Jump to content
SAU Community

Recommended Posts

what is considered too high of a comp ratio in a turbo motor

Depends what fuel is being used.

For 98 pump fuel in Australian climate anything over 9.4 is getting a bit exxy with decent boost pressure.

I bought Arias 9.4 forgies before cc'ing the 25 head which was a mistake coz the volume of the bowls were smaller than std brining the comp up to 9.6 :P

I've had to stuff around trying to source a thicker head gasket to bring it down to 9.2.

Tip of the week, get ur bowls in the head checked before you buy pistons. :down:

  • 2 weeks later...
we just got a new set of mahle 40 thou oversize forged pistons from hi octane. They look very schmick!

Will be the first set of pistons used in one of the many rb30 builds weve done that hasnt come from CP.

Judging by the specs on the piston box i imagine it will net a comp ratio of about 8.2:1, same as the flat tops CP provide for the conversion.

Have you had any headway with the Mahle's?

Cheers

Very good idea to put this thread up...1 question is, which is the better shape piston to use...dish top, flat top or crowned top pistons?

When I was building Cooper S engines 20 plus years ago I was lead to believe that dished tops were better because they captured the explosive combustion better not allowing as much blow force to spill over the outside diameter of the piston...(a bit like an upside down parachute) therefore producing more downward force on the piston creating more power...probably only marginally, but every bit helps. Also the shape of the dish helps keeps the piston more balanced and even under combustion load preventing less piston skirt wear on the bore. As I said, this is what I was lead to believe.

And Mahle pistons were the ducks nuts (the best) of pistons to be used...then.

Edited by The Ant

wisco's 8.5.1

Stock GTR Head gasket , stock head , cams and intake manifold

520hp on 20 PSI (street @ 8500rpm) , 637hp on 32psi C16 (race @ 9500rpm)

see for yourself. (I know its a VL not a skyline , but I also have a R32 GTR)

Behind a 2 speed power glide.

  • 4 weeks later...

ok i am looking around for piston options at the moment and thought i better keep piston info in the one thread instead of starting a new one again.

I am loooking for a fairly cheap option in forged pistons, firstly i was thinking cp pistons but havent got great confidence in there rings at all and have these in my other rb26 motor and rings are causing some minor blowby issues i dont like. Ive been told the rings have been updated but still abit uncertain.

other options are the Mahle pistons and ACL Race Series pistons.

does anyone have the part no. for the 2 brands of pistons above to suit a rb26/30 and what sort comp ratio do they work out too.

ACL Race Series

Ive found part no. 6MKRY9608 from the ACL race series website and they qoute 8.5:1 cr on rb30et engine so i assume its gonna work out too low using a rb26 head?

anyone with suitable part noumbers of pistons to suit rb26/30 engines and have a reasonably good cr ratio can u post them up.

Mahle Forged Pistons

Mahle i found a link on ebay qouting this info...

Part#: NIS264425I05

APPLICATION: NISSAN SKYLINE GTS-T RB30DET TURBO

RB30DET 87.00 Bore, 85.0mm, 152.5mm, 21mm Pin, 32.1mm, 4.5cc dish, 312g, 8.5:CR

does that info sound correct or do u think they are qouted for a rb30et instead?

anyone running Mahle pistons besides R33_Racer? any opinions on them?

Using either the RB25 or RB26 head on the RB30 block you need to use RB30E pistons the RB30ET pistons will result in a far too low compression ratio to be much use as a street car.

i realise that i was qouting the mahle website which said rb30det so i assume it might mean with a double overhead cam head setup, maybe it was a typing error from the ebay seller. The acl ones also had 2 versions for the rb30et, one being a 7.5:1 cr and the other a 8.5:1 cr.

Mahle Forged Pistons

Mahle i found a link on ebay qouting this info...

Part#: NIS264425I05

APPLICATION: NISSAN SKYLINE GTS-T RB30DET TURBO

RB30DET 87.00 Bore, 85.0mm, 152.5mm, 21mm Pin, 32.1mm, 4.5cc dish, 312g, 8.5:CR

does that info sound correct or do u think they are qouted for a rb30et instead?

anyone running Mahle pistons besides R33_Racer? any opinions on them?

Pistons with a 4.5cc dish sounds about right for 8.5 to 1 using a 62 cc combustion chamber. We run flat tops for 9.0 to 1.

Cheers

Gary

What is everyone using for a compression ratio calculator? Perhaps I am using the wrong formula?

I have been using the four stroke version on Wiseco's website.

http://www.wiseco.com/Calculators.aspx

Here are the specs on the Wiseco pistons I have.

