924Board.org

[simsport]

Hi
For me the key issue is resstance to Det. The turbo pistons have a squish band, indeed the JE variety I have (8.5:1)come with a much more pronounced squish which , I believe, promotes better combustion and reduces the risk of Det. The JE pistons are also lighter and the std rods can be lightened and balanced.

The std pistons will suffer from Det damage (to be fair all will) if the engine runs too lean, overboosts or, and I think this kills many road cars, the cooling to the 4th cylinder is not good enough. I have seen a number of blown turbo engines with the 4th piston damaged alone or more than the others.

When you boost any engine you will add heat and the trick is getting that out. A decent intercooler is a must for higher (above 6 or 7 psi) boost levels and if you can improve water flow and heat transfer via a decent alloy rad thats alll the better.

Cheers
Simon
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Blown is always best!

[Carrera RSR]

Yep, it's cylinder 4 which seems to have failed on mine recently. Despite compression results of 145 to 155 on cylinder 1 to 3, cylinder 4 is now showing 120. Haven't had a chance to pull it apart to find out what when wrong yet. In meantime I'm having a spare engine rebuilt, will need to look at the cooling to avoid future issues on 4.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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[MikeJinCO]

I might be a little out of school here, but in my past life I dealt with antifreeze in commercial applications. Many of them both decrease the heat transfer rate and increase the viscosity which will reduce the coolant flow rate. Obviously a large affect on cooling the motor. I would investigate the properties of your heat transfer fluid and also look into the use of some of the diesel engine coolant additives used in HD diesel applications. Some say it prevents formation of air bubbles on the cylinder walls etc (this might be what water wetter does) which for diesels prevents cyl wall erosion but also would decrease the cooling.

This is one of those areas where a lot of material improvements have been made since these cars were designed.
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Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[WEASEL149]

Cylinder 4 is definitely a problem.

I remember someone saying it's a good idea to run spark plugs that are one or two ranges colder in cylinders 3 and 4. This would help with detonation.

The standard coolant crossover pipe is crap and was the cause of my first head gasket failure years ago.
I have since replaced it with a larger bore copper version with the heater circuit routed differently so it doesn't affect flow. Can't remember whether the pipe was 28 or 32mm.
It's basically the same thing Bass GT did but retaining the heater as my car is purely for the road.

I've never seen anyone dry-deck the head/block on one of these engines.
I've seen it done to turbo-charged classic mini engines. It would probably improve the flow through the head and allow reliable use of a copper head-gasket, as the gasket would only be sealing combustion pressure.
On top of that you would get the heat-conducting ability of the copper gasket taking heat away.
Throw in some O-rings or similar and the headgasket blowing would be a thing of the past, although your tune would have to be spot on to prevent any piston damage. Dry-decking is probably overkill and too much hassle for a roadcar but may be worth it on a track car.

The other thing I will say is don't be tempted to run a weak coolant mixture as corrosion inhibition is extremely important. Apparently, 16 thou of corrosion can reduce heat transfer by up to 40%.
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1979 UK 932

[Lizard]

Weasel,

While the corrosion numbers may be correct, its doesnt help the fact that coolant doesnt cool as well as water. On a car like Simon's I would suspect that he is running just water wetter and water.

Always run the coldest plug possible that doesnt foul.

The problem may be that the coolant is too hot at the rear cylinder. But if that is the case, pull the WP and remove the impeller and change to an electric pump that can move the water faster. As well, block the thermostat recirc and ensure that your average temp doesnt get too high.
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3 928s,

[WEASEL149]

[morghen]

i'd be interested to clear the subject of the crossover pipe...come up witha bolt on solution and post it on the "how to" as improvement for the 924 turbo.

Do we know if just installing a bigger diameter pipe do the trick for a fast road car?
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Supercharger and EFI kits
https://www.the924.com

[WEASEL149]

[morghen]

[WEASEL149]

[john h]

My heater valve failed many years ago and wouldn't stop the heater warming up the interior even when shut.

Most of the year we don't need a heater in NZ so I just bypased the heater lines altogether, I've never had a problem as far as I know of No 4 running hot and the car used to be thrashed merciless on the track for many years after that mod and I have yet to blow a head gasket.
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Remember a Porsche is not just for Christmas,

if you take it to pieces slowly it can provide anguish all year long!

[Lizard]

Weasel,

The electric waterpumps are now great for road use.

If you wanted to have the most detonation resistance you could even reverse flow. Pump it in the head, and out the block (cooling head first). Then the water wouldn't be as hot when it hit cylinder number 4. In fact #4 would run cooler.......
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3 928s,

[Carrera RSR]

Reading with interest with a rebuilt engine going in my car soon.

My heater valve was replaced a couple of years ago but still never closed all hot water from passing through the heater matrix. Found I sweated my nuts off during Summer sprints/trackdays with suit and helmet on! Only need heat on very few occaisions so will look at all the options

larger exit pipe and replumb heater
electiric pump option (more insight into reverse flow needed, always a reaction to any actions!!)
blank off the heater altogether, wear a sweater on colder days!!

With the heater/blower being rather weedy anyway it never really helps with demisting/warming the interior anyway.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252

[WEASEL149]

[WEASEL149]