Is there anyone in the UK that can shave pistons to lower cr
Discussion
The 965 engine is quite different to the 964 engine.It used the old tech injection CIS and the compression was way to low at 7.0.1 The 993 turbo has a ratio of 8.0.1.My car at the moment has a ratio of 11.3.1.I am planning of turboing it so I need to reduce the ratio to 8.0.1.I don't think bigger gaskets are going to work as the 964 is notorious for oil leaks from the bace gasket and head seal.I have sourced a set of pistons and cylinders with the later sealing ring.Adding space to either of these areas would be asking for trouble.Your opinons are very much appretiated.
in an ideal would 500hp would be nice.I am undecided on which intercooler I am going to use either air to air or water to air.Turbocraft in the USA told me 500hp is fairly achievable.As to which management system I will go with.I am open to suggestions? Just bolting a supercharger on a standard engine and running 0.6 bar give 400hp,so I think 500 should be obtainable
I thought about 993 turbo pistons but the have a different size gudgen pin.There are people in the USA tracking their 964 turbo converted cars,so shaving the pistons to reduce the CR is not really bodging.There is a minimum amount of metal required at the top of the piston,shaving it of to the desired cr will still leave way more than this.I have just found a post that says after shaving there is still .280"dome thickness.
If the piston dome is solid metal rather than having a corresponding shape forged into the underside of it then it can certainly be machined down as long as the crown thickness stays sufficiently large. It is however quite a time consuming business continually taking a bit off, checking the resultant chamber volume and repeating until you get down to the target CR. It's a very simple matter with flat top pistons of course where it can all be calculated in advance. I've done both types on many occasions on my lathe. You just have to know how to hold pistons properly which is not quite so simple as just clamping the skirt in the chuck! It usually needs a jig to be made first which locates the piston on the datum faces inside the bottom of the skirt where all the machining ops are referenced from in production.
Actually with complex piston crown shapes it's easier albeit slower to do the job on a milling machine rather than a lathe. The piston rests on a mandrel fitting inside the skirt onto the datum faces and the piston is clamped down using a bar through the gudgeon pin hole. The same setup as is used for machining valve cutouts into the crown. Once the desired CR target has been reached with the first piston the rest can all be flycut to the same height. Finally they will all need balancing back to the identical weight again as the density of the metal removed will never be quite the same for every piston. Usually forged pistons are pretty good in this respect though so the balance shouldn't alter too much.
With all the measuring and checking required it'll still be the best part of a day's work I would estimate.
Actually with complex piston crown shapes it's easier albeit slower to do the job on a milling machine rather than a lathe. The piston rests on a mandrel fitting inside the skirt onto the datum faces and the piston is clamped down using a bar through the gudgeon pin hole. The same setup as is used for machining valve cutouts into the crown. Once the desired CR target has been reached with the first piston the rest can all be flycut to the same height. Finally they will all need balancing back to the identical weight again as the density of the metal removed will never be quite the same for every piston. Usually forged pistons are pretty good in this respect though so the balance shouldn't alter too much.
With all the measuring and checking required it'll still be the best part of a day's work I would estimate.
Edited by Pumaracing on Wednesday 26th October 14:20
I had to take the dome off a set of pistons a while back. I had them wire eroded off. There is no requirement to "grip" the pistons, they were just set up on a vertical angle plate and held through the gudgeon pin. Wire erosion, doesn't exert any force so it is quite stress free as far as the metal is concerned.
One thing I also did that was useful was to leave 0.2mm more on the piston crown that I really wanted. This enabled me to flat off the excess on each crown to balance combustion chamber volumes.
The only problem is finding somewhere with a wire erroder. Might be worth trying tool makers, die makers.
Steve
One thing I also did that was useful was to leave 0.2mm more on the piston crown that I really wanted. This enabled me to flat off the excess on each crown to balance combustion chamber volumes.
The only problem is finding somewhere with a wire erroder. Might be worth trying tool makers, die makers.
Steve
I struggle to believe an SC will give you an extra 150bhp with just 0.6 psi! Maybe 50bhp, maybe more peak hp with a rotrex type? Bear in mind Porsche only got to 400bhp with the 993 & 500bhp with the 997 - thats with variable valve timing & lift, vnt turbo's, direct injection & a cc increase. My advise - never, ever believe american hp claims.
In years gone by, Ive just had the top whacked off in a lathe.
Took around 6mm off my metro turbo pistons ( they had a huge dish and this still didnt reduce crown thickness )
Yes squish would be totally horrible, but it's cheap and worked just fine.
Ideally you would machine some sort of dish though.
Engine management is largely going to be down to who is mapping it and installing it. No sense buying a system nobody wants to work with or isnt familiar with.
You'll want good local support where possible.
Intercooler, air or water, as big as you can
Took around 6mm off my metro turbo pistons ( they had a huge dish and this still didnt reduce crown thickness )
Yes squish would be totally horrible, but it's cheap and worked just fine.
Ideally you would machine some sort of dish though.
Engine management is largely going to be down to who is mapping it and installing it. No sense buying a system nobody wants to work with or isnt familiar with.
You'll want good local support where possible.
Intercooler, air or water, as big as you can
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