3.046 inches (86.5mm)

1.260 inch (32mm) Comp Height

16cc dome

The RB26 head I have measures out at 64cc's. The head gasket being used is a Cometic MLS 87mm. Cometic rates the thickness at .051 inches (1.295mm). My block has been decked and the pistons sit at a zero deck.

When I plug these numbers,

Bore Diameter = 3.046

Engine Stroke = 3.346 (85mm)

Head Volume = 64cc

Gasket Bore Diameter = 3.425 (87mm)

Compressed Gasket Thickness = .051

Deck Clearance = 0

Piston Effective Dome Volume = 16cc

With these numbers I get a CR of 9.69:1

My plan is to shave nearly 11cc's off the piston dome to bring the dome volume to 5cc. According to the calculator this would give me CR of 8.490:1. I have to face the reality of 91 octane premium fuel in my part of the world.

  • 4 weeks later...

i am running Mahle pistons in my 26/30 hybrid. i think they are 8.5:1 - no idea on kw output from them yet, as during the run-in my turbos went pop and meant my rebuilt engine had to get re-rebuilt.

on the good side, the pistons were completely undamaged despite the bore getting damaged, so i can vouch for their construction.

on the bad side, new rings had to be ordered from the USA as there are none available here (apparently).

will post again once it has had a tune for power.

3.046 inchs = 77.3mm

86.5mm = 3.4 inchs

that could be a problem

What is everyone using for a compression ratio calculator? Perhaps I am using the wrong formula?

I have been using the four stroke version on Wiseco's website.

http://www.wiseco.com/Calculators.aspx

Here are the specs on the Wiseco pistons I have.

3.046 inches (86.5mm)

1.260 inch (32mm) Comp Height

16cc dome

The RB26 head I have measures out at 64cc's. The head gasket being used is a Cometic MLS 87mm. Cometic rates the thickness at .051 inches (1.295mm). My block has been decked and the pistons sit at a zero deck.

When I plug these numbers,

Bore Diameter = 3.046

Engine Stroke = 3.346 (85mm)

Head Volume = 64cc

Gasket Bore Diameter = 3.425 (87mm)

Compressed Gasket Thickness = .051

Deck Clearance = 0

Piston Effective Dome Volume = 16cc

With these numbers I get a CR of 9.69:1

My plan is to shave nearly 11cc's off the piston dome to bring the dome volume to 5cc. According to the calculator this would give me CR of 8.490:1. I have to face the reality of 91 octane premium fuel in my part of the world.

3.046 inchs = 77.3mm

86.5mm = 3.4 inchs

that could be a problem

Ops, I transposd that one in to the post wrong. It should be 3.406 for the bore diameter. Inputing that number would give a 8.174:1 CR.

Comp Ratio Calc For 4-Stroke Only

Bore Diameter 3.046 Inches

Engine Stroke 3.346 Inches

Head Volume 64.00 Cubic cm's

Gasket Bore Diameter 3.425 Inches

Compressed Gasket Thickness 0.0510 Inches

Deck Clearance 0.0000 Inches

Piston Effective Dome Volume -16.00 Cubic cm's

Gasket Volume 0.470

Deck Volume 0.000

TDC 3.399

Swept Volume 24.382

Compression Ratio 8.174 :1

Thanks for catching that error.

Not counting the typo in my copying, my numbers entered into the calculator still yeild the same 9.969:1

Comp Ratio Calc For 4-Stroke Only

Bore Diameter 3.406 Inches

Engine Stroke 3.346 Inches

Head Volume 64.00 Cubic cm's

Gasket Bore Diameter 3.425 Inches

Compressed Gasket Thickness 0.0510 Inches

Deck Clearance 0.0000 Inches

Piston Effective Dome Volume -16.00 Cubic cm's

Gasket Volume 0.470

Deck Volume 0.000

TDC 3.399

Swept Volume 30.485

Compression Ratio 9.969 :1

I am still not understanding how it is possible to get around a 9:1 CR with flat top pistons.

Changing the Piston Effective Dome Volume to 0 will yield a 7.968:1 CR according to the calculator.

Comp Ratio Calc For 4-Stroke Only

Bore Diameter 3.406 Inches

Engine Stroke 3.346 Inches

Head Volume 64.00 Cubic cm's

Gasket Bore Diameter 3.425 Inches

Compressed Gasket Thickness 0.0510 Inches

Deck Clearance 0.0000 Inches

Piston Effective Dome Volume 0.00 Cubic cm's

Gasket Volume 0.470

Deck Volume 0.000

TDC 4.375

Swept Volume 30.485

Compression Ratio 7.968 :1

What am I missing here?

