S15 turbo - what hp is it good for?

**Roo** · 24-12-2003, 08:17

Hi,

I'm thinking of using the Garret GT28 as fitted as std to the S15 as an upgrade to my S14a and wondered if any of you know what hp can be produced from this turbine? I know the S15 puts out 250 hp as std but don't know if the GT28 has a higher total output capacity than the S14 or if it's just working a bit harder in the later car. I'd be grateful for any advice or comments. I'm looking to achieve something over 300 hp from my car but don't want to cross the "expensive mods" line !

Merry Christmas,

Roo

**Papa Lazarou** · 24-12-2003, 08:26

I'm not 100% its exactly the same as the HKS GT-SS turbo, but I think it is.

The HKS GT-SS has been used for 340-odd hp on an SR engine with mild cams, 550's, FMIC etc. It differs from the standard T28 in having the 76 trim GT25 turbine wheel as well as the ballbearing core.

**TomM** · 24-12-2003, 09:21

The s15's T28 is reputedly not good for higher pressures.

Originally posted by Papa Lazarou
The HKS GT-SS has been used for 340-odd hp on an SR engine with mild cams, 550's, FMIC etc. It differs from the standard T28 in having the 76 trim GT25 turbine wheel as well as the ballbearing core.

Isn't Phatty getting 340bhp from a standard T28? In view of that, wouldn't a GT-SS plus cams hit a bit higher?

**Papa Lazarou** · 24-12-2003, 09:29

Depends on the boost I guess. About 1.2 bar is the most its really worth running on one of these turbo's.

I've not heard of 340hp on a standard S14 turbo.

HKS claim dyno'd figures of around 325 ps with 256/256 cams and 1.1 bar boost with a GT-SS.

**Vez** · 24-12-2003, 09:29

Whats the lag like on these, is it better or worse than the standard T28?

**Roo** · 24-12-2003, 09:36

Thanks for the comments guys - it looks like the GT28 might be just what I'm looking for. I've got a Power FC to install, Walbro and 550cc injectors. De-cat, front pipe and FMIC on the way from Bren and was thinking that 260 deg cams (or thereabouts) would probably give me around the 320 to 340 mark. Any thoughts on degree of lift I should go for on the cams? I think the std lift is 9.5mm but don't know what is a sensible upgrade - some cam manufacturers start at around 10mm but go up to 12mm or so. My guess is that the 12's would be way too "wild" for my state of tune, particularly as I'm not planning to go to forged pistons. For that reason, I'm not too worried if the GT28 can't run big boost pressure as I think the std pistons are only good for approximately 1bar.

All the best,

Roo

**Papa Lazarou** · 24-12-2003, 09:46

Should be less lag (i.e boost should build up faster again after coming off the throttle), with spool up maybe a bit quicker.

I know Billy has the GT-SS turbo, wonder if he's tried it out yet.

Many of the aftermarket companies make mild cams which are designed to be used without other modifications to the head - e.g HKS Step 1, Tomei "Poncams" etc. More agressive cams than this will require at least valve springs to be changed.

Seems that about 1.4 bar is a common maximum boost level used on standard pistons. But know of cases of up to 25 psi being used without them being damaged.

**archenemy.co.uk** · 24-12-2003, 09:46

Originally posted by Roo
Thanks for the comments guys - it looks like the GT28 might be just what I'm looking for. I've got a Power FC to install, Walbro and 550cc injectors. De-cat, front pipe and FMIC on the way from Bren and was thinking that 260 deg cams (or thereabouts) would probably give me around the 320 to 340 mark. Any thoughts on degree of lift I should go for on the cams? I think the std lift is 9.5mm but don't know what is a sensible upgrade - some cam manufacturers start at around 10mm but go up to 12mm or so. My guess is that the 12's would be way too "wild" for my state of tune, particularly as I'm not planning to go to forged pistons. For that reason, I'm not too worried if the GT28 can't run big boost pressure as I think the std pistons are only good for approximately 1bar.

