I read many threads where people put 9:1 compression for Supercharging applications, but I haven't found any topic on dynamic compression for boost.

What is the desirable DCR for Supercharging?

 

Here is how to figure out what it would be. I got this off B&M's blower web site a number of years ago but I can't find it again. I did write the formula down.


The Math for this is [(boost)/14.7)+1] x compression ratio = FCR
FCR-[(altitude/1000) x .2] = corrected compression ratio

I hope that helps

 

IF you have the available CFM from the blower to raise the boost to the detonation threshold, the following has been proven both empirically (in the lab and on the track) and in theory. In terms of producing max hp and performance, the lower the CR the better, down to about 7-8:1 for a typical OHV gas engine. Get the CR in this range, raise the boost as high as possible without causing detonation, and that is where you want to be. The cam is chosen based factors other than DCR (as the term has come to be used here). What is normally being done with "DCR" is that it is being used as a surrogate marker for cylinder pressure in a running engine. IOW, this is a situation where DCR is not terribly important. What is important is cylinder pressure under boost. The cam timing will have some effect on this, but it is not the "governing factor" as it is with an NA combo. The blower presure is far more important. Unfortunately, there are no easy ways to calculate cylinder pressure under boost or guidelines as to the limits with various fuels. So, you have to base your choices on experience or do some experimenting yourself.

A myth about such an engine is that it will be a "dog off boost". This is nonsensical for a couple of reasons. What is true is that efficiency (i.e. fuel economy) will be poor at light loads compared to a high CR engine.

The thing to keep in mind is that for a given combo, you need to choose the CR primarily on the amount of CFM available from the blower and the intended fuel. I have tried to come up with some generalizations, but I am not confident enough in them to "publish" them, even here. Based on experience with SBC and SBF's, I can offer opinions based on that experience and will do so if asked. You need to give me the following info if you want my opinion. Blower chosen, type of engine including heads to be used, fuel choice, use of intercooling and/or water injection.

Hope this helps.

Rich

 

Thank you. The plan is to build a SC engine with an ATI procharger and 3-core intercooler at 12#. The bottom end would be forged internals on 2-bolt studded mains and possibly a main stud girdle.
-One question is to use 19cc dish or 22cc dish.
-Total Seal Top Rings are a possiblity but heat expansion from the supercharger needs to be accounted for.

The top end would be a light port on stock heads and possibly Ferrea 6000 valves with viton seals. As well as a light port and possible polish on the intake manifold. Plans are to use the stock TB but may go to a 52... I don't think it would be worth the money versus gain.

The engine has only 15K on it so I was considering using a flex hone to seat new rings versus boring out the cylinders. 350 or 381 are possibilities but I'm not sure how tough 12# is on a daily driver 381.

Fuel choice is 91 Premium pump gas from California... 10% Ethanol.

Since it is California, smog testing is a concern.

Will a Static CR of 7-8:1 affect emissions? Or is it mostly the overlap of the cam?

*Note - I mentioned 12# as my goal is 500-550 RWHP.

 

~8:1 would be fine. CR per se does not have a big effect on emissions. I believe that NOx emissions go up and HC goes down as CR increases and vice versa. But AFAIK these are not large effects.

Rich