Higher Compression better or worse

[liquordrunk]

I was wondering since im not to keen on compression ratio's. How does it work as in higher the compression the more power you can make?

 

[L2RTSiAWD]

Yes you will make more power with a higher compression ratio but then you run into the CR being too high to run on pump gas, Since you are turbo you will make more power running a little lower compression 8.5-9.0 but running more boost.

I think it's 3% HP increase for every point of CR increase.

 

[Shadowfax]

Yes you will make more power with a higher compression ratio but then you run into the CR being too high to run on pump gas, Since you are turbo you will make more power running a little lower compression 8.5-9.0 but running more boost.

I think it's 3% HP increase for every point of CR increase.

typically it depends on base CR vs your new CR- like boost- it's diminishing returns given a certain setup.

I ran 11:1 comp on a 5.0L motor, stock is 9.0 - without any other changes, it was around 20HP - 225HP stock, 20HP is roughly 10%, or 5% per point. However that was the safe max on pump, had I gone to 11.5:1 or 12:1 I wouldn't have gotten my 5%, I may have had I ran higher octane, or decreased timing... but like upping the boost- there comes a point when it's not really worth any more power unless you are willing to make certain setup changes to handle it- and then it will typically make part throttle operation sketchy.

I've seen guys run 10:1 comp on the 6G72 with boost and they'll make 12psi no problems, and yeah, higher comp will net quicker spool, more power, but I've always just looked to Dre and his 509WHP on 93 octane in a DSM. His timing wasn't in the 30's, nor was he running higher than stock comp.

It's in the tuning, and IMHO, raising comp on a boosted car typically isn't worth it. It costs a fair bit for pistons, specially forged pistons at higher comp, and you'll have a harder time supporting it when you go for more and more power. If you need more power off boost, comp is a good thing to raise, but it's cheaper and easier to live with slightly anemic off boost, and get a bigger turbo and have a lot more power on boost.

I'm the biggest example in history for resisting the call of larger turbos. I sent a Stealth TT to the edge of 11's with the stock turbos. I spent $10,000+ in doing it. I could have picked up some DSM 13G's and hit the same power for pennies on the dollars I spent.

If you haven't increased the size of your turbo, specially you 2nd gen'ers, then do that BEFORE opening your motor. Don't get a head, or a new intake plenum, or worry about a 75mm tb, or a valve job, or higher comp pistons... get a bigger turbo, then see how badly you want another 20HP

 

[justin0469]

so in reading thru other compression ratio threads , i get the idea that compression ratio is simply how much the mixture as a whole has been compressed by the piston. ie: 10:1 means that the mixture has been compressed to be one tenth the volume that it was during the intake cycle and that the smaller the first number the higher the compression. What is the third number sometimes shown in compression ratios? or am i seeing things or seeing typos and there is no third number?

 

[justin0469]

so also under that understanding, the last number will always be a 1 right?

 

[liquordrunk]

Well how about talk to marco runnin high compression. its not to hard to run higher compression since if you are a experienced tuner you can run jsut high of boost that your able to tune for. Since with higher compression youll need to mess with the timmin and back it off some to make the power. so it takes a little more skill to tune thats all. Also higher compression makes you lean on hte side of messing with detonation alot easier
higher compression less boost around the same horsepwer
lower compression ass loads of boost
this is on pump gas now most guys run lower compression so they dont knock on pumpgas at higher boost level.
i run 9:1 on my 2.3

 

[justin0469]

Well how about talk to marco runnin high compression. its not to hard to run higher compression since if you are a experienced tuner you can run jsut high of boost that your able to tune for. Since with higher compression youll need to mess with the timmin and back it off some to make the power. so it takes a little more skill to tune thats all. Also higher compression makes you lean on hte side of messing with detonation alot easier
higher compression less boost around the same horsepwer
lower compression ass loads of boost
this is on pump gas now most guys run lower compression so they dont knock on pumpgas at higher boost level.
i run 9:1 on my 2.3

uhh...what? who are you talkin to? if that was in reference to my questions, i dont think i get how that answers them

 

[CanadianTalon]

higher compression less boost around the same horsepwer

, 10:1 vs. 8.5:1 on my ITR took less than a pound of boost to make up the difference. Turbo didn't spool ANY quicker. I didn't really care because it made more than enough power limited to 13-14 pounds of boost on pump gas to shred the tires in a FWD car, but in an AWD car where you can put that power down there is no way I would run more than 9:1 for a street motor.

