Pelican Parts Forums - Camshaft Selection -- First Draft

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HMM I guess my post after the first set of (as you say wrong) calcs never made it to the server. These look much better. I was working through them and got a soln similar to what you have here.

This is great information.

When changing to a longer rod I like to reduce the overall duration and widen the lobe centers. Usually a 2 degree wider change in lobe centers and up to 5 degrees less duration.
The problem with available 911 camshafts is there big jump from one grind to the next. Unless a custom ground set is made the only change you can easily make is the lobe centers.

Quote:

Originally Posted by jluetjen (Post 1533711)

Selecting the Appropriate Cam for Your Engine.

o For Autocross and DE use on an engine with carbs, MFI or individual throttle bodies, target an overlap of 60 degrees or less.
o For Full Race use on an engine with carbs, MFI or individual throttle bodies, you’ll want to target overlaps ranging from 40 degrees up to over 80 degrees.

•
o If you have access to some key measurements, it’s possible to take these thoughts one step further by calculating your engine’s dynamic compression ratio. You can do this by calculating the swept volume from when the intake valve closes and adding any clearance volume in the combustion chamber, and divide this by the clearance volume. Typically the dynamic compression ratio for engines on pump fuel is 7.2:1 up to 7.5:1. For racing engines this dynamic compression ratio is generally more then 9:1.

While there are still volumes more that could be learned about the “black art” camshaft selection and design, hopefully you will find these rules helpful in peeling back the some of the mystery of selecting a camshaft.

How does one calculate overlap on say a DC40 Cam profile?

Quote:

Originally Posted by joetiii (Post 4992046)

How does one calculate overlap on say a DC40 Cam profile?

Take the duration at 1mm for the intake, 266 degrees. divide by 2 and subtract the intake centerline.
266/2=133. 133-102= 31. We find the intake valve opens 31 degrees before top dead center.
Now do the exhaust side.
249/2=124.5. 124.5-102=22.5. The exhaust valve closes 22.5 degrees after top dead center.
Add the two events together and you have 53.5 degrees. The amount of time from the intake opening to the exhaust closing.

Fwiw, this is the least important parameter to me. I put the 4 most important events where I want them, and let the overlap fall where it may. 4 most important being intake closing, exhaust closing, exhaust opening, and intake opening, in that order.

Quote:

Originally Posted by camgrinder (Post 4992110)

Fwiw, this is the least important parameter to me. I put the 4 most important events where I want them, and let the overlap fall where it may. 4 most important being intake closing, exhaust closing, exhaust opening, and intake opening, in that order.

While I agree the order of prioritys I think it is also important to keep in mind what the end user wants. The wider LSA may be more ideal to some that are looking for the broadest posible torque vs the narrow LSA that will be more peaky and have better peak tq & hp.

Great thread!!!