Hi there,
Why is it recommended to lower the compression ratio of an NA car prior to adding boost of some kind?
I heard it was to avoid knock, but if you have a charge-cooler do you still need to do it?
Cheers, Paul.
lower comprssion ratio eases the EGTs quite a bit which helps to avoid detonation, while it is possible to boost higher comp ratio engines its best just avioded
1989 Turbo Trans Am #82, 2007 Cobalt SS G85
Quote:
lower comprssion ratio eases the EGTs quite a bit which helps to avoid detonation, while it is possible to boost higher comp ratio engines its best just avioded
The thing which confuses me is that even with a lowered compression ratio, under boost conditions the 'effective compression ratio' is still going to cause the same problems, isn't it?
After all, off boost you are still running the same NA you always did, which didn't detonate....?
Cheers, Paul.
with a lower compression generally 8:5 to 9:5 thats the range most ppl run the EGTs are not nearly as high as they are with a 10:0 or 10:5 or higher, moral of the story is compressed air generates heat, too much heat is very bad for engines, you lower the heat you make things easier on the engine. Off boost with a lower compression engine you will make less power NA, minor trade off for the gain up top
1989 Turbo Trans Am #82, 2007 Cobalt SS G85
Quote:
with a lower compression generally 8:5 to 9:5 thats the range most ppl run the EGTs are not nearly as high as they are with a 10:0 or 10:5 or higher, moral of the story is compressed air generates heat, too much heat is very bad for engines, you lower the heat you make things easier on the engine. Off boost with a lower compression engine you will make less power NA, minor trade off for the gain up top
My point is, under boost conditions you are looking at effective compression ratios of 14:1 and higher for > 6psi. So why would lowering the engines compression ratio slightly avoid detonation when you are going to up the boost psi (and effective compression ratio)?
The formular is:
effective compression = ((boost psi / 14.7) + 1) x motor compression
Cheers, Paul.
like rodimus prime said, lower compression lowers the heat generated by the engine, higher compression the motors gonna run hotter which increases chances of preignition/detonation
cavaliers been laid to rest
The saturn is being born
a new cavalier is in the mist
Higher compression ratios lower EGTs, not raise.
Lower compression ratios lower temps due to compression, etc.
<img src="http://home.nycap.rr.com/xenox/jbody/jbody_shocker.jpg">
yea i guess why the Evo has 8:8 CR, the cobalt has a 9:5, etc, you will NEVER see a factory boosted car with anything more than 8,9s in CR
higher compression ratio does not lower EGT
1989 Turbo Trans Am #82, 2007 Cobalt SS G85
Rodimus Prime wrote:yea i guess why the Evo has 8:8 CR, the cobalt has a 9:5, etc, you will NEVER see a factory boosted car with anything more than 8,9s in CR
higher compression ratio does not lower EGT
exactly as he said you never see a factory vehicle running high compression with boost. now if your thinking about dragsters and such they only run for a period of time start up, burnout, stage, finish. all out 1/4 mile machines werent ment with streetability in mind which is why they run mass boost with high compression which in the end they probably have several motors cause they know it wont last long.
http://www.cardomain.com/ride/827643
you can't intercool compression
think of it this way
for the sake of argument say you have 2 engines both at 2.2 liters
10:1 engine running 8psi of boost and making 200hp
9:1 engine running 10psi of boost making 200hp
whats the difference?
the lower compression/ higher boost engine will have lower exhaust gas temperature. Why? Because turbos can be intercooled, allowing intake air temp to be reduced, thus bringing down EGTs.
If you lower compression, the engine on its own makes less power yes, but thats ok because we have a turbocharger to take up the slack.
the compression that the turbo does can be thrown thru an intercooler before it makes its way into the combustion chamber. Since this air is normal temperature (non intercooled is MUCH hotter), the engine runs without a hiccup.
The disadvantage to this is that by lowering compression, you rely more on the turbo, thus if you have a laggy turbo you'll notice more with lower compression.
"static compression" is what you're after, but also at a sane temperature level. And yes, this is all to avoid detonation and increase reliability.