 

[liquordrunk]

with higher compression you can make more power but its keen to detonation. hince you are not as able to run high as boostas you can on lower compresion with out gettin knock on pumpgas is what i meant. you can make GOBS ofpower on higher compression to spol your turbo faster is the main reason most do it. higher compression = more hp = more tuning. lower compression more boost equal or lesstunnin. when you run higher compresion (more than 9:1) youll have to tune the timming on your dsmlink or standalone or retardit with the cas. its just a little different tunning. But big horse power can be made from runin higher compression

 

[CanadianTalon]

Did you hear what I said in the post above yours? Spool was exactly the same between 10:1 and 8.5:1 for me, no change AT ALL. Higher compression doesn't do anything for you except limit how much power you can make on pump gas safely.

 

[liquordrunk]

thats right cancadian i havent seen much difference from when guys run higher compression but i have seen a few that spool a bigger turbo a few hundred rpm faster but boosting on pumpgas safely is the concern with higer compression you absolutely right.
thats why i said that if ya runn higher compresion then youll have to mess with the timmim curve and since most guys are not that keen on tunnin their car its a little harder to run higher comp for some but you are right none the less

 

[CanadianTalon]

thats right cancadian i havent seen much difference from when guys run higher compression but i have seen a few that spool a bigger turbo a few hundred rpm faster but boosting on pumpgas safely is the concern with higer compression you absolutely right.
thats why i said that if ya runn higher compresion then youll have to mess with the timmim curve and since most guys are not that keen on tunnin their car its a little harder to run higher comp for some but you are right none the less

Do I need to repeat myself a third time?

My datalogs that told me boost comes on exactly the same > what some dude told you, or what you think you've seen

Higher compression does not, can not, and will not spool the turbo quicker. It's a myth, kinda like how some people think a 50 trim will make more on pump gas than an SC61, etc.

Here's the only test I could find.

 

[DSM90AWD]

It's a myth, kinda like how some people think a 50 trim will make more on pump gas than an SC61, etc.

you're going to make alot of 50-trim owners (like me)

BTW.. a nice primer on DSM Piston/Compression from Extreme$Motorsports

**note that using a thicker/thinner than stock HG or decking the block/head will also change compression so could have affected your results

 

[justin0469]

Higher compression does not, can not, and will not spool the turbo quicker. It's a myth,

I could be wrong because I am a newb at this but wouldnt it slow it down because compressed air is more dense?

 

[DSM90AWD]

I could be wrong because I am a newb at this but wouldnt it slow it down because compressed air is more dense?

Turbos are fueled by exhaust gasses not compression in the cyllinder. The more air/fuel you put in the cyls and exhaust out the valves in the faster the turbine spins.

Higher compression with the same amount of air/fuel will produce more downward force on the pistons = more tork, but should have little (if any) impact on turbine speed. This is one of the reasons 2Gs put in the 8.5:1 pistons, to make up for the power loss from the whimpy T-25 turbo.

But in keeping with the original question posted, there is no CR that makes the most power. I've met several people whom did the 2G pistons on 1G rods (7.8:1 to 8.5:1 CR) and said was the worst investment they ever made to their cars.

 

[justin0469]

ur right, now that you said it like that i get it, i wasnt thinking right. do the 1G rods give more compression because theyre longer or what?

 

[DSM90AWD]

ur right, now that you said it like that i get it, i wasnt thinking right. do the 1G rods give more compression because theyre longer or what?

No. Both Rods produce the same stroke but are different in other dimensions:
http://hometown.aol.com/rcm1061/myhomepage/auto.html

It's the size/shape of the piston that's different btw 1Gs and 2Gs giving diff effective compression ratios.

