924Board.org

[dreamgts]

I will soon be needing a new cylinder head gasket, since this is to be changed I was wondering if a less thicker gasket can be used to increase CR, The engine is a ROW so has I believe 9:3 to 1 CR, the head is modified a bit and has been ported, new cam(Newmans-UK) and will have slighlty bigger valves if I find anything appropriate. Fuel supply will be twin webber 45 DCOE. If a slimmer head gasket is possible, would it affect reliabiity? and anyone knows form where I can get one or how much CR will be gained?
Thanks
dreamgts
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2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
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[ideola]

Yes, running a slimmer head gasket will increase CR. There are so many variables that go into determining CR that there is no way anyone can tell you what you will end up with until you have all of the critical dimensions measured; in particular, how much material you are going to remove from the block and the head.

Generally speaking, you want to remove as little as possible in order to preserve the correct geometry for the timing belt tensioner, otherwise, you have to run an adjustable cam gear, and possibly even a smaller belt. But those are different issues...

All of that said, I can provide you with Cometic MLS head gaskets in various thicknesses ranging from .027" on the thinnest (compressed) all the way to .120" on the thickest (compressed). Anything above the stock .051" thickness requires a 5-layer gasket, while those from .051" and thinner are 3-layer. Beware of other resellers advertising Cometic gaskets for the 924, as they only have access to the old-school copper gaskets. The MLS are only available from me and are special order / made-to-order, usually requiring about 2 weeks to turn around. They do not require any annealing spray (unlike the copper versions), and have special embossments which improve sealing for high compression and forced induction applications.

On a Euro spec motor, if everything is left UNTOUCHED (i.e. stock deck height with NO material removed, stock pistons, stock head, stock bore, etc.) the .027" head gasket will yield approximately half a point higher CR, i.e. from the stock 9.3:1 to somewhere around 9.8:1. If you remove material from the block that figure will be higher. But this will at least give you a "rule of thumb" figure to determine what the max possible CR increase is.
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erstwhile owner of just about every 924 variant ever made

[dreamgts]

Ok a .027'' will do fine if thats the max it will go slimmer, the 2 week to turn around is not a problem since I would be needing it in around 4 to 5 weeks most probably due to other work being done. I have some other questions though since you mentioned other things, will skimming the head also increase CR since I read that it wont on other threads, and also does the adjustable cam gear mean the cam gear needed when one changes the camshaft to a modified camshaft? Also if the head can be skimmed to increase CR can one find a shorter timing belt. I would also like to know what would the gasket cost including shipping to Malta if thats possible, also payment method please. The gasket would definetly do. May I also ask if a half a point higher CR would do any difference to aid a bit of performance? Im not expecting a rocket with the half point either though just to make things clear.

Thanks
dreamgts
_________________
2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
4 SUV'S
2 SEDANS
1 OPEL PANEL VAN
WIFES SUZUKI ALTO

[ideola]

The purpose of the adjustable cam gear is to make up the difference in timing that results from changing the geometry of the cam sprocket to crank sprocket when you deck the head and/or the block. It's not necessarily related at all to a custom cam prodfile.

The NA head has no combustion chamber, so decking it does yoo no favors at all and has zero impact on CR. On a 931, it will have an effect on CR becausae decking the head will effectively reduce the chamber volume.

Increasing CR will increase torque/HP, but by how much is anyone's guess on our motors. There are general rules of thumb that indicate about a 10-20 HP gain for every full point of CR increase, but there are so many variables at play, it is nearly impossoble to say what the effect is without scientific before & after testing. My theory is anything that helps out thelow-end on our motors is a good idea, and a thinner gasket / higher CR is a cheap way to get there.

Now, you mentioned bigger valves. You better have your valve reliefs enlarged AND deepened by the difference between stock thickness and whatever gasket yoyu choose if yoyu want to keep non-interference.
_________________
erstwhile owner of just about every 924 variant ever made

[dreamgts]

I guess one thing goes with another, I will check with the engineer works regarding interference, I havnt decided on valve size yet but would like to know if the half point gasket would make a difference in the non interference of the valves with pistons. Or else how much can one deck without having to enlarge and deepen the valve releifs, should I ask also what is the measurement between the valve and the valve releif on a standard engine?

dreamgts
_________________
2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
4 SUV'S
2 SEDANS
1 OPEL PANEL VAN
WIFES SUZUKI ALTO

[dreamgts]

Im not sure if this has been done to any 924 NA head, but is it possible to fill up/weld any part of the combustion chamber and re shaping it, I think this would also increase CR?? or am I wrong? If this can be done then there is no worry about any interference either .

dreamgts
_________________
2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
4 SUV'S
2 SEDANS
1 OPEL PANEL VAN
WIFES SUZUKI ALTO

[fiat22turbo]

There is no combustion chamber in the head. You'd have to have one created from scratch, including making new areas for the valve seats, shaping the area in the ports and behind the valves, etc.

