Dynamic compression

What do some of you think about dynamic compression? You think it's something to worry about, or just get the static compression where you want it and go with it? The motor I'm putting together right now comes out to about 9.7:1 dynamic (13.25 static).... and I'm told good racing motors usually only get up to about the 9:1 range, and anything street should be lower. Then other people say don't worry about it.... Any informed opinions?


Thanks
Chuck

Your motor doesn't "see" the static CR, so it's irrelevant in terms of the operating characteristics. 9.7:1 DCR on street gas = go boom. It will not be possible to prevent detonation which will rob power and decrease engine life, perhaps substantially and dramatically.

Rich Krause

Oh, I forgot to mention this isn't going to be a pump gas motor....

Later
Chuck

Originally posted by chucks97ss
What do some of you think about dynamic compression? You think it's something to worry about, or just get the static compression where you want it and go with it? The motor I'm putting together right now comes out to about 9.7:1 dynamic (13.25 static).... and I'm told good racing motors usually only get up to about the 9:1 range, and anything street should be lower. Then other people say don't worry about it.... Any informed opinions?


Thanks
Chuck

If you have a 13.25:1 static what cam are you running to get a 9.7:1?

Bret

I'd rather not say. Suprisingly it's actually pretty big (in LT1 standards). Over 260 duration at 50 and over 670 lift...


Later
Chuck

And here I thought I had some sort of grasp on Dynamic Compression.

Is it correct to say that volumetric efficiency is directly related to
DCR?

In other words a motor with a static compression (mechanical
compression) of 10.0:1 and a volumetric efficiency of 100% (let's
assume forced induction or tunnel ram at a given RPM) would at
some point see equal the static compression of 10.0:1 (at that given RPM).

I'm having a tough time with the theory, "will NEVER see the static
compression value"

Thanks for the clarification.

You are correct in your assumption that VE has a direct effect on dynamic compression... but your thoughts on 100% VE creating dynamic compression equal to static compression at some given point I believe are incorrect. Static compression is a measurment of compression assuming 100% sealing from BDC. You would have to have a cam that closes imediately at BDC to achieve a dynamic compression equal to your static compression... Which is why the theory "will NEVER see the static compression value" is more than likely correct. Hopefully that answers your question?


Later
Chuck

Originally posted by Zero_to_69
And here I thought I had some sort of grasp on Dynamic Compression.

Is it correct to say that volumetric efficiency is directly related to
DCR?

In other words a motor with a static compression (mechanical
compression) of 10.0:1 and a volumetric efficiency of 100% (let's
assume forced induction or tunnel ram at a given RPM) would at
some point see equal the static compression of 10.0:1 (at that given RPM).

I'm having a tough time with the theory, "will NEVER see the static
compression value"

Thanks for the clarification.

Try the Search function. Search for dcr in the Advanced Tech Forum.

Enjoy the read. There's some good stuff there.

So what do some of you more seasoned builders think? Is it important to worry about dcr, or not? I have some people screaming to me that it is and I'm crazy to run such a high dcr.... and others who are laughing at those people saying, no, not to worry about it. Any informed opinions are welcome...


Later
Chuck

Chuck, you're building a beast. Just be sure to feed it right.

Originally posted by chucks97ss
Oh, I forgot to mention this isn't going to be a pump gas motor....

Later
Chuck

Then don't worry about a 9.7:1 dcr, you will be fine on the race gas. Just don't think you can run a batch of 92 for a trip downtown, aint gonna happen.:)

I'd want more duration with your compression but you should be fine either way.

-Mindgame

From what I have been taught, yes.. it is important, and there are certain ideal DCR's to shoot for. If your builder is willing, they should be able to tell you what they think aboot it & why they're going for whatever DCR/CR.

Originally posted by SkarodoM
From what I have been taught, yes.. it is important, and there are certain ideal DCR's to shoot for. If your builder is willing, they should be able to tell you what they think aboot it & why they're going for whatever DCR/CR.


