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Old 06-02-2006, 05:15 AM   #1
STI8U
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Default What Compression Ratio to run?

I'm doing a Tomei 2.2L stroker kit in my EJ207 and want to know what CR I should run.

Options are:

8.2:1 (0.7mm head gasket)
8.0:1 (1.0mm head gasket)
7.9:1 (1.2mm head gasket)
7.7:1 (1.5mm head gasket)

Plans for my car are an HKS GT2835 twin scroll turbo (450HP) - probably running 1.3bar to 1.6bar (no higher).

I've been told not to go too thick on the head gasket, as it affects quench (i.e. ability to resist detonation).

I'm leaning towards either 8.0:1 or 8.2:1, but the Tomei contact in Japan recommends 7.9:1 for the GT2835.

I'm also considering Kelford 264 AVCS cams which will shift the power band towards the higher RPMs, so perhaps 8.2:1 is the way to go to keep the engine responsive down low?
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Old 06-02-2006, 07:36 AM   #2
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I'll give you my opinion. I originally had my 2.5l with stock Sti head gaskets and WRX heads. The result, according to Ron at Axis Power Racing, was a CR of 9.17:1. The vehicle with a VF22, was super responsive and boost hit like a freight train.
I got stupid with the boost and turned it up w/o remapping the fuel. This resulted in running it lean and breaking ringlands in number 2 and 4 pistons. Of course I got scared and lowered compression to 8.5:1 using thicker .051" Cometic head gaskets. Needless to say I don't really like the lower part of the revs.
SO, I would agree with you and go with either the 8.0 or the 8.2. I am not sure how big the GT2835 is but being twin-scroll the boost with definetly come on quick. That, paired with the "high" compression of 8.2:1 would be a nice touch, IMO.

Oh, and I just saw that you have a Spec-C type RA, so your gear ratio is killer. MAN, wanna donate any parts you've taken off to a good cause?

My .02, hope it gives you some form of direction.

Jose
WPB, Florida
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Old 06-02-2006, 08:15 AM   #3
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Quote:
Originally Posted by Pacobeagle1
I'll give you my opinion. I originally had my 2.5l with stock Sti head gaskets and WRX heads. The result, according to Ron at Axis Power Racing, was a CR of 9.17:1. The vehicle with a VF22, was super responsive and boost hit like a freight train.
I got stupid with the boost and turned it up w/o remapping the fuel. This resulted in running it lean and breaking ringlands in number 2 and 4 pistons. Of course I got scared and lowered compression to 8.5:1 using thicker .051" Cometic head gaskets. Needless to say I don't really like the lower part of the revs.
SO, I would agree with you and go with either the 8.0 or the 8.2. I am not sure how big the GT2835 is but being twin-scroll the boost with definetly come on quick. That, paired with the "high" compression of 8.2:1 would be a nice touch, IMO.

Oh, and I just saw that you have a Spec-C type RA, so your gear ratio is killer. MAN, wanna donate any parts you've taken off to a good cause?

My .02, hope it gives you some form of direction.

Jose
WPB, Florida
The HKS GT2835 has a 53.1mm/71.1mm compressor wheel and 51.8mm/56.6mm turbine wheel...

Here's HKS own information on the GT2835 turbo - the red line is the GT2835...as you can see the turbo is not hitting 1.3bar until 4000RPM or so. I'd imagine the highest you would run this turbo would be 1.6bar, anything more and you will run outside of its efficiency range.



So let's say I want to run 1.6bar through the GT2835 - would the 8.2:1 CR be safe for this (I will be running 800cc SARD injectors)...
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Old 06-02-2006, 08:24 AM   #4
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Definitely go with 8.2:1 or even higher. The gt28 series really isn't made to flow a ton of air. Those turbos are better suited for medium boosted SR20DETs or stock WRX 2.0. If you want to run higher boost and not run out of efficiency in the upper rrrr's upgrade to a 3071r or another similar trim.

8.2:1 should give you the best balance between low end responsiveness and not overly high for upper end power and safety. Anything lower and I'd say build those heads for a much higher redline, get a huge turbo and shoot for honda characteristic power like some of the other guys around here do.

