Reducing Compression for Turbo engine

Post by **PrecisionBoost** » Sun Dec 18, 2005 3:58 am

I just thought I'd bring this up.... my original guess of adding 1mm to the head gasket thickness was almost bang on.

I found a company in the UK that supplies a special gasket for the 2.0L 16V that is meant specificly to reduce the factory compression down to a level that is acceptable for addition of a turbo.

They are advertizing a 1.9mm Kevlar gasket for a couple hundred dollars.

kinkyllama · Post by **kinkyllama** » Sun Dec 18, 2005 6:48 pm

What else do you need when you lowering the compression? If i remember right it would be highly effective to have adjustable cams... but not required?

Mr_Efficiency · Post by **Mr_Efficiency** » Mon Dec 19, 2005 12:50 am

Yeah, if you raise the head to much it can throw off your timing, why do you have to lower the compression, is it really that high? theres guys on other forums boosting cars with 9.5:1 comp with no problems you just cant go to high on the boost.
I know its better to take out the compression, I'm just saying its not a necesity to boost a car.

Post by **PrecisionBoost** » Mon Dec 19, 2005 2:51 am

Sure you can do it with 9.5:1 and you will make more power than if it were 8.8:1 but the margin for error is significantly higher.

A few mistakes in tuning the 8.8:1 won't be as serious as mistakes with the higher compression.

With 8.8:1 you don't need as high of octane either.... you could probably get away with mid grade with a low boost application. (6psi).... although premium grade would make more power.

Given he lives in Arizona..... I think he should go with lower compression.... they get some awfully hot days out there in the summer.

I don't think you will have to worry that much about rising the cylinder head 1mm..... hard to say.... I've never done the calculation to see how much it rotates.

Hmmmm lets see if I can do an estimate..... I'm going to guess the cam gear is around 120mm in diameter (roughly 4 3/4" )

I don't have one with me to measure..... but that dimension seems about right.

Ok....... so..... the circumference is found as 3.1415X120mm =377mm

Divide that by 360 degrees and you get 1.03mm per degree.

So.... if you raise up the cylinder head by 1.03mm it should equal roughly one degree of rotation of the crankshaft gears.

Hmmmm..... one degree doesn't seem like much.... I don't think that you would have to worry about having adjustable cam gears.

It's going to move both cams 1 degree clockwise...... I can't remember which way the cams normally rotate.... Clockwise or CounterClockwise

So it will either advance the cams by 1 degree or retard them by 1 degree.... depending on the normal rotation direction of the cams

Stefan · Post by **Stefan** » Wed Dec 21, 2005 10:16 am

A lot of people have doubled up on head gaskets, added spacer plates, etc. when turbocharging the C20XE - none have worried about cam timing or fitting adjustable cam pulleys...

Who was the supplier of the head gasket? If you don't want to make it public, pm me.

Stef

Post by **Efratech** » Wed Dec 21, 2005 6:43 pm

Stefan wrote:A lot of people have doubled up on head gaskets, added spacer plates, etc. when turbocharging the C20XE - none have worried about cam timing or fitting adjustable cam pulleys...

Who was the supplier of the head gasket? If you don't want to make it public, pm me.

Stef

So i can add 2 gaskets and go turbo... how long have they last Stef?

Post by **PrecisionBoost** » Wed Dec 21, 2005 8:05 pm

Stefan wrote:Who was the supplier of the head gasket? If you don't want to make it public, pm me.

I don't remember... I just happened upon the site.... I don't think it was courtney....hmmm.... not sure.

I personally would go with a custom Multi Layer Steel gasket..... Tom said he can get a variety of thicknesses.

I thought Kevlar might be a bad idea.... so I didn't bother keeping the post.... I thought Kevlar broke down over 500 degrees.

Do you know much about Kevlar gaskets?

I know they are used on the intake sometimes.... but this was the first time I had heard of one being used as a cylinderhead gasket.

The info I was interested in was the fact that they specified this 1.9mm gasket specificly for reducing compression on the X20XEV for a turbo conversion.

I will see if my browser history still has it in memory.

Chris

Post by **Efratech** » Wed Dec 21, 2005 8:14 pm

Can you throw some numbers on how thick ill need it?

Remember that the XE has a 10:5 compresion.

Audacity Racing · Post by **Audacity Racing** » Wed Dec 21, 2005 8:17 pm

to get teh xe to 9:1, you'd need to add 1.2mm onto the stock gasket

Stefan · Post by **Stefan** » Thu Dec 22, 2005 10:23 am

Efrain A. wrote:So i can add 2 gaskets and go turbo... how long have they last Stef?

Yes, Courteney in the UK used to do turbo conversions on the astra using the two head gasket and spacer plate approach. Most of these were good for up to 260hp iirc. Their life depends on installation and what you use the car for. I have known people to poorly fit them up and blow gaskets within a couple of days...
For a 10psi boost they'll be okay as long as they're properly installed and detonation or long periods at high engine temps are avoided.

