Boost, Power, Pistons & Headgaskets..

[B-Man]

Howday Folks,

I've been doing a bit of forum searching and it seems that there is concensus that standard RB25DET bottom end internals can run up to 300rwkw (if tuned and driven appropriately) Maybe 275 rwkw is a safer limit ? ......

So without doing forgies, cams, head gaskets, etc What sort of boost is required to get into the 275 rwkw domain ?

Also, does boost required for more kw differ between turbo's ?? ie If you can you run a T3/4 at 20PSI to get X rwkw, can a T88 running 10PSI can get the same (X rwkw) power. If u get my drift ...

I understand that by upgrading the bottom end with stonger gear (eg forgies etc) then you can run more power, more safely -

By changing the head gasket to metal or composite - the lowers the compression right, so you can run more boost ? Is this right ? What boost is safe for standard internals ? Or should I be asking what power is safe for standard internals, in which case I have answered my own question.

Thanks in advance.

[GTS-t VSPEC]

brendanf,

Boost and power are two separate things, they are however related. A stock turbo is only efficient up to about 14psi, after this point is doesn't make much more power. However a big turbo like a T88 with flow heaps more air than the stocker even at 8psi. It is the amount of airflow that counts not the actual pressure.

Having said that, I know that the ring-lands on the RB25 psitons are weak and would not like pushing more than 18psi of boost no matter which turbo you have. Yes some people have done more than this, but it depends on how safe you want to be.

There are many ways to get to 275kW, all will mean a different turbo but this can be achieved at various boost levels. A lower compression ratio does mean that the engine can handle more boost without detonation, but for high horsepower then a highish compression is still desired.

At 300rwKW the rods may start to bend, so replacement or shot-piening may help.

See'ya:burnout:

[Steve]

The reason that running a lower compression enables you to use higher boost is that the compression ratio is directly related to how much the air is heated on the compression stoke - and the more boost the higher the overall pressure. Compressing a gas (air) causes it to heat up. Heat causes detonation - as an air fuel mix, when heated enough will burn (more like an explosion) without the presence of a spark.

To achieve more power at lower boost (thereby keeping detonation at bay) you need to consider how much air you need to flow and at what sort of rpm - this is where a bit of a quandry occurs - to get a turbo to spool up, you need to move air out of the exhaust - so either more revs, bigger capacity engine (displacement) or better flow characteristics are required.

One turbo that I know of that might fit your needs is the GT35r hybrid that is built by Per4manz in perth to quote an e-mail I got from them,

'Yes we can offer GT35r hybrids, mostly using the smaller turbine AR to make them come on boost earlier. Can also supply different turbine/ compressor selections to match them to different engines.

Most of the turbos we use come on boost @ 3-3500rpm for the serious hp applications or even sooner for street combinations.

Last 2 rb25's that I have done have made 320hp @ 1 bar (std engine) and 400hp @ 1.3bar (modified) at the wheels.

Can only supply propriety compressor maps and these don't have much detail (surge limits & max flow only) The gt35s flow @ around 80% efficiency @ 1 bar depending on a lot of variables

Cost of one of these is $2477 inc gst'

Their e-mail address [email protected] they may be worth a try.

Best to do a bit of research and see what you can find - that way you wont be dissapointed later on after you have made your choice.

Sorry I rambled on a bit

Steve

[B-Man]

kewl - thanks guys - I think got the answer I was looking for . Boost is not flow - Bigger turbos = more flow at less boost. Bigger turbos require more revs to get spooling to get the big flow happening.

Boost is just plenum pressure measured in PSI or Bar.

But wouldn't big turbo = more flow = more boost ??

OK, Here's where I am coming from: I just got a bunch of mods done. Injectors, Fuel pump, FMIC, GCG500HP Hi FLow Turbo, Front, Dump & Hi FLow Cat. Oh, and custom plenum with larger throttle body (reversed for easy FMIC install)

I'm getting 235rwkw @ 18PSI boost. But I was sort of expecting 250 rwkw. DOn't get me wrong the car goes like a rocketship and the power is right accross the rev range - Turbo starts boost at 2000rpm and full boost by 3200-3500.

I've been told that the turbo is good (effecient to) for 24-26PSI. Since I am getting 235rwkw , if increased boost from 18 to 22 PSI, I should be getting towards the 250rwkw level.

However, as GTS-t VSPEC says, the std rings crap aout at 18 PSI.

So to get my desired 275 rwkw (without doing the bottom end) , I would need to put a bigger turbo on right ?? Not increase the boost.

