There are turboed cars that came 7.8:1 stock, with very modest boost. No its not a blower, but I figured I'd throw that in.

For a street car for what I know though I'd go for more compression myself, to have a little more low end, since I wouldn't be able to run alot of boost (pump gas).

 

I think just the opposite.... I'd go with less compression and more boost to a point. If dropping static comp from 9.0:1 to 8.5:1 allowed 2 psi more boost it's be a good trade in my opinion as the power loss due to the lower compression would be more than recovered with the extra boost. My low limit would be 8.5:1 with a street engine though, from there I'd look at intercooling, water injection to keep the engine out of the detonation zone. JMO.

-Mindgame

 

I am not sure where the idea that a CR in the 8.5:1 range makes a car a "slug" off boost. I am at 8.5:1 and it is not a slug. Just off idle, it feels a little weaker than way back when it was stock. But by ~2,000rpm it's "Katie bar the door"! Maybe this idea has it's origin in NA cars in which people have installed big cams?

Anyway, the evidence is clear that more boost/less compression is a good tradoff. Of course, the blower has to be big enough to make more boost for this theory to work. It has to do with the volume of the combustion space. Under higher boost, a larger combustion space contains a greater mass of fuel/air. This more than compensates for the less efficient combustion from lower CR.

Rich Krause

 

You didn't hear that from me Rich.
The centrifugal type blowers are a bit more efficient heat-wise than the roots type blowers I'm really familiar with. To run really good amounts of boost with those we had to run lower compression ratios. 8.0 and lower was not uncommon for an 8-71 or similiarly equipped engine making really good power on slightly-better-than-today pump gas. Things have changed a bit over the years.... we now have the lysholm screw compressors and the centrifugal types are much better than they were years ago.
Hard to say but I'd venture out in saying that DCR, intercooler efficiency, a/f mixture and spark intensity play as big a part in the matter as anything else. How else would some of these engines be running 8.0:1 with 24+ lbs of boost. Alot of that comes down to tuning I'm sure.

-Mindgame

 

I’m on to one of those that fits in the ‘too low a compression will kill your low end’ category… the fact is that you do loose a little of the crispness to the throttle response and the exhaust note, but I’m not sure that I see that as a problem. There have been factory turbocharged 4’s in with cr’s in the high 7 range that were plenty fun to drive, and v8’s in the low 8’s that were fine also…

The thing is that this doesn’t really answer what I was thinking about. In a blown application you want to run a ‘safe’ compression ratio for the amount of boost that you’re planning on running, but you don’t want to run substantially under what you can get away with, since at some point this will hurt responsiveness, idle vacuum, fuel mileage and ultimate power production if you can’t make enough boost to push it close to the limit.

Seems like DCR is a step in the right direction wrt finding this for NA applications, but where do boosted applications fit in? For that matter, if I decide to build a new 350 for my truck, I’d like some idea of what kind of DCR #’s I’d want for running 87octane (sucks when you’ve got to fill up a 36gallon tank with 92…, but being a performance freak (geek?) I wouldn’t want to be leaving any substantial power and in this case gas mileage on the table)

I guess I may start a new thread on this in tonight or tomorrow, maybe get more people that read here in on the discussion.