Stock XE gaskets are 1.3mm when compressed. Spacer plates are normally 1.6mm thick and made from stainless steel. This gives and extra 2.9mm of thickness over the stock single gasket. Gasket and spacer bore are 87.7mm.

I'll try and upload the dxf file for the spacer onto my space on your server in the next few days (maybe tonight if I get the chance) you can then give it to your tuner to get one laser or plasma cut.

Erfinder: I asked about the gasket as a friend wanted one. I couldn't find a supplier (that I knew of). I wasn't going to recommend the steel one as he doesn't want to get the block and head surfaces re-machined for the proper surface finish required by the steel gasket.

Stef

Post by **PrecisionBoost** » Fri Dec 23, 2005 10:29 pm

I couldn't find it in the history on my browser here at work.... so it was probably on my home computer.... I will do my best to find it for you.

One of these days I'm going to measure the cylinder head chamber volume so that it's easy to come up with all the required info..... I keep forgetting to do it..... perhaps I will get some time over the holidays.

Post by **PrecisionBoost** » Fri Dec 23, 2005 11:46 pm

I can't find the link at home..... but I did find this...

http://www.migweb.co.uk/forums/showthread.php?t=36487

I also found a guy talking about how he used a 1.9mm head gasket on his calibra to lower the compression to 8.8:1

So.... perhaps I was confused... perhaps the Kevlar 1.9mm gasket was for a C20LET to move the compression down a tiny bit.

If the stock gasket was 1.3mm and the lower compression one is 1.9mm then the difference is only 0.6mm

Audacity Racing · Post by **Audacity Racing** » Fri Dec 23, 2005 11:52 pm

check www.racingdownloads.com for a set or racing calculations in excel. there is a head milling calculator in there and if you LOWER compression, it gives you a negative number and that's the added distance you need to add. that's how i firgured my number up there for the XE

Post by **PrecisionBoost** » Mon Dec 26, 2005 7:35 pm

here is a great site for these questions.........

http://not2fast.wryday.com/turbo/compre ... sion.shtml

I came up with my own mathmatical forumlas bases upon simple volume and compression calcuations and it turned out to be the same as I found on this page. (very close )

A 1mm spacer should bring the stock 9.6:1 compression down to 8.8:1

based upon my calculations an XE an extra 2.9mm would bring the compression ratio down from 10.5:1 to roughly 8.1:1

Post by **PrecisionBoost** » Mon Dec 26, 2005 8:00 pm

Ok... some math....

Total Volume of cylinder / Compression volume = compression ratio.

The total volume of the cylinder is simply 1/4 of the engine displacement plus the compression volume

The compression volume is made up from a series of things.... compression chamber volume, gasket volume, spark plug displacement, valve displacement, piston volume, deck to piston clearance volume.

But fortuantly.... we don't need all these pieces of info to calculate the change in compression for a given gasket thickness.

Since Total Volume ( Tv ) = cylinder displacement (Cd) + comp volume (Cv) we can substitute into our first equation and move things around a bit.

Tv = Cd + Cv

Tv / Cv = Comp ratio (CR)

so...

Tv = ( Cv )( CR )

Substitue Tv= Cd + Cv

Cd + Cv = (Cv) (CR )

Further reduction of formula gives...

Cd = (Cv) ( CR - 1 )

Since we know the compression ratio and the cylinder displacement we can find the Compression volume

( 1/4 )( 2.0L ) = Cv ( 9.6 - 1 )

0.5L / 8.6 = Cv

Cv = 0.0581395L = 58.1 CC (cubic centemeters)

(1L =1000CC)

So... now that we know the Compression Volume we can add a spacer and then move backwards to solve for the compression ratio.

Lets add a 1mm spacer

Let's also assume that the spacer is slightly bigger than the bore.... so let's make it 87.5mm in diameter and 1mm thick.

The volume this spacer adds to the Compression volume can be found as follows:

Spacer Volume = ((3.1415)( 8.75cm / 2 ) ^2 ) ( 0.1 cm )

Sv = 6.013 cm^3 = 6.012 CC = 0.006013 L

So the new volume equals the stock Compression volume plus the spacer volume

New Volume = Cv + Sv = 58.1 CC + 6.013 CC = 64.113 CC = 0.064113 L

Putting this new compression volume back into the equation gives us...

(1/4)(2.0L) = Cv ( CR - 1 )

0.5L = 0.064113L ( CR - 1 )

0.5L / 0.064113L = CR -1

CR = 8.78873

So the new compression ratio with a 1mm spacer is 8.8:1

This is just a "quick" estimate as there are many more factors which need to be put into the real calculations.... which is done on that link I gave on the last post.