[Mk2] [Turbo] Compression ratios and head gaskets

[Quibble]

I stumbled across this on another forum. Some of you will already know all about this sort of stuff but I thought if you're relatively new to these engines like me, and still trying to do your homework and learn about them, then this might be helpful.

"Ok so Ive gotten several questions and concerns about pistons and compression ratios with the 3sgte pistons on the market and I wanted to make a post to clarify everything.

CP, JE, and SRP all have 9.0:1 pistons. Its the only compression ratio they offer, and I often get customers who just prefer 8.5:1.

Arias and Ross both list a 8.5:1 piston but heres the part to pay attention to.

CP/JE/SRP all list their compression ratios assuming an aftermarket .040" (1mm) thickness headgasket.

Ross/Arias both list their compression ratios assuming a factory .056" (1.4224mm) thick headgasket.

Well, if you measure the compression ratio with the CP/JE/SRP pistons using a factory headgasket thickness like Ross/Arias do, you get an 8.5:1 piston, and likewise, if you measure the compression ratio with a Ross/Arias piston using a 1mm aftermarket headgasket thickness, you get a 9.0:1 piston.

So, all of these pistons are the same compression ratio, just listed in different ways. I got with my guy at JE and we used their high tech fancy shmancy inhouse software to get the compression ratios using different thickness headgaskets and this is what we came up with...

Here are the headgaskets we worked with that are available for the 3sgte:

TRD: 0.8mm
Cometic: 1mm
HKS: 1mm & 1.6mm
Apexi: 1.1mm & 1.5mm & 1.8mm & 2.1mm

(CP/JE/SRP/Ross/Arias Pistons)+(TRD 0.8mm Headgasket)=(9.2:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Cometic/HKS 1mm Headgasket)=(9.0:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Apexi 1.1mm Headgasket)=(8.8:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Factory 1.4224mm or Apexi 1.5mm or HKS 1.6mm Headgasket)=(~8.5:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Apexi 1.8mm Headgasket)=(8.375:1)
(CP/JE/SRP/Ross/Arias Pistons)+(Apexi 2.1mm Headgasket)=(8.1:1)


Wiseco's 9.0:1 piston however is 9.0:1 with a factory .056" (1.4224mm) headgasket, so it truely is a 9.0:1 at the same headgasket thickness where the rest of them are 8.5:1

If you want to get close to an 8.5:1 compression ratio with a Wiseco Piston you will need to use the Apexi 1.8mm Headgasket. That combo will put you right around 8.6:1 compression. or you can use the Apexi 2.1mm headgasket to get to 8.4:1 ... more clearly illustrated below:

(Wiseco Pistons)+(TRD .8mm Headgasket)=(9.4:1)
(Wiseco Pistons)+(Cometic/HKS 1mm Headgasket)=(9.2:1)
(Wiseco Pistons)+(Apexi 1.1mm Headgasket)=(9.1:1)
(Wiseco Pistons)+(Factory 1.4224mm or Apexi 1.5mm or HKS 1.6mm Headgasket)=(~9.0:1)
note: i put the "~" because all of my calculations come up saying the wiseco piston is 8.8:1 with a factory headgasket thickness, but they advertise 9.0:1 so its about 9.0:1
(Wiseco Pistons)+(Apexi 1.8mm Headgasket)=(8.6:1)
(Wiseco Pistons)+(Apexi 2.1mm Headgasket)=(8.4:1)





disclaimer: this is slightly simplified, in order for it to be exact, you would have to do this for every bore size as well and go out a couple more decimal places on the compression ratio but for the purpose us car guys are using it for, this is as accurate as we need.
__________________________________________________ _______________
Now for the sake of saving time and making things easier for you guys, I will go ahead and list the part numbers for everything ive talked about in this post.