TheFlyingSquirrel (PJ) wrote:you can't intercool compression
think of it this way
for the sake of argument say you have 2 engines both at 2.2 liters
10:1 engine running 8psi of boost and making 200hp
9:1 engine running 10psi of boost making 200hp
whats the difference?
the lower compression/ higher boost engine will have lower exhaust gas temperature. Why? Because turbos can be intercooled, allowing intake air temp to be reduced, thus bringing down EGTs.
If you lower compression, the engine on its own makes less power yes, but thats ok because we have a turbocharger to take up the slack.
the compression that the turbo does can be thrown thru an intercooler before it makes its way into the combustion chamber. Since this air is normal temperature (non intercooled is MUCH hotter), the engine runs without a hiccup.
The disadvantage to this is that by lowering compression, you rely more on the turbo, thus if you have a laggy turbo you'll notice more with lower compression.
"static compression" is what you're after, but also at a sane temperature level. And yes, this is all to avoid detonation and increase reliability.
A very nice job of explaining this.....
TheFlyingSquirrel (PJ) wrote:you can't intercool compression
think of it this way
for the sake of argument say you have 2 engines both at 2.2 liters
10:1 engine running 8psi of boost and making 200hp
9:1 engine running 10psi of boost making 200hp
whats the difference?
the lower compression/ higher boost engine will have lower exhaust gas temperature. Why? Because turbos can be intercooled, allowing intake air temp to be reduced, thus bringing down EGTs.
Ok, i think i understand what your saying. Forgive me if i sound antagonistic, i'm just trying to understand all this stuff:
I can see how a 10:1 engine will have higher EGTs than a 9:1 engine *off boost*, but (in your example above), both engines have the same effective compression ratio under boost conditions - so wont the EGTs will be the same under boost?
Cheers, Paul.
Quote:
I can see how a 10:1 engine will have higher EGTs than a 9:1 engine *off boost*, but (in your example above), both engines have the same effective compression ratio under boost conditions - so wont the EGTs will be the same under boost?
Did you not read what I typed? Whatever thoughts you have about this put them aside for a minute because its preventing you from understanding.
I'll explain again
The 10:1 engine will have HIGHER EGTs ON BOOST. Off boost, egt between the two will be relatively the same since they're n/a engines and IAT is the same, altho the 9:1 should have slightly lower EGT due to (duh) less compression.
"Effective compression" isn't the only factor in determining EGT. The intake air temperature makes a pretty substantial difference, but once inside the engine you can't do jack about the temp.
This is key now... pay attention:
the air heated up by the turbocharger gets too hot for comfort. So we intercool it, getting it back down to sane numbers.
Then it goes into the engine. When the piston rises to squish the already compressed air, our temperature is going to go up again.
The more the piston squishes this air, the higher your temperature goes. Higher temperatures = detonation aka pre ignition.
Since this air is already compressed, it doesn't need to be compressed more by the piston, which is why people drop compression to raise boost. You lower temperature while raising the amount of air that can make it into the compression chamber.
lets review:
high compression + 8psi of boost = TOO HOT. EGT thru the roof, detonation and bye bye motor 200hp
lower compression + 10psi of boost = acceptable temp, EGT ok, no detonation, reliable motor 200hp
if you still don't understand read this thread because I asked this same question a long time ago and got a very good answer.
Read this
Lets not all suck each others !@#$@ just yet...
Higher compression ratios DECREASE egt's if everything is kept constant except compression ratio (and hence expansion ratio). Some tuning might be required, but when all is said and done, the engine with higher compression will have lower EGTs.
You guys keep thinking that greater compression equals greater temperature increase, and this is true of course, but greater temperature increase due to higher compression also equals greater temperature decrease due to higher expansion. The hot expanding gases spend more time giving up their power to the drivetrain with higher compression. This is where a part of the gains come from. This is basically what I'm trying to get across.
Compression ratio vs. EGTs has got to be in the top 3 of all time misconceptions about engine dynamics. If you still don't believe me,
do some research or
ask some engineers.
