Why aren't the base circle on aftermarket cams ground to stock specs?
Why smaller, and 7.4 p-rods?.
Why not stock and 7.38's?.
Hmmmm

 

Highlift cams have a smaller base circle so they will work in conjuction with the lifters recommended operating range. Also if you used the same OEM basecircle diameter with higher lift. The cam would not fit in motor or be difficult to install (lobe hitting cam bearing, lifters what ever).

 

That makes sense but why then are the .550< lift cams also on smaller base circles. Is it just because it is cheaper since the "blanks" are only produced in smaller sizes?

Why then not use Stock "blanks" and grind those?
The reason behind my thinking is that this would eliminate the preload guessing on a lot of smaller baby cams and provide better valvetrain tolerances.

BTW what would be the max lift on stock base circle cams.??

 

The higher lift OEM LS6 cam has a smaller base circle than the lower lift OEM LS1 for the same reasons I stated above. Have you ever measured the base circle of a .500 lift aftermarket cam? I have not. But as long as the preload is within safe limits with stock valvetrain it is OK by them. Some engine builder say you make more power with less preload. So maybe the aftermarket thinks less preload is better.

I do not really know the answers.

 

Ok so now you guys have me confused I thought the stock length p-rod was 7.4? Is this not correct? If there is guessing in how much preload that you need how do you get it right

 

7.4 is the correct LS1 length. Just make sure your lifter compresses between .045 - .090 when torgued down. Fully torqued down 3/4 to 1 1/2 turns past the point when the pushrod cannot be spun easily between fingers. You may have to use a different length pushrods when using an aftermarket cam to get your desired preload. The stock 1 1/2 turns or .090 preload will result in a quieter valvetrain than the 3/4 turns or .045 preload.

 

.045 is going to gain more power correct......But will be louder is this also correct? How safe would it be to run on .045. I race quite frequently...(meaning I beat on the car pretty hard)

 

You are correct and it is safe to run .045 according to several people.

 

I think that is the way to go then.....Thanks for your help.....You would happen to have or know anyone with a set of stock lifters would you. I am trying to find some but not many out there.

 

No. Why do you need a set?

 

Cause I over reved my car ( you just posted in My thread and said could be bent pushrod.) I just wanna make sure that it is not a lifter so I was going to get a set. If it is a pushrod where would the sound come from....(top, middle, Bottom) Mine sounds like it is coming from the bottom more then anything.

 

FYI you cant take out the lifters without taking the heads off...

Its probably abent pushrod(or more), unless its real loud or something, then it could be lifter preload, but probably not the lifter itself.

Is it just a light tapping or a loud clatter? Is it when the engine is hot or cold?

 

I have a little problem with this
You must be talking about a adjustable rocker
I would like to hear how this is done on a non adjustable rocker
stock, Harlan sharp, jesel ss ?
Thanks
John

 

22lbs torquing just as OEM rockers.

 

Thats what I thought .
The only way to tell 3/4 turn or 1 1/2 turns would be with a adjustable rocker
as comp pro magnums
The only way to properly set pushrod length,would be to use a solid lifter and a adjustable pushrod to set the pushrod length.
Or just put in a 7.4 for stock heads or a shorter one if heads are milled.
With the non adjustable is this all correct.
I am setting up my jesels ss's and would like someone with experience with push rod length to chime in.

Thanks

 

If I may take a stab at this:

Remember the entire cam has to fit through the bearing holes. The lift of a cam is determined by the difference from the base circle to the apex between the leading and trailing nose. If you want really high lift, you need a smaller base circle so the finished lobe height remains smaller than the cam bearing dia.

There's some great cam articles in here (somwhere) that will help visualize the physical properties of the cam.

The problem with high lift, small base circle cams is the increased side loading of the lifters, non roller lifters that used to be standard fare in old V8s were limited on ramp steepness because of the flat tappet lifter relationship with the cam lobe. Roller lifters allow for a much steeper ramp...remember, the max lift of a cam is really a non issue, the cam spends only an instant in time at max lift. Most of the flow (and most of the perf gains) occur in the first 20 degrees or so of cam/valve opening. the steeper the ramp, the faster the valve opens. The effect is more flow quicker. Bigger valves help this too, they allow more air to folow instantly and more are to flow at maximum flow.

I know this is sorta jumbled, but hopefully it will make some sense.

 

Here's som emore valve train info. This is from an email I got from my good friend. He's a good friend of Nick Arias and used to be the owner of Howard's cams and many other race shops. He still holds one class land speed record and plays with motors in his spare time away from his aero parts machine shop. He's the guy that is porting and massaging a set of 5.3 heads for my LS1.

The text below was in response to me proposing a Comp Cams 224/224 cam.


"Drivability. As you add cam timing, low end torque suffers. It is a question
of how much the torque is moved up in the RPM range. The newer (high peak
output) engines rely on high idle vacuum for good low RPM throttle response.
The timing numbers you show are not "radical" in terms of current cam timing
but are no doubt at least 6-10 degrees over stock. It is this addition in
overlap timing that causes the idle and low RPM vacuum losses due to pumping
losses because both valves are open during the beginning of the intake
cycle. In a racing engine, this allows us to add to the compression ratio.
You can't add the mechanical compression ratio without a bunch of effort.
Although it might be worth taking the heads off and cutting about .030 while
the manifold is off. The ideal situation would be to add lift only. This
allows for more "area under the curve" which gets more air/fuel in/out. The
ports flow more CFM at higher valve lift which is why added lift works so
well. Until the recent use of hydraulic roller lifters, lift and duration
were somewhat interdependent. The diameter of the lifter is fixed. As lift
was added the tangency of the lobe to lifter bottom moved from center to off
the edge. That destroyed cams and lifters. So timing was added to make the
event take longer. There is actually a maximum lifter velocity per degrees
of rotation on a flat bottom lifter. This was circumvented by using roller
lifters. While they solve the maxV problem they incur a huge weight penalty.
We use enormous springs on the valves and sometimes helper springs on top of
the lifter itself. A drag engine often sees spring pressures of 250-400 lbs
at seat and 700-1000 lbs at open. So along comes the hydraulic roller
bottom lifter. Again it solves the maxV problem but it is still a hydraulic
lifter and will tend to pump up and is also very heavy compared to a flat
bottom. Very high lifts with high valve spring pressures will also collapse
a hydraulic lifter. Recall all hot rod engines had solid lifters too get
around that problem. So you see it is a GIANT compromise. Keeping everything
light (Ti retainers etc.) is key to life span of the valvetrain. Matching
valve springs are a must. I witnessed an engine on a dyno, pickup 45 hp from
a valve spring change. Clearly the originals were dead but 45hp?"

 

Nice response from your friend. makes real sense and brings to light why some lifters keep collapsing.
It also kinda explain why my Stealth II (small dur. high lift, fast ramp is making a healthy amount of power, rivaling some big cams)