Porsche Enthusiasts Club Forum

That's good to know. I probably worry needlessly about the coolant system. The rad I've bought should be way over the top to be honest - it certainly has the thermal mass you were talking about with the 951 intercooler so should even be decent when stood in traffic.

I'm hoping to build my own cold air intake but haven't got round to that part yet. Hopefully, colder air to turbo will equal colder air out compounded by the more efficient compressor, then further reduced by the 951 intercooler.
I'd like to get the engine running on sequential injection but wasted spark to start with then maybe a bit later upgrade to coil on plug or coil near plug.
I agree totally with what you said about ignition being very important and also gaining power and reducing temps through a good tune and supporting mods. My initial target was to increase power to 230hp at 0.7bar - whether this is possible remains to be seen.

It's interesting reading what you said about 295hp 944 running the same boost as 420hp one. It just goes to show what careful and efficient tuning can achieve. I would eventually like to surpass Carrera GTS Clubsport power level at the same boost. This should be possible with the advances in technology.

I will leave the head alone after your advice. It's actually a bit of a relief and one less thing to worry about - probably do more harm than good anyway.

I have an idea on how I will approach the new inlet manifold and intake pipework. One thing I have learned is that big isn't always better with the pipework - within reason at least. The intake runners will be tapered velocity stacks and the plenum will end up larger than standard. I don't think I'll be going for a tapered plenum though. I'd rather the air enter the plenum centrally to increase the chances of equal distribution.

Regarding the exhaust, I think I'll tackle this again further down the line. I would like to move to a full 3" system eventually. Is it true that the 2.5" system is good for up to 300hp?

The other thing I forgot to mention was moving to a larger oil cooler...

As of Friday I was adamant I was going to order the Mahle pistons. Having read more here over the weekend I decided not to phone the engine builder yesterday or today as planned. Jon, you mentioned you get frustrated when customers don't work to your plans or suggestions. I believe this is happening to my engine guy. He just wants to build an engine he's happy with, get paid, everyone's happy. But this JE piston discussion is still niggling with me.

How, who, where do I need to go to at least get a quote on some JE pistons to my requirements? Once I know the alternatives fully I can put this in front of my guy once and for all.

It's probably my fault Steve. Maybe me trying to dig up info has confused matters for which I apologise.
I can understand your builder wanting to go with original OEM pistons as they're what Porsche fitted but you know which way they went with the GT, GTS and GTR - maybe this is worth pointing out to your builder.
You don't plan on keeping the engine to a standard 0.7bar OEM spec, so why go with the OEM pistons?
After seeing the old photos of Joakim's engine with the JE pistons I would personally be happy with using them - he confirmed to me his car has seen 1.9bar so the only issue as usual is the tune.
I will always state my honest opinion, regardless of who disagrees and I stand by choosing forged pistons over cast if I personally had the choice.

Wes, I feel you and John are right regarding JE pistons, which is why I want to know where and how much. I will then go back to my engine guy tomorrow and speak with him in person. If he's not happy with my choice then I'll take my business elsewhere. The difficulty I have is I have already undertaken some level of work with him (a full strip and clean of block and head and some machining) and paid for this plus a reasonable cash deposit for purchasing more components. My fear is moving my business elsewhere will forfeit some/all of my cash. I've haemorrhaged far too much cash in the last six months and I'm trying to minimise any more!

ebsracing.com sell JE pistons but you'll have to contact them for 931 as they only list for 911.

It may be worth trying to source some OEM forged Kolbenschmidt pistons. EBS used to sell them but they're probably difficult to find now. This could be a way of appeasing your builder though with them being OEM.

I can understand how your builder feels. I feel a bit bad because i was chucking other options at you and confusing the issue.
I know what you mean as well with the money situation as i have also spent too much on my build.
Hopefully you can stay with your current builder and come to a suitable conclusion with pistons.

Forged pistons it is. Now I need to track some down ........... Jon, any advice on supply, can you source some for me?

Not a problem.

What compression ratio and bore size are you going for?

JE have done the 924 Turbo pistons for me before to any size and compression ratio.

JE also can provide the piston rings and wrist pins, so the other question is, do you have ring material preferences and are you sticking with standard conrods?

