Rb25 Rebuild

[Dasmbo]

Hi so far has cost around $3200.00 for parts hey its my baby.

Im one of those people who like to keep all the cars engine original.

Why i have asked all the questions. This is my first rb inline six engine i have touched and i want to get it right

and find out what problems i might run into now and the future.

{wont get permission of the missus to do it again}

Dont worry about me building my own engine im sure ill get it ok.

[discopotato03]

Absolutely your call but I wouldn't go any lower in the compression ratio than standard , if anything a tad up would be better in a road RB25DET IMO .

The issue is that you lose torque and response (engine) off boost and road cars spend much of their time at low or no boost . It also shows up as worse fuel consumption as well which is not insignificant with fuel prices as they are .

Low CRS are , were , once the way to avoid detonation with crude engine management and inadequate if any intercooling . I know people are going to say what about RB26s being ~ 8.5 CR , the difference is that they have individual throttles so they have less restriction when all you have is atmospheric pressure trying to fight its way into the cylinders - ie pre boosting phase . ITBs give you a higher DYNAMIC CR where with singles require you to have a higher measure or static CR to achieve a similar outcome . This is why RB25DETs are 9.0 .

Todays superior computers and injectors greatly increase the accuracy of engine management and there are plenty of reasonable intercooler kits out there for 33s and 34s .

As I said your call but I wouldn't have a rebuilt RB25DET with less than the std CR , lower is just another way of losing that low down nice crisp responsive power most people crave . You can have big turbos and small ones , they just bolt on but you can't fix a low CR problem easily with the engine in the car . Actually scratch that because I suppose you could use a Neo head with its smaller chambers but thats more money .

The right pistons should be around the same money as those 8.5 ones .

Cheers A .

[Harey]

+1 Disco!

A mate had his engine rebuilt with a lower compression 9.2 down to 8.5 and with the GT30 turbo it made it less responsive and only made the same power. He then changed to a GT35 which made it even less responsive but obviously more top end power once on boost. As far as a street car it was better with the original higher comp engine.

Low comp is only if you really want to lean on the engine and as said above these days its still not necessary. My understanding is the RIPS RB30s are mostly standard compression.

[Rolls]

In the grand scheme of things 9:1 is low compared to new engines anyway. I honestly don't know why they went 8.5:1 with the rb20s, its not like they were running more than 7psi standard.

Edited August 1, 2011 by Rolls

[StevenCJR31]

In the grand scheme of things 9:1 is low compared to new engines anyway. I honestly don't know why they went 8.5:1 with the rb20s, its not like they were running more than 7psi standard.

the engineers may have opted for 8.5:1 to suit the fuel octane available at the time

[Rolls]

the engineers may have opted for 8.5:1 to suit the fuel octane available at the time

Good point, but surely even 91 octane could handle 7psi with 9:1 compression? Maybe lower static compression and more timing provides better economy/power though.

[Dasmbo]

Hi thanks for the input i cant change the pistons allready have themabout all i can do is hope the head and block decking might raise it a little

I have a standard turbo had it hi flowed also have gtr syle manifold lightened flywheel i will just have to live with what i have purchased.

Another Question i also have a rb20 cylinder head would this be of any benifit raising CR...????

[Rolls]

Another Question i also have a rb20 cylinder head would this be of any benifit raising CR...????

It would but the head won't flow as well, plus I doubt it just bolts on, probably needs various modifications.

[Dasmbo]

Looking for timing belt so many to choose from i have read that some are noisy any recomendations ????

[Aleckr33]

I find my Gates timing belt very noisy.... I'll be going Nissan OEM next service.

[boostn0199]

Go stock one can pick up whole kit with genuine belt idlers tensioners water pump thermostat 250 delivered from US.

[Dasmbo]

Hi i have my part back from machine shop and now im looking for a supplier for plastigauges cant find any other than u.k.

anyone know where i can buy local

thanx

Sam....

[Simon-S14]

uh repco? lol

plenty places have it. i get it from repco all the time

[boostn0199]

Im looking at piston oprions for my rb25 and saw that it was mentioned to keep cr at 9 but allthe cp wiseco pistons ive seen so far are 8.5 which pistons suit for a 9 cr?

[JustinP]

You can get your pistons customised to your requirements. I got some 9:1 CP forged pistons made for my Neo build.

Im looking at piston oprions for my rb25 and saw that it was mentioned to keep cr at 9 but allthe cp wiseco pistons ive seen so far are 8.5 which pistons suit for a 9 cr?

[boostn0199]

Yeah i know you can go custom but alot of people are just using off the shelf pistons like cp etc and just wondering are they all just going 8.5 or what?

[Gerg_R31]

Dynamic compression ratio has nothing to do with itb, itb gives a more efficient atomising aka better air fuel mixture

a dynamic compression ratio is the "true" compression ratio, static cr follows the crank stroke, dynamic follows the crank stroke between intake valve closing and TDC

i.e my rb30de 10.7:1 static cr, 10.8mm lift cams, 252° duration @ 0.050" intake valve closes 60° after BDC which robs me of 12% of my 85mm stroke. which means my effective dynamic stroke is 68mm. do the maths and ill get a 8.75:1

same cams and timing with 8.5:1 & 15 pound boost brings it to 13.25:1

When we machine turboed stuff usually 8.2:1-9:1 for dedicated race cars, 8.5:1 - 9:1 for street cars, 10.5:1-12.5:1 for methanol cars

[boostn0199]

Dynamic compression ratio has nothing to do with itb, itb gives a more efficient atomising aka better air fuel mixture

a dynamic compression ratio is the "true" compression ratio, static cr follows the crank stroke, dynamic follows the crank stroke between intake valve closing and TDC

i.e my rb30de 10.7:1 static cr, 10.8mm lift cams, 252° duration @ 0.050" intake valve closes 60° after BDC which robs me of 12% of my 85mm stroke. which means my effective dynamic stroke is 68mm. do the maths and ill get a 8.75:1

same cams and timing with 8.5:1 & 15 pound boost brings it to 13.25:1

When we machine turboed stuff usually 8.2:1-9:1 for dedicated race cars, 8.5:1 - 9:1 for street cars, 10.5:1-12.5:1 for methanol cars

Ok, so should I still aim for 9:1? If you I did go with the 8.5:1 pistons how much would I have to deck the block and head if im running 1.2mm gasket to get it to 9:1? Just dont get why Im reading 8.5:1 is bad and most the forged pistons off the shelf are in that compression.

[Gerg_R31]

http://www.skylinesaustralia.com/forums/topic/378823-engine-internals-comp-ratios-general-info/page__pid__6040826#entry6040826

that i wrote up a few days ago which has the formula for working out your comp ratio which is more then grinding x amount off

Think of 8.5:1 as minimum and 9:1 as maximum, speak to your tuner, find out what ratio he is comfortable with, the turbo your running

[boostn0199]

Thanks mate thats a great write up!