Actually I am the one building it. If it makes any difference to you I am currently a student at SAM. Right now it's summer break for a couple weeks, so I'm just spending a little time researching a couple things and making sure I'm putting together what I want this motor to be. So far I'm pretty happy with what i've got. Hopefully I'll have it on the dyno for some preliminary tuning starting at the end of July...

Mindgame- Yeah, no worries there. I never planned on this being a pump gas motor. I'm just trying to put together a n/a LT1 that runs for once. One person (who has been the one on my case the most about this dcr thing) told me that almost all good racing motors come out to a 9.0:1 dcr. That's fine, I can understand that. But he also tells me he built a motor once with a 10.0:1 dcr and it's still running fine (and I'M SURE there are plenty other motors even higher than that driving around out there). So I'm a little hesitant to take his advice that my 9.7:1 dcr is out of line if you know what I mean. That's why I'm just trying to see what a few other people say. Pat Topalinski (shop forman at the school) told me he wouldn't worry about dcr at all. The guys at Race Prep (the shop that did the heads for me... who are also former students from SAM) told me that also. And I'm about 75% sure Jud is going to tell me the same thing when I ask. If the race gas can handle it, there's no reason why the added compression is going to hurt me, let alone make more power...

Oh well, anyways, just venting a little...

As for the added duration... yeah, I thought about that too, but I kinda want to see how this one does first. It's a real nice in between cam that I think is going to work pretty well with street driving. We'll see what happens....

I've heard the general rule with static compression is that for every full point increase, i.e. 10:1 - 11:1, you gain 2% more HP and 10% worse mileage, with all other things equal. If that is the case, wouldn't it be better to knock the motor down to a 12:1 or even a 12.5:1, and get some of the power back by some more rpms, instead of making it a strictly race gas motor? Is there that much more HP to be had by the 13+:1 SCR? Also, Mindgame said you couldn't put some 93 in it and putt downtown. Is the DCR so effective at pointing out the Streetability of a motor, that you couldn't retard ignition timing enough to prevent detonation? If so, I would imagine you should definitely worry about DCR. Last, I believe the Mr. Riddick had luck running a 9.5 DCR LT1 on pump gas with some trick cooling techniques. I think it was a dyno motor though, and didn't see temps above 120. Wasn't it running Evans NPG?

Originally posted by chucks97ss
Mindgame- Yeah, no worries there. I never planned on this being a pump gas motor. I'm just trying to put together a n/a LT1 that runs for once. One person (who has been the one on my case the most about this dcr thing) told me that almost all good racing motors come out to a 9.0:1 dcr. That's fine, I can understand that. But he also tells me he built a motor once with a 10.0:1 dcr and it's still running fine (and I'M SURE there are plenty other motors even higher than that driving around out there). So I'm a little hesitant to take his advice that my 9.7:1 dcr is out of line if you know what I mean.

I respectfully think the guy is full of poodoo.:)

No one will contest the importance of calculating a dcr for a street build which is gonna be octane limited...... in other words, it's gonna have to run on 92-93 octane petrol. Talk to a slew of good engine builders and they usually tell you that 9:1 is a good rule for engines with good cooling and efficient combustion chambers. Older less efficient engines might want to err on the safe side of that with maybe 8.5:1. At least this is what I've been told and seems to be the recommendation of Don Losito, Riddeck, Bischoff and a few other guys I've asked about it.
The first place a sbc is gonna detonate is between the center pair of cylinders (where they put the two exhaust valves side by side in more conventional sbc heads). Some of the better heads spread the exhaust for that very reason. So there are other considerations as to what will affect the "limit". Plumbing coolant to those areas would also help... ala circle track.

With race gas it's a whole other game. You have a new octane limitation, so the 9:1 thing goes out the window. Looking back, I've probably run as much as 11:1 dcr on all out race engines without any signs of detonation.