<img src="http://home.nycap.rr.com/xenox/jbody/jbody_shocker.jpg">
Wow so my final compression ratio is 15.9:1 at 10 psi with 9.5:1 compression no wonder why it only runs on 91 or above good write up. someone else had a nice explanation about retarding timing v adding extra fuel, one guy pointed out that too much fuel will increase compression as fuel takes up space and cannot be compressed. Will unburned fuel increase compression enough to make a difference or will performance loss be identifed by characteristics of flooding? I know this can be monitered by a wideband but many of us are on a budget and rely on; watching for changes in the shortband, reading plugs, and egt's.
a narrow band is almost useless there points at where it could read rich and be lean, not very accurate because of the narrow voltage. adding too much fuel will kill power and maybe cause bogging out and stalling it.
i run 93 in my cars but if you can get by with 91 so be
cavaliers been laid to rest
The saturn is being born
a new cavalier is in the mist
The fuel is the deciding factor, plain and simple. If you try to run pump gasoline in a diesel with around 22:1 compression, it will blow itself up. Gasoline burns hotter and faster than diesel. The engine's tougher and the EGT's wouldn't even have a chance to get dangerously high. But the engine would live a short life.
On a build, you decide the level of risk you'll accept. With lower compression and reasonable boost levels you can make more mistakes in tuning and the engine is more likely to live through it. OEM's lower compression to reduce the chances of damage occurring while the vehicle is under warranty. For example, pre 94 2.2 engines had no knock sensor, so GM used less compression and milder timing. When they added knock sensing they moved to higher compression and more aggressive timing. They cover their rears quite a bit in this respect.
PJ, your explanation makes it sound like my stock 95 Cav engine, non intercooled, too small turbo, 7+psi, and 87 octane runnin turbo car will never live. I'm going on 2 1/2 years with this setup and I don't baby it. Exhaust manifold isn't blue, the seals aren't cooked out of the turbo, and I haven't promised my first born to the Gods of Mechanical and Thermal Dynamics to make it happen.
A large part of tuning is really about trying to find the balance between fuel and spark timing. The whole trick is to extract maximum power while keeping the fuel from detonating. Cooler intake air can help, as more of the heat generated during compression will be transferred to the incoming air and fuel and effectively reduce the temp of the combustion chamber. Retarding spark timing is key... hotter combustion chamber temps from higher effective compression ratios decrease burn time.
Mixtures need to be extremely rich to make notable differences in compression. Liquid fuel actually absorbs heat energy and changes phase to become gaseous as compression ratios increase. If this weren't true, it would be almost impossible to start an engine in extremely cold temperatures. What really happens is that a shortage of available oxygen simply makes for reduced power for the amount of fuel used. These cases are obvious; exhaust smoke too black to see through, lowered EGT or even coolant temps, large carbon deposits on spark plugs and raw fuel spitting out the tailpipe. It doesn't take special equipment to detect "waay too much fuel," the birds dropping out of the sky will do it for you.
-->Slow
"Higher compression ratios DECREASE egt's if everything is kept constant except compression ratio "
^ this is true...in some sense.
You're going to have a higher Temperature at point 3 (looking at a basic Otto cycle, point 3 is just after "heat addition" (ignition)), but a lower termperature at point 4 (at maximum displacement)
I personally don't know if this warrants a decrease or increase in egt's...as there are so many other factors to be considered.
2000 z24
1985 z28
http://www.cardomain.com/ride/825536
Ok, so I think I have worked it out:
Its all due to pressure (as some of you were no doubt indicating
). I was wrongly equating compression ratio with pressure, which is obviously not right.
The point at which an engine starts to knock is at a specific cylinder pressure (psi).
If an NA engine with 10:1 comp ratio makes 147 psi at TDC and is right near its knocking point, adding boost will cause knock, which is bad.
So, you lower the comp ratio to, say 9.5:1, then the engine makes 139.65 psi at TDC you have space for 7.35 psi of boost before you incur knock.
This is probably a bit simplified, but at least it all makes sense now!
Thanks for all your replies,
Cheers, Paul.