My motor is going to be a 2.2 bored and stroked to 2.5. I'm using Wiseco 8.5:1 compression pistons with ported and polished heads that have had the combustion chamber reshaped and slightly enlarged. I need to measure valve clearance to see if I can go with a thinner then stock headgasket. I'd really like to keep compression closer to 8.5-8.9:1. Those heads should be capable of 9000 rpm. I'm planning to rev to 8000-8500 but don't want it to be too soft down in the lower rpms.

Brad
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Old 06-02-2006, 08:35 AM   #5
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OK so my car is a JDM STI Spec C Type RA, 2.0L running 8:1 CR stock.

I melted a couple of pistons a couple of weeks back, hence the 2.2L stroker is going in.

The car has a 8000k redline and gearing in late generation Spec C and Type RAs is the same as the 6-speed on the JDM/USDM STI.

Tomei (who make the stroker kit) recommended 7.9:1, but I feel 8.2:1 with a nice thin gasket to increase quench is the better choice as well. This is a daily driver so responsiveness is important...

edit: Sorry, my bad, JDM 6 speed is different from USDM 6 speed. The Spec C has the same ratios as the JDM STI...

Last edited by STI8U; 06-04-2006 at 06:36 AM.
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Old 06-02-2006, 08:57 AM   #6
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Sorry I was writing my post at the same time you posted your last one.

Those sizes should be a great combo for 8.2:1 and 8k redline. If you were running a larger turbo I would definitely say the lower the compression the better. That way you could take advantage of the larger wheels and housing and run higher boost. If your not spooling until 4000 you should be fine running that turbo at current 1.6bar with .2 litres more displacement and 2 points higher compression. You should spool a few hundred up to 1000 rpms quicker but be able to sustain 1.6bar to redline. I'd bet in the higher rpms you'd be pretty close to beginning to work outside the efficiency range.

In plain english I think you'll be fine but to best take advantage of your setup a slightly larger turbo may be needed. Is your 28 ball bearing? If not you could upgrade to one of the 30r ball bearing turbos.
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Old 06-02-2006, 02:16 PM   #7
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Don't ruin your quench with a thicker head gasket.

I think you can go higher than stock 8.2, but staying below a static CR=9 seems to be preferable unless you have aftermarket cams that will lower the dynamic CR.
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Old 06-03-2006, 02:15 AM   #8
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Quote:
Originally Posted by MY99 2.5GT
Sorry I was writing my post at the same time you posted your last one.

Those sizes should be a great combo for 8.2:1 and 8k redline. If you were running a larger turbo I would definitely say the lower the compression the better. That way you could take advantage of the larger wheels and housing and run higher boost. If your not spooling until 4000 you should be fine running that turbo at current 1.6bar with .2 litres more displacement and 2 points higher compression. You should spool a few hundred up to 1000 rpms quicker but be able to sustain 1.6bar to redline. I'd bet in the higher rpms you'd be pretty close to beginning to work outside the efficiency range.

In plain english I think you'll be fine but to best take advantage of your setup a slightly larger turbo may be needed. Is your 28 ball bearing? If not you could upgrade to one of the 30r ball bearing turbos.
Yes the GT2835 is ball bearing - the reason I am going with this turbo is that it is a twin scroll bolt on (i.e. it has a twin scroll turbine housing with appropriate downpipe flange). Looking at the Garrett GT2871R compressor map (closest thing I could find to the HKS turbo), 1.6 bar @ 7000RPM has the engine flowing 40lb/min assuming 2123cc, 85% VE, intake depression of 1psi and pressure loss of 2psi. On the GT2871R map, this plots at around 74% efficiency and around 121k RPM (max RPM is 126k RPM), so it looks OK.

With 1.6 bar @ 7500RPM, the turbo is hitting its maximum RPM but still operating around 72% efficiency...
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Old 06-03-2006, 11:50 AM   #9
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Quote:
Originally Posted by STI8U
Yes the GT2835 is ball bearing - the reason I am going with this turbo is that it is a twin scroll bolt on (i.e. it has a twin scroll turbine housing with appropriate downpipe flange). Looking at the Garrett GT2871R compressor map (closest thing I could find to the HKS turbo), 1.6 bar @ 7000RPM has the engine flowing 40lb/min assuming 2123cc, 85% VE, intake depression of 1psi and pressure loss of 2psi. On the GT2871R map, this plots at around 74% efficiency and around 121k RPM (max RPM is 126k RPM), so it looks OK.