So we have

Series 1 - OEM Mahle cast piston S1 specific = 7.5:1 / 0.7 bar / larger volume turbo K26 2470R 6.10 = 170bhp
Series 2 - OEM Mahle cast piston S2 specific = 8.5:1 / 0.65 bar / smaller volume turbo K26 2664G 4.10 / DITC = 177bhp
Carrera GT - OEM Kolbenshmidt forged CGT/GTS/GTS Clubsport = 8:1 / 0.75, 1.0 ,1.1 bar respectively / K26-2660 GA 6.10 / DITC/ Top mount IC = 210/240/270bhp

My car - JE Pistons forged = 8.0 - 8.5:1 tbc / 1.0 bar / S1+951 250 hybrid turbo / no DITC but S1 fuel & ignition mods/ 951 FMIC = target 230/240bhp

Based on what you know of my cars intentions and current/impending modifications I am guessing somewhere between 8 and 8.5:1. But your experience and knowledge would again be very welcome. Would you go up from OEM 86.5 to 87mm or all the way to 88mm block limit? Stock rods although I'll look at lightening and shot peening. Total Seal or similar rings. I have hopefully secured a US group buy for some ARP conrod bolts which are NLA from Porsche and should be replaced when rebuilding.

I did call you earlier John and message/number left

I will call you tomorrow morning Steve? If thats a bad time, PM me back with when is good for you.

8:1 compression ratio would be my choice for a fast road and track car.

Seeing as at the moment the pistons would be a bigger investment than the current value of a core block, I would probably recommend going a couple of sizes oversize, even if just to make sure that in the future if you need a rebuild, you will certainly be able to get a block and therefore re-use the pistons you are about to invest in.

Usually I would take the opposite approach and advise you just go to the next oversize available to you within standard limits and treat the pistons as something to replace when you go oversize on the next rebuild (which hopefully might not be for years, but you never know).

This advice is also biassed because I know you have more than one block laying around, just cover them in a smearing of grease and wrap them in oily rags for a rainy day.

Alternative to JE are these Diamond pistons pictured next to the stock cast Mahle. Diamonds can be had for £600 including coatings, plus shipping & UK duty (or free if stuffed down your pants en route back from US!!)







Mated to Crower rods, 50% weight saving comes at a cost of £720 + shipping/duty



Just thought some would like to see some internal porn shots!!

More on an interesting 924 turbo engine build + car re build clicky!

Going to have a look at that link, looks interesting.

Never used those pistons before, will do some digging, always nervous to take the plunge in using new pistons (which is why I can partly understand your builder being iffy about JE Pistons), but I might actually see if I can get hold of a set of these.

Maybe if they do 944 Turbo sizes I will buy a set and use them in one of my fleet in an otherwise known quantity tuned car, that way if they fail I can do some forensics to see what they are like.

The thing is, if I try them and they fail, who is to say in the same situation that JE items would have survived?!?

Off to read that thread.. I could be some time!

Now that was an interesting read.

Such a shame he didnt modify the front panel to receive a 944 turbo intercooler before it went off for paintwork!

Even if he didnt want it now, at least the option would have been open for him.

He has an interesting concept though. The whole "What if" thing, a similar idea of what I am thinking of, along the lines of "What if Porsche continued the 924 turbo, CGT and CGTS into 1985? What would it be like", which is what I have in my head as my 924 turbo project.

Not 100% sold on the piston choice yet... Will do some research.

I'm loving this thread. I understand very little but still enjoy it

The Diamond pistons and Crower rods definitely look the part.
Diamond have a good rep in the USA but I don't know anyone personally who's using a set.

Going back to the intercooler pipework and also making a custom inlet manifold...

I've done tons of reading on the net and books etc. From what I can gather, massive pipework running to the cooler and from it gains nothing, apart from maybe milliseconds of lag due to extra volume and pressure drop which is bad.

A few of the turbo books I have go into gas velocity calculations. Going above Mach 0.4 gas velocity increases drag and therefore you get flow losses. My idea was to keep pipework smaller, increasing gas velocity and reducing intake volume but not exceeding Mach 0.4.
Going by this I'd say 2.25" (57.15mm) tubing from turbo to intercooler would be sufficient then gradually increase in size after the cooler to my throttle size of 65mm. So intercooler pipework wouldn't be exceeding 2.56".

The next thing to decide when building the intake is the plenum size/shape and where/how the air enters it.

1) Single plenum tapered to go narrower towards the back with the air entering from the front of the car. This is the most favoured route by most DIY'ers and tuners as it's not too difficult and if executed correctly, should give even air distribution.

2) Dual plenum. Same as first but there is a tapered plenum fed from the front which in turn feeds a primary plenum chamber through a diffusor slot that generally runs the full length of the chamber and exceeds the size of the throttle plate in area. Harder to make for the DIY'er but is proven to work well.