Might be easier to just do the bottom end and then increase the boost to 26 PSI - Then I retain the investment in the 500HP turbo.

[rev210]

alcohol injection, namely isopropyl alcohol. Will allow you to go to the boost level of 22psi or so on stock internals. I know of a guy who has run 28psi quite a few times on an internally standard RB20DET, using one.

sounds like you have a great little turbo there, I'd be reluctant to move to the GT35r given the drop in response you will experience.

Perhaps you can do other things to further improve the turbo's response.

[GTS-t VSPEC]

brendanf,

As rev suggested it is possible to run higher boost, as long as you cool the intake temperature down sufficiently, it is detonations that destroys the engine not the actual boost.

As Steve suggests the modifed engine was able to take the higher boost, and at 18psi I don't think you've reached the potential of your turbo. It is also difficult to get the most out of any turbo without doing cams aswell, there is a huge amount of power locked up in your turbo, you'll need to either do forged pistons or cams to unlock much more, if you want to retain your current turbo.

See'ya:burnout:

[Sydneykid]

Hi brendanf, there area couple of things that we would normally do to an RB25DET that aren't on your list;

an adjustable exhaust camshaft pulley (can't do the inlet camshaft because of the VVT). Worth a few kw's.

airfilter upgrade, either a panel filter in the standard airbox or a pod in a custom airbox. Also worth a few kw's.

a set of camshafts (Tomei 254's would be ok, still keep a good wide power band). Good for about 12 kw's.

an electric fan, the water pump pulley driven one soaks up about 3 kw.

we get a few extra kw's from running a good low viscosity synthetic oil (eg; Castrol Formula R Synthetic)

I also wrap (exhaust thermowrap) the intercooler piping from the I/C to the throttle body, as this helps to keep the engine bay heat out of the pipework.

Continued in next post

[Sydneykid]

Continued from previous post.............

We use GCG turbos a lot and the one you have is a ripper. I wouldn't be changing it, a bigger one will only reduce the power band. You may get more max horsepower, but the torque curve may be compromised so much that the car actually accelerates slower.

I don't know what sort of FMIC you are running, we use a standard R33 GTR one and it has good inlet air temperature control at 1.5 bar (22 psi). Keep a check on you inlet air temp, that will tell you when the boost increase becomes inefficient.

We get quite a bit more horsepower running on Elf (Turbo Max 101.7 ron) as we can wind a bit more ignition timing into the engine.

Hope that is of some use.

[GTS-t VSPEC]

brendanf,

If you going to do cam gears and camshafts then do both inlet and exhaust gears, get rid of the VVT for high horsepower applications. With a decent set of camshafts you will make alot more power than the 12kW sydneykid suggests, it is the only way to make full use of the turbo's power, I'd suggest a 256/264 combo or both 264's for good power with a wide power-band.

If I were you, go the forgies, along with shot-piening the rods and balance the crank, then do the cams. You will be looking at about 320rwkW with that setup.

See'ya:burnout:

[Steve]

Hey Brandanf, opinions flowing thick and fast now - regarding the more air means more boost - not necessarily so as you can fit more air down larger passageways (bigger turbo) with less pressure - the more the air passageways are restricted the higher the pressure with the same amount of air flowing.

Here in lies the yet another opinion - I agree with the idea of doing cams and cam gears - although I dont believe it is totally necessary to replace the inlet cam and loose your vvt - my understanding is that vvt give you a better torque curve, but you may not get quite as high a top end power figure. Another area you could look at is giving the head a decent port and get the plenum/manifold matched - this will further allow better airflow and enable you to make best use of the turbo/plenum/cams because the better it flows the less boost pressure will be required to get the air in and out.

The unfortunate thing and a good thing about asking questions like the one you posed is that there are alot of different opinions around and some will be contradictory - you just need to decide what is right for you

Good luck

Steve

[B-Man]

It's all good folks - Thanks heaps for all the advice. It's always good to get the technical juices flowing in the old grey matter. I am and have been learning heaps from these types of threads and I'm sure other's are too. So keep the ideas coming in. I know there are different opinions of stuff, but I reckon the folks who know what they are on about respect all the other ideas too - As you can tell, I am still learning about all this turbo stuff, I find it fascinating. It's a pity it costs so damn much to 'play' he he. I'm not in the motor industry so independant ideas and opinions are so valuable to ppl like me and others on the forums so from all the ppl like me to you gurus - THANK YOU !