When they say its flat, it is flat. from front to back, the valves and seats are only slightly countersunk. The combustion chamber space is created via the areas in the tops of the pistons.

So you'd have to use different pistons as well in order to maintain or gain CR.

I wouldn't think you could weld, etc on the tops of the pistons since they move at high rates of speed, you could end up with bits of welded metal floating around. Plus the pistons would need to be balanced afterward.






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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[tyfighter123]

Who did your head work? Looks great.
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Porsche 924 1977 N/A
Mustang GT/CS 2007
Porsche 924S 1987 (parts car)(cut up and recycled)
Porsche 911S 1976
Porsche 931 1980
Porsche 931 1980 (parts car)

[dreamgts]

Yes Stefan I guess it is flat, flat as can be actually. My pistons arnt completly flat though, they have counter sunk valve shapes on the top, are these counter sunk to avoid interference? or is this because its a ROW engine?

dreamgts
_________________
2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
4 SUV'S
2 SEDANS
1 OPEL PANEL VAN
WIFES SUZUKI ALTO

[fiat22turbo]

Yep. Valve reliefs in the pistons allow the engine to not become a grenade when someone pulls the pin aka timing belt.

You have higher static compression than most of the US cars, so the dish is smaller on your pistons than they are on mine.

For me to make similar power, just means that I need to artificially increase the compression ratio somehow (BAE turbo manifold sitting on the shelf along with an M60 from a Mercedes 230, need to do a lot more tuning and work before that development starts)

The head work was done by the local Napa machine shop in Portland, they've done great work on the motors for our family for quite a while. I think I've had at least one motor for each car I've owned run through that shop (certainly the heads thanks to turbo 4's that like to pop gaskets when not maintained properly) My Uncle has the machine work for his customer's lift-truck engines done there along with his race motors.
_________________
Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[dreamgts]

So if I increase the CR by half a point by placing a .027'' thick gasket, and if I increase the valve sizes, shall I be risking interference?? Should I therefore settle for the standard valves and the .027'' head gasket only? I dont wont to interfere with the lower end at this stage since the lower end looks as good as it gets, besides this project has taken more time than supposed to. I might tackle the lower end on a spare engine block which I have but thats for another day. I understand that by increasing the CR by half a point(9:3 to 1 to 9:8 to1) I might add between 5 to 10 bhp which Im happy with since no drastic work or expense is involved. although I would have been much happier if the CR went up to at least 10:5 to 1 or something in that region.

dreamgts
_________________
2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
4 SUV'S
2 SEDANS
1 OPEL PANEL VAN
WIFES SUZUKI ALTO

[fiat22turbo]

Maybe only if the timing belt breaks. That was the only reason for the valve reliefs on most motors. Timing belts are cheap/easy to replace on these motors, so I wouldn't sweat that too much.

It also wouldn't be hard to check during assembly. Just put the head on the block with a gasket, rotate the cam to TDC and see if the head is lifted as the engine is rotated. The valve reliefs could be machined larger to accommodate if necessary as well.

I'm sure Ideola or someone else has the specs for the engines and could potentially calculate what your changes would garner as far as space in the chamber for all of the moving parts.
_________________
Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[dreamgts]

Thanks Stef, I will just have to wait for someone to work it out then. I cant find anything about clearances on the Haynes book either or else I have skipped it somewhere. It wont be worth it if I have to disassemble the lower end on the fitted block just to gain 5 to 10 BHP, although they are tempting if real figures and in anycase should this be done I would opt for the block to be decked etc etc.

Thanks
dreamgts
_________________
2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
4 SUV'S
2 SEDANS
1 OPEL PANEL VAN
WIFES SUZUKI ALTO

[ideola]

Too many variables for me to feel comfortable throwing out a half-assed assertion. However, for another customer here in the US, I am helping him source all of the components for a high compression (10.3:1) NA motor with big valve head (44mm intake / 37mm exhaust). This involved custom Diamond pistons with the stock compression height, but with a shallower dish (for higher CR), and larger valve reliefs to accommodate the big valve head. When we discussed the design of the pistons with Diamond, they advised that the only safe thing to do was to increase the depth of the valve pockets by .020" to account for the shallower dish. Also bear in mind, a higher lift cam can have an impact here, as well as the thinner gasket. Many many variables. You also have to watch out for valve float at high RPM, so stiffer springs are a good idea. The only way to be for certain is to assemble the long block and measure the clearance with modeling clay.

Here are some shots of this customer's pistons:








The careful observer will note that the custom piston sits higher in the bore (look at the first pic that shows the relative height based on wrist pin position), and that the dish is MUCH shallower, while the valve reliefs are MUCH larger in diameter and deeper. This design will provide better quench than the OEM setup, which is a good thing for higher compression setups. Also note how much smaller the skirt area is, and how much less material is on the custom slug, resulting in significant weight reduction.

[!tom]

Sweet!

What's the budget required for this setup?
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78 924 NA
5-lug