Just as the title states, I am curious as to how compression ratio effects idle vacuum. I will go out on a limb and guess that as cr increases so does idle vacuum(the closer to 0psia). I'm sure someone will correct me if I'm mistaken.

 

that make sense to me, increased cr=more thermodynamically efficient=less air/fuel for a given hp, idling takes a certain amount of hp, so a higher cr=less air/fuel=lower manifold press for that given amount of hp required to idle..I don't know how Much of a difference it actually makes though, it takes so little hp to idle in the first place. we all know that ignition/cam timing will alter idle vacuum conditions much more..and I imagine intake manifold and exhaust design would too..

 

A buddies DSM motor went from ~12.5 inches at 10:1 static compression to ~10.5 inches at 8.5:1 static compression. I don't know the theoretical answer behind it, but I do know that in his situation less compression equaled less vacuum.

 

Hey thanks for the replies, that scenario kinda supports my theory.

It would be nice if one of the more knowledgable people here gave a good tech based answer.

 

Bump for a good technical answer......

 

bump for info

 

bump

 

No one?? If I am being retarded and this has been covered a million times let me know and I will go and search some more, however I was unable to locate any info on this before.

 

I don't see how CR would effect idle vacuum since the vacuum is before the compression...unless you changed CR by modifying the stroke, then there would be more vacuum since there is more/less chamber volume...i dunno if that makes any sense outside of my head.

I know high overlap cams will create less vacuum at idle though

free bump anyways

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ToasterOven &raquo;</TD></TR><TR><TD CLASS="quote">free bump anyways</TD></TR></TABLE>

Thanks.

 

Maybe someone will help a nub out eventually........