameter direct follower lobe with 220 degrees @ .020 lift- total lift .400 and a 2 inch dia. direct follower lobe with the same # can have different area under the curve. Maybe I am not using the correct words- but correctly designed the 2 inch cam will have the valve open longer at .4 lift then the smaller cam. Who has knowledge/ software to look at this stuff? I think the correct term is "area under the curve' Again I am not sure of the correct term - thanks to all for your thoughts and comments- scott
  8. Oct 12, 2015 #7
    The numbers you are looking at and the software are almost all proprietary. I will look in my library of stuff as the functions that you are looking at are very valid concerns for race tuning. Many of them are addressed in some of the texts of internal combustion but once you move into the ethers of tuning people play there cards very close. As Ranger Mike previously stated there is not even agreement in the industry as to how to report. By general consensus most cam grinders report the duration at 0.050" lift. This works "kindof" as the intake airflow is only starting to be consistent at this point. Exhaust flow in like fashion is established at this point.
    It is possible to make an educated guess as to the ramp angles and the true lift profiles if you look at the look at the degrees of duration at 0.050" as compared to the degrees that the grinder lists. This is usually measured at a significantly smaller lift and hard to pin down. The lower the lift the higher the bragging rights for duration. Realistically claiming a measurement at 0.005" lift when the valve lash is 0.008" is bogus and most builders call the grinders on it.
    Each of these measurements you are looking for does actually have an affect on the engines performance, however it must be considered as a whole. I do not know of many actual machine shop types on this forum however there are several engine builders or designers. If you have access to a performance race track I think you will find better input from visiting the track. Drag race especially there are loads of "custom" grinders who all provide specific data for their product. If you hang a little with the people that are not quite pro you can gain lots. The serious pros work in house and are very closed mouth. The semi pro are often into having fun and the competition these are the ones who buy custom stuff and are willing to talk about why they like it.
  9. Oct 12, 2015 #8
    Separate from my previous post but here is my two cents.
    1) If you are looking for longevity I like cams that come off the seat easy and land easy. The slower acceleration at the beginning in my experience reduces wear and closing under control I see less damage to the seats.
    2) For a short use engine, get the intakes open as quickly as possible to move air and the exhaust open as quick as possible to create a more solid pressure wave. The healthier the pressure wave the stronger the reflected wave which increases scavenging. This only applies to short life engines as the mechanism can beat itself to death pretty quick.
    3) For a Turbo engine reducing the area under the curve during the period of overlap increases the exhaust pulse ( average pressure) but reduces the cylinder cooling. for short periods of high boost with longer terms of mid range boost low overlap is beneficial.
    4) Directly affecting your original question. Depending on the throttle response requirements the larger the cam base circle the easier and more forgiving the design. However, for the truly high end driver operating at the limit of their equipment a larger cam base circle does have a disproportionate affect on rotating mass. The response to throttle will generate more complaints for being "laggy."

    In the end I tend to go with " it depends" on your question more than widely variable answers.
  10. Oct 14, 2015 #9
    Gentlemen; Thank you for looking at my post. I guess I am not correctly asking my question. I think what I am really after is to compare 2 same size lobes made on different base circles. Lets assume both lobes are 220 degrees duration @ .050 lift with a total lobe lift of .5 inch. Lets then say 1 lobe is made on a 1 inch dia. camshaft ( a 1 inch base circle?) and the other lobe is made on a 2 inch dia. camshaft ( a 2 inch base circle?) I want to know what the average lift of each lobe would be.( is that a correct question?) I also want to know what type of software is used to make eccentric lobes? Regardless of spring rates, acceleration ramps etc I want to know how much more area would on the lobe with the bigger base circle. I am thinking this will translate into something I think is called "area under the curve".( is this a correct term?) I am not interested in anything else. I do not want to talk about ex. pulses. or turbo engines or throttle response or rotating mass or the inertia factor of a 4 inch dia. 50 lb camshaft vs the inertia factor of a 1 icnh dia. 10 lb camshaft. If the terms area under the curve or average lift are not correct terms can some one tell me what a better term is? thanks- scott
  11. Oct 14, 2015 #10


