Do all aftermarket LS1 cams have smaller base circle?
05-03-2005, 06:06 PM
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Do all aftermarket LS1 cams have smaller base circle? May be a stupid question, but I have tried search and not gotten much info. Are all aftermarket cams .03" smaller base circle than stock?
Gotta cam going in, need to know what lenght pushrods to get
Gotta cam going in, need to know what lenght pushrods to get
05-03-2005, 08:27 PM
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Originally Posted by T/A rocker
May be a stupid question, but I have tried search and not gotten much info. Are all aftermarket cams .03" smaller base circle than stock?
Gotta cam going in, need to know what lenght pushrods to get
Gotta cam going in, need to know what lenght pushrods to get
05-03-2005, 10:12 PM
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Originally Posted by jrp
the info i have on the stock cam is 1.553", my custom cam lobes are 1.47 int. and 1.45 exh.
are you just running stock unmilled heads?
are you just running stock unmilled heads?
05-04-2005, 12:16 PM
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As you increase lift the basecircle must get smaller. The lobe can't get bigger or you couldn't put the cam through the bearings. Generally speaking, the nose of most lobes is .005" below the height of the cam bearing. If cam lift (not lobe lift) increases by .060", the basecircle has to get .060" smaller in diameter. That means the radius from the center of the cam to the edge of the basecircle (at any point) is .030" shorter. Therefore, your pushrod length would have to be increased by .030" to achieve the same geometry. Make your self a drawing and it will become very obvious.
The important thing to remember is that on a pushrod engine the basecircle must get smaller as lift increases. That is not true on many OHC designs. Cam designers want to keep the lobe as large as possible because it provides better lifter motion, lift dynamics,etc. If you check cams with different lifts on the intake and exhaust, you will find that the basecircle on the two lobes differs by a few thousandths. Hope this helps.
The important thing to remember is that on a pushrod engine the basecircle must get smaller as lift increases. That is not true on many OHC designs. Cam designers want to keep the lobe as large as possible because it provides better lifter motion, lift dynamics,etc. If you check cams with different lifts on the intake and exhaust, you will find that the basecircle on the two lobes differs by a few thousandths. Hope this helps.
05-04-2005, 02:35 PM
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Originally Posted by GuitsBoy
I think you mean if you increase cam lobe lift (not valve lift) by .030, then you need to decrease the base circle radius by .030. This would make the diameter .060 smaller. The added lift = the decreased radius, not the decreased diameter.
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05-05-2005, 08:06 AM
#10
Originally Posted by Mark Campbell
If cam lift (not lobe lift) increases by .060", the basecircle has to get .060" smaller in diameter.
Also, lift increase of .060 results in a .060 smaller radius (or .120 smaller diameter)
Just wanted to clarify some milseading information in your post that you may not have caught. Sorry, not trying to bust your *****. Its damn good info you gave!
05-05-2005, 09:20 AM
#11
Here is an actual look at a 2002 F-body intake lobe versus a Comp XER 230 lobe. Both full lift at the lobe measurements in relation to the cam journals will be approx the same as Mark pointed out. Each lobe is at its max height in relation to the cam bearing journal to help keep the base circle as large as possible. The stock intake lobe has .281" of lift at the cam. The Comp XER 230 has .354" lift at the cam. The difference is .073" which is approx the extra length of pushrod need to have the same preload as long as the heads used are the same, not milled and have the same gasket. You can look at it as a radius change more so a diameter change if this helps. Most folks would use a 7.450" pushrod and have slightly less pre-load, but well within specs.
Basicaly, the lift is gained by making the base circle smaller, not the actual height of the lobe in relation to the cam journal. Look at a stock cam and how close the top of the lobes are in relation to the cam journals.
Now from that equation above and figure in heads milled .025" and .040" thick head gaskets you now have an approx pushrod requirement of 7.434", so maybe a 7.40 or 7.425" puushrod would be best. In the end, the preload determines the pushrod length. But knowing the cam lobe lift numbers as well as milling and gasket thickness will get you in the ball park.
Hope this helps.
Mike Norris
Basicaly, the lift is gained by making the base circle smaller, not the actual height of the lobe in relation to the cam journal. Look at a stock cam and how close the top of the lobes are in relation to the cam journals.
Now from that equation above and figure in heads milled .025" and .040" thick head gaskets you now have an approx pushrod requirement of 7.434", so maybe a 7.40 or 7.425" puushrod would be best. In the end, the preload determines the pushrod length. But knowing the cam lobe lift numbers as well as milling and gasket thickness will get you in the ball park.
Hope this helps.
Mike Norris
05-05-2005, 09:56 AM
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Good explanation Mike. Yesterday was one of those days and my explanation ability left something to be desired. If I was talking " I would have explained that my tongue was in front of my eye tooth and I couldn't see what I was saying!!" Apologies to Guits Boy! I looked at what I was writing and kept reading it wrong. Thanks for sticking with me and forgive the tone of my reply--One of those days! Sorry!
Mark Campbell
Mark Campbell
