When it is time to rebuild my motor (396), what is the highest compression I can run with it? I would consider running race gas, or a mix of race gas/pump gas. I want to stay naturally aspirated with the 396, but was just wondering what kind of compression I could run on the street. If anyone would like to elaborate on what rods, pistons, heads, cam etc. would be good for the setup that would be awesome! I just thought of this 5 mins. ago so sorry if I am being vague. I just want a general idea of what can be done and with what parts. Thanks!

Jeff

 

well if you are willing to use race gas the sky is the limit on compression....on the other hand if you are careful about cam timing i have seen motors that have 13:1 static compression run on pump gas(87!) the secret is having enough overlap in the cam that at low rpm the cranking compression is much lower, due to the overlap bleeding off excess pressure. At higher rpm, due to faster valve events, cranking compression increases. so you can kinda get the best of both worlds.....for most people though arounf 11:1 is about the best you get on pump gas

 

13:1 on 87 octane.... i find that hard to beleive, maybe on 94 octane in a wildly tuned engine


pretty much highest i know of is a pretty mean lt1 (actually about all it has in common is lt1 block and reverse cooled) made by jim lab for an rx7, 12.2 compression i beleive
http://www.rx7club.com/forum/showthr...0&pagenumber=1

 

I am under the impression that CR above 13-14:1 do not produce any more power. Of course, you need to run race gas to use CR in this range. There are a number of points regarding combustion chamber and piston crown configuration that need to be taken into account to run a very high CR. If you want to use pump gas I would say not >11:1.

I would be interested to know why increasing CR beyond a point produces no further power gains. Is it a limitation of gasoline as a fuel, or something else? Or is this a myth?

Rich Krause

 

13:1 if you run a mix of race/pump gas. 87 octane with 13:1 is not gonna happen unless you pull lots of timing in which case the car would make actually less power than a 12:1 pump gas motor.

I would just go 12.0-12.2:1, run 93 octane, and call it good. You are not going to make that much more power with 13:1. FYI I run 12.2:1 on 93 pump gas and have no problems.

Jason

 

hey guys that car was in car craft about a year ago...i believe it was a malibu. anyhow it came with a fairly detailed discusssion about exactly how you can make that happen. The car ran 9's i think too.

Like i said before its very simple....your static CR really has doesnt have a whole lot of bearing on cranking compression, unless someone out there has a camshaft with square lobes on it. If a camshaft has a lot of overlap in the case of the motor with 13:1 compression, the cranking CR might only be 10 or 11, because at low RPM the exhaust valve is remaining open for a small time during the compression stroke...because of overlap. There is more to the cam timing than that, this guy used a custom grind which im sure he specified the LSA, intake opening/closing, exhaust opening/closing etc. as the motor revs (this thing was a high revver too if i remember correctly) because the opening and closing events are happening at an increased rate there is less and less intake charge lost through the exhaust valve before it shuts....thereby increasing the cranking CR.

i'll try to dig up the article for you guys

 

As some of the posts have pointed out, cam choice has a lot to do with maximum static CR. I always think "small cam" and hence my 11:1 comment. As Jason (and others) have done, if you have a big cam you can (and should) use a much higher static CR.

Rich Krause

 

Just to avoid any likely confusion, that should read "the intake valve is remaining open...."
Also worth noting.... timing events, in non-variable valve-event engines are independent of RPM.

That said, this whole discussion is about dynamic compression ratio (DCR). Understanding DCR gives you a much better understanding in relation to, "How much compression can I run" type questions.
There are, as with everything, rudimentary calculations. I've seen a few posted on this board. One thing to consider though is that any calculation which doesn't take into account, altitude and temperature (at the very least) is not worth the crunching. A change in altitude alone... let's say, 2,000-3,000 feet is an approximate .4 points change in DCR.

In my research of this subject amd how it relates to reverse-cooled LT engines, I've found that they can generally tolerate DCR's in the range of 9.1:1 with proper tuning, MBT.

Let's take an example.... a 355 cid LTx with a Comp Cams 290 HR (CC306), running a static compression ratio of 12.5:1 . We all know people running a setup like this on 92 octane. With some calculations and assuming an altitude of 1000 ft, 6" rod length and 72º IVC we calculate a DCR of approximately 9.1:1. Too high for a conventionally cooled sbc (which should stay below 8.3:1) but not a problem in this case.
I've tested my old LT, running Evans NPG and a high volume water pump, at DCR's of 9.5:1 without any problems. The engine was extremely thermal efficient albeit at a loss to volumetric efficiency. There's always a trade off.
I've also run as high as 10:1 with water injection with excellent results.
All that aside, there is always a limit and a law of diminishing returns.... octane limit plays a big part as does proper tuning.

So, through quite a bit of rambling... I'm rather confident in saying that an LTx can run (NA) 9.0:1 DCR all day long without a hitch. Always safe to err on the safe side though.

It would be a good idea to do some research on dynamic compression as it will help in every aspect of planning an engine build.... cam selection, target static compression, etc.
I've purposefully grazed the surface (don't have much time) so if you'd like to get more into this we can.