All the best,

Roo

I think you may have already crossed the "expensive mods" line

**bren** · 24-12-2003, 10:05

Havent the S15 turbos got ceramic blades and therefore are only good for a maxlimum of 1bar?

We are running 359bhp at 18psi on standard cams. And thats with a 1.8mm metal headgaskets that will have lifted the head and knocked the cam timing out.

I personally wouldnt bother with uprated cams at your level. From the power we're getting with our S14 without them they dont seem necessary.

The standard pistons are good for a chunk more than 1bar on a T28. We run upto 1.5bar peak with no probs and we run 50bhp nitrous jets on top of that. The nitrous gives us a whopping great extra 90lb/ft of torque at 3k rpm so our pistons are under enormous strain and seem to just shrug it off.

Ive certainly not managed to blow the engine despite numerous balls out track days, 1/4 mile runs. top speed events etc.

**Papa Lazarou** · 24-12-2003, 10:25

The S15 has an inconel turbine wheel I believe (special high temperature metal). Info on it here http://www.burnsstainless.com/TechAr...l_article.html

**MarkH** · 24-12-2003, 11:34

Why is the turbo limited to 1.2 bar, I thought the sky was the limit with Ball bearing turbos, or are other factors kicking in here!

**Vez** · 24-12-2003, 11:37

Its more to do with how hot the density of air gets as it is compressed under higher and higher presures!

**MarkH** · 24-12-2003, 11:44

So it is recognised that this particular turbo is limited in its efficiency, and operates at its optimum at 1.2.bar?

**SR20DET** · 24-12-2003, 11:56

Im pretty sure the S15 turbo has the same cr@p ceramic internals some skylines do have. Theyre prone to cracking if the boosts only upped a little bit. I wouldnt fancy some ceramic bits in my engine...

Daniel

**Papa Lazarou** · 24-12-2003, 12:04

It depends on the size of the engine. Smaller engine it will be able to run higher boost pressures. I believe in GTR applications they are typically used at about 1.4-1.5 bar.

**TomM** · 24-12-2003, 12:16

Originally posted by MarkH
So it is recognised that this particular turbo is limited in its efficiency, and operates at its optimum at 1.2.bar?

No, not really; the s15 turbo is limited by the fact it has been known to destroy itself if you run pressures that are too high for too long.

IIRC it was Jason Pigeon that blew his up at 17psi;
Link

Vez is correct that all turbos have a maximum effective pressure; Phatty (and Abbey) found that above 16psi, the standard T28 doesn't really gain much 'cos it heats the air too much. IIRC the maximum recommended pressure for the s15 turbo is below this.

@Bren: I've noticed in the past that you're not a fan of cams. Surely it depends on the power level you're hoping for long-term? You're going for 600bhp, so for the time being, there's no point in wasting money on cams. If I want to run circa 350bhp long-term, then I'd probably get nice gains in various things (spool-up, mid-range torque, maybe a touch more power too) with a well-chosen set of cams.

**MarkH** · 24-12-2003, 12:31

Originally posted by Papa Lazarou
It depends on the size of the engine. Smaller engine it will be able to run higher boost pressures. I believe in GTR applications they are typically used at about 1.4-1.5 bar.

I thought it was the other way around!

I thought the bigger the CC the more exhaust power you had to the turbine enabling faster spool up and greater PSI!

Are you saying that the turbo becomes more efficient with lower CC engines because there is less heat from the exhaust to spoil the efficiency?

**steve(s14a)** · 24-12-2003, 12:41

Originally posted by MarkH
I thought it was the other way around!

I thought the bigger the CC the more exhaust power you had to the turbine enabling faster spool up and greater PSI!

Are you saying that the turbo becomes more efficient with lower CC engines because there is less heat from the exhaust to spoil the efficiency?

Yes. More CC = more exhaust gas hence quicker spool up.

But more exhaust gas going through the turbo = more heat hence boost can not be set as high due to temperature considerations.

**MarkH** · 24-12-2003, 12:45

Cheers mate,

I like a bit of techy talk!

Does anyone know of forums that specialise in techy chat for car tuning?

**steve(s14a)** · 24-12-2003, 12:45

I think you're in one!

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