cam timing

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Don
Posts: 46
Joined: March 24th, 2007, 3:40 pm

cam timing

Post by Don »

Just wondering........why do we retard the cam timing 2-4 degrees? Also read somewhere for vintage Vees use stock cam timing, why the difference?

thanks, don
brian
Posts: 1348
Joined: June 26th, 2006, 12:31 pm

Re: cam timing

Post by brian »

in the begining, we couldn't use off set keys and relied on sloppy gears to retard the cam. Now we use offset keys or adjustable gears. It might be easier to understand if we think of advancing the crank versus retarding the cam. By retarding the cam we start the intake event sooner in terms of crank angle to make up for engine speed. Much like timing, we lead the events relative to the crank angle to time the charge for efficiency at higher rpm. Advancing the ignition timing is the same principle, to allow time for the explosion to reach maximum effect at the optimum crank angle. Hope this helps.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
FV80
Site Admin
Posts: 1195
Joined: June 27th, 2006, 9:07 am

Re: cam timing

Post by FV80 »

Don,
You also asked about the diff between a "regular vee" and vintage - the vintage guys (mostly) still run a fan shroud and their Max RPM is generally MUCH lower than ours - more like the max of an original beetle - therefore standard (or closer to standard) would work better for them. Retarding the cam to get more power at 6000+, won't do any good for an engine that never goes past 5500.
Steve
The Racer's Wedge and now a Vortech, FV80
SR Racing
Posts: 1205
Joined: June 24th, 2006, 1:58 pm

Re: cam timing

Post by SR Racing »

I THINK Brians explanation is correct, but I really didn't understand it. :lol:

Let me give it a try. At high RPM the engine is drawing in lots of air. This air (especially in our long intake runners) gets "stretched" like a rubber band. With a retarded cam we delay the closing of the intake valve slightly. This allows that "streched" air (that has mass) to still get into the head. This is actually called "supercharging effect". This is why some race engines can achieve MORE than 100% efficiency at certain RPM. There are of course no free lucnches. We sacrifice some low end torque in doing this, but it is in an RPM range where we never operate so we don't care.

Jim
Don
Posts: 46
Joined: March 24th, 2007, 3:40 pm

Re: cam timing

Post by Don »

Thanks very much. Always nice to know what is going on behind my head.

cheers, don
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