highest street compression?

[1fast97gsx]

who here is running higher than stock compression and on what turbo, what boost, and what type of gas? I'm planning on raising mine up some more to make more power, but I don't know at what point it's considered too high? I'm planning on trying 9:1 ... although when I bought my pistons ( 8.5:1 ) from AMS they told me 9:1 was too high and they insisted I go with 8.5:1 which I did. I regret doing it now because the car isn't making as much power as it should, but I'd like opinions from other people on here about running a 9:1 compression as well. Are any of you doing it on pump gas? How much boost on a decent sized turbo?

 

[Maglin]

Don't go over 8.5:1 compression unless you plan on running racing fuel all the time. Your turbo shoves more air in and that creates more pressure which effectivly raises your CR. On pump gas you will probably have to run lower boost cause of knock from detonation or pre-ignition caused from the higher CR. Also when a company or multipule companys tell you not to do something and they are leading companys in that field. Take their advise. I know I do, even if it is with a grain of salt.

The only difference you will notice is off boost performance will increase a little. Not much and the top-end power loss with be higher than the low end power gain. Not to mention that you will suffer from lower gas milage cause of the more work the engine has to do (due to the high CR) to bring the pistons to TDC.

If I'm wrong here someone correct me then. From what I've read on forced induction and what I know building small block chevys over the years is what I've stated.

 

[CanadianTalon]

Don't go over 8.5:1 compression unless you plan on running racing fuel all the time. Your turbo shoves more air in and that creates more pressure which effectivly raises your CR. On pump gas you will probably have to run lower boost cause of knock from detonation or pre-ignition caused from the higher CR. Also when a company or multipule companys tell you not to do something and they are leading companys in that field. Take their advise. I know I do, even if it is with a grain of salt.

The only difference you will notice is off boost performance will increase a little. Not much and the top-end power loss with be higher than the low end power gain. Not to mention that you will suffer from lower gas milage cause of the more work the engine has to do (due to the high CR) to bring the pistons to TDC.

If I'm wrong here someone correct me then. From what I've read on forced induction and what I know building small block chevys over the years is what I've stated.

If he can't make as much power as he wants on an 8.5:1 motor running a safe 20-22 pounds of boost on pump gas, I don't know what will. MM&FF just did a test, comparing an 8.1:1 and a 10.1:1 motor. Naturally aspirated the compression drop took it from 401 to 365 horsepower, a loss of 9%. Guess what happened when they strapped on a KB blower? Power dropped from 600 to 533 horsepower, a drop of 12%. Sorry, but I don't think 7.5:1 compression is worth loosing 6% power, all across the board, just so you can run a couple PSi more boost. Keep in mind a small bore motor like a DSM is much more resistant to detonation than a large bore motor like a 350 chevy. Do you think Mitsubishi would send stock EVO 8's out of the factory with 8.8:1 compression and 19 pounds of factory boost if it was on the ragged edge of detonation? If they sent it out with 19 pounds of boost on 8.8:1 it is ATLEAST safe to run 19 pounds of boost on 9:1 compression with pump gas, and even more with a good tune.

If the 6 percent loss in power per full point drop in compression is true like it was on the 4.6L mustang motor, we can assume that you are making roughly 3% less power under boost with 8.5:1 than you are with 9:1. So instead of making 400 HP @ 9:1 you would be making around 388 HP @ 8.5:1, albeit with less low end not only due to a higher base horsepower from the inreased compression but slower turbo spool as well. IMHO 8.5:1 is a great median for a street motor. Not as low as the 7.8:1 stock 1G compression, but you can still run a goodly

 

[Lunch_Box]

Don't go over 8.5:1 compression unless you plan on running racing fuel all the time.

Where do you get this from, I am sure there are lots of people with 9:1 compression driving around on the streets with pump gas all the time.

 

[AMS stampy]

ok, I dont know whom you talked to but you can run 9:1 on the street no prob. Hell I do it my daily driver. now the performance difference is mild and you can still make gobs of HP on 8.5:1. I Appoligize if you were mislead by an employee here but you have some bad ass pistons there. I can tell you with the utmost confidence that the biggest problem in this field is companies that mislead customers on HP numbers. People often question me when I give them what they are really going to put down. just because one person in one town did it doesnt mean its going to be applied to the general public. now on to your problem. What HP are you making and what did you expect to make. a small mod list (the pertinent stuff) would be nice too.

Eric

 

[Maglin]

I stand corrected. The info I got was from some of the leaders in Forced Induction, but this was also like 8-10 years ago and also this was with much bigger motors ie. American V8's. Sorry for the bad data. I know on N/A motors more is better to an extent.