Edited by Stealth-Z

A flat top piston with a 1mm ( compressed head gasket - as in a std OEM ) will achieve 8.1:1 - 8.2:1 with your combustion chamber size. As does the CP flat top piston.

To achieve around 9.0:1 you will have to have an around 6 to 7cc dome on your piston.

Ops, I transposd that one in to the post wrong. It should be 3.406 for the bore diameter. Inputing that number would give a 8.174:1 CR.

Comp Ratio Calc For 4-Stroke Only

Bore Diameter 3.046 Inches

Engine Stroke 3.346 Inches

Head Volume 64.00 Cubic cm's

Gasket Bore Diameter 3.425 Inches

Compressed Gasket Thickness 0.0510 Inches

Deck Clearance 0.0000 Inches

Piston Effective Dome Volume -16.00 Cubic cm's

Gasket Volume 0.470

Deck Volume 0.000

TDC 3.399

Swept Volume 24.382

Compression Ratio 8.174 :1

Thanks for catching that error.

Not counting the typo in my copying, my numbers entered into the calculator still yeild the same 9.969:1

Comp Ratio Calc For 4-Stroke Only

Bore Diameter 3.406 Inches

Engine Stroke 3.346 Inches

Head Volume 64.00 Cubic cm's

Gasket Bore Diameter 3.425 Inches

Compressed Gasket Thickness 0.0510 Inches

Deck Clearance 0.0000 Inches

Piston Effective Dome Volume -16.00 Cubic cm's

Gasket Volume 0.470

Deck Volume 0.000

TDC 3.399

Swept Volume 30.485

Compression Ratio 9.969 :1

I am still not understanding how it is possible to get around a 9:1 CR with flat top pistons.

Changing the Piston Effective Dome Volume to 0 will yield a 7.968:1 CR according to the calculator.

Comp Ratio Calc For 4-Stroke Only

Bore Diameter 3.406 Inches

Engine Stroke 3.346 Inches

Head Volume 64.00 Cubic cm's

Gasket Bore Diameter 3.425 Inches

Compressed Gasket Thickness 0.0510 Inches

Deck Clearance 0.0000 Inches

Piston Effective Dome Volume 0.00 Cubic cm's

Gasket Volume 0.470

Deck Volume 0.000

TDC 4.375

Swept Volume 30.485

Compression Ratio 7.968 :1

What am I missing here?

when we get a motor that holds together long enough to inspect after a season of racing ill let you know how the mahles look....but after the last quick blow up with them in it, they seem like brand new, considering the amount of bearing material that was being rammed up its clacker!

is anyone on here actually measuring the static compression of their motor or just using a calculator or what the specs say you should get?

Mine has been apart for some time now. I have measured the parameters for the compression calculators. My first set of custom Wiseco pistons were flat tops with -5cc valve relief cuts. They were suposed to produce a CR of around 9:1. After measuring, the CR came out to less than 7.

PistonsRodsBearings2.jpg

  • 3 weeks later...
Pistons with a 4.5cc dish sounds about right for 8.5 to 1 using a 62 cc combustion chamber. We run flat tops for 9.0 to 1.

Cheers

Gary

If your pistons are flat top then that means no dome/dish correct? Using the engine compression ratio calculator I'm having to enter values of -1mm deck clearance (piston sitting 1mm above deck at TDC) to get comp ratios of 9:1.

Does that sound right to you?

Is there a preferred choice in pistons in regards to dome versus compression height?

flat tops with higher compression height or domed pistons with lower compression height.

I've been looking at other possible sources of pistons, the SR20, 3SGTE, 1J and 2J series engines also share 86mm bore. Is it just a matter of finding something with the same wrist pin diameter? Or can that be overcome with use of different bearings?

I'm aiming for somewhere between 250 and 300rwkw, which I think forged pistons are overkill. I'd use the factory RB30 pistons but I want higher compression ratios. Would it be simpler to just deck the block and/or shave the head to remove approx 1mm?

Also, ceramic coating the pistons/combustion chambers, is that going to affect the combustion chamber volume or is the amount of change negligible?