3) Single plenum with the pipework entering on the passenger side centrally/symetrically. This is a simpler route and makes it easier to get even air distribution across the intake runners.
I've seen this on Steve Bassington's monster 931 race car and that made 393hp at it's last known RR session.
I'd like to have a go at this but would ideally like a generous curve on the pipe before it enters the plenum.
The brake servo and refill does get in the way with this route.

I'm undecided on plenum volume and shape. I've read that between 0.8 and 1.5 times engine size tends to work well for 2 litre turbos but this gives between 1.6 and 3 litres. Using something like 4" or 5" pipe for the chamber would mean it would end up around the 3 litre size depending on the length of the finished plenum.

Regarding plenum shape I could always shape it so it tapers down to the throttle.

With the runners I wanted to use full radius off-the-shelf velocity stacks to make fabrication easier/simpler. They're not easy to find in useable/correct sizes. Mine taper down to the inlet port - whether this will be of any benefit I don't know.
I know longer runners benefit low speed running and off-boost torque but space limits what you can do length-wise.

Hopefully, losing some volume and getting better airflow through my intercooler will improve my set-up over the last one. Only problem is, I'm making that many changes, I won't know what is down to the intake system and what isn't

Does anyone have any thoughts/advice on a custom intake manifold and intercooler pipework? Jon?

Intake and intercooler plumbing is a science on its own.

Every aspect of it has its own calculations, so behaviour can be predicted very well, but also changes in design can impact many aspects, including fuel atomisations, air distribution, intake turbulance, intake velocity, intake temperature as well as how much power the car will make and at what engine speed and load.

It needs a lot of thought, and really needs to be matched to every other component used as well as with the desires of the car owner/driver.

There are some rules of thumb worth remembering.

Keep every change in angle to less that 12 degrees in any length of a pipe equal to the pipes width, In other words, ideally make sure that a 2.5 inch pipe does not change direction by more than 12 degrees in a 2.5 inch length. Also make sure that any changes in pipe diameter do not change in size at a rate of more than 12 degrees. This relates to a pipe and not a bellmouth, which behave differently.

Testing of flow normally shows that if you break the above rule, the airflow will try to seperate from the pipe wall, causing the airflow to not only see a different size pipe (smaller) but also to cause vortexing of the air where it meets the pipe where the rule is broken.

This rule tends to relate to a pipe with a dull matt unpolished finish, a polished internal pipe will seperate at a lower change of direction.

Really your intake manifold needs to be modeled, flow tested (including velocity and flow examination using smoke) and should evolve with the help of the flowbench, but thats probably not going to be possible with a home brew version unfortunately, much else is going to be a shot in the dark and you might be lucky, or you might not.

Use of something like the ricardo wave engine modeling software also helps, but even I can't afford that and have to rely on favours from people in factory motorsport to help out with that, but even then a clear plastic intake will need to be tweaked on a flowbench to get perfect.

Just to put it into perspective, I have been working on an intake design for 944 turbo's for about 5 years now, its way better than the competition, but its evolution is not finished so it can be fabricated as a final design and put onto a car.

Pulse length tuning is also important you just have to make your calculations based on the turbo being the "ambient air source", get this right and you can add power wherever you want in the rev/load range as well as giving your turbo an efficiency spike in that zone.

Lastly, keep the big picture in mind. If you want a 400 bhp car and if you design everything in your package around that figure, and the translation of that figure into other units, then its not amazing when you actually get 400 bhp. This relates to everything in the package, from pistons, con rods, intercoolers, ducting, pipework, turbo, injectors, fuel pump, ignition system.. everything.. To be honest, set your target and apply science to the project, and I garauntee you will exceed your performance goals.

As an example, if we aim to make 400 bhp, we tend to actually get more, such as 420 bhp, not just this, but is is repeatable and reliable.

Everything within the engine and the support hardware is predictable.. Even quickly predicting how an increase of 2psi of boost will effect the intake temperatures without a massive calculation, just basic engineering and physics principles.

The diamond pistons indeed do look the job, I like to see the short skirt on them, but I always ask why the short skirt? A shot in the dark? Closer tollerances being used? Is it due to the longer stroke and therefore the difference rate of piston accelloration and rock relative to time at any set rpm? or was it some guy at Diamond saying "hey, that bore size and pin position is very similar to this honda piston we did so we can use most of the CNC program or solidworks model to mill it and we can use the honda forged blank" Sometimes its better to have a custom piston which is exactly as per your design, so when you have it in your hand, you know every aspect of it is because of a purpose, not just a manufacturers short cut which should work out...

Its a field full of claymores this tuning business )