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  12. Oct 14, 2015 #11
    Scott, Thanks for the clarification. I see what you are getting at and the answer is now " no difference." By Defining the curve specifically as you did the area under the curve ( good term for point of interest) is exactly the same as another curve of identical dimensions. What varies is only the approach and deproach angles and the accelerations involved. In reality if a cam grinder were to be told a limit for valve average opening They could make it flow exactly the same.

    That being said with the same area under the curve the 2" base circle will have a higher red line and marginally better top end acceleration. This is due to the lower angles and the gentler accelerations delaying valve float and providing a marginally better timing for valve to fully seat. If you are only concerned with area use the simplest and cheapest to machine. It is in the tuning and fine adjustment outside of area that a larger circle starts to have benefit.
  13. Oct 14, 2015 #12
    I think you are wrong- if you were right all of the race engine builders would not be going to larger camshafts. I want to see the what the actual changes are and what maximum realistic diameters are. The dia. of the roller or flat tappet also has an effect Again, I am not interested in valve float or or or or I only want to see the # and know what software is used so far no one is willing to give me that info thank u scott
  14. Oct 14, 2015 #13
    Scott, thanks for your consideration of what I have shared. I think that you are missing the point brought up. You are restricting the input to only one parameter and insisting if that parameter is the same what is different. I think that if you reviewed some of my posts and some of the others you would realize that I am involved in racing and I do build as well. The performance of an engine is a very complex relationship. Any parameter changed will have both negative and positive effects in several other areas. By restricting your search to only one parameter and one relationship there is an inordinate limit on the serviceability I think that an open evaluation of the comments made will show that several advantages exist of a larger base circle and many of us are going there.

    Once upon a time every one built blown nitro rails and ran the timing at around 33o btdc because everyone knew that is where an engine belongs. When Don Garlits said what the heck and advanced further he ended up at around 50o btdc and totally dominated. It was hard on the motors, but then again it was a blown nitro motor. It was never going to have a long life. Most of the auto racing world have learned from his lead.
  15. Oct 15, 2015 #14
    No you are missing my point
  16. Oct 15, 2015 #15
    Disinterested Observer here, but... You keep on asking "where's the software?" -- I have to ask, what makes you think such software exists? If I were in the cam grinding game I suppose I would be writing my own "software" in the form of excel spreadsheets or my own programs in {language of choice}. And it wouldn't be posted on the web for anyone to grab.

    It's just kinematics, right? In the old days they did it with paper drawings and sliderules.
  17. Oct 18, 2015 #16

    I am pretty sure I get your point. I know your looking for the area at the peak. That makes it hard for me to lead other directions. Just for your consideration let me share a video. This is Tom Nelson of Nelson racing. I am relatively sure he is not on this forum however a little bit ago he produced a video talking about One particular high performance engine. In it at about time mark 2:30 he talks about cam core circle. Brief but note the advantages he lists.

  18. Oct 19, 2015 #17
    NO I AM NOT LOOKING FOR AREA AT THE PEAK. No where did I say anything like that in any post I made. Yes I have seen Nelsons stuff. I am well aware of the benefits of a larger base circle. Please stop following me or responding to my posts- thank u- scott
  19. Oct 27, 2015 #18
    Gentlemen, I am very disappointed at the responses to my question.Only one person understood what I was asking. He quickly realized how difficult an issue this is.( he gave up) Here is an example of snappy answers to stupid questions; you want "area" at the peak? I am only a h.s. graduate. I know the "area" at the peak is a "point". No where did anything I ask talk about this. When I saw this forum last year, I was ecstatic. I thought wow! Here is a place I can ask difficult questions. Instead, I got comments like "You want the area at peak lift'. This forum has left me heart broken. scott
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