On thing I have learnt, is that it pays to do as much research as possible before comitting $$ to mods - It's hard & costly to undo a mistake.

Back on topis .....

If doing cams & headwork, then wouldn't it make sense to do forgies, rings, and bottom end stregthening at the same time (if funds are available) ?? I understand that just to fit a metal or composite head gasket is like a $1500 job....

Oh and regards to VVT, I understand that EMS can hook into this (mine not currently configured tho) so then EMS can contol VVT. ( I have EMS in piggyback mode to take advantage of the idle control for air con and power steer, etc. .. yes the custom plenum has all the right controls for this too.....) Maybe this is worth investigating too....

Cheers folks,

[rev210]

do the cams, not the head work. Doesn't require an engine strip.

[GTS-t VSPEC]

brendanf,

When it comes to metal headgaskets, I don't think it is necessarily a wise investment, if you want to lower compression you can do this with a set of pistons, and all up if would cost you $1500, so you'd have forgies and the lower compression. Have seen many of those expensive metal headgaskets cause leaks, a waste of money in my opinion.

If your still using the factory exhaust manifold then headwork is wasted, the factory exhaust manifold is very restrictive, so it should be replaced if you want decent power, then you can match the ports to the new manifold, most of the gains are on the exhaust side.

Research is very important, arm yourself with the facts then you know if someone is bullshitting you.

See'ya:burnout:

[Xeron]

With metal head gaskets a sealer must be used and it has to be done very well, if you can find someone thats good at it then its a wise investment. But im a strong believer in forged internals, although the prices of hks jun etc pistons are crazy, but a good set of forged dished pistons will allow you to run alot more boost. And why not shot peen those rods while ur down there too

[sidewaymambo]

Does the VVT we are talking here is like NVCS?

[Steve]

Brendanf, it all depends on how much more you want to spend - doing internals will at least give you some peace of mind, but its not a cheap exercise.

To do cams you wont need to take the head off, so it will be cheaper.

If you intend on getting head work done, you wont need to pull the engine. If you want cams and headwork, do them both at once it will be cheaper.

It may well be worth getting a manifold and plenum setup done before headwork so it can all be matched.

If you really want to decompress the engine, it may be worth getting RB26 crank, rods and pistons which are said to be able to withstand 550bhp reliably - and it will drop your compression ratio to 8.5:1

More thoughts

Steve

[whatsisname]

Originally posted by brendanf

Oh and regards to VVT, I understand that EMS can hook into this (mine not currently configured tho) so then EMS can contol VVT. ( I have EMS in piggyback mode to take advantage of the idle control for air con and power steer, etc. .. yes the custom plenum has all the right controls for this too.....) Maybe this is worth investigating too....

Am I reading you right here Brendan, your EMS isn't controlling the VVT? If so I sure hope the stock ECU still is, otherwise you're losing a huge amount of bottom end & mid range torque

My list of mod's is similar to yours, although I still run the stock injectors (@ higher f/p) & have the stock plenum.

Here's a couple of minor power producing mods I do have (that you didn't list) that showed noticeable rwkw increases for me;

1. An adjustable exhaust cam gear set @ 4' retard (bigger gains are had with a larger turbo &/or after a cams upgrade)

2. Replacing the standard clutch fan for a 16" Davies Craig thermo.

Actually, upon reading back through this thread, everything Sydneykid has suggested is what I was getting @! Right down to his suggestion of v/good quality synthetic oil.

[B-Man]

whatsisname - dude, 257 at the wheels on 13 PSI is excellent - What sort of turbo are you running.

Yes, the standard ECU is controlling the VVT - I understand that if the EMS controls it tho, I'd be able to kick in the VVT at a more appropriate time, considering all the other mods. (more dyno time needed) We are looking at EMS doing this as next step.

Also, do you boosted and modded guys run on the richer side of stoich, right thru the rev and load range ? (even under very small load or even idle ??)

Thermo fan thing will be done ASAP - have been reading other threads about this - (gotta determine if this is a DIY thing or not...)

Cam Gears - yeah, I like the look of the HKS ones - very pretty - but gotta wait for clear perspex cam cover fisrt .. j/k - I'll prob get that done at same time as cams - makes sense......

I think what I'll do is do some small, low cost stuff, like the thermo fan, and replace the crappy pod to turbo shitty plastic pipe with aluminium & sil custom jobby. Do a catch can and aluminium air box - Whilst I do some more research on cams, heads and internals....

[B-Man]

Here's some pics ....

[B-Man]

another