Jump to content
SAU Community

Recommended Posts

hi mates. just got a question about my setup. its na pistons and the head has been skimmed i rekn atleast 3 times. i had it done twice (after i got the head rebuilt then had to do it again after welded the bit for conversion), and i rekn the guy i got the head off may have skimmed it coz he had used it on a conversion also.

how much would 2 or 3 head skims raise the compression?

and i got the bottomend rebuilt. has na replacement pistons. shud they sit level with the top of the block on the outer rim of the piston? i believe mine do.

reason i ask about this is my last setup broke 4 out of 6 ringlands and doesnt show signs of leaning out. guy says its too much compression. but he also says the rb25 head on 30 bottomend RAISES the compression. but it doesnt hey? rekns id be about 11:1 with his idea that it raises the compression and something like 16:1 on boost

so just wanna see if the head skims and decking the block ( i assumed it only got skimmed to make it perfectly flat again) would cause me compression issues.

ive already bolted the head on again and am about to get it tuned again by different place. havent checked compression. was just going off the guide thinking it wud be 8.2:1 plus abit extra from the head skimming

thanks mates

Edited by joe blo

http://www.skylinesaustralia.com/forums/topic/378823-engine-internals-comp-ratios-general-info/page__fromsearch__1

that is a link to some info i wrote up a while ago, it has the correct formular to work out your static compression ratio

as the rb heads have a tappered chamber, and how much was taken off each time it was ground will vary or "how long is a piece of string?"

Do a compression test and post the results, some forum users on here may be able to tell you a rough area on where your comp ratio is with that

for my motor 10.75:1 i get 225-230psi dry comp 240 wet comp test

8.2:1 is a standard NA RB30E block and RB25DET head.

With 20 thou taken off mine was estimated to be 8.35:1

So with 60thou taken off I'm estimating around the 8.65:1 mark. But this is a very avergage estimation. I'm assuming with tapered chambers etc the more you take off the greater it goes up.

But, I am sure that you compression will be well under 9:1.

The reason your have blown your ring lands off 4 pistons, is because of the tune, and driver abuse. I'd even put money on that.

http://www.skylinesa...__fromsearch__1

Do a compression test and post the results, some forum users on here may be able to tell you a rough area on where your comp ratio is with that

for my motor 10.75:1 i get 225-230psi dry comp 240 wet comp test

OK well the compressions for my RB30/25 are all around 132psi so has anyone got a similar engine with a known CR to compare ( I would guess about 8:1 but its not that important now)? For reasons too boring to relate my budget engine had dodgy pistons replaced with some good cond but evidently less than ideal pistons that were available at the time. Consequently its only making 230awkw (with a GT3540) at 15 psi instead of nearer 300. I have just done a leakdown test - all between 5 and 8% so I will wind the boost up as much as possible with a new tune and live with whatever power it makes until I can afford a rebore with 9:1 pistons.

At 8.2:1 I got 155psi per cylinder.

What sort of gauge are you using?

At 8.2:1 I got 155psi per cylinder.

What sort of gauge are you using?

At 8.2:1 I got 155psi per cylinder.

Just the handheld compression gauge I bought many years ago - the tuner got the same figure with whatever he was using

What sort of gauge are you using?

just a quick Q, wouldn't a comp test give you dynamic comp ratio and not static?

Comp test doesn't really give you a compression ratio just wanted to compare with someone who did actually know his compression ratio and also the psi.

The two tests gave me the answers I was seeking - the leak down test is ok so the motor is basically sound and the low compressions are the product of the choice of pistons, head gasket and head with the main culprit being the pistons so as above I iwll just make the best of what I have with the best tune possible until I can afford new pistons, balance etc ...probably a year or two away!

  • 1 month later...

I would think everyones different camshafts would mess with your comp readings from engine to engine.

^id have to agree with this comment ^ Edited by MRHD66
  • 4 weeks later...
  • 2 years later...

What would be the highest compression you can get with RB30 stock bottom end and a neo head? has anyone achieved 10:1 or higher with say a 0.8mm head gasket and shaving the head, how much can it be shaved?

I wanna get the maximum out of the combo, as in area under curve for the street...

What would be the highest compression you can get with RB30 stock bottom end and a neo head? has anyone achieved 10:1 or higher with say a 0.8mm head gasket and shaving the head, how much can it be shaved?

I wanna get the maximum out of the combo, as in area under curve for the street...

Have a search - someone built a very high compression N/A RB30 a while ago.

Found it already - its back in this thread^^

Gerg_31 got 10.75:1

maybe I should be more specific, I want to get 10:1 compression ( 200plus psi on a comp test) or very close, what gasket and shaving is required ?

pretty sure the 10.75:1 was from a 3.2 stroker 26/30..