So go with 9.0:1 as it seems some of the people here and AMS says it's totally ok. Maybe I'll change my plans on my motor build if this seems like a decent ticket on pump gas.

 

[ItsStockOfficer]

You compression is a scapegoat for your low power numbers. Find the real reason before you start to bi*** about something that doesn't matter.

Canadian Talon, your example is incredibly stupid. A blower flows a fixed amount of air based on engine speed and blower size. Of COURSE it made less horsepower! This isn't even applicable! You might as well have said they lowered compression on a N/A motor and made less power. With a turbo you raise dynamic compression by raising boost, allowed by the increased detonation resistance of the lower static compression engine. And you gain MORE power by the increased boost then you would have otherwise enjoyed on the lower boosting high compression engine.

And Maglin, I agree with you. Perhaps on 93 it's a different story, but on 91, 8.5:1 is the max compression ratio you can run with a good/aggresive pump gas tune

 

[14.5 drift]

ok, I dont know whom you talked to but you can run 9:1 on the street no prob. Hell I do it my daily driver. now the performance difference is mild and you can still make gobs of HP on 8.5:1. I Appoligize if you were mislead by an employee here but you have some bad ass pistons there. I can tell you with the utmost confidence that the biggest problem in this field is companies that mislead customers on HP numbers. People often question me when I give them what they are really going to put down. just because one person in one town did it doesnt mean its going to be applied to the general public. now on to your problem. What HP are you making and what did you expect to make. a small mod list (the pertinent stuff) would be nice too.

Eric

pte scm61, pte fmic, greddy-s bov, custom piping, 95 eprom, dsmlink, pte 780 injectors, tial 38mm wastegate, hks evc4, hks tt, 3" hrc downpipe, 3" test pipe, Thermal r&d catback, act2600, fidanza flywheel, ported 2g exhaust mani, emissions hoses removed, 190lph fuel pump, 272's, 1g throttle body....



and he's still not happy....................

 

[myblack98gst]

I run Wiesco 9.0:1 bored .020 over, and block and head decked everyday on 93 octane.

I got it tuned to where I am getting virtually no knock at 20 psi..

I see no problem with either compression 9:1 MAY give slightly faster spool up with a large turbo (but minimal difference) and the off boost power will be slightly better.

 

[CanadianTalon]

You compression is a scapegoat for your low power numbers. Find the real reason before you start to bi*** about something that doesn't matter.

Canadian Talon, your example is incredibly stupid. A blower flows a fixed amount of air based on engine speed and blower size. Of COURSE it made less horsepower! This isn't even applicable! You might as well have said they lowered compression on a N/A motor and made less power. With a turbo you raise dynamic compression by raising boost, allowed by the increased detonation resistance of the lower static compression engine. And you gain MORE power by the increased boost then you would have otherwise enjoyed on the lower boosting high compression engine.

And Maglin, I agree with you. Perhaps on 93 it's a different story, but on 91, 8.5:1 is the max compression ratio you can run with a good/aggresive pump gas tune

No, it isn't incredibly stupid. They lost about 9% of the power naturally aspirated, 12% with a twin screw blower, and the same 12% with a centrifugal blower. Lower compression or not they were still getting the exact same amount of airflow into the motor, so tell me how that is stupid. What you're saying doesn't even make sense, they wanted to do a comparison back to back of how much power you lose by dropping compression, not to compare how much boost and power you can make for a given octane of fuel.

And FWIW, we did sort of the same thing on a friends ITR last year, replaced the stock 10.6:1 compression pistons with 9.5:1 pistons, and the power dropped from 314 WHP to 294 WHP, weather conditions were off by a few degrees so that didn't have a huge effect, and they were done in the same shop so you can't say it was the dyno. Spool was significantly slower as well. That's about as big of a gap as 7.8 to 9:1 compression, and it made a noticeable difference in how the car drives.

I still say that if you are running a large turbo that will move lots of air at low boost pressures, 9:1 is fine. You can still run lots of boost, and given that he lives in illinois getting 93 octane shouldn't be a problem for him.

 

[myblack98gst]

what the canadian says makes sense..... at the exact same boost level and same turbo a higher compression motor will make more power than a low compression, just because the static compression of the motor is higher so that adds onto the total after the turbo is added on..

a low compression motor has to run more boost to get the same power level as a high compression..

a perfect example of this is a honda.

