xkhalil333x

Hello all and thank you in advance.... I have a b16a that i bought already built. It has 7:1 compression ratio (stupid) pistons, springs, retainers etc. It uses a custom turbo kit with a garret potato turbo and a haltech plat 1000 ecu along with injectors, fuel pump etc etc. We tuned it and it ran easy at 20psi with no pinging. Now the problem is that it layed down 230 whp lol. When i saw the read out i almost shat myself. My b18cr stock with a turbo kit made 240whp. Anyhow my mechanic said that the 7:1 is way too low. I was thinking i should change it to 9:1... what kind of power do you think i can gain by just doing this with the same boost pressure etc?

Tony the Tiger

With a Garrett GT28RS aka disco potato, even 9.0:1 is too low... Stay with 9.5:1 and you can still max out this turbo without pinging.

Due to having such low compression, the engine is not even working properly and it affects everything. There is no eficiency at all, so you could already be maxing out the turbo because each lbs/min of airflow is yielding much less power. By increasing the compression, you should end up with typical numbers at 20 PSI, which would be 300+ WHP.

91jdmhatchback

Ok really, 9:1 is to low? I mean i understand with the tuning programs available and the fact the heads flow great on Hondas allow them to do things that most other engine can't do. but to say 9:1 is to low for a boosted application is just asking to get laughed at.

Personally 7:1 is a bit low but it's more then workable and will not hurt his efficiency much. there is a reason the gt28rs is fine to 20+ psi.

Dyno numbers don't mean a lot to me. as a mustang reads quite a bit lower then a dynojet or pack dyno more often then not. So take the numbers with a grain of salt and concentrate on trap speeds and how it is actually running.

Also a b18c is way different form a b16 btw.

Spawne32

iTrader: (3)

less cfm = more compression, never use PSI as a basis for determining your compression ratio

8YourV8

Um, what?

xkhalil333x

Ok thanks guys... So should i go with 9:1 or 9.5:1 with my 20psi?

91jdmhatchback

What are you talking about? I'm referencing the fact that the compressor map for the gt28rs graphs flow to 22 psi. which means the turbo functions just fine at those pressures, so the drop in static compression would not be that big of a deal.

Go read a compressor map, once you can figure out what your looking at reread what i said and it will make sense.

91jdmhatchback

Well for one you haven't given enough information. If for instance your on a low reading dyno your numbers wouldn't be all that bad for a low compression 1.6L using a turbo with an advertised flow rate capable of supporting 250-360 hp.

How do you know you even have a good tune? I have seen huge gains from going to a good tuner.

8YourV8

Just because the turbo is good for 22 psi doesn't mean that 7:1 cr is helping him in any way... i would say yeah, if he was using a bigger turbo, but he would probably be able to reach much higher numbers at 15 PSI and 9.5:1 with his current snail. Way more efficient, and reliable.

Mackzero

7:1 compression is too low. Period. If you are going to stay with your current turbo, just go 9:1. We build most of our turbo motors 8.5:1 but 9:1 will be fine. Its all in the tune after that. Dynos should be used to tune the motor for flat spots in the power curves... Don't put too much stock in your peak numbers. The higher compression will help with your torque down low also. Good luck.

turtleman321

its about to be 2011 if your tuner knows what he is doing then compression shouldnt be an issue.

7:1 is ungodly low. I would run 10:1 then you wont have to run 20 psi to make desirable numbers

xkhalil333x

yes my tuner know a lot. he is not the one who put the 7:1 in but the one who discovered the issue. ill do 9:1

Tjabo

It seems to me that there may be some potential for BIG numbers in a 7:1 compression build, but it's going to take a lot of PSI to do it, i.e. a more modern turbo. The low compression can't help make a nice, quick spool, but once the turbo is making boost, the motor will be very accepting of a lot more boost than people ordinarily use with higher compression builds.

In a theoretical sense, ridiculously low compression builds like this have always had more power potential on pump gas than higher compression builds because the detonation threshold is much higher. It's easy to get off track though if you think in terms of how much boost is "okay." For 7:1 compression, the "okay" boost numbers are really high, but the turbo choice is different than one for the "normal" build.

91jdmhatchback

Well I'm glad someone could bring something to the table that wasn't complete hearsay or ignorant rhetoric.Thank you sir.

TheShodan

iTrader: (14)

I wouldn't say that really. The power amount is the same for an engine for a given turbo. If one is going for an effective compression of about 17:1 (which is what a turbocharged car can make on average), on a 10:1 engine it would take maybe 12psi to make "X" power, but would take 30psi on a 7.0:1 to make the same "X" amount of power. But it won't make more power than a 10:0:1 motor. The difference is the acceleration rate based on where the turbo is in its efficiency range.

0PSI2HI

If it were mine i'd be bumping that compression way back up. Your leaving a lot on the table w/ 7:1, pump gas or not.

Tony the Tiger

9:1 is way on the low side for a GT28RS.

You can use the full limits of a GT28RS well above 9.1:1 compression without even running into detonation issues. There is no reason to go lower with a GT28RS turbo setup, especially with a B16 when off-the-line torque and power is so critical. Do you like a slug that makes under 90 lbft of torque from 2000-4500RPM?

You can't generalize the word "boosted".. One guy can be boosted with a GTX42R good for 1000WHP, but the other guy is boosted with a T28 good for 260 WHP. They are both "boosted", but sure doesn't require the same motor build nor compression ratio.

Going lower will only lose response, increase BSFC under a given power, consume more mass air to make the same power output, and lower output out of spool and during onset boost. The same mass air will no longer support the same amount of power due to reduced efficiency, so the turbo will max out the compressor much sooner and require more flow from the compressor to deliver identical power as a higher compression motor.

High vs low compression, do a search here. It's very easy to understand about compression versus power vs octane.


Here's a video of my old GT28RS setup.
http://www.youtube.com/watch?v=CGVEhW3cqLw

Spawne32

iTrader: (3)

i agree with tony, again, everyone takes too much stock in PSI ratings ONLY, the GT28RS does not move enough CFM to make use of 7:1 compression pistons. Especially at low RPM's.

xkhalil333x

so what compression do you recommend?

Spawne32

iTrader: (3)

9:1 at LEAST, depending on how your tuning it, and what kinda limits you wanna push with compression should determine if you want to go higher. Im running 10:1 on a supercharged application on 5-10psi...but the most my supercharger will ever flow in CFM at its maximum speed is 300cfm. Where as a turbo at 10psi might move 1000CFM.

Tony the Tiger

9.5:1 would be ideal, also to account for the day if you decide to go slightly bigger on the turbo, or if you must run some sort of low octane gas like 89.

There are many of those who ran this turbo on a stock bottom-end, which would be 10.0:1 or higher, and still could run this turbo to its full potential.

K7-1Ktrevor

loved that video, the power in your teg seemed to be perfect for the street, like it was a all motor car.

Bond

lol acceleration from a stop with a 7:1 compression must be like starting in 2nd gear. Wouldn't his spool time to hit say 20PSI be a long ways up the powerband due to such a low CFM producing motor?

91jdmhatchback

You obviously have never been in a 7:1 compression car. I have a d16 running 7:1 and get 12 psi by 3k on a sr t25. I remember the same idiots saying it will never spool and the car won't have out of boost power. LOL

You honestly think 7:1 vs 9:1 makes a **** of difference for low speed drive ability?

91jdmhatchback

Thanks for the wicked advise to search.