Porsche Enthusiasts Club Forum

Looks like my intake manifold is progressing. We've used two manifolds to create one modified plenum, cut open and then rewelded to be tapered with the S2 throttle body placed at the front of the plenum. The old TB centre mounting has been shaved and filled. We will also use the S2 boost tube cut down and will be a straight run between the IC outlet and the TB. No pics yet as I'll wait until is finished. It may go against some of the thought process above but my guy has some experience of 80's F1 turbo engines, understands runner lengths and plenum volumes. It won't be as optimised as you're readings Wes, but I am sure it will work OK. Will certainly look very OEM I hope as well as give a far more direct intake than the original.

There was a re manufactured GTS intake manifold on ebay.de earlier this year, I believe made by Mittlemotor in Germany. Was on a buy it now at circa £1100. I was very tempted but really couldn't justify it in one hand, but knew it would make a great addition to my build.

Jon, whats your views on replacement of the con rod bolts in a bottom end rebuild. They are NLA from Porsche, any other suppliers? I was let down with a group buy on 924board for some ARP rod bolts. They are a listed part but only on special order. Best price at 10 sets plus circa £150.

I would go for it with the ARP fasteners, just remember to relieve the con-rod and tighten them to stretch, not torque.

Some progress in the engine bay







Fuel injector pipes modified
Fuel metering unit turned 180 degs and relocated on new cross member which will also carry coolant from 944 expansion tank to rad. Air outlet from metering unit has been cut and turned 180 degs to enter the turbo intake
Turbo to IC pipe adapted to reuse original mounting bracket
Two intake manifolds have given their lives into one modified tapered unit, throttle body moved 90 degs, modified IC to TB pipe to be finished it will carry either the cast S2 '924 turbo' badge as is or we will graft in the recess to place in the S1 badge.
Alternator mounting has been modified to give clearance to intake pipework
Low profile rad fan installed

Lots more to do ........

Hi Jon, just for your inquisitive mind, here are the printouts from the dyno runs on my failed engine

3 runs in the S2 prior to the engine being pulled. With FMIC and DITC it gave 193, 195 and 197bhp over the runs. I just have the 193bhp print out.


3 runs in the S1. Without an IC and S1 ignition. It gave 165,167 and 168bhp.


Only the block and head were swapped. Both cars used there own fuel and ignition systems. My S1 car has a fully refurbed/renewed fuel and ignition systems within the last few years.

Any thoughts?

So the second one is without FMIC?

Interesting comparison, shame it is not just a "with FMIC" and "without FMIC" as that would have been really interesting data to see.

The dyno graphs are a little bit "cocked" unfortunately, I would have given the opperator a hard time for using different scales for horsepower and torque, which makes the graph harder to compare the relationship between the two.

I quick check I always do when looking over a graph is to make sure that HP and Torque cross at 5252rpm, as a quick checksum to make sure the graph is scaled properly, otherwise the analysis of the curves need to be mentally transposed or a spreadsheet knocked up to visually analyse the graphs.

Would also be nice to have intake pressure and air fuel ratio overlaid onto the graph, as this makes for interesting analysis.

I would have expected the S1 turbo, with FMIC to make more power than the S2 turbo with FMIC. Shame the S1 graph is without FMIC.

The good news is, if the graph of the S2 turbo with FMIC is right, at only 0.6 bar of boost, 1.2 bar of boost with the right ignition timing and fueling, and if the turbo is matched correctly, should make a very fast car.

Interesting that at only 0.6 bar of boost, its not making much short of what I would expect to see from a 944 turbo when scaling for the engine capacity is taken into account, it is actually doing very well. Remembering of course this is running on a very basic fuel and ignition system compared with the 944 motronic and a much lighter car.

Not posted for a while so here goes.

It seems a few of us are now looking at pistons options. I now need a set as all my top rings are broken.

So what would be the optimum piston design for one of these engines?
The 931 has a pretty uncommon combustion chamber shape. Probably a compromise between low compression and quench.

I think most will want to leave the head chamber untouched so the piston has to be designed around that.

The S1 design seems to go for quench, low compression and pushing towards the plugs. I've not measured the crown thickness at the bottom of the dish.

So starting from scratch what could be done to improve the pistons, or would it be best to just make a copy with better thicker rings in forged 2618 or 4032?

Back in the day the 924 turbo was breaking some ground, not many cars had been turbo'ed up until that point, apart from 911 turbos and Saab turbo's, so piston design was not state of the art.

But in the early days a few turbo cars had similar pistons, its more of a hemi piston, which theoretically is good, especially for a 8 valve engine where you are looking for "swirl" during combustion, rather than "tumble" that you look for with a 16 valve engine.

With rings it is all down to personal preference and how it will be used, I tend to use a nickel ceramic bore coating even on cast iron blocks, as the bore finish is more durable than even iron and ring break-in is very fast, while ring wear throughout the lifetime of the engine is also great. But with this bore type you need to use a completely different ring material.

I usually stick with the ring recommendation of the piston manufacturer, as there are issues with the ring land design, thermal properties combined with the bore material and expecty duty which need to be taken into account.

Even better, I tend to get the piston company to supply the rings, so then no one can pass the buck when it comes to who's fault and engine failure is. I even get the piston manufacturer to decide on clearances between the piston and bore, as well as the ring gap.. no excuses then.

The piston crown is not an area on the 924 turbo which I think needs a re-design, but I do incorporate piston skirt alterations and various piston upgrades when I specify a 924 (or 944 turbo) piston, as some things in the world of piston design have moved on since the late 70's

Hi Jon, please check your emails re JE piston order

Hi Steve,

Sorry been flat on my back for the last couple of weeks and away from my office and email

Going to be away for a bit longer, but hopefully my office computer and therefore email and notes will be coming to my home tomorrow and I can read your mail and get back to you.

Sorry again

Thanks Jon.

The current engine has now been pulled and engine bay is stripped ready to head off to the paint shop and get a re paint along with some remedial work on the body.

A spare stripped block is ready and waiting in the engine shop for these pistons.

The modified intake is mostly complete and will get its final fettle once the re built engine is back in place.

A hole that needs filling with a rebuilt engine

Jon, Wes

Any thoughts on the fitting the ARP con rod bolts as per Dan's ARP group buy? http://www.924board.org/viewtopic.php?p=338681#338681 Appears to be some key fitting/stretch guidance missing. You guys seem to know the fine engineering arts!!

I am sure my engine builder has a view but I want to ensure the bottom end is bullet proof and no weak links!!

Woo.. Interesting thread, especially Ideola getting hot under the collar and complaining about being asked for an important installation question about a product he sold!!

Looks like he has provided the answer as being .0092-.0096 as the stretch figure.

Quite typical of ARP being a pain to get a technical detail though, a problem with having several people employed to answer questions and sell products with a specific task of keeping dumb questions from taking up the technical engineers time, which is understandable.

Ideola though should not be in the business of selling parts if he does not want to support them. What a willy

I think Jon already mentioned somewhere that you should relieve the con rod bolt holes if needed (chamfer holes) to make sure the bolts sit flush.

The other thing to mention is that when you change con rod bolts you should really check the con rod and big end cap for roundness. If it's moved out of round then they need honing.
I would actually do a before and after check ie. assemble each con rod and cap with the old bolts torqued to factory specs then check roundness and record for each, then do the same again with the new ARP bolts tightened to the required stretch.
It's common to find that they move at least a couple of tenths of a thou when changing bolts.