how to lower 9:1 compression

[95AWDTALIPS]

I bought Weisco pistons at 9:1 CR and 0.40 over . I was wandering how much boost I could push on a GT-40. Is their any way the machine shop could do something to lower the compression. I was told I could double up on head gaskets but I don't know about all that with lots of boost. Any other I deas while I got it in there to drop the CR. Also, pay no attention to the list of MOD's on this profile. My serious goal is 550-600awdhp. Trust me I have all supporting mod's but, yes it is a 7-bolt and many of you are probably now wandering "why", lets just say I have faith in my car. And the whole CW issue I believe was just a rare case that a few people found out they had and posted about it and it spread like wildfire, lets not get started on that subject though.. My only question is with that compression and the max possible boost (on pump gas prefferably, but I will soon be water/alcohol injected) can I hit my mark? Thanks.

 

[Turbo Talon DL]

What would the reason behind wanting to lower your compression ratio be? Do you not like off-boost power and better gas mileage? Do you not like being able to run your turbo more efficiently, less boost and still make the same HP?

 

[iwantawd]

gt-40 is pretty big. i havent looked at the compressor map but he may not reach his best efficiency area running lower boost with higher compression which is not what you want to do.

 

[95AWDTALIPS]

Well, you can't run as much boost on a higher compression. With my 800cc injectors my gas mileage is not going to be too great anyway. At 20psi my GT-40 will not be anywhere close to its capabilities. Many people say they would rather have more boost then higher compression anyday. Am I wrong for wanting to run lower compression. I just don't see me hitting my goal on 20psi or does the higher compression substitute for loss of boost?

 

[Black94TSI]

Well, you can't run as much boost on a higher compression. With my 800cc injectors my gas mileage is not going to be too great anyway. At 20psi my GT-40 will not be anywhere close to its capabilities. Many people say they would rather have more boost then higher compression anyday. Am I wrong for wanting to run lower compression. I just don't see me hitting my goal on 20psi or does the higher compression substitute for loss of boost?

http://forums.evans-tuning.com/viewtopic.php?t=1612

Here's a good comparison, 10:1 vs. 9:1 cost about 15 WHP on a 250 WHP setup (and look at all that off boost response, I mean, the 10:1 motor makes like 2 WHP more before boost hits!), and that's going to be easily made up with by a pound of boost, and I'm pretty sure a full point in compression will let you run more than 1 pound of boost on pump gas. 600 AWHP isn't happening without race gas or Methanol no matter your compression, so i'd just leave it as-is, and hit it with a lot of boost, meth, and timing if you want to make 600 AWHP.

 

[95AWDTALIPS]

That link kind of confused me, is it saying I should run low compression or there isn't much difference between compression ratio's? I wont be happy if I can't push around 30psi. I know 9:1 at 18-20psi max isn't going to cut it. I saw a site where it showed a little graph on how much boost you can push at different compression ratio's with the boost being intercooled and it was like 10psi more you can push at each 1:1 lower compression .Basically I would like to lower my compression without changing the pistons, how would I go about this?

 

[MNGSX]

You can o-ring.. the head and run a SCE pro copper head gasket.. They come in thicknesses from .032 to .093.

You can actually do both the head and block but usually not with the block in a car.. Also they must be staggered from eachother.

The head is easier to do and cheaper to do as more ppl have a head o-ring tool...

IMHO Use a single stainless o-ring in the head with the copper gasket... Use some copper spray because it is'nt the titan version....

www.scegaskets.com

 

[95AWDTALIPS]

My motor is in the machine shop right now and I was told that I could have the combustion chamber bored out also. I'm really trying to get my compression as close to 8:1 as I can without coming out of pocket for new pistons, I guess I'm just beating this subjuct to death huh? I just want to boost the crap out of this motor as much as I can without detonation. Do you guys think 500hp is possible with 20psi on a GT-40?

 

[MNGSX]

Just do what I said in the last post.

That gasket & o-ring will take the cylinder pressures that come with the boost you want to run for sure.. It will also probably be cheaper than hoging out the combustion chamers then cc'ing them to match... Just have the head 0-ringed and buy a SCE copper gasket. Should be cc'd anyway but stock is really close and easy to dial in as compared to hoging them out...

