Porsche Enthusiasts Club Forum

My head is already mated to the block, however I have asked for an iphone pic of the head chamber that was taken prior to rebuild, I'll post when I get it. This shows all the redundant metal around the valves and spark plug removed leaving a small quench 'land' to match the flat area of the Omega pistons. They removed enough material to enable a 8.3:1 CR, which I said was the north end of measure if closer to 8.0:1 was not achievable. To achieve 8.0 they would have needed to remove material off the top of the piston which would raise the distance of the piston to head and reduce the quench, which was not ideal. They also didn't want to machine into the piston dish and mess up the squish within the bore (Squish/quench - I think I got that around the right way!!) I forgot to ask the quench distance they achieved. Try to remember tomorrow.

Dropped off a few parts today, and I'll need to drop off a few more tomorrow. Hopefully I'll have a finished engine back in my hands by the weekend.

Scribed line around the head chamber shows the piston bore

Looks as though they've taken more out than what I have, Steve.
The quench land on the piston measures ~12mm. I hope they haven't removed too much of the quench pads on your head but it's difficult to tell from the photo.

3cc is possible without going into the quench pads, maybe 4cc max.

Thanks for your help Jon with quench size, very much appreciated.
3cc out of the head chambers and 0.046" gasket means 8.7:1 CR.

From what you've said before Jon and very old posts on .org I believe the 931 head gasket used to be thinner until it was rationalised and now the compressed thickness is over 0.060".
Going by this, I don't think a lot of people realise they need to leave the pistons proud of the deck to maintain quench.

You would not believe the extents we had to go to on the 3.2 944 turbo engine development with gasket thicknesses.

On that engine we used bronze rings to seal from the bore to the head, and used a laser cut standard head gasket with the bore sections removed to seal the outside of the waterjacket and oil passageways between the head and the block, this meant we had a block to head with absolutely no compliance, and needed to lap the sealing rings to very precise tolerances and use shims under the rings to ensure the gasket material crushed by the correct amount when the head was torqued down, this also had implications of the distance from the head to the piston and squish/quench, so before pistons were ordered we needed to make sure every measurement was precise and do a lot of experimentation and research into the ideal crush thickness of the outer head gasket so we could come up with measurements for everything else, including the sealing rings and shims.

Not a lot of people ever realised just how much work went into the 3.2 engines, Mark Otway and I spent many hours running through the design process for the engine before any metal was actually ever machined, then during the build process Simon and I spent many hours discussing and double checking every tolerance during the dummy and final builds.

You have to go through much of the same with any engine build, and the details are the most important part. You would not even believe how many brand new Porsche engine components are not within their own specified wear tolerances before they have even been used.. You have to measure everything, assume nothing, and that is where the difference between a high performance and mediocre performance Porsche engine, and an engine which will last 200,000 miles and one which will burn oil for most of its short life.

Well done for caring about the details.

I think you are right Wes with regards to the quench pads. There is more material gone than I'd like to have seen but they did point out there is material still there. Is it enough? Well the engine is now on a route of no return. It is going to be completed and put back in the car as is. I still have the original matching number block and head which can be rebuilt once the pain of this build has passed. I have learned a lot and to be honest if/when I start again I will go a different route with the next.

The area near the outside extremes of the bore is the most important, and it does look as though they've left some there. To be honest it's difficult to retain all the area AND lower the CR significantly without touching the pistons and/or doing much more work to the head.

Ideally, you'd want a custom CNC machined piston crown and head chamber that virtually mirror each other but I would imagine you're looking at £'s for that.
Omega do the 3D billet pistons at ~£185+VAT each but they'd probably want a minimum order again.

The Omega pistons are one of the things I'm most happy with to be honest out of all the parts I've sourced, and I think it's worth going that touch higher on the CR to use them over the other alternatives I've seen.

Up to 1 bar I can't see you having detonation problems with the 951 intercooler, but I could be wrong. There are ways to get round the problem if it arises.

I wouldn't worry about your build. IMO you've made a lot of wise decisions. I have also learned a lot over the last 18 months.
I'm still likely to change direction with this build with certain details so watch this space

I sense a 300+bhp itch that needs scratching

Good news is my engine is done.

