Compression ratios

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Jenno007

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Hi guys,

I'm currently deciding between 8.5, 9 and 9.5 compression ratios for my build
Will be a fast road / track car running pump fuel (98 octane)

Will be running either a gtx28 or gtx30.

I was planning on going with 9:1 but then I came across this thread...

http://nefariousmotorsports.com/forum/index.php?action=printpage;topic=2969.0

I was wondering if these guys are right with what they are saying?
 
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.
 
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/>
 
antwan64og said:
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.
Click to expand...

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.


<tuffty/> said:
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/>
Click to expand...

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.
 
Just a thought on the head gasket. Surely a 2.0 TFSI head gasket would fit as the blocks are pretty much the same.
 
Jenno007 said:
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.
Click to expand...

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/>
 
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Also... go with pocketed pistons... especially if running cams

<tuffty/>
 
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/> said:
It will happen soon enough lol

<tuffty/>
Click to expand...
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!
 
Scullies said:
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!
Click to expand...

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/>
 
Jenno007 said:
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.....
Click to expand...
 
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
 
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.
 
superkarl said:
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.
Click to expand...
Any truth to the thread I linked in the OP, saying a lower ratio will give a better spool?
 
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.
 
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Jenno007 said:
Any truth to the thread I linked in the OP, saying a lower ratio will give a better spool?
Click to expand...
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
 
superkarl said:
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
Click to expand...
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
 
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
 
gabin said:
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
Click to expand...

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 crap petrol available in ypour country you could use a thicker head gasket as a band aid fix
 
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gabin said:
I've got some issues with my CR and my mechanic told me the best way is to make a custom headgasket
Click to expand...
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
 
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:)
 
gabin said:
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:)
Click to expand...

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?
 
badger5 said:
8.5:1 one is my preference currently.. but my current strokers are also GT35 builds
Click to expand...
That's true. A gtx30 should be able to run similar boost levels though?

Just to be on the safe side I'll go 8.5:1
 
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.
 
Booth1982 said:
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.
Click to expand...

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?
 
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?
 
that it was low, and basically could be the cause of all my problems. the pinging etc etc.
 
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
 
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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