Its how I would do it in your shoes..

I think 20psi is a bit low for 500.. Although boost levels are'nt lbs/min of air which is what makes power not pressure alone.....

 

[1fast97gsx]

My motor is in the machine shop right now and I was told that I could have the combustion chamber bored out also. I'm really trying to get my compression as close to 8:1 as I can without coming out of pocket for new pistons, I guess I'm just beating this subjuct to death huh? I just want to boost the crap out of this motor as much as I can without detonation. Do you guys think 500hp is possible with 20psi on a GT-40?

Have you ever driven or played with a 9:1 motor versus an 8:1 motor? Lets put it this way, saying you can only run 18-20 psi on 9:1 is just plain retarded. I ran a 9:1 motor with an mls headgasket which lowered it just a tad ... I'm assuming around 8.8:1 or so. This is exactly what the evo8 has as well. I personally ran 23 psi on 93 octane no problem and evos are doing it as well. A 8:1 motor may be able to run around 24 with a perfect tune but that's nothing to worry about. Higher compression with less boost may still make more power than lower compression with more boost. It depends on the eficiency level of the turbo, your timing, your tuning, the type of gas you're running and a few more deciding factors. Just run 9:1 with an mls headgasket and it'll be fine.

 

[99gst_racer]

Why did you get 9:1 pistons if you didn't want a 9:1 CR?
And why not just return them to where you bought them and swap them out for some 8.5:1 pistons?

If you keep them and you still want to lower your CR, then I would recommend MNGSX's advice.

 

[definitiveno]

Have you ever driven or played with a 9:1 motor versus an 8:1 motor? Lets put it this way, saying you can only run 18-20 psi on 9:1 is just plain retarded. I ran a 9:1 motor with an mls headgasket which lowered it just a tad ... I'm assuming around 8.8:1 or so. This is exactly what the evo8 has as well. I personally ran 23 psi on 93 octane no problem and evos are doing it as well. A 8:1 motor may be able to run around 24 with a perfect tune but that's nothing to worry about. Higher compression with less boost may still make more power than lower compression with more boost. It depends on the eficiency level of the turbo, your timing, your tuning, the type of gas you're running and a few more deciding factors. Just run 9:1 with an mls headgasket and it'll be fine.

I don't think that you are right on that. Correct me if I'm wrong but a lower compression motor can actually hold more air in a cylender, and if you reach equal amount of effective compression in both motors (lets use extreme scales, 7:1 motor vs 10:1) the one that can process more air in a given cycle has more potential to make power supporting a proper tune.



When the motor is at bottom dead center, each cylinder displaces close near .5 liters, the compression ratio means it is going to compress that .5 liter of space by 7 and 10 times it's original space (7:1 motor vs 10:1). The engine that has a 10:1 compression will have measurably less space in the chamber at top dead center (full compression stroke) then the 7:1 motor. Apply more PSI from your turbo charger and you can equalize the effective compression occuring before ignition. One has more air and sequentially more fuel.....

This doesn't mean that it is useful to run really low compression in every instance though, there are happy mediums for every thing. Not all turbo's run efficiently in higher boost applications, not only will you get heat from compressing air with your piston but you will be preheating the air prior to engine compression with the manifold compression via your turbo. This might be too much heat in the long run and cause detonation.





Atleast this is how I see it in my head. Again, feel free to correct me if I'm off base in my thinking....

 

[95AWDTALIPS]

The main reason I went with 9:1 compression is that that's what my friend did when he rebuilt his motor and I decided it was a good choice and ordered them before I knew the boost capabilities of each CR. I don't want to return my pistons is I'll have to pay for return shipping and restocking fee and plus my dsm has been down for 7 months and I don't want to have to wait too much longer, it's killing me. I'm going to the machine shop today being they are open since the hurricane hit this past weekend and take all of your guys's advice and see what they can do for me. Thanks guys, I'll be checking in here and there to see if anone else has some other ideas they would like to share. Thanks again guys.

