-5s or -7s

[Lith]

Nah you are about right there The -9s should be a lot more popular than they are....

[SamuraiSam]

Originally Posted by frostmotorsport

not sure I agree with that.... must mention it to my engine builder who slapped me about the ears when he rebuilt my 4G63 - as we'd been running 23PSI on a standard motor and stretched not only the head bolts but the threads in the block!!

if I understand simple physics correctly - and I think this is what Lith is getting at.... - cylinder pressure is dictated by the amount of air stuffed into the cylinder (in this case by boost) x the compression ratio - so a small increase in boost pressure = a relative (and large!) increase in cylinder pressure ergo more stress against the head bolts/gasket etc.

I know det killed my valves and pistons over time, and will certainly contribute to cylinder pressure - but it was the excessive boost we ran that caused the head to lift and kill everything....

now awaiting correction....

No, you should fire your tuner. The boost you ran had little to do with your head bolts stretching and lifting. You had poor head bolts (were you using stock items?) to lift in the first place and it was the excessive in-cylinder pressures caused by detonation, caused by too much ignition advance that caused your head to lift. The fact that you admit to having detonation at all is proof of this. If your head hadn't lifted you would have blown out a piston or cylinder wall or the head itself. Proper ignition timing mitigates large amounts of intake manifold pressure. Honda guys run 30psi on big turbos with standard ARP studs and don't have any problems lifting the head or detonation issues at all.

Lith, I have no information for you on the -9 turbos. I don't know anything about them, sorry.

[SamuraiSam]

Originally Posted by SteveN

No, DETONATION does that as the fuel cant cope with the current situation, not boost pressure itself.

You think an extra 15psi or even 30psi of pressure makes any difference when the cyl pressures are hundreds and hundreds of psi anyhow?

People are scared of boost without even knowing what they are scared about. No need.

Correct. Proper ignition timing for the fuel used and load the engine is under completely mitigates intake manifold pressure being a 'scare tactic'. I like boost and nobody should be intimidated by running a lot of it. Nitrous does the same thing, any sort of forced induction is simply trying to pack more oxygen molecules into a small space so that you can generate higher in-cylinder pressures and then ignite that mixture and burn it slowly to generate controlled cylinder pressures over the few degrees of crankshaft revolution you have as part of the combustion stroke.

[SamuraiSam]

Originally Posted by frostmotorsport

the majority of people have voted for -5's - which surprises me, as the RB26 is quite laggy with stock tubs.

LOL. NO, it's not. There is no such thing as lag.

Originally Posted by frostmotorsport

Interestingly enough, I've been driving a MY98 WRX of late and even as a non-STi - it would EAT a GTR point to point. any GTR. they are just so much more chuck-able and the turbo spools almost from idle..... quite impressive.

It might "Feel" like it does - but it wouldn't. Absolutely no way in hell. Compare the two cars using quantitative analysis. The 0-60 on a GT-R will smoke the 0-60 on a Impreza. The Impreza has a tiny turbocharger with a super low boost threshold that 'feels' fast and falls on its face at about 5500rpm.

[frostmotorsport]

you seem to have forgotten that roads (well, ones around here anyway) have corners.....

[Lith]

Originally Posted by frostmotorsport

you seem to have forgotten that roads (well, ones around here anyway) have corners.....

That gives two places to walk over the WRX. Straights, and corners

[SamuraiSam]

Hehe, correct.

Improper ignition timing=detonation. Detonation is when you have thousands of psi flame fronts colliding with each other. Uncontrllolled explosions generating many times over what std. combustion pressures are which is what kills engines and lifts heads.

[Lith]

Originally Posted by SamuraiSam

Improper ignition timing=detonation. Detonation is when you have thousands of psi flame fronts colliding with each other. Uncontrllolled explosions generating many times over what std. combustion pressures are which is what kills engines and lifts heads.

Yep, agreed. A chap I know has for some time now been running ~24psi on a stock internal (aside from big cams) 4G63 through a GT3076R running water/methanol injection for det suppression etc and no problems at all. So far. Touch wood That would probably have really high controlled cylinder pressures, on the turbo its running the average pressure throughout the rev range would be far more than a stock turbo'd EVO running 23psi. Decent head studs don't go amiss though, when dealing with high boost imho.

[frostmotorsport]

you make good points - except the ones about WRX's - I've owned and driven lots of fast cars and I'd take a good rex over a GTR anyday for a tight piece of tarmac. DOn't get me wrong, I love my GTR and they're a great car, but in the hands of someone skilled, a WRX is quite a weapon! no Evo but! but a weapon nonetheless.

If you want to be proved wrong, I'm happy to step up to the challenge! I have all 3 at my disposal right now!

[frostmotorsport]

in fact - in Glenns famous words .... do a search!!

any BMI/Option video/internet source will show GTR's struggling to keep up with AE86's / WRX's / Evo's / on a number of circuits - they may make loads of HP but they struggle with the bendy bits!

extrapolate that to a tight, twisty bit of B-road tarmac - you'd be chewing taillight!

[Lith]

Thats why GTRs have been known to dominate in Targa NZ and Tasmania?

Anyway, was a bit of a wind up - I know where you are coming from. Its completely unrelated to the fact I'd go GT2560R-9s over GT2860-5s or -7s on a stockish RB26 which in itself is quite off topic haha

[frostmotorsport]

lets be honest tho... targa is not what I would call tight & twisty! care to explain why 2560's and not 2860's?

[Lith]

Look at the compressor map of the GT2560s that I posted - or compare with

GT2560-9s:

Compressor = 44.5mm inducer, 59.4mm exducer
Turbine = 53.8mm inducer, 42mm exducer
Compressor efficiency at 30lb/min = 75-6% from pressure ratio of 2-2.5bar

GT2860-7s:

Compressor = 44.5mm inducer, 60mm exducer
Turbine = 53.8mm inducer, 42mm exducer
Compressor efficiency at 30lb/min = varying between choke and 65% from pressure ratio of 2-2.5bar

On paper the GT2560-9s have almost identical turbine and compressor sizes but has far better compressor efficiency in the areas that you'd want a nicely modified RB26DETT to be floating around in.

In theory you should have no more lag, but the ability to both make more power - or the same power easier.

See for yourself on Garretts own website:

Garrett GT2560-9s

Garrett GT2860-7s

If it were me, I'd be looking seriously at the GT2560-9s unless you really want a high power thing. The GT2860R-5s can flow more than the GT2560R-9s but until you get up into the higher revs etc they have no major compressor efficiency advantage, and should give nothing away to the GT2860R-7s. I think they would easily give the best of both worlds.

[Monster GT-R]

Thats a good point

[Samuel Cross]

You can see what type of power the -9's make on and RB26 on the following thread

S.-