-Mindgame

Brian,

Yes, Chuck runs NPG and also recommends a BASF coolant I'm not real familiar with.... believe Mercedes uses the same coolant. Then again, Chuck does alot of things that he doesn't recommend people doing at home.:)
If you're in that line of work you can afford to push the limits through research and just plain knowing your shiznit. I do believe his coolant temps were higher than 120. When I talked with him about evans, he mentioned that his coolant temps were ~220* but the point he made was that coolant temperature was not a clear indication of critical area temperatures along the water jacket and combustion chamber. Of course he's in R&D, does alot of testing, rigging engines with sensors and stuff like that so he can make more "educated" choices than you or I can.
We would do well tol err on the safe side.

On static compression my thought is this.... if the compression is too high to run pump gas, then why not just go with a compression ratio that makes the most power for the application. There's a bit of power there.... not much but here we're usually talking about engines in class racing where you're seperated by a few tenths. Every bit helps in that case but it's probably not worth much one way or the other for a race car that's gonna see street time (non competitive).

-Mindgame

Originally posted by Mindgame
I respectfully think the guy is full of poodoo.:)


Yeah, i shouldn't be rude... but he started at school the same time I did, and he just does a ton of reading. So it's not like he know's this stuff from experience. He's a real nice guy and I appreciate him trying to help me out... but it's almost like he always has to disagree with whatever I say just to be mister mocha. lol, oh well, who cares life goes on :)

Thanks for all the help... you are saying the same thing pat did. If it's a race gas motor then I'm fine with the compression. I'm sure dcr has it's place in the street motor world running with octane limitations like you mentioned (and I tried telling him this earlier)... but for me, if I can run 9.7 without detonating on race gas, there's no reason for me to want to lower my compression.

Thanks for the thoughts...

Later
Chuck

He just wrote me this, so I thought I'd post it if any of you were interested in seeing a little bit of his opinion... This is his general thought on the matter.

"My point is that people who build real pro stock engines are of the opinion that you can have too much compression for certain cam specs."

It's not like he's an idiot. He definitely know's his stuff... I'm just not sure where he's getting all this from as far as the whole dcr thing goes. Or what his reason for believing what he has read is...


Later
Chuck

This has become a really muddy discussion, and I am not up to writing an essay (for a change). As was suggested, a search may be in order as this has been extensively discussed here in the past.

The one point I want to make strongly is to dispute the idea that CR doesn't matter. Note I didn't say "DCR", though that's the crucial thing. Static CR is one of the two major factors that determine DCR, as well as intake closing point. So they are both important. If DCR doesn't matter, than static CR doesn't either, 'cause they are directly related (all else being equal).

As to how much is enough, but not too much, other than the generalizations that Mindgame posted fore street motors, you won't find answers without be much more specific. Fuel used, engine operating temp, air density, rpm, VE, quench, etc., all will play into the equation. And I don't think most of the people here (at least not me) can give you a definite answer. I'd say stick with what's known to work, unless you have a large R&D budget!

Rich Krause

Originally posted by rskrause
This has become a really muddy discussion, and I am not up to writing an essay (for a change). As was suggested, a search may be in order as this has been extensively discussed here in the past.

The one point I want to make strongly is to dispute the idea that CR doesn't matter. Note I didn't say "DCR", though that's the crucial thing. Static CR is one of the two major factors that determine DCR, as well as intake closing point. So they are both important. If DCR doesn't matter, than static CR doesn't either, 'cause they are directly related (all else being equal).

As to how much is enough, but not too much, other than the generalizations that Mindgame posted fore street motors, you won't find answers without be much more specific. Fuel used, engine operating temp, air density, rpm, VE, quench, etc., all will play into the equation. And I don't think most of the people here (at least not me) can give you a definite answer. I'd say stick with what's known to work, unless you have a large R&D budget!

Rich Krause

Yeah, that was my plan... Not sure if I ever said anything about static compression not being important, but if I did, I must retract by saying yes I do realize it is directly related. ;) So hopefully I clarified myself good enough.

We're just going to try it and see how it goes. It's a pretty well balanced setup, so if all goes well it should be a runner. I'll keep you all posted. But for now I've got the basic question/thoughts answered/confirmed that I was after...


Thanks
Chuck