Edited by AngryRB

That was just to give an idea, it will get the drop in tomei cams down the track but when you have a good RB30 stock block and a neo head, what do you suggest to get every drop of torque under the curve from the block and head combo, so hence comp and quench area, im sure a lot of guys will be interested in this...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • jeebus. glad you weren't under it while performing the stunt. Also thanks for the link to the wheel measurer, exactly what I needed
    • In the older stuff there were very significant differences 2wd to 4wd, for example Stagea had strut front end for 2wd and double wishbone for 4wd so it was not minor to swap. From poking around the 2wd v37, it *looks* like it might be more possible; some of the parts specifically have "2wd" stamped on them which suggests the platforms are more similar. You'd still want to start with a 4wd half cut to swap stuff from though. I'd suggest if you don't have a tune on the ECU you don't really need one on the trans either. Throttle mapping is in the ECU side (and you can always use a Roar Pedal if you want the throttle to actually respond to your foot), and really if you are happy with the stock power you probably accept the stock trans behaviour too....its all made to be "sporty" not racey.
    • So, updates. I have not washed the car since it came back from Tassie. I've driven it around a bit but not got around to actually sorting it out. I DID raise it because I cracked the rear bar leaving a hotel which was very distressing. Interestingly, the car drives more compliant now that it's raised a fair bit (5mm front, 15mm rear). Also noticed that my FR height was 10mm lower than FL. So that's now sorted out, too. I also bought this and had it printed: https://www.etsy.com/au/listing/1576422240/wheel-and-tire-fitment-tool-universal?ref=shop_home_feat_1&dd=1&logging_key=08f604d9fa4cc383550ba985e6ac85cd5cac7fbb%3A1576422240 Now, if I was smart I would have taken my brake calipers off to actually use this correctly but it was evident enough to me that in the region where the caliper was... there was nothing to hit suspension/guard/arm wise. So I'm going with "it'll be fine" after using the tool to hopefully very precisely measure the wheel clearance. Also while doing this, I had the very VERY bad idea of jacking one of the wheels/suspension arms up while the rest of the car was on jack stands. I did this to see how the arm would travel. This all was well and good until the car slid off the stands and went through a fence. So don't do that. Incredibly nobody was hurt and there was only minor damage to the rear bumper as the car didn't have far to slide, and had 3-4 wheels on it. The only damage turned out to be the fence itself which was easy to fix, and a little bit of damage to the fibreglass rear bumper trim. I had already planned to try a touch up paint kit to fix the time I drove into my garage door to see if it'd help in the interim before I get it fixed properly. I used the Dr Colorchip kit after looking online and seeing everyone talking about it. Yes it's made for chips and not huge broken missing pieces and I'll be 500% recommending it for stone chips after using it for stupid things like me. This took about ... 10 minutes and looking at the half assed photo the 30 second job I did on the bumper corner was almost perfect just by using the tiny little brush and painting it in. The sealact stuff to remove over-painting is really useful, so if/when I do it again I'll likely slather the touch up paint well over it and then clean it up with the cleaning solution. The wheels should arrive in a couple of weeks. I am still kinda confident after doing a stupid amount of measuring (and borrowing a set of 18x10.5+15) that they will not fit because I overlooked something, somehow and flew too close to the sun. ALSO R34 GTR guard liners do not fit on a GTT. I bought the undertray brake duct guides and had the wonderful problem of them not fitting my intake, my oil cooler and the liners themselves were even worse. Attempting to fit them won't work in general - You would have to cut them up as another poster mentioned as the bodywork is different on the GTT. At least I can try to resell them. So instead of cutting those up, I cut up my old already-cut-up GTT liners and extended them by using some PP plastic and drilling some 8mm holes for some nissan clips for the 'extra' bit. Because I was happy to cut them I was able to mount them pretty damn forward so I now have some semblance of guard liners, and the brake vents seal the bumper from the bottom. It sort-of-looks like this, to give some idea - If you look at the GTR and then the GTT this is when I realised that I needed to seriously measure as the inside of the rim area is entirely, entirely, entirely different and could not take any internet measurements for granted.   
    • If you are Ecutek tuned then these TCU tunes are anywhere from $550 to $700 usd. If you are not ECUTEK tuned, then it gets costly!    
    • Sorry to hear your HFM BM57 was faulty,  did you contact HFM I would hope they would be at least grateful for the information if there was some sort of manufacturing fault, you would hope they would be sympathetic even if your item was 2 years old if it had never been fitted. May I ask where it leaked from ?
×
×
  • Create New...