Dynamic Compression Ratio (DCR)

This is out of my league, but I will take a stab at a few of those questions. The RWHP, generally, is a better measurement for dictating what's safe and what's not for a stock motor, pending safe tune. You could make that statement a lie by simply saying "OK, lets put a cam in and rev a bone-stock motor without FI to 12,000 RPM's to make a "safe 450 rwhp"; boom. That being said, if you have 10 psi with a blower, then put a big cam in, the higher lift and duration will let the engine flow more which reduces boost as that is only a measure of restriction anyway. Same thing happens with long tube headers and high flow cylinder heads. THey flow more, they reduce boost numbers. As far as "is it safe to increase boost again after that", if you were at 500rwhp with just 10psi pre-mod and then put that big cam in, it may lower the boost, but your rwhp is going to go up if it's a decent cam and been tuned accordingly. You have to figure that you're moving more air, which needs fuel, which creates a stronger combustion which will accelerate the piston/rod/crank and at a certain amount of energy is going to let go (stock) whether you are at 5psi or 10psi. I think (guessing) what you are trying to get at is - is all horsepower equal? Can you get away with more horsepower if the boost is lower via higher flowing components. And MOST definitely, there are a lot more educated answers to be had. There are a few guys here with seriously built engines. Hopefully they will give their input.

Throck said:

I've been studying the dark art of cam design and found some information & calculators. These only raised more questions. So I decided to start a thread (the search didn't find a previous thread) dealing with DCR and cam timing events so inquisitive people like me might have our questions answered by the more knowledgable members of this forum.

The sites I found are:
Static Vs Dynamic Compression Ratio | Piston Ratio

http://victorylibrary.com/tech/cam-c.htm

Wallace Racing: Dynamic Compression Ratio Calculator
(stock 3v specs can be found at http://www.suttonhp.com/file.asp?F=...E5CDF.pdf&N=4.6 3v engine specs.pdf&C=q_and_a).

I know VCT engines (like ours) are a little different, but that just means we need to add the IVC to the cam retard at max/min to find extremes.

My questions are:

1. For a stock engine with boost, keeping the dynamic boost compression ratio under a certain # should be more accurate than staying under a certain RWHP (if pre-mod RWHP is known to be safe)?

2. If a cam drops this number (which would likely reduce parasitic drag from the blower and compression), is it safe to turn the boost back up to where the dynamic boost compression ratio was for a stock cam?

3. How much will porting heads change these calculations? (notice head flow #s are not included in the variables).

4. I may be asking the wrong questions...what numbers are actually important?

5. Does cylinder pressure relate to conrod stresses (the weakest link in our engines).

Thanks again for any information and hopefully this thread will help everyone become better informed.

Click to expand...

1) Changing the dynamic compression will not likely have much of an impact at all on RWHP safe limits. It would likely lower the possibility of predetonation, but the internals aren't going to get stronger.

2) Most of the guys who use cams to modify the dynamic compression ratio are leaving the intake open below BDC and into the compression stroke slightly. What this does is reduce the volume of air in the cylinder, so there is not as high of pressure at TDC, thus effectively reducing the compression ratio which helps in lowering the potential of pre det. In the case of a boosted motor, it would increase the back pressure, raising the boost to a level higher than what it was originally.

3) Head porting won't effect the dynamic compression ratio. It's all in the cam profile and timing.

4) In relation to dynamic compression ratio's: Unless you can find someone to build a set of custom cams, and have a tuner willing to play around with it long enough, it may not be worth the trouble to play with it.

5) There are several critical cases for rods.
a) The power stroke where the rod is under a lot of compression.
b) inertial forces where the rod is in tension (usually during the
intake stroke). Boost actually lowers this stress a little, and
changing the dynamic compression ratio has no effect on it.
c) Predetonation .... huge loads on rods. This is where changing
the dynamic CR would come in to play.