[/so many questions]

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

    • As discussed in the previous post, the bushes in the 110 needed replacing. I took this opportunity to replace the castor bushes, the front lower control arm, lower the car and get the alignment dialled in with new tyres. I took it down to Alignment Motorsports on the GC to get this work done and also get more out of the Shockworks as I felt like I wasn't getting the full use out of them.  To cut a very long story short, it ended up being the case the passenger side castor arm wouldn't accept the brand new bush as the sleeve had worn badly enough to the point you could push the new bush in by hand and completely through. Trying a pair of TRD bushes didn't fix the issue either (I had originally gone with Hardrace bushes). We needed to urgently source another castor arm, and thankfully this was sourced and the guys at the shop worked on my car until 7pm on a Saturday to get everything done. The car rides a lot nicer now with the suspension dialled in properly. Lowered the car a little as well to suit the lower profile front tyres, and just bring the car down generally. Eternally thankful for the guys down at the shop to get the car sorted, we both pulled big favours from our contacts to get it done on the Saturday.  Also plugged in the new Stedi foglights into the S15, and even from a quick test in the garage I'm keen to see how they look out on the road. I had some concerns about the length of the LED body and whether it'd fit in the foglight housing but it's fine.  I've got a small window coming up next month where I'll likely get a little paint work done on the 110 to remove the rear wing, add a boot wing and roof wing, get the side skirt fixed up and colour match the little panel on the tail lights so that I can install some badges that I've kept in storage. I'm also tempted to put in a new pair of headlights on the 110.  Until then, here's some more pictures from Easter this year. 
    • I would put a fuel pressure gauge between the filter and the fuel rail, see if it's maintaining good fuel pressure at idle going up to the point when it stalls. Do you see any strange behavior in commanded fuel leading up to the point when it stalls? You might have to start going through the service manual and doing a long list of sensor tests if it's not the fuel system for whatever reason.
    • Hi,  Just joined the forum so I could share my "fix" of this problem. Might be of use to someone. Had the same hunting at idle issue on my V36 with VQ35HR engine after swapping the engine because the original one got overheated.  While changing the engine I made the mistake of cleaning the throttle bodies and tried all the tricks i could find to do a throttle relearn with no luck. Gave in and took it to a shop and they couldn't sort it. Then took it to my local Nissan dealership and they couldn't get it to idle properly. They said I'd need to replace the throttle bodies and the ecu probably costing more than the car is worth. So I had the idea of replacing the carbon I cleaned out with a thin layer of super glue and it's back to normal idle now. Bit rough but saved the car from the wreckers 🤣
    • After my last update, I went ahead with cleaning and restoring the entire fuel system. This included removing the tank and cleaning it with the Beyond Balistics solution, power washing it multiple times, drying it thoroughly, rinsing with IPA, drying again with heat gun and compressed air. Also, cleaning out the lines, fuel rail, and replacing the fuel pump with an OEM-style one. During the cleaning process, I replaced several hoses - including the breather hose on the fuel tank, which turned out to be the cause of the earlier fuel leak. This is what the old fuel filter looked like: Fuel tank before cleaning: Dirty Fuel Tank.mp4   Fuel tank after cleaning (some staining remains): Clean Fuel Tank.mp4 Both the OEM 270cc and new DeatschWerks 550cc injectors were cleaned professionally by a shop. Before reassembling everything, I tested the fuel flow by running the pump output into a container at the fuel filter location - flow looked good. I then fitted the new fuel filter and reassembled the rest of the system. Fuel Flow Test.mp4 Test 1 - 550cc injectors Ran the new fuel pump with its supplied diagonal strainer (different from OEM’s flat strainer) and my 550cc injectors using the same resized-injector map I had successfully used before. At first, it idled roughly and stalled when I applied throttle. Checked the spark plugs and found that they were fouled with carbon (likely from the earlier overly rich running when the injectors were clogged). After cleaning the plugs, the car started fine. However, it would only idle for 30–60 seconds before stalling, and while driving it would feel like a “fuel cut” after a few seconds - though it wouldn’t fully stall. Test 2 – Strainer swap Suspecting the diagonal strainer might not be reaching the tank bottom, I swapped it for the original flat strainer and filled the tank with ~45L of fuel. The issue persisted exactly the same. Test 3 – OEM injectors To eliminate tuning variables, I reinstalled the OEM 270cc injectors and reverted to the original map. Cleaned the spark plugs again just in-case. The stalling and “fuel cut” still remained.   At this stage, I suspect an intermittent power or connection fault at the fuel pump hanger, caused during the cleaning process. This has led me to look into getting Frenchy’s fuel hanger and replacing the unit entirely. TL;DR: Cleaned and restored the fuel system (tank, lines, rail, pump). Tested 550cc injectors with the same resized-injector map as before, but the car stalls at idle and experiences what feels like “fuel cut” after a few seconds of driving. Swapped back to OEM injectors with original map to rule out tuning, but the issue persists. Now suspecting an intermittent power or connection fault at the fuel pump hanger, possibly cause by the cleaning process.  
×
×
  • Create New...