FYI..
90-91 7.8:1 Comp and Big Rods
92-94 7.8:1 Comp and Small Rods
95-99 8.5:1 Comp and Small Rods

 

[justin0469]

ohok, i just saw where it say "2g pistons on 1g rods" that gave more compression so i was wonderin how that worked

 

[Irrational]

My rule of thumb my old boss told me is- air is what is making your power. yes if you compress it more it will make a little better combustion. But for a pump gas street monster, you want as much air possible witch means don't raise the compression much higher then stock if at all. because if you do you will limit yourself on air due to detonation. every mod you do for the most part is to get more air in the cylinders or to support that air with fuel/water/alcohol what ever your poison its to get as much air and as little fuel as possible without knock.If you do build the engine.it's to support more power.you get more power by adding more air (i.e. Boost)and supporting that with more fuel. note- this is a pump gas engine im talking about so dont flame me saying shep runs 5345656 to 1 comp


On a side note about head gaskets's like stated above. I have herd bad things about running thicker head gaskets to lower compresion.People say it causes hot spots witch lead to the big boom that we all know so well. I also have seen some turbo Honda's put out some real nice dyno number's with thick head gaskets's so no one will ever win that argument. The reason those Honda guy's are going to the thick head gaskets's is to lower the compression so they can put run more boost witch makes more air go in the cylinder's (I'm sure you new that I'm just tying to help) and they really are making more pump gas power.
Danny

 

[bluegs-t]

Do I need to repeat myself a third time?

My datalogs that told me boost comes on exactly the same > what some dude told you, or what you think you've seen

Higher compression does not, can not, and will not spool the turbo quicker. It's a myth, kinda like how some people think a 50 trim will make more on pump gas than an SC61, etc.

Here's the only test I could find.

i didn't want to bring this post back up but i do have a concern. there is this guy, on my local board, that really believes that raising the compression will make the turbo spool faster. he will not just let it go as agreeing to disagree; however, he will not post any substantial evidence to prove without a reasonable amount of doubt that it helps. he posted a log of back-to-back runs, 20secs apart, saying that there is too much inconsistency in those runs in spool-up and that he proved his case.

 

[yesItsdsm]

I think alot of people fail to realize that there is "power" to be made from higher compression WITHOUT boost.. so some may mistake this added power for a quicker spool. I agree with those who actually have datalogged with higher/lower compression and have proved that higher compression doesnt decrease spool times.

 

[bluegs-t]

I think alot of people fail to realize that there is "power" to be made from higher compression WITHOUT boost.. so some may mistake this added power for a quicker spool. I agree with those who actually have datalogged with higher/lower compression and have proved that higher compression doesnt decrease spool times.

agreed. as well, they tend to make a little more torque and mistake that for quicker spool-up. i did a search on google and found a link that made perfect sense: http://www.sr20forum.com/archive/index.php/t-99016-p-2.html. the guy stated that higher compression helps with spool-up in time, as opposed to rpm. basically, the higher compression motor will get you to full boost quicker than the lower compression motor; however, the lower compression motor will still spool the turbo at that same rpm. it makes perfect sense to me, and i coudln't agree more about the datalogs. as a matter of fact, i posted the one from evans-tuning:http://forums.evans-tuning.com/viewtopic.php?t=1612. this is what he had to say about that: "It looks to me like the blue bar is above the red one in that graph to start with. Is that a sign that the turbo was spooling faster or was there inconsistencies between the two runs? If there was no effect on spool you shouldn't of seen the other line. Obviously something happened after it spooled because the blue (i assume high compression) was running more boost than the red. Weird how compression ratio would effect that.....

Raising compression is going to create more heat. More heat is going to expand the exhaust gas. The volume of exhaust gas leaving the cylinder is going to increase. An increase in exhaust gas is going to decrease spool times. I've already said it could be negligible but none the less it's being effected.

I'm not going to lie though, it's an impressive comparison."