Compression ratios

antwan64og

Registered User
What power are you shooting for racing with and what boost are you expecting that to be on track given your fuel?

Usually
Lower cr (say 8.0) and crank up the boost (more heat).
Higher cr (say 9.5) and less boost (less heat).

In an ideal world you could run straight methanol have a compression ratio of 15:1 and not need an intercooler because it runs so cool.
 

<tuffty/>

Badger 5 Edition...Its all about the flow...
Staff member
Moderator
If going full 83mm then 9:1 or 8.5:1 should be considered as the correct sized headgasket for 83mm is thinner which lifts comp ratio a bit more... last couple of builds at Bill's have been on 8.5:1 pistons IIRC...

Mines on 9.5:1 with a std 1.8t gasket as the bore is 82mm... strokers tend not to like as much timing as 1.8's from what I have seen

<tuffty/>
 

Jenno007

Registered User
What power are you shooting for racing with and what boost are you expecting that to be on track given your fuel?

Usually
Lower cr (say 8.0) and crank up the boost (more heat).
Higher cr (say 9.5) and less boost (less heat).

In an ideal world you could run straight methanol have a compression ratio of 15:1 and not need an intercooler because it runs so cool.

I would like at least 400bhp on pump 98 with the money I'm spending on. I will go WMI in future, when I build the head, once the bank account recovers.

I was more talking about the spool side of things, the guys on the thread I linked said a lower compression engine will spool earlier because you can run more timing.

Very interesting what you say about a pure methanol engine, I never knew this.


If going full 83mm then 9:1 or 8.5:1 should be considered as the correct sized headgasket for 83mm is thinner which lifts comp ratio a bit more... last couple of builds at Bill's have been on 8.5:1 pistons IIRC...

Mines on 9.5:1 with a std 1.8t gasket as the bore is 82mm... strokers tend not to like as much timing as 1.8's from what I have seen

<tuffty/>

My mechanic said that he will source the head gasket and go with a metal head gasket. Does this sound like a good idea?

I am getting 83mm pistons. I might just go for the 8.5 compression pistons to be on the safe side of things.

If Bill is going for 8.5:1 pistons, it sounds like the smart thing to do.
 

<tuffty/>

Badger 5 Edition...Its all about the flow...
Staff member
Moderator
I would like at least 400bhp on pump 98 with the money I'm spending on. I will go WMI in future, when I build the head, once the bank account recovers.

I was more talking about the spool side of things, the guys on the thread I linked said a lower compression engine will spool earlier because you can run more timing.

Very interesting what you say about a pure methanol engine, I never knew this.

My mechanic said that he will source the head gasket and go with a metal head gasket. Does this sound like a good idea?

I am getting 83mm pistons. I might just go for the 8.5 compression pistons to be on the safe side of things.

If Bill is going for 8.5:1 pistons, it sounds like the smart thing to do.

All 20v head gaskets are metal... you can either buy a cometic gasket and walk funny for a while from the cost or get an OE one for an ALT engine code (2ltr NA)

<tuffty/>
 

Jenno007

Registered User
ahh just had a quick google, they are for valve clearance. It looks like the pistons have a small pocket from the picture, so I'm guessing they are
 

<tuffty/>

Badger 5 Edition...Its all about the flow...
Staff member
Moderator
what you going to use? Cat or IE

fwiw - I run 9.0 wiseco stroker pistons and cat cams 3658's

I'm not fan of valve relief pistons but then if something goes wrong with my timing, its going to be BAD!

Cat Cams... using Bill's profile

My CP pistons have a deeper centre inlet pocket but not anything else... when I had the gear change incident exhaust pockets may have saved the valves but to be fair I was extremely lucky that the engine didn't full on lunch itself

<tuffty/>
 

antwan64og

Registered User
I was more talking about the spool side of things, the guys on the thread I linked said a lower compression engine will spool earlier because you can run more timing.
QUOTE]

Do you need earlier spool?
A quicker onset reaches traction limits sooner, a linear map is usually preferable as its nice and predictable.

For example in track mode Prawn uses low boost, at 5500rpm that only makes 250hp.
In full boost mode it makes almost 300hp at 5500rpm.

Other set ups make 300hp at 4500 rpm.....
 

Jenno007

Registered User
I should have said I have a s3 unlike Prawns a3, so I'm not as worried about wheel spin. Plus I'm a real rookie so I won't really get in on the power until I'm in a straight line and the car is settled. This isn't going to be a car to get the quickest lap time, this is a car to have fun, and be a sleeper of sorts (everyone here in Australia has JDM cars) .

I just want the longest rev range possible mainly for the street, and since I'm going to leave the head for later I figure I'd like the earliest spool possible, while not sacrificing top end, hence the cr discussion
 

superkarl

MAN OF STEEL
i wouldnt lower it past oem 9:1.
You're building a stroker so theres a good amount of low end torque there.
The gtx28 and 30 arent the most responsive units in the world, so keep it high.
 