 

[98talonAWD]

I agree with what definitiveno said.

im running 7.5:1 compression with my ITS dual ball bearing T67 .82 AR Q trim( its a custom turbo with a garrett T67 compressor wheel.). and have spray as well. i have the same setup as SBR's AWD black car, just bigger cams. i am going to spray a 125 shot direct port and run around 42-44psi of boost pressure. i had 8.8:1, and a 9.0:1 motor before that. so im trying all compression ratios and seeing whats the best for my setup. this turbo likes to run a lot of boost and eventually i will use a ITS T80 or something similar which likes even more boost. usually the bigger the turbo the more boost they like to run, especially the dual ball bearing turbos which are more efficient.


i guess we'll see

 

[MNGSX]

The really awesome and easy to work with no spray and no o-ring needed ICS titan is availble in these thickness levels.. 043”, .050”, .062” & .080 If one of those drop the CR enough for you just buy a head gasket...

 

[Turbo Talon DL]

I don't think that you are right on that. Correct me if I'm wrong but a lower compression motor can actually hold more air in a cylender, and if you reach equal amount of effective compression in both motors (lets use extreme scales, 7:1 motor vs 10:1) the one that can process more air in a given cycle has more potential to make power supporting a proper tune.



When the motor is at bottom dead center, each cylinder displaces close near .5 liters, the compression ratio means it is going to compress that .5 liter of space by 7 and 10 times it's original space (7:1 motor vs 10:1). The engine that has a 10:1 compression will have measurably less space in the chamber at top dead center (full compression stroke) then the 7:1 motor. Apply more PSI from your turbo charger and you can equalize the effective compression occuring before ignition. One has more air and sequentially more fuel.....

This doesn't mean that it is useful to run really low compression in every instance though, there are happy mediums for every thing. Not all turbo's run efficiently in higher boost applications, not only will you get heat from compressing air with your piston but you will be preheating the air prior to engine compression with the manifold compression via your turbo. This might be too much heat in the long run and cause detonation.





Atleast this is how I see it in my head. Again, feel free to correct me if I'm off base in my thinking....

There is more space, correct, but the more you can squeeze the fuel/air mixture, the hotter the combustion cycle, which means it pushes the piston harder. The 9:1 is going to squeeze the air into a smaller space, thereby moving all the charge closer together and closer to the spark, allowing for an overall better burn. I had a 9:1 motor that went south shortly after it was born, it did pretty well on the 14b. Higher compression ratio can help spool the turbo slightly faster as well. I just feel that low(er) compression motors feel more sluggish than a 9:1 motor. My dad's volvo feels more sluggish than my car off boost. His is also an auto too. I dont think that car can stay out of boost during normal driving. I dont know about anybody else here, but can you go up a hill at about 45-50 mph and stay out of boost? I can, I stay at about 0, but not in boost.

 

[95AWDTALIPS]

98talonawd, how much boost were you able to push on your 9:1 setup with that turbo?

 

[definitiveno]

There is more space, correct, but the more you can squeeze the fuel/air mixture, the hotter the combustion cycle, which means it pushes the piston harder. The 9:1 is going to squeeze the air into a smaller space, thereby moving all the charge closer together and closer to the spark, allowing for an overall better burn. I had a 9:1 motor that went south shortly after it was born, it did pretty well on the 14b. Higher compression ratio can help spool the turbo slightly faster as well. I just feel that low(er) compression motors feel more sluggish than a 9:1 motor. My dad's volvo feels more sluggish than my car off boost. His is also an auto too. I dont think that car can stay out of boost during normal driving. I dont know about anybody else here, but can you go up a hill at about 45-50 mph and stay out of boost? I can, I stay at about 0, but not in boost.

Every car should be designed systematically(figure out what gas your using, how much power you want, a turbo, pick out the sweet spot or how much psi, then figure out how much compression you can use without detonation). I believe you should have an idea of what you want to do with your car but for the sake arguement I was referring simply to making horse power.

While higher compression has it's perks you and I both appreciate (I have a 9:1 motor also) You and I are not running GT40 wheels. Depending on wich trim GT40 95AWDTALIPS is using his sweet spot may be at 30 PSI. If so he may not be able to utilize the most efficient settings in pump gas configuration.

What i'm leading up to here is my GT30R and your 14b is an apples to oranges compared to the GT40

 

[MNGSX]

More air into the motor beats more static compression...

