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

    • My experience with Rising Sun Exports Before agreeing to the sale I tried to do as much research as I could (obviously), his Facebook reviews are 98% and he goes Live at least once or twice a week. I contacted 2 people in the UK who had used him for their imports, both had positive feedback. His explanation and talk through of the import process was thorough, answering any query no matter how stupid it was. It felt as soon as the money was sent, communication dropped off. I asked for shipping updates every 2 weeks or so, not wanting to pester him, he never had any updates. I wasn't informed the car had been dropped off at the port, I only found out by his Facebook story. I asked for the photos taken at the port, knowing he would need some for insurance purposes. I received a few 5 second clips and that's it. When asked again, he said his staff had them. Weeks later I asked again, he tells me he doesn't have any, but does have 50 photos from the original advert. I never received them. I eventually got the documents sent via WhatsApp after I mentioned the port was requesting them. I purchased a CarVX report, to find out the vehicle is a Grade R with recorded accident damage, first recorded in 2017 when it was first auctioned. He never told me the grade, then again I didn't ask. His response was "Grade R means nothing, it wasn't chassis damage". Still, I would have liked to have been informed about it. Jon prides himself on being open and honest when it comes to inspecting cars, it's his main job doing so at the auctions for customers. When the vehicle arrived in the UK I noticed a few little cosmetic issues. It's a 21 year old car so it wasn't going to be mint condition. The side skirts are cracked on each corner and the sealant is failing. The front grill on the bonnet/hood isn't secured very well, mounting studs are missing. Both minor things, but again, it would have been nice to be told. During a Facebook Live walk around video of the vehicle, he mentioned it has a front Whiteline anti roll bar/sway bar. While on the inspection ramp, I noticed the stock item has been installed. When first questioned, his response was "the ARB? Switched? Since when, it never had them". Since sending video and photo evidence I've not received a response. I'm probably being over critical of the overall condition of an old car, but all I wanted was honesty (which he claims to have). I'm aware I wasn't his only customer, he's busy doing XYZ but other reviews praise him for great communication with regular updates and photos, I felt I didn't receive the same treatment. 
    • I was able to get some underside photos while the car was on the ramp The suspension is all Altezza/IS200/IS300 so getting part's will hopefully be less of a headache
    • Welcome to my 2004 Toyota Mark ii IR-V Fortuna (series 2) With a 1JZ-GTE powerplant under the bonnet (hood) it'll give me plenty of scope for power upgrades. For those who aren't familiar with imports, the 1JZ-GTE is a 2.5L 6 cylinder VVTi engine with a single turbocharger. This has the factory R154 5 speed gearbox, along with a aftermarket 2 way LSD differential (brand unknown). Under the arches are a set of CST Zero 1 alloys, 18x9 +30 225/40 up front and 18x9.5 +15 265/35 on the rear. The car was quite low in Japan and there's evidence of the wheels catching the rolled arches/fenders. The tyre's aren't great so I'm in two minds whether to replace both or just the tyres and put up with the wider wheels on the rear. The car still uses stock brakes with the addition of some braided hoses. The exterior is stock with the exception of a BN Sports front bumper and a replacement Fortuna grill  Moving to the interior, the steering wheel has been replaced with a dished MOMO steering wheel (which will get swapped for my Momo Tuner for the time being) Defi Link Gauges are mounted above the climate controls and on the A pillar, the Oil Temp,Oil Pressure,Water Temp and Boost gauges should help with spirited and track driving  The stock seats have been replaced with some retrimmed Recaro bucket seats. Being a larger build these are a little snug, unfortunately the orange isn't for me so I'll look into replacing these down the line. Other modifications include a twin plate clutch, Blitz intercooler, Evolve alloy radiator, a stainless exhaust with decat, HKS EVC-S boost controller and coil overs
    • Apologies for the long read My R34 GTT was up for sale at the beginning of spring due to a few repairs creeping up. The strut tops needed replacing, roof and bonnet (hood) painting (yay for 3 stage pearl) and the underside stripped and treated. I sold the car which allowed me to be in a much better place financially. Leading up to the sale I was already thinking about the replacement. In an ideal world it needed to be a good all-rounder. Something I can mess around with, modify, do track days, do the school run, go on long drives etc.  Options included but not limited to... Laurel C35, Evo 8/9, Civic FD2, Impreza Hawkeye, Aristo and even an Audi S4 Avant (I've already got the Mazda 6 wagon). But there was always one car at the top of the list The Toyota Mark ii JZX110 I found an advert on a Facebook group for an example in Japan, from a seller called Jon at Rising Sun Exports. A few messages back and forth and Jon calls me from Yokohama one morning (or afternoon in his case). He briefly explained the import process, the costs involved and a repeat of the advert. After much deliberation, the price was agreed and the sale was locked in. I've never imported a vehicle so I jumped into the unknown head first. The money transfer was completed through wise.com (fees apply), very easy to use and the money was with him within a day or two.  The car suspension was raised for the vessel and the car dropped off at the port. It was 7 weeks later when the bill of lading was received and the freight invoice sent to me, followed by the export certificate a couple of days after (both digital copies) In the mean time the port had been in contact. I needed an agent to deal with the NOVA (notification of vehicle arrival) along with the tax/duty invoice, this was £75.00. The port also had a fee of £100, I'm guessing to cover the cost of the 10 day 'free storage' and for moving the car off the boat etc. They need a copy of the vehicle invoice, freight invoice and export certificate to allow the vehicle to pass through customs. The vessel arrived on Tuesday 5th August, the tax/duty invoice was generated and sent over. This is generally tax 20% and duty 10% of the vehicle value. Although the invoice came in at a higher amount than I had calculated. Once HMRC had received the payment the vehicle could be released from customs. I thought once the tax/duty was paid you could collect at any point, that's not the case. Your agent will need to book a collection slot, I requested Thursday 7th which was accepted, with a 9:00am slot allocated. It was a 5:00am start from Norfolk heading to Southampton. We eventually found the compound, upon presenting the bill of lading and some ID they released the car (they drove it out of the compound to the front ready for us to load up). The email from the port stated each slot had a 10 minute window, which seemed abit farfetched but the staff said it's not a strict rule. We were there for approx. 30-40 mins in total. A week prior to collecting I contacted my garage and explained the situation, I was able to get an inspection slot that afternoon. For the registration, DVLA require the car to be insured, for this I used a company called Adrian Flux who can insure the vehicle using the VIN number. 
    • Hey guys, looking for these side skirts if anyone can help me out. 
×
×
  • Create New...