Jenno007

Registered User
i wouldnt lower it past oem 9:1.
You're building a stroker so theres a good amount of low end torque there.
The gtx28 and 30 arent the most responsive units in the world, so keep it high.
Any truth to the thread I linked in the OP, saying a lower ratio will give a better spool?
 

rallye_turbo

Registered User
I have an engine in my S3 that has 8.5:1 ratio (Wiseco pistons) and I am in the process of putting on latest BBT Hybrid Turbo.
I will then get it mapped at Bill's. It would be interesting to compare with what is already out there in terms of power & torque.
 

superkarl

MAN OF STEEL
Any truth to the thread I linked in the OP, saying a lower ratio will give a better spool?
imo the guy that is claiming that, is right if the car is able to make boost, because you can technically run more, and more timing. But were talking about low end driveability here where there is little to no positive pressure being created , therefore the engine is going to be slightly more responsive with a higher CR. I think.

To be honest its a tedious debate. The differences are probably so minute you'd be dissapointed for stressing about them.
Stroker + stock compression, is going to be a winner anyway, i doubt many would be unhappy
 

Jenno007

Registered User
imo the guy that is claiming that, is right if the car is able to make boost, because you can technically run more, and more timing. But were talking about low end driveability here where there is little to no positive pressure being created , therefore the engine is going to be slightly more responsive with a higher CR. I think.

To be honest its a tedious debate. The differences are probably so minute you'd be dissapointed for stressing about them.
Stroker + stock compression, is going to be a winner anyway, i doubt many would be unhappy
Thanks for the feedback. He was saying that you can run more timing off boost with a lower cr. I don't know if there is any truth to that, but I'm sure as you said the difference will be tiny.

I think I will go with what tuffty said Bill does, and run 8.5 as the head gasket will increase it.

Cheers
 
Ino on other engines different thickness headgasket are used too to help lower or raise CR but on the 1.8t its rarely heard of!?:s
 

gabin

Registered User
Antwan, could you please share some info regarding on how you can control CR using the pistons?

I've got some issues with my CR and my mechanic told me the best way is to make a custom headgasket
 

antwan64og

Registered User
Antwan, could you please share some info regarding on how you can control CR using the pistons?

I've got some issues with my CR and my mechanic told me the best way is to make a custom headgasket

The CR is calculated by the following formula:
3afd933b679b7b20f1561c841867f901.png
, where
06fef6cf9cd6d4b3c27115712d7f9f89.png
=
cylinder bore (diameter)
0a9faac9096f735c3f42c9c14414aaac.png
=
piston stroke length
5ed8c6d87dd12484eeb722ffd6b1361c.png
= clearance volume. It is the volume of the combustion chamber (including
head gasket). This is the minimum volume of the space at the end of the compression stroke, i.e. when the piston reaches top dead center (TDC). Because of the complex shape of this space, it is usually measured directly rather than calculated.

The shape of the top of the piston afects the clearance volume.

How have you got a CR problem?
The sensible use of thicker headgaskets is for when the cylinder head comes off and the cylinder head and or block has to be skimmed/decked to make it flat and seal properly.
A thicker head gasket can be used to get the clearance volume back to within spec.

I guess if you only have **** petrol available in ypour country you could use a thicker head gasket as a band aid fix
 

superkarl

MAN OF STEEL
I've got some issues with my CR and my mechanic told me the best way is to make a custom headgasket
How and why can you have 'issues with CR' ?
Are you sure your mechanic hasnt told you that your compression is poor? 2 different things.
And a headgasket is not how to remedy poor compression
 

gabin

Registered User
No Karl, in fact I have 10 on 3 cylinders and 9.5 on one of them..I'm preparing a build now, getting money for the parts:)
 

antwan64og

Registered User
No Karl, in fact I have 10 on 3 cylinders and 9.5 on one of them..I'm preparing a build now, getting money for the parts:)

Really?
Aren't Normal results like 160 for an old motor to 190 for a rebuilt motor.

How did he measure your compression ratio?
Cylinder head off and use CC's of fluid to calculate it?
 

Booth1982

Registered User
don't mean to Hijack. when a compression test is done on all 4 cylinders, what should the readings be in Bar Preferably. just curious, I had a test done and the differ slightly, just hoping its not bad.
 

Jenno007

Registered User
don't mean to Hijack. when a compression test is done on all 4 cylinders, what should the readings be in Bar Preferably. just curious, I had a test done and the differ slightly, just hoping its not bad.

A compression test, and a cylinders compression ratio are two different things.

Anyway, what results did you get? Did they test the cylinders both dry and wet?
 

Jenno007

Registered User
That is incredibly low. 8 bar = 116 psi. You would like to see around 180psi.

What did they say when they did the compression test?
 

Jenno007

Registered User
yeah I don't know what effects it would cause apart from a loss of power. All I know is that it's very low and isn't great news. Basically you're looking at a rebuild
 

Booth1982

Registered User
I thought the BAM engine's compression ratio is 9 and thet it should be low? please enlighten me. im obviously way off.
 
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