With each tuned to make the most safe power on a given fuel the lower compression engine on higher boost will reach the same cylinder pressure as the higher compression engine..

Turbo and intercooler efficiency matter too... IMHO 9:1 is too high for any serious amount of boost on pump gas..

 

[98talonAWD]

i never had that turbo on a 9.0:1 motor... i had like a baby turbo.

 

[1fast97gsx]

I don't think that you are right on that. Correct me if I'm wrong but a lower compression motor can actually hold more air in a cylender, and if you reach equal amount of effective compression in both motors (lets use extreme scales, 7:1 motor vs 10:1) the one that can process more air in a given cycle has more potential to make power supporting a proper tune.



When the motor is at bottom dead center, each cylinder displaces close near .5 liters, the compression ratio means it is going to compress that .5 liter of space by 7 and 10 times it's original space (7:1 motor vs 10:1). The engine that has a 10:1 compression will have measurably less space in the chamber at top dead center (full compression stroke) then the 7:1 motor. Apply more PSI from your turbo charger and you can equalize the effective compression occuring before ignition. One has more air and sequentially more fuel.....

This doesn't mean that it is useful to run really low compression in every instance though, there are happy mediums for every thing. Not all turbo's run efficiently in higher boost applications, not only will you get heat from compressing air with your piston but you will be preheating the air prior to engine compression with the manifold compression via your turbo. This might be too much heat in the long run and cause detonation.





Atleast this is how I see it in my head. Again, feel free to correct me if I'm off base in my thinking....

Your thinking is correct for the most part, however the amount of boost you can run at 8:1 and 9:1 isn't a very large difference so it's not worth trying to lower it. A higher compression motor can still take in the same amount of air, it just compresses it more. Knock is caused by preignition when we talk about a high compression motor and this happens when the combustion chamber get too hot and explode before the spark. This is why we get timing pulled and loose power. The way to avoid this is to turn the boost down a bit or run a higher octane fuel. It isn't really the amount of boost that's causing the preignition however, its the amount of airflow the cylinder sees. The more air gets compressed, the hotter it gets, which is why a higher compression motor can't see as much boost on pump gas. Once again the difference isn't really work losing sleep over. Do you really care if you can run 23 psi safely or 25 psi safely on the street? On a built motor you can run 30+ and make 500+ whp safely on race gas and that's what matters. At 20psi on a 9:1 motor and 20 psi on a 7:1 motor assuming the turbo is in it's efficiency range, which will make more power with the exact same mods and a perfect tune on both? The 9:1 will for sure. Off boost response is better on a high compression engine as well. I agree when people say a turbo isn't in it's efficiency range at 23 psi or so if we are talking about these crazy turbos that need to see 30 pounds of boost, but then neither motor will support it properly on pump gas anyways. The point is to run lots of boost race gas is needed anyways, and with a proper tune the higher compression motor will make more power with all other variables set aside. This leads me back to my original question, why do you really want to lower it?

 

[95AWDTALIPS]

I don't know. I guess I'm just assuming I can make more overall power with lower compression and higher boost. I guess I just have to see what happens. I know on the "DYNO" link at the top of the home page has a guy making like 500+hp and says he's using pump gas so I didn't figure it was impossible. I don't think he had 9:1 compression either though.

 

[1fast97gsx]

Like I said, on pump gas you can run more boost with lower compression so it all depends on the efficiency of the turbo. Who wants to make big power on pump though?

 

[definitiveno]

Like I said, on pump gas you can run more boost with lower compression so it all depends on the efficiency of the turbo. Who wants to make big power on pump though?

 

[4pistons]

You are trying to determine how much boost your engine can tolerate by only analyzing one factor - static compression. It is only one of many factors that affects your "dynamic compression", which is the ACTUAL compression your combination produces and is way more important to detonation and performance. It is determined by ignition timing, camshaft and cam timing, connecting rod length, and boost, just to name a few. In fact, static compression probably has the least effect on peak cylinder pressure than all the rest of them.

It is a very complex topic and the subject of alot secrecy among big time racers and professionals. Hot Rod magazine did a very good in-depth article on it in their August 2005 issue but I can't find an online link to it. Bottom line is it can't be answered by discussing static compression only since it's only a small part of the whole picture.

Rick - '91 GSX