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Unread 12-07-2012, 01:25 PM   #1
fifai
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Default Is a 2.5L turbo 16v a good idea ?

I've got a 2.5 block that I can build with 951 piston or 924S (CR 9.7) pistons...

Is it worth to use a 944S 16v head to build a 2.5L 16v turbo ?

What CR use 8 or 9.7 ?
What kind of turbo can be used ?
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Unread 12-09-2012, 06:29 AM   #2
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I reckon it would be a great project. Wouldn't be running 9:7.1 though. Maybe for low-ish boost on E85. Safer to run it at 8's.
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Unread 12-09-2012, 07:17 AM   #3
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Quote:
Originally Posted by fifai View Post
I've got a 2.5 block that I can build with 951 piston or 924S (CR 9.7) pistons...

Is it worth to use a 944S 16v head to build a 2.5L 16v turbo ?

What CR use 8 or 9.7 ?
What kind of turbo can be used ?
Personally I wouldn't use either pistons. I'd get pistons made for the head as the "squish" or "quench" of the 951 pistons with the 16v head wont be as good and it wont match up (piston bowl to the head combustion chamber). Probably could get them modified if you're on a budget.
Means you can lower the CR.
I mean there's nothing wrong with using the pistons and head together but like i said you wont use the full potential from the head with out the proper "squish"

Quote:
Originally Posted by 333pg333 View Post
I reckon it would be a great project. Wouldn't be running 9:7.1 though. Maybe for low-ish boost on E85. Safer to run it at 8's.
I don't see running 9:7 CR an issue if you use moderate (15 psi) boost as the head is a better design then the 8v as the plugs are centered other then on the side.
e85 would be good/better to use if you're looking for big power. but stock 951 rods.. 400rwhp is your limit.
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Unread 12-09-2012, 03:10 PM   #4
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Sure, I meant more the c/r value than the pistons themselves. You'd have to get the turbo ones modified to work anyway. Agreed, get some custom pistons/rods and know what you're dealing with.
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Unread 12-09-2012, 04:36 PM   #5
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Quote:
Originally Posted by 333pg333 View Post
Sure, I meant more the c/r value than the pistons themselves. You'd have to get the turbo ones modified to work anyway. Agreed, get some custom pistons/rods and know what you're dealing with.
reliability will be better, less heat with lower CR though.
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Unread 12-10-2012, 10:30 PM   #6
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I've seen a disposition to mark 951 pistons with an old valve mount on a drill... I'm looking for the picture...

The idea to put a 16v head on a 951 bloc is to do a "test" engine in parallel to the installation of the gt30 in the car.

The aim is to keep clutch, engine internals stock, I'm still with the 924s gearbox too... be under average 400 rwhp can be a good next step for my 924 ! (now with k26/6 19psi guru is already quite fun ! )

I've lots of pieces (pistons, bloc,manifolds...) from 2.5L NA and turbo, that's why 3L don't interest me now.

To build another (with the stock one i've rebuild in my 924) engine with mid power range i've possibility to use a NA 8V head with bid valves or an 16v head... and maybe stock 16v head is cheaper than 8v machined and more exotic too !
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Unread 12-13-2012, 01:05 PM   #7
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16V heads work well on 2.0 big power Jap engines and Cosworth engines over here in the UK. I don't see why this wouldn't work on the 944. Does the 16V head bolt up to the 951 block?
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Unread 12-13-2012, 01:15 PM   #8
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- the 944s head is p'np on 2.5L blok (NA and turbo)
- the 944s2 head is p'np on 2.7L and 3L blok

differences between both is there :

"Building a 3.0L using a 968 block and wanting to use a 951 8 valve head? Lindsey Racing can take care of the water passage alteration necessary to join the two. Relocating the water jacket to mate to the 968 block"

http://www.lindseyracing.com/LR/Parts/LRHEADS.html

Click the image to open in full size.

Click the image to open in full size.
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Unread 12-13-2012, 01:29 PM   #9
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Some pictures from race 924 GTP engine (2.5L 16v 400hp @ 15psi)

Click the image to open in full size.

Quote:
Lastly, in 1981 Porsche entered a 924 Carrera GTP (Aka: 944GTP Le Mans) which Porsche Motorsports build a new highly modified 2.5 liter I4 engine. This engine sported 4 valves per cylinder, dual over head camshafts, twin balance shafts and a single turbocharger K28 to produce 420 hp (313 kW; 426 PS). This last variant managed a 7th place overall finish and spent the least time out of any other car in the pits. This new configuration engine is the predecessor of the 944 platform(s) and the later quick 944S 16V M44/40 powerplant.
http://webspace.webring.com/people/e...ls1/turbo.html
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Unread 12-16-2012, 05:24 PM   #10
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i'm not sure but i've found that :

951 pistons are 20-25 cc
951 head chamber are 56 cc

but

944s pistons are +/- like 928s4 ones so maybe 20-25 cc too !
http://forums.pelicanparts.com/uploa...1298138880.jpg
http://forums.rennlist.com/rennforum...c-numbers.html
and 944s head maybe like 928s4 one or 944S2 one : 41 cc !

So i think that it won't be good for turbo application ! i'm scared that the CR will be the same that 944s one, so 11 or 10.9 ... ?
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Unread 12-16-2012, 06:31 PM   #11
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but with a 10.9 TC for 944s, 7.8cc for JDC, 41 cc for head i fund 13.5cc pistons ? ? ?

So the mix will give something like : 9.3/9.9 due to 20 or 25 cc 951 piston

it works with 951 pistons (20/25 cc), 7.8cc for JDC, 56 cc for head i fund 7.8/8.4
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Unread 01-08-2013, 06:23 PM   #12
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Good info ! the 951 piston I've are 25 cc ! Rods are ARAST... so stock should be ok.

I've done some measure and its seems easy to dig 5 cc in the top of piston in ordrer to have an 8,5 TC .... !

The 944S head is on the road (maybe still at post office ) ...

Now i've to look at 968 exhausts valves and maybe if not too expensive a 968 exhaust cam...

I think that it's now official i plan to build this engine... i give me some years to do it (as i change my house, have an work place to build... )
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Unread 01-11-2013, 06:15 AM   #13
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Now I wonder what sort of exhaust valve to use ?

It seems that 968 one are sodium filed... ?

But refs on the PET are the same on 944s 944s2 968 928S4 ...

What sort are used on 3L 16v turbo ?
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Unread 01-22-2013, 06:33 PM   #14
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What are the valve specs on the head you have? Can you use the S2 or 968 valves?
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Unread 01-22-2013, 06:48 PM   #15
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The head is in a box... and i've no possibility to look at this quickly.

By the way, refs and dimensions are the same (according to workshop manuals) for :

intake : 944s 944s2 928S4
Exhaut : 944s 944s2 928S4 968 ...
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