10.6:1 @ 7psi > 8.5:1 @ 7psi
BUT
10.6:1 @ 7psi < 8.5:1 @ 30 psi

 

[1fast97gsx]

pte scm61, pte fmic, greddy-s bov, custom piping, 95 eprom, dsmlink, pte 780 injectors, tial 38mm wastegate, hks evc4, hks tt, 3" hrc downpipe, 3" test pipe, Thermal r&d catback, act2600, fidanza flywheel, ported 2g exhaust mani, emissions hoses removed, 190lph fuel pump, 272's, 1g throttle body....



and he's still not happy....................

cams and dsmlink have not yet been installed so I'm sure that will wake up the car as well. Just planning ahead for the winter.

 

[1fast97gsx]

ok, I dont know whom you talked to but you can run 9:1 on the street no prob. Hell I do it my daily driver. now the performance difference is mild and you can still make gobs of HP on 8.5:1. I Appoligize if you were mislead by an employee here but you have some bad ass pistons there. I can tell you with the utmost confidence that the biggest problem in this field is companies that mislead customers on HP numbers. People often question me when I give them what they are really going to put down. just because one person in one town did it doesnt mean its going to be applied to the general public. now on to your problem. What HP are you making and what did you expect to make. a small mod list (the pertinent stuff) would be nice too.

Eric

Mod list includes:

ross pistons, eagle rods, bs ellimination, decked block, bored .020", 1g head ( also decked ), mls headgasket, arp studs, pte scm61, pte fmic, greddy-s bov, custom piping, 95 eprom, safc ( to be replaced with dsmlink asap ), pte 780 injectors, tial 38mm wastegate, hks evc4, hks tt, 3" hrc downpipe, 3" test pipe, Thermal r&d catback, act2600, fidanza flywheel, ported 2g exhaust mani, emissions hoses removed, 190lph fuel pump, 1g cams, 1g throttle body

I did a compression test today and my numbers aren't really very consistant. They jump from like 160 to 180 to 170 to 155 .. kinda all over the place. I'm sure that's not helping me make power either so I might try a new set of rings and hope the old ones for some reason just didn't properly seat. I know it's not a head gasket problem and I don't think these compression numbers are going to make this much of a difference in performance.

I haven't yet dynoed the car because I'm still running the safc and 1g cams ... but on the highway I literally stay even with a close friends 2002 LS1 camaro with slp headers and a dump as well as K&N and a few other bullshit mods. He's lucky to have 400 crank hp and I'm having a hard time hanging with him ... also my car "felt" faster with the 16g last year when it was doing high 12s.

I just figured with these boost numbers I should just blow the 16g away, but from the compression test my average compression comes out to be like high 160s. Last year I did a compression test before the motor teardown and it was 175 across the board. I know the mls gasket is thicker ... but exactly how much affect does it have on compression?

 

[ItsStockOfficer]

No, it isn't incredibly stupid. They lost about 9% of the power naturally aspirated, 12% with a twin screw blower, and the same 12% with a centrifugal blower. Lower compression or not they were still getting the exact same amount of airflow into the motor, so tell me how that is stupid. What you're saying doesn't even make sense, they wanted to do a comparison back to back of how much power you lose by dropping compression, not to compare how much boost and power you can make for a given octane of fuel.

And FWIW, we did sort of the same thing on a friends ITR last year, replaced the stock 10.6:1 compression pistons with 9.5:1 pistons, and the power dropped from 314 WHP to 294 WHP, weather conditions were off by a few degrees so that didn't have a huge effect, and they were done in the same shop so you can't say it was the dyno. Spool was significantly slower as well. That's about as big of a gap as 7.8 to 9:1 compression, and it made a noticeable difference in how the car drives.

I still say that if you are running a large turbo that will move lots of air at low boost pressures, 9:1 is fine. You can still run lots of boost, and given that he lives in illinois getting 93 octane shouldn't be a problem for him.

It's stupid becuase on a turbo car, or any car, on lower compression you should increase air flow. If you lower compression to gain airflow headroom, then don't use it, you can't much complain about losing peak power, now can you?

 

[sweet97]

Precision turbo has some info on compression. Thay state that raising compression increases power around 3% but leving he compression low and adding boost wil increase power 3% for just 1 psi increase in boost. It's early and I'm not fully awake so check them out yourself. Of course street drivability has to be taken intoconsideration for off boost situations. There is a limit to what boost that can be run for a specific CR. Lower boost allows more boost which equals equal power.
type in www.precisionturbo.com and you will see around 8 names on the left boxed in. Precision is on the bottom. read what they say about CR and power. Mark

 

[AMS stampy]

well the compression isnt awful but you most certainly need to have the link install and get that thing TUNED!!! you would not believe how much that will wake the car up. get it on a wideband meter and dial that puppy in.

