924Board.org

[Martijnus]

Hi, I'm kinda new here... looked around sometimes.
I own a Porsche 924, could be a weissach 1981, has the interior but I can't find the proper numbers to confirm it.
Doesn't matter by the way..

I'm converting the kjet to MS-I v3, in combination with individual runners and GSX1000 k2 throttlebodies (and injectors).
Because of this I am planning to use my spare cylinder head which I will modify for the larger-bore runners (42mm dia)
Hope I will not f*ck up my coolant channels when I modify...

Anyway, I want some more power out of the engine, but it's for regular use, so high end horsepower is not important. I would like to keep (or better, improve) the current torque.
I've raised the compression ratio on my XR600 bike from 9:1 to 11:1 and it got a much better mile per gallon ratio. I'm hoping to get the same result on my porsche, not extreme compression ratios but a bit higher.
In combination with the new injection, intake and (eventually) exhaust sytem it will run cheaper I hope (but wait... the conversion costs a lot... I know, but it's a psychological thing for me )

I'm planning to use a 931 head gasket on my NA block, because it is thinner and have a slight bit machined off my head.
The latter is the problem, how much can be machined before my valves will hit the pistons. Has anyone done the same thing?

So, how much thinner is a 931 head gasket compared to my 924NA gasket?
and how much can be taken off the head in combination with the thinner gasket?

Thanks for the info!
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[endwrench]

Machining the head will not get you much of a compression change. It has no volume. You will need to change pistons or machine the block.

Email me and I will send you some pics of a sectioned head like the one in my avatar so you can see the coolant passages.

Todd
_________________
'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

[Martijnus]

[Martijnus]

and suddenly, it came to me.
Indeed, it has NO effect on the compression ratio if the head is machined.
the only gain is that the "holes" in which the valves are seated are a bit less of volume, which means i've got 0,01 cc gain...

indeed, the block itself has to be machined, how stupid of me that I didn't see it.
Thanks for opening my eyes.

Slimmer head gasket WILL increase the compression, for sure.
a quick calculation shows that:
the "volume" of the gasket per cylinder is:
43,25 x 43,25 x pi x 2 (assuming the gasket is 2mm thick) = 11,75cc.

if the 931 gasket is half the thickness the volume will be half; 5,87 cc.

i've calculated that the theoretical volume of the combustion chamber is 59,76 cc, which will be 60 if the valve "holes" are taken into the picture.
let's say it's 60 cc.
This is including the gasket.

with the 1mm thick gasket the volume will be: 60 - 5,87 = 54,13cc

The compressionratio is normally 9,3:1.
Now it is: (displacement + volume of combustionchamber)/(volume of comb. chamber)= compressionratio.
=
496+54,13 devided by 54.13 = 10,16.

el gaino mucho in my eyes!

This is in optimal condition, with OEM gasket of 2mm (i'll have to measure that) and 931 gasket of 1mm (anyone idea how thick these are?)

the piston has (how do you call those?) savings? for the valves, which influence the calculations, but i think 10:1 is not so wrong...

worth the effort...I do not want to go much higher, because of the side effects, hotter oil, more load for the conrods, starter motor has to accept the new compression and so on...

anyone any comment about my calculations? like to hear it
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Martijnus]

I've decided to only replace the gasket by a thinner one. I could use the original one and machine the cylinderblock, but I think a thinner gasket has less chance to rip to shreds (Am I right?)
I think this is the reason the 931 has a thinner gasket.
over [url=http://www.924.org/techsection/1engine.htm#2 - engine mods]HERE[/url] I found out that a thinner gasket is indeed often used to raise the compression.
10:1 is enough I think.

Only risk is the valves to hit the piston. Only thing I can do is get rid of the valve clearance to simulate a warm engine and measure or try out if the valves hit the piston. This is ofcourse no guarantee but it's hard to test such things.

The valve timing will also change, not much, but it will change. It will retard a slight bit (because of the shorter distance between crankshaft and camshaft).
Depending on the diameter of the belt pulleys the timing will retard **degrees. Can try to make the timing adjustable, but I haven't looked how the whole system works exactly and if this is possible.

What does everyone think of my plan to only use a 931 gasket?
I think it will improve my mpg and performance a little bit.

I've read about the 931 head which has a better flow... would love to have one of those but can't afford it
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[leadfoot]

Calculations are good, take the 931 gasket thickness with a grain of salt though, I bought a 931 head gasket kit and the thickness is the same as a 924N/A @ 2mm thickness, you would need to confirm this... or go for a copper/metal head gasket as these are generally 1mm thickness...
With that in mind you might have to make sure there are good sealing firerings and in conjuction with some head studs should seal adequately on a non boosted application alleviating the need for o ringing the block as is generally the case for copper gaskets. The triple layer steel ones from ACL might be different but you would need to research this further before commiting.
Think about larger valves whilst your at it, 2mm larger on the intake and exhaust is roughly what you can go with the stock bore.
Good luck with the project.
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Martijnus]

[endwrench]

Decking the block wont hurt anything. It will slightly retard the cam timing though. I decked mine .050" for a calculated full point of compression increase from 8.5:1 to 9.5:1. You should also have no problem with valves clashing with the pistons unless you are running some mammoth lift cam. I checked mine with clay and had gobs of clearance with a .435" lift cam. I didn't measure it at the time but I would not be afraid to put a .535" lift cam in. Plenty of room. I'll go check my mail now.

Todd
_________________
'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

[morghen]

he already has biger valves....he is from europe

good luck with the project...try to do some tests(speed,power) before and after you change from 9.3:1 to 10:1 lets see by how much this improves performance.
_________________
Supercharger and EFI kits
https://www.the924.com

[leadfoot]

Morgan it depends on the year, early heads had smaller valves, what I was suggesting though was an increase on the later head valve size...
Chrenan had his valve increased to 44mm intake and 37mm exhaust same as the gts heads.
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Martijnus]

about the bigger valves... i've got european pistons, with "cut-outs" (do these things have a name?) for the valves...
If I install bigger valves, won't they hit the piston?

I'm not installing bigger valves...but I went into the garage to clean up (needed a screwdriver...took me 2 hours) and saw my pistons... question rised
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Martijnus]

in the meantime, some pics of my brand new inlet.

homebuilt from stainless steel, 4 individual throttlebodies (gsxr1000 k2) with gsx injectors will be fitted, got them already.

only thing I didn't do myself is the welding... though I'm proud because I've built it with my prehistoric tools and skills.






_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Lizard]

looks good, lets hope that the bolts and gasket can take out all the warping
_________________
3 928s,

[Martijnus]

think so, it's flexible enough... as long as it isn't warped around the runners it will be fine..

and it isn't warped around the runners

original idea was to make shorter runners, so that the throttlebodies were as close as possible to the cyl.head... but the thermostat gave problems with the tps... few cm's more on the runners won't matter...

wonder how it will look with 4 seperate K&N filters
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Chrenan]