Cam Overlap effect on static compression

RET_COP · #1 (**permalink**) 08-25-2009, 05:53 PM

I have a cam with 70 degrees overlap installed at 110.5. Would you consider 70 overlap to be significant and likely have an effect on the overall compression ratio. I have a static compression of 10.5:1. I'm hoping that it would be reduced a little.
Duration@.050 is IN 226, EX 236---.600/.600 lift---351w

\Also while were on cams would it be worth shaft mounted rockers if I were to ever change cams. Exactly how is the lash done for this. I never had shaft mounted.
Thanks, Lou

Rick Parker · #2 (**permalink**) 08-25-2009, 06:24 PM

Lou: Shaft mounterd rockers are nice, BUT really only provide a benefit in valve train stability (why they were designed in the first place) if you are consistantly RPMing the engine hard. A very good alternative is to use ARP 7/16 rocker studs and their design of locks. http://http://www.arp-bolts.com/Catalog/Catalog.html

If you are using aluminum heads and 91-93 Octane fuel you should be ok on your compression, although it is not a real radical cam. There are some head gaskets that are a little thicker than others and can reduce or raise the compression a small amount.

philminotti · #3 (**permalink**) 08-25-2009, 06:48 PM

Lou-

I'm by no means an expert, but static compression ratio is, by definition, static. Valve events have no bearing on the calculation. Which is why the SCR is really not terribly useful when picking a cam.

You need to calculate the dynamic compression ratio. I like this calculator: http://www.kb-silvolite.com/calc.php?action=comp

As you can see, it's really the intake valve closing point, or the number of degrees past BDC that the intake valve closes, that most effects the DCR. So yes, a more "radical" cam can have the effect of bringing a high SCR into a more manageable range, at least as far as octane requirements go. Naturally, there are other factors such as emissions, fuel consumption and driveability that are also affected.

Hope this helps.

phil

RET_COP · #4 (**permalink**) 08-25-2009, 07:14 PM

I realized that the thread title should have addressed dynamic compression, sorry for that misprint.
Thanks for your responses, Ill look at my intake closing point if I can find the card. I am running iron heads also.
The shaft mounted looked nice and I wanted something different, but I'll look at the cost factor too.
regards,Lou