Take care,
Chuck Riddeck
Progressive Race Engine Development

 

FWIW I did a little poking around and this is the best DCR calculator I've found so far:

http://www.rbracing-rsr.com/comprAdvHD.htm

By plugging in the numbers Chuck gave in his example I get a DCR of 9.09:1 so it appears to be fairly accurate. Unfortunately it doesnt take into account temperature but everything else is there including boost pressure for your FI guys.

Chuck, you have me a little confused here I realize the Evans NPG makes a more efficient cooling system but I fail to understand how it affects VE, could you elaborate?

Also I noticed that rod length plays a noticeable role in calculating the DCR. For instance in your example subsituting a 5.7" rod increased the DCR by .04 (from 9.09 to 9.13). Now we all know that the real world difference between a 5.7 and a 6.0" rod is almost non-existant but the DCR calculation seems to say otherwise. Am I missing something?

Thanks!

Edit: This seems to be a good introduction to DCR for those who may not be familiar with it.

 

even with a dcr, the key term here is that is a ratio, its a volumetric quantity, and yes variations in altitude and in temperature will affect the given cylinder pressures, the compression ratio (including dcr) will be the same under all operating temperatures and pressures.

this is because the swept volume of the cylinder and combustion chamber size are not going to change. for example if the the DCR is XX.x at sea level and 60 deg......it will also, be XX.x at 4000' and 100 degrees. Compression ratios dynamic or not are a function of volume which doesnt change.....the atmoshperic conditions can only affect the actual cylinder pressures.

If you put 1psi into a cylinder with 10:1 CR (dynamic or not, this is an instantaneous event) and crank it over, the piston will compress the 1psi charge to 1/10 its original size....the same applies if it was 100 psi.

thus spake the laws of physics and the gas laws

 

Way to go Jason!
I guess some people are actually interested in learning. I'm impressed.
Looks like a good calculator for NA calculations. No parameters for compressor efficiency and or density change, so I wouldn't spaz out over the boost input. These are significant factors when considering knock limit in a force inducted engine.
Other than that, looks great.

Sure thing.
It's important to understand that an increase in coolant temperature decreases loss of potential power to the cooling system.
The other side of this is the fact that increased charge temperatures reduce volumetric efficiency.

Generally speaking, increases in thermal efficiency are most beneficial to economy while increases in volumetric efficiecy are the key to maximum power output.
By the same token, inreased compression ratio can help both economy and power by improving thermodynamic efficiency.

In a little more detail...
An engine is more 'thermodynamically' efficent when it is hot due to the fact that there is less corresponding heat transfer between the burning charge and the combustion chamber. As in the perfect otto cycle, the combustion is adiabatic.

However in order for maximum power to be realised, maximum VE needs to be found, ie the charge needs to be as dense as is combustibly possible. Therefore the smaller the delta T between inlet tract and charge the better, with the enthalpy of the charge remaining as constant as is possible before it is intoduced into the combustion chamber.

Does that answer your question?

You may have missed a little but that's easily remedied. In calculating DCR at the most basic level there are some basic relationships you need to understand.

1) static compression ratio, which is the ratio of swept cylinder volume in relation to combustion chamber volume.
2) dynamic compression ratio, is the swept volume at the IVC event.

So, rod length is a factor in that it dictates the amount of piston rise (from BDC) at a given IVC event. In essence, the volume will always be less than total swept volume.

If you'd like a math breakdown on this we can do that too.

HTH

Chuck

 

I think so, I only had to read it over about four times before I completely understood, lol...

So in laymans terms with the NPG system you were able to run higher coolant temps without detonation (resulting in increased thermal efficency, one of the benifits of NPG coolant) but at the same time you were sacrificing VE since the incoming charge was being heated more than usual. This also jives with something I read on the Rehr-Morrison website stating that the optimal coolant temp for a drag race motor was around 120F.

Got it, so am I correct in saying that while the rod length is a factor in determining DCR there are better ways to alter it if nessecary? Say by using a cam with a different IVC or by changing the SCR? For example lets say you built a motor that had a borderline DCR (say 9.14:1). If you wanted to lower it would you be better of subsitiuting a longer rod or selecting a different cam?

Thanks Chuck!

 

You've got it Jason. The NPG coolant ran at approximately 240ºF.



Depends on the goal. "More cam" (one with a later IVC event) will effectively reduce DCR but the engine makes power at a higher speed.
If that's not within the bounds of the intended design, then you simply alter SCR to get what you need.

Take care,
Chuck

 

OT, but everything I've read says that its worse at heat dissipation. This stuff is used in the airline industry at a much lower cost, I forget exactly for what though.

 

I should have mentioned this before as I've seen a few posts that could benefit from a little understanding of DCR and the IVC event.
Why don't you try looking at the effect of running a cam (all things considered equal) with a shorter lobe displacement angle. What does that do to the DCR? mmm, homework.

Take care,
Chuck Riddeck