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Discussion Starter #1
I know that RB26 normally run around a 8.5 compression. The question is how much boost you can run until you reach the limit of pump fuel.

I see cossy run 2bar of pressure. and I see some Evo in the US run 2.5bar of pressure. Are these all done on pump fuel? If so, what is preventing RB26 to run 1.8-2 bar of pressure? I see most big tune road GTR back off around 1.6bar or so.

If the turbo is efficent say at around 2bar, what factor is limiting RB26 from runing high boost. is it the motor compression? the UK fuel? A properly tune/mapped road GTR on a good turbo should be able to run 2bar all day long with no ill effect correct? This is most likely for single turbo application as most twin seems to leave its effective area at these pressure.
 

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XS Engineering in California is getting something like 800whp on pump gas.
 

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Discussion Starter #3
No, not Whp, but boost.

From what I know, it is safer to run a bigger turbo at low boost than a smaller turbo at high boost to achieve a certain power figure.

Basically RB26 + GT35R @ 2.5bar = 650bhp
the same RB26 + T04Z @ 1.5bar = 650bhp

The T04Z will have less chance of det because there isn't as much pressure in the cylinder in theory. Or at least that was what I read. You can lower the chance and run higher boost with race gas, then the limitation will be put on the engine parts.

But I would have to say most RB26 with forge pistion and H-shape con rod should be able to take high boost. However, the only person I have heard running high boost is TweenieRob with is old GT35R kit @ 2 bar.
 

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that doesn't make any sense. Yeah the single is running at a lower pressure, but it's pushing a greater *volume* of air. So in terms of how much air and how much fuel goes into a cylinder, at 650bhp, it's going to be the same. Meaning, the air/fuel pressure inside the cylinder will also be the same, and det potential also the same. You can only get lower cylinder pressures and run the same amount of air and fuel (equalling power), by increasing cylinder volume, in other words - bigger displacement.
 

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Discussion Starter #5
Yea, I know what you mean. That was my thought also.

However, from what I read, you want to lower the Compression ratio of the engine to run higher boost. Something to do with higher boost and higher heat and things like that. And the higher heat can cause det, etc.

Rob blow a ringland from his GT35R at 2.3 bar. There was so sign of det as he said, so that was purely a pressure issue at that point.

The thing with the GT35R is that they seems to be most effective around 2bar, and I don't see much people running that kind of pressure.
 

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I see what you're saying. Hard to say why Evo's can run so much boost, I wonder what the comp ratio on a 4G63 engine is?
 

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I see what you're saying. Hard to say why Evo's can run so much boost, I wonder what the comp ratio on a 4G63 engine is?

Stock is 8.8,mine is 8.3,im running my Evo6 at 2.3 bar,no probs regarding det on 98oktan pump fuel :smokin:
 

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that doesn't make any sense. Yeah the single is running at a lower pressure, but it's pushing a greater *volume* of air. So in terms of how much air and how much fuel goes into a cylinder, at 650bhp, it's going to be the same. Meaning, the air/fuel pressure inside the cylinder will also be the same, and det potential also the same. You can only get lower cylinder pressures and run the same amount of air and fuel (equalling power), by increasing cylinder volume, in other words - bigger displacement.
Basically RB26 + GT35R @ 2.5bar = 650bhp
the same RB26 + T04Z @ 1.5bar = 650bhp
Neither of those make any sense to me. Boost pressure is the result of the turbo trying to flow more air than the engine can, and the restriction causes pressure. 1 bar through a massive turbo is the same as 1 bar through a small turbo (temp / efficiency differences aside). On the same engine a big turbo and a small turbo will flow the same (mass) of air if they are at the same boost pressure (Again temp / efficiency aside) since if boost is present it is the engine VE which controls the flow, not the turbo.
 

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Discussion Starter #9
I am going out on a limb here.

The T04Z can flow about 650bhp (crank) at 1.5bar, I am sure it has been done many times before.

The GT35R cannot flow 650bhp at 1.5 bar, as someone here has made only 530bhp with it at 1.6bar. But the compressor map show that it is efficent well over 2bar, and people with EVO have prove that by making 650 wheel horsepower with the same GT35R.

It may be a different configuration, but I am pretty sure bigger turbo can make the same power as smaller turbo at the same pressure (bar). However, bigger turbo also give up the response when compared to smaller turbo. So this is the old arguement again "bhp vs. boost threshold"

The Evo I saw running 650 whp is running a 2.3 stroker kit with compression at 8.5.

I am sure most GTR is around this 8.5 also, and therefore I cannot see why it cannot run around 2 bar. Unless other part of the car cannot take the stress, but I would assume most forge piston + H-shape conrod should be able to handle that.
 

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It may be a different configuration, but I am pretty sure bigger turbo can make the same power as smaller turbo at the same pressure (bar). However, bigger turbo also give up the response when compared to smaller turbo. So this is the old arguement again "bhp vs. boost threshold"
I take it from the other things you have said you mean 'more' - not 'the same'. I would agree that it could definitely make the same flow / power at the same boost level.

If you mean that the larger turbo makes more then the T04Z must be either more efficient at that flow wasting less energy, converting less into heat, or be less restrictive through the exhaust gas route (Improving VE).

650bhp requires a certain mass of air to flow every second. This doesn't change depending on which turbo you have fitted. The mass of air flowing is a function of resistance (VE), pressure and temperature. If the pressure is the same then either the temperature or the VE must be changing.

