Rb26 Raised Compression

[legzy86]

Hey fellas just looking for some advice towards the build of my R32 GTR.

So a little bit about the build first.

*Extensive head work ( ported, +1mm valves, Tomei type 'C' springs and retainers, Tomei 280 x 11.50 cams)

*Nitto rods

*Standard block 86.5mm bore, grout filled to bottom of welch plugs

*Standard inlet manifold modified with single throttle body

*Precision 6766 split pulse T4 1.00 AR

*Split pulse manifold with twin Precision wastegates

And all other supporting mods ( nitto oil pump, ARP retainers etc.)

My question is what compression pistons are people using and what is available? I was originally putting a set of arias in that I had ( 8.2/1 I think) But since the local United is now selling E85 I'm more inclined to use higher compression pistons. I know that CP do 9.0/1 and SPS can make anything to order. I was thinking something around the 9.5 to 10 area. Does any company produce these as a shelf item? And also my current fuel system is the stock GTR pump as the lift pump then twin 044s from the surge tank. Obviously going to E85 I'm thinking a lift pump upgrade is in order. What size pump would be suitable for this kind of setup? Any help would be appreciated.

Cheers Simon

[GTR_JOEY]

i know this isnt what ur asking but first thing I would do is not use a jap valve spring,

from my experience and what ive seen they are too soft, for high lift cams... they might be ok on tomei cams with a low ramp rate on the lobe but personally Id be getting something different

in regards to your fuelling question.. im using an intank nismo that feeds twin walbro 400's... but im only making 420rwkw... that 6766 will make more power then my set up

I think anywhere from 9-9.5:1 is a nice number

Waits for PIGGAZ's wisdom haha

[legzy86]

Yep didnt want to hear that lol. Its a bit late now so I will just run them and see how it goes as its money already spent. ( Fingers crossed)

[r32-25t]

Actually the problem isn't the valve spring usually, it's the way the machine shop cuts the valve seats and sets the clearances

[Joshbigt62r]

Actually the problem isn't the valve spring usually, it's the way the machine shop cuts the valve seats and sets the clearances

but by saying that maybe true, why does it seem to affect jap brand springs "usually" more often than the other big brands available??? Can you please explain mate? A few mates of mine have come across valve float with tomei type c's, apexi spl v-max's and also atomei type b's from memory. Of the 4 gtr's 1 went to ferrea and other 3 went to some brand I cant think of off the top of my head but I know that JHH RACING use and recommended my mates the brand I cant think of and problem was solved. I should add high revving rb26 strokers of all sorts. Keeping revs sensible and I guess you probably wouldn't have a problem?

Josh

[r32-25t]

The jap springs are designed for a head with factory height valve seats, once you cut the seats it's moves them up in the head, which moves the valve up and the retainer with it and that lowers the spring tension at rest which is what causes float.

Also put some boost over the top of the valve and the problem becomes even worse

[r32-25t]

But to answer your actual question, raised comp in an rb26 is always going to be a good thing

[legzy86]

The jap springs are designed for a head with factory height valve seats, once you cut the seats it's moves them up in the head, which moves the valve up and the retainer with it and that lowers the spring tension at rest which is what causes float.

Also put some boost over the top of the valve and the problem becomes even worse

Just like you have said it comes down to if the seat pressures have been checked and are within specs which any decent head machinist would do. Not just slap it together and hope for the best. Do you know of any brands that do shelf items higher than the CP 9.0:1. I cant find anything.

[Piggaz]

Has the chamber in your head been modified at all? Head been decked?

[legzy86]

Has the chamber in your head been modified at all? Head been decked?

Head surface was skimmed

[r32-25t]

Just like you have said it comes down to if the seat pressures have been checked and are within specs which any decent head machinist would do. Not just slap it together and hope for the best.

then a new problem arises! That is when you get the seat pressure right the spring binds at lift

[r32-25t]

Both me and Paul (piggaz) have already been thru this very recently

[legzy86]

Both me and Paul (piggaz) have already been thru this very recently

Sh&ts just never ending!! lol. Well they are in there now so I will give them a go. If I have to change them so be it and if not BONUS!! Cant believe I havent heard of this before now. I know of 3 cars running tomei top ends with no dramas whatsoever which is why I went that way. What is not going to give me headaches? lol

[Joshbigt62r]

Performance = head aches and lot of $$$

Well the heads not currently bolted to anything yet is it? Be easier to get it sorted now. Pull em out and sell them and chuck the same springs as what "Piggaz & r32-25t" are running in.

The chances are you'll see the exact same problems as so many people that have walked this path before you have. You may get lucky and not but while its a common issue I reckon you'd be mad not to address the problem now and stay that one step ahead.

Its your choice atcthe end of the day though mate.

Josh

[discopotato03]

I'm curious to know what the car will be used for though it sounds like a top endy drag app .

Always with long period cams the dynamic CR drops because the trapping efficiency is poor at low engine speeds , valves open too early and close too late to get much effective compression happening .

Raising the static CR means what air the valves seal in is compressed more so you get higher compression pressure .

Also your call but big cams make for lots of reversion (inlet air going in and partially back out of the inlet valves at lower engine speeds) and having a throttle plate per inlet port helps here .

If I was contemplating a high horsepower RB26 and not using twin turbos or ITBs it may have been worthwhile researching turbo Neo heads with the RB26s valve train . Reasons being you get smaller chambers for less heat absorption area and you can have higher compression ratios without raised piston crowns which screw up breathing to a degree . Also the basic but useful variable inlet cam ability is not a bad thing either . IMO the value in a GTR head is it's valve train and manifolds , beyond that its just another RB TC head that started out in 1989 .

From memory GMS had the CR around 9.6 but that changed when CAMS tried to rein in the GTRs power wise , I think they had less boost with a higher CR to minimise the power loss .

Edited September 11, 2013 by discopotato03

[legzy86]

I'm curious to know what the car will be used for though it sounds like a top endy drag app .

Always with long period cams the dynamic CR drops because the trapping efficiency is poor at low engine speeds , valves open too early and close too late to get much effective compression happening .

Raising the static CR means what air the valves seal in is compressed more so you get higher compression pressure .

Also your call but big cams make for lots of reversion (inlet air going in and partially back out of the inlet valves at lower engine speeds) and having a throttle plate per inlet port helps here .

If I was contemplating a high horsepower RB26 and not using twin turbos or ITBs it may have been worthwhile researching turbo Neo heads with the RB26s valve train . Reasons being you get smaller chambers for less heat absorption area and you can have higher compression ratios without raised piston crowns which screw up breathing to a degree . Also the basic but useful variable inlet cam ability is not a bad thing either . IMO the value in a GTR head is it's valve train and manifolds , beyond that its just another RB TC head that started out in 1989 .

From memory GMS had the CR around 9.6 but that changed when CAMS tried to rein in the GTRs power wise , I think they had less boost with a higher CR to minimise the power loss .

The previous build was 2860-5s and poncams and was hella fun on the street but going for a mainly drag setup this time round with the odd street cruise. Like I said earlier I'm not too keen on wasting the thousands already spent on the current setup. Thanks for the tip though! Maybe do this when I eventually convert my RB30et powered VL to twin cam.

Cheers Simon.