Dynamic Compression Ratio?

[killi6]

So... I've been talking engines with my coworker. He's fascinated with my progress on my '74 Satty... and why shouldn't he be? Anyway, he learned about dynamic compression ratio last night... told me, which I'd never heard of.

I am looking around for pistons with a taller compression height than stock for my 318 in order to boost compression and power a little without buying expensive pistons. I know, I'm cheap, but if I can get a set of 1.745 comp height 'stock' pistons for the same prices as 1.720 stock replacements (100 bucks), thereby pushing the top of the cylinder up to only 0.065 down from the deck, I figured that would be good. My initial calcs with a 0.028 gasket are roughly 9.2:1 compression over the 8.5:1 with the 1.72 pistons and a 0.040 gasket.

My question is this... well, first of all if I'm missing something or am retarded or whatever, please feel free to comment. My question is when you hear all of the rules of thumb regarding max compression ratio to avoid detonation with pump gas... are they referring to static compression ratio or dynamic? Because dynamic is lower and with the 340 cam I was looking at from Comp, it's MUCH lower.

What do you guys think? I figured I'd raise my compression height for more compression and put in the 340 cam... on top of dual exhaust out the manifolds... for my rebuild. Am I stupid or what?

Thanks in advance...

 

[Meep-Meep]

Raising compression is a good idea but must be within the restrictions of the available fuel (of course). Lots of factors contribute to detonation and compression is just one variable. Dynamic compression takes the intake valve closing point into consideration and a number in the 8.5:1 range is starting to get to the limit of 91 R+M/2. A static ratio of 9.5:1 is a good starting point for 91 gas. Also engine temp, bore diameter, chamber shape, quench, mixture strength are all factors and need to be considered.

 

[boltupal]

Also keep in mind , the taller piston is going to weight more , so your going to have to balance the reciprocating assembly.

 

[killi6]

Ah, good point boltupal... I was planning to balance it anyway, but wasn't thinking it was that vital, but duh, if i'm putting different sized pistons in then of course.

I found rules of thumb of roughly 4% power increase per 1.0 increase in compression ratio... but depending on the cam's 'closed ABDC' degree, the dynamic compression ratio can vary quite a bit.

 

[451Mopar]

Dynamic compression is just a calculation of compression ratio from the approximate closing point of the intake valve. This gives a way to estimate cranking cylinder pressure. The estimated cranking cylinder pressure can be used as a rough guide for estimation fuel requirements (cranking cylinder pressure will change depending on altitude.)
Usually if your cranking cylinder pressure is under 150 psi, you may be giving up some low end torque, but the engine should run fine on pump gas. Around 160 psi, you may need pump premium, but this is a good number for many combinations. When you get 170+ psi, the tuning and chamber / quench will be a factor on how well the engine runs on pump gas. There have been some engines with close to 200 psi cranking pressure that run on high quality pump gas, but these engines have aluminum heads with good combustion chambers, and minimum quech clearance to prevent pinging.
The basic formulas can be found here:
http://victorylibrary.com/mopar/cam-tech-c.htm

 

[Meep-Meep]

Even with only a 4% power increase for the higher compression I think the drivability benefit when everything is right is way more than that. Better vacuum, crisper throttle response, carb tuning is easier, etc..

With regards to chamber shape and quench, a hemi is pretty darn good, but my 66 Charger blows 215 PSI with what seems to be a very mild cam and pings like an SOB on pump 91 when you jump on it. Drive around like an old lady is OK but who wants to do that in a hemi 4 spd car?? I didn't build the engine so for all I know someone put 12:1 pistons in it.

As I said there are many factors to control pinging. My 440 ran with a stock iron intake and a 750 Eddy AFB and made some great power but required a splash of race gas (estimated octane to keep it happy was 95 R+M/2). On the same engine I put on a 6 BBL setup and it now will not ping on pump 91, but makes less power. Not one thing was changed other than the intake and carbs. Granted the 6 BBL has been verified to be lean, which you think would make the pinging problem worse, but the lean condition is most likely responsible for the power loss. Only thing I can figure is change in fuel distribution.

 

[451Mopar]

I have ran my 12.4:1 compression 500" stroker on pump gas, but the engine has 74cc aluminum heads, flat top pistons, 0.040" quench, and a big roller cam.
I think my dynamic compression is around 7.5, and I am running at 6,000 ft altitude. Also, the engine is not loaded hard at low RPM with a 5K stall converter and 4.10:1 gears.
For comparison, I have a 360 with 9.5:1 compression, stock 1975 Iron Heads (but milled and ported with big valves) KB232 quench dome pistons set at 0.040", and a mild RV cam (Hughes 215/223 @ 0.050", 0.501"/0.539" lift with 1.6:1 rockers), and I think it has a higher Dynamic compression than the stroker above. It does run on pump gas just fine with the quench dome pistons, but it is also loaded harder in the low RPM range with a stock converter and 3.23:1 gears.

 

[67 coronet]

Even with only a 4% power increase for the higher compression I think the drivability benefit when everything is right is way more than that. Better vacuum, crisper throttle response, carb tuning is easier, etc..

With regards to chamber shape and quench, a hemi is pretty darn good, but my 66 Charger blows 215 PSI with what seems to be a very mild cam and pings like an SOB on pump 91 when you jump on it. Drive around like an old lady is OK but who wants to do that in a hemi 4 spd car?? I didn't build the engine so for all I know someone put 12:1 pistons in it.

As I said there are many factors to control pinging. My 440 ran with a stock iron intake and a 750 Eddy AFB and made some great power but required a splash of race gas (estimated octane to keep it happy was 95 R+M/2). On the same engine I put on a 6 BBL setup and it now will not ping on pump 91, but makes less power. Not one thing was changed other than the intake and carbs. Granted the 6 BBL has been verified to be lean, which you think would make the pinging problem worse, but the lean condition is most likely responsible for the power loss. Only thing I can figure is change in fuel distribution.

i would say increased intake runner volume and velocity are what happened there meep.mix gets into comb chamber much easyer,more of same mix.just enough to make the difference between ping or not.

 

[Meep-Meep]

If anything the velocity might have slowed down, but the carb placement might might do what you said - allow more of the fuel mix to get in because the path is shorter. The singe 4 BBL has some sharp turns that might make for fuel separation, where the air would move in but the fuel doesn't. I'm accepting just about anything logical as an explanation and the bottom line is more research will be required. All it takes is one cylinder to be less than I deal and you get a problem. Measuring EGT or O2 at each cylinder is ultimately the answer to find out what happened. Thanks for the perspective!