With regards to compression ratio i might be wrong but i believe a Cosworth YB is 8:1 std and most of those running 1.5 bar+ will be more like low 7. SteveN seems a bit of a Cosworth guru so might be able to clear that up?
 

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If you want more boost than you can safely do on pump fuel and 8.5:1 its not hard is it, lower the comp a bit.

Might be able to run whatever boost at whatever comp, but the in-cyl pressure might be stupid high for what your getting out of it, or the ignition is so retarded that the EGTs and general temps are mega high.

Higher boost at whatever lower compression gives much more power than whatever boost was safe at the original higher comp, through airflow and more advanced ign in many respects.
Providing the turbo wasnt already at its limit that is!

Main advantage of boost is torque, and powerband.
Ie its one of the VERY few mods that will give you a noticable torque increase, and it doesnt **** your powerband low down like wild cams, bigger turbos, or whatever would.

Do a search for dyno sheets ive posted in the past, they tell their own story really.
 

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Discussion Starter #12
So is there a way to calculate what is "safe" boost level at a certain compression, especially for a RB26?

Say we take the following engine:

RB26 @ 8.8:1
RB26 @ 8.5:1
RB26 @ 8.3:1

All assuming that the internal is made to withstand the power, so con rod bolt won't break and piston come flying out. Safe map of couse.

Lets use GT35R just in this case as it has been use before, and we know that it is still effective over 2 bar.

Given this options, how do we calculate or decide what is consider as safe boost level. Not just a case of "becase everyone is not going over" X bar, then I shouldn't go over that either.
 

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Discussion Starter #14
I see,

so it is kind of a guessing game until things blow up. Once it does, then you know you have got a bit too far. :chuckle:

Better safe then sorry I guess.
 

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The only true way to tune a car to the limit is literally tune it till it dets slightly, then back it off a little.
But thats risky for obvious reasons.

Regarding compression, you cant EXACTLY tell, but from experience of various cars and specs you would be able to get a ballpark figure for a compression ratio if you have all the other specs decided.
IMO the compression should be a comprimise along with the rest of the spec, tho comprimise is a bad word for it, basically all the mods should work alongside each other, rather than compensating for a particular one.

Many mods I see on cars are self defeating, which makes for all these cars with plenty of bhp, but horrendous drivability compared to what wouldve been possible with a no more expensive, but better thought out, spec.
 

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My Evo engine for comparison ran a version of a GT35 & made circa 650bhp at the flywheel on an engine dyno.

It was a 2.15, using 97ron Esso SUL, 2.0 bar of boost & standard rev limit.

It made good low down torque, as well as across the rev range. It could have taken more boost & more revs as it was a quiet at as mouse, but it was built for track use so a big margin of safety was included.

I would have liked to have seen what it would have done with race fuel & a smidge more boost & revs however, but it's sold now.

Its a tough one with Skylines & the relatively short stroke/high rev nature of the engine - its never going to be a torque monster, however I think people should try harder with what you can get out of a normal fuel pump :)
 

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the relatively short stroke/high rev nature of the engine - its never going to be a torque monster
In my eyes thats another myth.

Skylines are 73.7mm stroke. And pack 2.6litres
Cosworths are 77mm stroke. And 2litres.
Mitsi Evos are 88mm stroke. And 2litres.
SR20DETs are 86mm stroke. And 2litres.
Capacity has a bigger effect on torque than stroke length anyhow, esp over half a litre more.

Heres a coupla dyno printouts from YBs, just first two I could find with 300bhp per litre or more.




Id deffo call them torque monsters, even tho they low comp and far shorter stroke than the other 2litres, and over half a litre less than a GTR.

Id like to see some dyno sheets from the others that were similar, never looked on Evos, but deffo never seen SR20 or RB26 ones even close to that really.
 

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Are they still standard stroke & 2 litre capacity?

As a comparison, I only have to hand the initial mapping results from my engine, but it was pushed a little harder later & these were the results:

2500rpm
75bhp - 134ft/lbs

3000rpm
134bhp - 232ft/lbs

3500rpm
288bhp - 430ft/lbs

4000rpm
372bhp - 472fl/lbs

4500rpm
422bhp - 493ft/lbs

5000rpm
473bhp - 494ft/lbs

5500rpm
515bhp - 489ft/lbs

6000rpm
559bhp - 488ft/lbs

6500rpm
593bhp - 477ft/lbs

7000rpm
611bhp - 465ft/lbs
 

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As others have said, determining safe boost/CR isn't as simple as some think.

If you have a restrictive exhaust, you're going to get more hot residual gases left in the cylinder at the end of the exhaust stroke, thus getting in the way of fresh charge and increasing cylinder temps. A smaller turbo exhaust housing will also have this exhaust-restricting effect, which is one (but not the only) reason why you can get more power at the same boost from a larger turbo.

With higher cylinder temps, you're more prone to det and so have to run your ignition more retarded, losing you power.

Without such efficient removal of exhaust gases from the cylinder due to these exhaust restrictions, you'll be getting less fresh charge into the cylinders even at the same boost pressure and inlet temps because when the fresh charge tries to get in there's some old stuff left over and getting in the way. That loses you power as well.

Smaller compressors (inlet side of turbo) also tend to not work so efficiently at high boost levels. A bigger compressor might be closer to its efficiency island than a smaller one at the same boost. This being the case, the bigger compressor will add less heat to the charge, so lower charge temps and therefore greater charge density, therefore greater mass flow of air at the same boost, therefore more fuel mixture entering the engine, giving more power.

There are many other reasons too.
 
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