Cometic 1mm 87mm Headgasket: C4314-040
HKS 1mm 87.5mm Headgasket: 2301-RT034
HKS 1.6mm 87.5mm Headgasket: 2301-RT035
Apexi 1.1mm 88mm Headgasket: 814-T201
Apexi 1.5mm 88mm Headgasket: 814-T202
Apexi 1.8mm 88mm Headgasket: 814-T203
Apexi 2.1mm 88mm Headgasket: 814-T204

Wiseco Pistons:
K615M86 // 3sgte // Bore: 86.00mm (Standard Bore)
K615M865 // 3sgte // Bore: 86.50mm (+.020" Overbore)
K615M87 // 3sgte // Bore: 87.00mm (+.040" Overbore)

CP Pistons:
252061 // 3SGTE // Bore: 86.50mm (.020" 0.5mm overbore)
252062 // 3SGTE // Bore: 87.00mm (.040" 1.0mm overbore)

JE Pistons:
SC7449 // 3SGTE // Bore: 86.00mm (Standard Bore)
SC7450 // 3SGTE // Bore: 86.50mm (+.020" Overbore)
SC7453 // 3SGTE // Bore: 87.00mm (+.040" Overbore)

SRP Professional Pistons:
283458 // 3SGTE // Bore: 86.00mm (Standard Bore)
282309 // 3SGTE // Bore: 86.50mm (+.020" Overbore)

[MR2 Rich]

Interesting post. I've got a fiver on Bob "compression ratio" Hatton putting his 2 cents in.

[Super_red]

Wow, is the factory head gasket really 1.4mm thick compressed? Is this the same for the rev1/2 and rev3+ gaskets?

[bobhatton]

Read and learn MR2 Rich, you need all the help you can get


The correct piston to head clearance on this engine is 1mm, so the head gasket can be used to get this right.
Head gaskets should never be used to change the compression ratio of an engine

[^Trickster^]

There are plenty of combinations available to get the compression ratio you require, the pistons and headgaskets are all much of a muchness now, and people get themselves too involved in compression ratio

What if the deck is skimmed? how much has been taken off? head skimmed? how many times have you done it, and how many times has it been done in the past?

In my experience anything between 8:1 and 9:1 on these engines is fine, ive driven so many varieties of built engines and compression ratio only becomes noticable if you run stupid low numbers like 7:1 or high 9.5:1 and thats only notciable when your tuning the car anyway, lower compression ratio gives you less power off boost but you can generally run more boost and vice versa for high compression ratio, its only about getting a happy medium

Most modern ecu that we are all running have very high resolution fuelling and timing maps and you can map more effectively than years ago.

I dont put any thought at all into compression ratio beyond pistons. My last block was 8.5:1 pistons and my new one is 8.3:1 pistons as ive had a heavy skim on the head.

I usually run a 1.4mm gasket to compensate for skimming, but this engine build is different and I could only get a 1mm gasket.

Graeme

[MR2 Rich]

Couldn't have put it better Graeme. Compression ratios are so over thought out by some "specialists".

[kev8611]

Couldnt agree more with you graeme.

We would be aswell saying the engine was built wrong from the factory by toyota/yamaha going by half the things you read on here

[ashley]

The 3SGTE came from the factory designed to run on Japanese fuel with higher octane levels than we have here in the UK. It's all very well saying compression ratio doesn't matter, but ANY decent engine designer/ builder who knows anything will never ignore it- you do so at your own risk.

Yes- Bob has some extreme views on just how low to go, but the theory behind his views is sound. Modern ECUs do give us the ability to fuel around detonation, but if you go too high on CR you will end up with late ignition and the problems this creates such as high thermal stress on the cylinder walls, and huge EGTs as you start to flush charge that is still in the ignition cycle into your exhaust manifold.

Not knowing the history of your block / head, how many times it's been skimmed etc is just an excuse, if you're building a high bhp engine properly you'll be measuring your CR anyway.

And (much as I hate to admit it) I 100% agree with Bob that using thicker head gaskets to lower CR is a bad idea, all you end up doing is reducing your squish which further damages the efficiency of the combustion process.

I agree- don't get hung up on CR, but you should plan what you run before you build your engine, and measure where you get to before assembly to make sure you ain't too high. 9:1 on an engine running 99 RON is just asking to crack a cylinder wall with anything over a bar of boost...why do you think we see so many cracked 3S blocks? They're just not well made enough to cope with retarded ignition....

