Flathead cam timing marks??

I believe it was Iskedarian who referred to overlap as the fifth cycle. As others said it allows for scavenging at higher RPMs. This is why a hot cam idles "rough". It also allows using higher compression because a lot of overlap will reduce compression pressure to a degree. That last statement may be more prevalent in overhead than flathead.
 
Frank Miller said:
It also allows using higher compression because a lot of overlap will reduce compression pressure to a degree. That last statement may be more prevalent in overhead than flathead.
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Yes, overlap will bleed off compression at lower speeds . . . the true 'dynamic compression' will be influenced by engine speed, valve timing, intake flow, etc..

The engine doesn't care WHERE the valves are (for this conversation) - just how they actuate - so the concept is the same as far as flathead, overhead, f-head, etc..
 
I try to use early gears on all my builds. I have also found cam back lash to be as hi as .022 in one case. Rarely use a timing cover gasket, just some Hi tac. You can do most anything to these engines, sometimes it works, sometimes it don't. But if you don't try you don't learn anything.
 
Frank Miller said:
I believe it was Iskedarian who referred to overlap as the fifth cycle. As others said it allows for scavenging at higher RPMs. This is why a hot cam idles "rough". It also allows using higher compression because a lot of overlap will reduce compression pressure to a degree. That last statement may be more prevalent in overhead than flathead.
Click to expand...

I thought I read somewhere that a full race cam runs "rough" at low idle because this overlap allows a tiny bit of the exhaust into the fuel charge giving it a not so "clean" dose of fuel at low revs.

As RPM's increase, this turns into the scavenging effect that also produces more power over a stock cam designed to operate over a wider range of revs.

I can be completely off base on this, but it stuck with me along the way.

Any truth to this?
 
You're not off base. Racing cam design has and likely always will be a trade off though. Most high performance set ups are going for the highest possible performance in the range that they will be operated in. In low rpm ranges, the performance level is not spectacular but it will get it to the starting line OK.
 
There is a point in time when the intake and exhaust valves are open at the same time, In racing cams this is quite noticeable, however even on stock cams this can be seen when rolling over the engine, and watching the valves. as the piston approaches TDN on the exhaust cycle, just before the piston reaches TDC you'll see the intake open. Both valves will be ope approximately the same height as the piston goes over the top. This is the "Split overlap" I speak" of. On dual pattern cams the valves may not be open at he same amt when the piston goes over.
Now, when installing a cam, this is a quick check.
 
People like the lopey idle but it is actually a poorly running car. Today's manufacturers use variable cams to perform well at low and high rpm.
 
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