Eric

 

[JASON ROSARIO]

As far as 9:0:1 im running them my car as a daily driver and I have no complaints. Im pretty sure my cr might be a little lower because of the mitsu 4layer head gasket so i might be at like 8:8:1. But as far as driveability the car runs awesome. Im running a ported big16g and spoolup is so awesome now im getting full boost a lil under 3000rpm. So all around performance is very good now which i like.

 

[Damn 2nd Gear]

1995 esi - apc intake, apc plugs, apc wires, apc hoses, apc wire loom, apc washer nozzles, apc hood pins, apc breather filter, apc sticker, apc shifter, apc light up dipstick, apc red cam gear, apc blue cam gear, apc red timing belt, apc coilovers, apc shocks, apc camber kit, apc caliper paint, matching apc oil/gas caps

those mods belong on a honda........ :laugh:

 

[Maglin]

1995 esi - apc intake, apc plugs, apc wires, apc hoses, apc wire loom, apc washer nozzles, apc hood pins, apc breather filter, apc sticker, apc shifter, apc light up dipstick, apc red cam gear, apc blue cam gear, apc red timing belt, apc coilovers, apc shocks, apc camber kit, apc caliper paint, matching apc oil/gas caps

those mods belong on a honda........ :laugh:

You know that APC sticker adds 20HP.

So I guess Maybe I wasn't totally wrong in the info that I had. I best gas I can get in ND is 92 octane so I'm definitly not going to go over 8.5:1.

 

[1fast97gsx]

1995 esi - apc intake, apc plugs, apc wires, apc hoses, apc wire loom, apc washer nozzles, apc hood pins, apc breather filter, apc sticker, apc shifter, apc light up dipstick, apc red cam gear, apc blue cam gear, apc red timing belt, apc coilovers, apc shocks, apc camber kit, apc caliper paint, matching apc oil/gas caps

those mods belong on a honda........ :laugh:

those mods are a joke ... I don't even know if they make light up dip sticks. It's my n/t beater

 

[1fast97gsx]

well the compression isnt awful but you most certainly need to have the link install and get that thing TUNED!!! you would not believe how much that will wake the car up. get it on a wideband meter and dial that puppy in.

Eric

What would you suggest to do about the compression? I don't really want to redo the engine again but if I have to I will. Should I leave it till something breaks? How closely can you guys get my car tuned just using a street tune? I don't care about the actual whp number as long as it's pulling as hard as it should so no point in putting it on the dyno. I want to dyno it on race gas and 28-29 psi with the nitrous to see my true hp number

 

[AMS97EclipseGSX]

I have 8.8:1, I'm sure the compression is actually with 8.5:1 with the head milled. I would say stay with what you got it is fine. Stock 2g compression is 8.5:1 and I have seen a lot of fast 1G's on 2G stock pistons. You will make good power on the 8.5:1 pistons.

 

[ItsStockOfficer]

I have 8.8:1, I'm sure the compression is actually with 8.5:1 with the head milled. I would say stay with what you got it is fine. Stock 2g compression is 8.5:1 and I have seen a lot of fast 1G's on 2G stock pistons. You will make good power on the 8.5:1 pistons.

Yes, becuase the more you mill the head, the lower your compression gets. I am definitley all for getting tuning advice from AMS!

 

[1fast97gsx]

Yes, becuase the more you mill the head, the lower your compression gets. I am definitley all for getting tuning advice from AMS!

milling raise compression .. he said he is running 8.5:1 pistons with a milled head netting him 8.8:1 compression. Sounds reasonable.

Just curious ... what are your compression numbers when you hook it up to a tester with 8.8:1? I'm running 8.5:1 with a milled block, and head was milled twice. I was shooting for 8.9:1 or so, but the mls gasket ( I'm assuming ) lowered it back down quite a bit. My average compression is still only in the high 160s though.

 

[ItsStockOfficer]

milling raise compression .. he said he is running 8.5:1 pistons with a milled head netting him 8.8:1 compression. Sounds reasonable.

Just curious ... what are your compression numbers when you hook it up to a tester with 8.8:1? I'm running 8.5:1 with a milled block, and head was milled twice. I was shooting for 8.9:1 or so, but the mls gasket ( I'm assuming ) lowered it back down quite a bit. My average compression is still only in the high 160s though.

Ok, misunderstanding, I apologize.