Bad news is that the Conti cam belt and tensioner pulley is at the limit of its adjustment. Described as OK but not ideal tension on a engine which if it skips a tooth or two could be very detrimental.

The Piper vernier pully is exactly the same as stock teeth/circumference, as is all the others. Only dimension change is a skim of the head and block but both are still well in tolerance. However enough for the builder not to be 100% happy.

Ideal option is to find a cam belt with one less tooth than the stock 121 teeth - cost circa £20. Alternative is to turn a new alloy tensioner pully with better bearings which will delay the build by another week and release me of another £120. Only another £120 on a long list of 'only' another extra!!

Any thoughts or suggestions.

Hmmm that's a bit of a bummer.

I would go for the new tensioner with better bearings - it's not worth gambling with the time and money that's gone into it.

Also, I would swap the Contitech belt for a Gates. I've heard more than one person slag the Contitech belts off and some have even snapped on a 931 engine (luckily flat-tops and non-interference in custom build so no damage).

I finished replumbing the vacuum pipes and fitted the badges properly. Seriously considering moving the WUR to the big space in the middle. I have a cut up intake manifold spare which I could cut the WUR mounting off and weld in to that space. Will make adjusting the control pressures on the WUR a doodle for Jon once its modified to be adjustable.

Jon, can you modify the WUR to be adjustable when the car is with you for tuning or do I need to get it done prior? I have an adjustable bolt already to fit.

Probably best to modify it prior to fitting it and yes a fine idea to move it to under where the throttle body would usually be.

We usually modify them as dual adjustable for cold and warm pressures.

For years we have also been able to offer a basic electronic fuel pressure modifier for the 924T, CGT, 930T and 3.3 964 turbo which increases fuel pressure by a global percentage based on the position of a knob on the dashboard whenever the boost is above 0.8 Bar.

However I have been working on a 3D mapable electronic fuel pressure modifier for the same models, which can alter fuel pressure based on RPM and boost/vacuum via a small display and control buttons (so no need for a laptop) which may be handy. Doubt we will ever sell many, as it is a tiny market these days, but it is something I want for my own cars. If your interested I will do some more work on it over the spring and early summer and you can try it out.

Right I have sent the manifold back into the workshop to have the WUR remounted to the middle of the intake runners and the fuel pipes adjusted for this.

Jon, ideally I'd like the WUR made adjustable prior to refitting, however this would mean resetting all the control pressures as soon as we start a fresh engine which I guess is not ideal. Therefore its probably best to get the car run in first, then bring the car down to you for some time to get the WUR modified and all the other tuning work done at the same time. At least it will be in its new position and be very easy to remove and reinstall How long would you need it for?

Finally! I have a fully finished engine. The final spec includes

Omega Forged 87mm pistons
ARP conrod bolts and nuts
Lightened flywheel circa 30%
Fully balanced
ARP head studs
Stock head gasket
Enlarged head chambers and now running 8.2:1 CR
Enlarged/ported/smoothed intakes to head
New guides, seals, relapped valves
Piper Cam & vernier pulley and all timed up
Custom copper washers to ensure the spark plugs are not protruding into the head chamber to avoid hot spots/detonation
Break in oil for initial running in on the rollers for its first 2-3 hours. Then switch to its regular engine oil diet

And all for the price of a concours 924 turbo!! This doesn't include the cost of turbo rebuild, Zircotec exhaust coatings, engine bay repaint and other refurbishment work at the front end.

Had a long discussion regarding squish and quench of which I couldn't regurgitate the theory here. However I am satisfied that the material lost within the head chamber has been done with care and attention and shouldn't cause any issues. This is bourn by the fact that they even considered the copper washers needed to pull the spark plugs out of the chamber to avoid hot spots and therefore detonation.

It'll get fitted next week and I plan to be driving within 2 weeks barring any further delays of issues. Big annoyance is I can't get started on it until Sunday.

Crappy phone pic. Not its usual orientation!!

Progress today

Good work steve, can't wait to see it in the flesh and hear the sweet sound of a well tuned engine

cant wait to see this beast up and running!

btw if anyone is interested i may have a complete 924 turbo engine available soon, as im thinking of fitting a 944t engine into my 924t race car.

cheers
nick