4agze head gasket size?

[johan_oern]

I'm trying to figure out which head gasket I should use in my rebuild.

It would seem that people recommend the HKS metal head gasket.
My block is not bored to over size and so bore diameter is still 81mm. All HKS gaskets however have a diameter of 82 mm.
Does it matter that the gasket is 1mm to big?

Also the HKS gasket are only available in 0.7mm, 1.6mm, 2.0mm thickness whereas I believe the stock head gasket is 1mm thick.
So which thickness should I use?

I will use a supercharger and later on I plan to increase boost up to 20-25 or so PSI.

The machinist told me that he will be shaving the block and head and regrind the valve seats. The compression ratio should thus be unaffected.

[mattysshop]

if he machining the head AND block the comp ratio may be slightly affected.. any reason why he is surfacing the block??

what pistons are you using? any head work? will the chambers be modified at all? what static compression ratio are you aiming for?

if your aiming for close to stock CR, just add together how ever much he machines off the surfaces.. (50thou is approx 1mm..) add it to the 1.0mm factory gasket, you might have to use a 1.6mm, although taking 30-40 thou off for surfacing is quite a bit!!

[johan_oern]

He's surfacing the block because it's not altogether flat. I don't know exactly how much he's taking out of it but it should be very little.

I'm using stock pistons.

The machinist will be porting the head. He's only smoothing out the sharp edges inside the ports and is chamfering the port openings. He's also fixing the valves.

Chambers will not be modified unless you'd count the valve seats job.

I figured I was gonna try to keep stock 8.9 CR but if you have any other recommendation please let me know. As I said I plan to increase boost in the future up to 20-25 or so PSI. Engine will be running on 99 octane shell v-power fuel.

[mattysshop]

what is the supercharger's efficiency (rpm vs cfm) using somthing like an M45?

any particular your having to run this sort of boost through a S/C?

the only reason i would decompress an engine to run more boost, is if the given charger particularly turbo charger's were not in there peak efficiency at a lower boost..

example GT28RS - most efficient at around 16-18psi according to the compressor maps, but if the static compression is 10.5:1.. would not be really safe to run that sort of boost.. but more like 10psi which the turbo really isn't that efficient..

so you would lower the compression somewhat to say 9:1 or 8.5:1 to have the turbo get into it's peak efficientcy range..

being supercharged you don't really have this problem at all, as it is all linier.. 20-25psi is a pretty big variance..

prolly at least a 1.6mm gasket to 'lower' the CR a bit.. possibly even 2mm.. will he CC the chambers for you? he should be able to tell you exactly what to get to give you the compression ratio you want.

[MRMOPARMAN]

instead of taking a stab in the dark, and guessing what you'll need. why not wait till the engine and head are finished. ask the engine builder what the deck height is now at, and ask what the head CC's are. put this info into a compression ratio calculator.. THEN you can work out which head gasket you'll need.

btw, what model GZE are you running, and what comp ratio you after?

[johan_oern]

what is the supercharger's efficiency (rpm vs cfm) using somthing like an M45?

any particular your having to run this sort of boost through a S/C?

the only reason i would decompress an engine to run more boost, is if the given charger particularly turbo charger's were not in there peak efficiency at a lower boost..

example GT28RS - most efficient at around 16-18psi according to the compressor maps, but if the static compression is 10.5:1.. would not be really safe to run that sort of boost.. but more like 10psi which the turbo really isn't that efficient..

so you would lower the compression somewhat to say 9:1 or 8.5:1 to have the turbo get into it's peak efficientcy range..

being supercharged you don't really have this problem at all, as it is all linier.. 20-25psi is a pretty big variance..

prolly at least a 1.6mm gasket to 'lower' the CR a bit.. possibly even 2mm.. will he CC the chambers for you? he should be able to tell you exactly what to get to give you the compression ratio you want.

I'm going to be one of the lucky people running the Kennebell twinscrew s/c kit designed by Mr2Swift. I'm not really sure about the compressor map at this time but I guess I could figure it out.

The boost figure I gave was a rough estimate actually. From what info I've been able to gather it seem that 20-25psi would give me 250-300hp which is what I'm shooting for.

Now that I read your input I get the feeling I might have missunderstood the whole concept of forced induction.
Please correct me if I'm wrong...
In the example with the GT28RS turbo you said it wouldn't be safe to run 16-18psi on 10.5:1CR. I guess this is because the volume of air/fuel going into the combustion chambers are so much bigger than the space of the combustion chamber. Forcing the fuel mixture into this tiny space would heat up the mixture and cause detonation. By lowering the CR to 9:1 or 8.5:1 the volume of the combustion chamber would be larger and the fuel mixture going into this larger space would not be compressed as much and thus not heated up so much as to cause detonation.
I hope I got that part right?

So to produce say 300hp I would have to know how much boost is required. I'm guessing 25psi.
Idealy the s/c I'm using should have it's peek efficiency range at 25psi correct?
Next I would need to know what CR is needed to safely run 25psi of boost. How do I know what CR is needed for what boost? Are there some magic rule or formula?

If I know the preferred CR I can compare that to the actual CR after the machinist is done and go from there when I select the thickness of the head gasket.

Oh and just to be sure, is it ok that the head gasket is 82mm while the bore is 81mm?

[johan_oern]

instead of taking a stab in the dark, and guessing what you'll need. why not wait till the engine and head are finished. ask the engine builder what the deck height is now at, and ask what the head CC's are. put this info into a compression ratio calculator.. THEN you can work out which head gasket you'll need.

btw, what model GZE are you running, and what comp ratio you after?

By deck height you mean how much he took out?

[MRMOPARMAN]

yep, how much the engine builder has taken off the deck of the block. its measured from the centre of the crankshaft line, to the deck of the block. you will need that measurement (among others) to determine the clearance volume of the motor

[johan_oern]

Ok. I'll ask the machinist to measure it for me Thanks

Now I just need confirmation on those other parts.