With 1.6 bar @ 7500RPM, the turbo is hitting its maximum RPM but still operating around 72% efficiency...
this being a turbo car... VE looks more like 100% to 110%
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Old 06-03-2006, 06:24 PM   #10
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Quote:
Originally Posted by ejsportcom
this being a turbo car... VE looks more like 100% to 110%
Is this correct? Every turbo sizing tutorial I've read states to use a value of 85% VE at high RPMs...
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Old 06-03-2006, 08:36 PM   #11
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85% is just a ballpark figure. Actual VE varies widely between engine designs


on most turbo cars you'll see a lower VE before boost (turbo restricting the intake) and once it spools you'll see >100% VE.

Last edited by solarin006; 06-03-2006 at 08:47 PM.
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Old 06-03-2006, 10:12 PM   #12
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GT32 is a non ball bearing turbo but is offered in a twin scroll setup. I don't know about the dp flange and such but I'm thinking you can get any Garrett turbo with either the 3 or 4 bolt flange.
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Old 06-03-2006, 10:52 PM   #13
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Here's a picture of the HKS GT2530, which is smaller than the GT2835...



You can see that the twin scroll entry is quite different from a GT32....

Aqua in Japan have managed 430PS @ 1.55bar on the GT2835, on a setup similar to mine (JDM STI Spec C, 2.2L stroker), which is exactly where I want to be...(+ another 20PS would be nice, should be able to get there with water injection which I'll be running)...
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Old 06-08-2006, 05:04 AM   #14
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Aqua told me they are running 8.5:1 in their demo car, which is running the HKS GT2835, 2.2L stroker, cams and puts out 430PS @ 1.55bar...

So based upon that 8.2:1 should be sweet for the 1.6 bar max boost I will run...
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Old 06-08-2006, 07:28 AM   #15
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Here's the dyno of the GT2835 twin scroll from Aqua - 392PS at the hubs @ 6000RPM holding to 7500RPM is pretty impressive and making 300lbft @ 3500RPM...

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Old 07-16-2006, 11:41 AM   #16
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Hi STI8U, we are actually in the same boat. I have the same Tomei 2.2L stroker kit with 272 Kelford cams. Turbo wise though is slightly different, I'm using a Blitz K-450R kit.

I decided to go with the 1.1mm gasket, my mechanic will be measuring the actual CR tomorrow so I will post them up here for your reference.
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Old 07-17-2006, 04:52 AM   #17
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Cool thanks, I'll get my CR measured as well as I went with the 0.7mm gasket...
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Old 07-17-2006, 02:31 PM   #18
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OMG i that graph looks awesome. I was going to just go with a GTRS/GT28 of some sort. The twin scroll looks awesome tho. I might have to hit that ish. Any idea how this turbo would respond on a built EJ205? Stock CR. Probably less responsive. Maybe I would need something smaller. That graph is sick for a non USDM STI.

cheers

garrett
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Old 07-17-2006, 02:39 PM   #19
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If you go with the Spec C cams (your other thread) there is at least some precidence for a CR of 8.7 with those cams and similarly sized turbos. That's about as good as it gets. Not many people here have experimented much with CR..........that's going to change.
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Old 07-17-2006, 04:42 PM   #20
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Quote:
Originally Posted by coxmaster
OMG i that graph looks awesome. I was going to just go with a GTRS/GT28 of some sort. The twin scroll looks awesome tho. I might have to hit that ish. Any idea how this turbo would respond on a built EJ205? Stock CR. Probably less responsive. Maybe I would need something smaller. That graph is sick for a non USDM STI.

cheers

garrett
I can't see why there would be any difference, the only difference between an EJ205 and EJ207 block will be strength, possibly different cooling characteristics might slightly alter performance?

The above dyno is of an MY06 JDM STI Spec C, so it has better flowing heads than any other STI...it also has HKS 272 intake cams with HKS variable cam control (it has stock exhaust cams, which are quite sufficient). If your built EJ205 had good heads with AVCS, you should be able to achieve similar results. The variable valve timing I'm sure helps with the first 30% of the dyno graph...