IMO of course

[Peter Gidden]

Not knowing the history of your block / head, how many times it's been skimmed etc is just an excuse, if you're building a high bhp engine properly you'll be measuring your CR anyway.

[^Trickster^]

My engines must not be built properly at all then, damn, how on earth do i make all that power for all these years of hard driving.

Must be luck

[Adam86]

This is one of my pistons and the results of running 1.2 bar. Always run on 99 fuel. Obviously you can see all the detonation damage and the end result.



My engine is being rebuilt with a compression ratio of 8.0-1 for a reason. So that along with a fuel and ignition map it will be a much better setup for our 99 fuel.

What's the point of going through the expense of an engine build and not seriously considering the compression ratio?

[Odin_S]

[quote="^Trickster^"]My engines must not be built properly at all then, damn, how on earth do i make all that power for all these years of hard driving.

Must be luck [/quote

But you are in the process of rebuilding your engine because of a cracked block....
Granted you are running high BHP

I am of the philosophy of a rebuilding engine to last as long as OEM if possible. I don't want extra variables, so if lower CR help I'm all for it, which is why i went for 8:0 CR. We in this country don't have the luxury of ethanol like the US so we can't build the 3S like them

The other route is of course the 5 block

[bobhatton]

Yes- Bob has some extreme views on just how low to go,

[bobhatton]

And (much as I hate to admit it) I 100% agree with Bob

[ashley]

My engines must not be built properly at all then, damn, how on earth do i make all that power for all these years of hard driving.

Must be luck

Not saying you don't know what you're doing mate (far from it!), but if you haven't been measuring/ designing the CR on the engines you build (which I don't believe as you've told us the CRs of your builds in your original post), then yeah- I would have to say it's been luck

[ashley]

Yes- Bob has some extreme views on just how low to go,

And (much as I hate to admit it) I 100% agree with Bob

[^Trickster^]

Adam if that was a stock engine with stock turbo there is no way on earth you detonated that heavily on 99 fuel at 1.2bar of boost! You ran lean or overboosted heavily and didn't realise, and trust me it doesn't matter what CR you run if you have atht same issue it will smash your built engine to bits just the same!

Odin well observed I am building another engine, I used 2 3s based blocks which i already owned for my previous 2 builds which bother did 20k of hard driving with 500+ hp, both cracked behind the water pump which is a common issue and not something that you can build around without a different block.

This build is 5s based to get around that issue which I had planned years ago if I cracked another block which I did at 700hp on the dyno after running that for over 20k mile at 500hp, I wouldn't build a 3s block ever again.

There is a reason I rarely post on this forum, and I won't be posting again. Plenty of people here to give advice, I'll keep mine to myself, sorry if i offended anyone.

[rev3turbo]

There is a reason I rarely post on this forum, and I won't be posting again. Plenty of people here to give advice, I'll keep mine to myself, sorry if i offended anyone.

Please do NOT stop posting, you amongst a few others are the posts I look out for an read.

[Adam86]

Trickster - it did happen and it happened lol.

1.2 bar, st205cc and a hybrid CT20 to be precise.

I should point out though that the car has done a fair few track miles and a few flat out runs where ive touch the rev limiter in 5th gear.

So a hard life lol.

[ashley]

There is a reason I rarely post on this forum, and I won't be posting again. Plenty of people here to give advice, I'll keep mine to myself, sorry if i offended anyone.

Apologies if I offended you with my opinions (I need to learn to keep them to myself), fwiw I agree with what you are saying- there's a safe margin for error with these engines and CR, and I believe 8.5 - 8.0:1 is bang in there unless you are going mental with boost. My only point is that guys need to check what they're building instead of just buying 9:1 pistons off the shelf and assuming everything will be ok.

I admit I don't like the idea of folk using hugely thick head gaskets to drop CR though, but agains that's not what you said you were doing.

Please don't stop posting- I too always enjoy reading what you say, and I apologise if I appeared to be attacking you- I didn't mean it that way at all