See http://www.aqua-fsp.com/democar.html
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Old 08-27-2007, 01:53 PM   #21
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So what gasket size did you end up using?
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Old 08-27-2007, 04:22 PM   #22
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Quote:
Originally Posted by MY99 2.5GT View Post
Sorry I was writing my post at the same time you posted your last one.

Those sizes should be a great combo for 8.2:1 and 8k redline. If you were running a larger turbo I would definitely say the lower the compression the better. That way you could take advantage of the larger wheels and housing and run higher boost. If your not spooling until 4000 you should be fine running that turbo at current 1.6bar with .2 litres more displacement and 2 points higher compression. You should spool a few hundred up to 1000 rpms quicker but be able to sustain 1.6bar to redline. I'd bet in the higher rpms you'd be pretty close to beginning to work outside the efficiency range.

In plain english I think you'll be fine but to best take advantage of your setup a slightly larger turbo may be needed. Is your 28 ball bearing? If not you could upgrade to one of the 30r ball bearing turbos.
I know I'm late to the thread, but do you trust 1.6 bar on pump fuel? That's getting up there, what type of timing figures would you run at that combination?

In reference to the original posted question. Higher compression ratios help with essentially every aspect of drivability. What it does is decrease the amount of horsepower you can make on a fuel of a given octane usually. If you were in a spec-fuel class or running only pump fuel, you will usually be able to obtain higher peak numbers from a lower SCR/higher boost combination. Your low end performance will suffer and so will your powerband in general.

A higher SCR/lower boost combination will have a superior powerband and usually have a better BSFC as well - use fuel more efficiently.

On a street car where you're not going for obscene power, I would go with a higher SCR and sacrafice some boost in an effort to have a car that's more fun to drive. If you want to lean on it get some VP Streetblaze 100 or C10 or something of the like and lean on the boost controller a bit.
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Old 08-27-2007, 06:09 PM   #23
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Quote:
Originally Posted by Mark@RealStreet View Post
I know I'm late to the thread, but do you trust 1.6 bar on pump fuel? That's getting up there, what type of timing figures would you run at that combination?

In reference to the original posted question. Higher compression ratios help with essentially every aspect of drivability. What it does is decrease the amount of horsepower you can make on a fuel of a given octane usually. If you were in a spec-fuel class or running only pump fuel, you will usually be able to obtain higher peak numbers from a lower SCR/higher boost combination. Your low end performance will suffer and so will your powerband in general.

A higher SCR/lower boost combination will have a superior powerband and usually have a better BSFC as well - use fuel more efficiently.

On a street car where you're not going for obscene power, I would go with a higher SCR and sacrafice some boost in an effort to have a car that's more fun to drive. If you want to lean on it get some VP Streetblaze 100 or C10 or something of the like and lean on the boost controller a bit.

This sounds more reasonable. So with this in mind what CR would you recommend and the amount of boost to utilize it?
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Old 08-27-2007, 06:22 PM   #24
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9.0:1

If you can find a tuner comfortable to tune with that CR......go for it!!

For the boost.........you boost untill just before it knocks.
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Old 08-27-2007, 07:10 PM   #25
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Given your specific options, 8.2:1 without a doubt. Personally, I'd mill the heads to grab some more comrpession out of it. You have to realize, you're dealing with something that is only 2.2L, it's a street car, you want to enjoy it. If you had a 500 inch V8, you can sacrifice some compression ratio because the torque is there anyways.

I'd probably run about 1.4-1.5 bar with 8.2:1, but a lot of it is in the tune. Talk with your tuner, see what he recommends. I'm conservative on pump fuel because it can vary from tank to tank - hell, if you are filling up at a station that doesn't have individual pumps you may be pouring a little 87 in the tank from the Saturn that just pulled away from the pump.

On a ground-up deal I like compression ratio, but I would definitely recommend race gas when doing something like that because the octane requirements go higher much quicker. The current engine I'm doing is an 8.5ish engine (2JZ-GTE with a shelf CP piston) but if I build another one I'll probably do something in the 10:1 area.

Last edited by Mark C.; 08-27-2007 at 07:25 PM.
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