Machine work.

[Warwood]

I want build a 2.7 16V, I have a set of 104mm 968 pistons. This brings up the problem, the 968 & 944 pistons have differing compression heights due to the 8.9mm difference in stroke. I could have special rods made that are 4.45mm longer than the stock rods to compensate. Or I could the blocks deck milled by 4.45mm. I don't know if a 944 can be milled by this much. But what the maximum amount that a 944 block can be milled down by? I am sure this would cause all sorts of other problems.

Your thoughts.

[DannoXYZ]

You'll need custom rods. And you do not want them to be exactly the differnce in stroke because you need to compute the actual compression-ratio.

The balance-shaft weights have to be modified to account for the difference in weight with the new pistons and rods.

You'll also need the 968 engine harness and DME in order to have chip-mappings for a 4v head (completely different from 2v head). Also gives you the knock-sensor function as well to deal with the higher compression. The cam-sensor is another piece neeed. You'll need the 968 flywheel, bell-housing, clutch in order to get the 60-1 trigger wheel to work with the 968 DME. Top it off with the 968 MAF.

[fast924S]

I agree with danno on what you need or you could go full stand alone. I wouldnt mill the block, I would have custome light weight connecting rods made

[Warwood]

I agree, but I am not building a 968. What I have in mind is more like a bored out 944S. But I didn't think about balance-shafts. I have the basics down on the peripherals. The pistons compression height is the only thing slowing me down from building the engine.

[pokey]

You can eliminate the balance shafts. It won't be as refined an engine, but it frees up more power.

[DannoXYZ]

The balance-shafts are worth 3-4hp max.

"The pistons compression height is the only thing slowing me down from building the engine."

You really should look at the final-results and work backwards. There's a tonne of headaches waiting for you if you don't. The process would look like this:

1. select octane-level of gas you'll be using
2. this will dictate the type of chip-mappings you'll use
3. Which dictates using either 944S, S2 or 968 harness and computers
4. which determines which compression-ratio you'll be running
5. which dictates the lengths of your custom rods

Did I mention the knock-sensor function???

[pokey]

The balance-shafts are worth 3-4hp max.

Yes, but eliminating them saves the headache of modifying them.

[AznDrgn]

One thing that nobody has brought up is that the cylinders in a S2/968 and I believe in a 2.7L are siamesed so you have enough material to bore out. Using a 2.5L block by the time you bore out that far the cylinder walls are going to be quite thin.

[Warwood]

This is a revolting development. I think I will shelve this idea until my "89 2.7L needs a rebuild.

Funny thing is that I am told that my 2.7L w/ 10.9:1 CR doesn't have a knock sensor. Wonder how they got away with out one on the 8V N/A cars?

[ronniebaldino]

Have you thought about just getting some head work done on your 2v head. I wonder what a 2.7 with a really well built 2v head and a good exhaust would be like vs your original plan?? Port polish valve job header test pipe etc.

[Warwood]

My 1989, 944 is fairly unmolested, with a 2.7L engine that has 129K miles on it, so I don't have a need to or a desire to rebuild it. Now when it has 225+K miles on it or probably more, that will be another story. Well unless it throws a timing belt that is.

But where as porting and polishing the head can improve it's flow, in general 16V heads will out flow 8V heads and when you port and polish a 16V head it's that much better. What the real shame is that the 2.7 and 3.0 heads don't fit on a 2.5 blocks.

I have a 2.5L 944S 16V head, cams, and intake, but the pistons I have are for a 968. I wanted to bore out the block to 104mm, and thus getting 2.7L 944S. But that just won't work as I have been told without sleaving the block. So I can wait until my '89 needs a rebuild and when that happens I will use a 968 head, pistons, cams (vario), intake and custom rods.

Take easy,

[ronniebaldino]

I would figure that it wouldn't be any more expensive to get a S2 or 968 motor compele and just swap them. Sell your 2.7 and the S head (these go for decent money)

I was not putting down the 16v head but I have also seen 400+hp 951 cars with the 8v head so it can deal with a decent amount of flow. I know that is forced but it speaks well of the heads ability to flow air.

[Warwood]

So what's the point? Naturally aspirated performance is different than forced induction. It's easier to force air though a head, than it is to let the piston suck it through it. The reason that the 944S makes 30 more horse power than a 8V 944, is that it's head is about a 20% greater in Volumetric Efficiency.

Did you read the June issue of Excellence? Missing Element, a turbo charged 968 turned out 435 Hp @ 6500rpm w/ 370 lb/ft torque @ 4000 rpm on pump fuel. The guys a Vision Motorsports claim 500+Hp on race fuel.

In the case of turbos, you get more out of them the more you put into them. If you start with a little power you will have a lot out. If you start with more you will get even a lot more out, so the idea is to build as much power into your engine before it gets blown.

[Rmills944]

True, but you aren't going to have the maximum horsepower out of the engine if you're going to turbo it because you will not be running the maximum compression ratio.

Running a turbo on a car with a 10.9:1 compression ratio is going to take one hell of a headgasket!

[ronniebaldino]

The 944 vs 944S power diff has alot more going on than the head. Keep in mind that the 944 motor did not get a HP jump since it was released. That means 83 - 87 Porsche had time to make little improvments to everything and find a HP hear and a HP there. Yes the head played into that but so did the addition of knock sensors and the ability to fine tune the mixtures better and set the timing a little more agressive. I am too lazy to check but didnčt the compession ratio go up as well?

I still say S2 motor swap.

[Warwood]

US '87-88 944 vs '87/88 944S = 7 tenths of a point differenc. '87/88 US, 944 10.2:1 CR was rated at 156Hp. And the 944S makes 188Hp w/ 10.9:1 CR, (but there were 944S Cup cars that were reportedly chipped at the factory out to 200+Hp and I haven't be able to confirm this though).

If you want to really convolute the argument, my '89 2.7L 944 w/ 10.9:1 compression makes 162 Hp. The '82 - '86 (and maybe '87 too) 2.5L ROW 944 w/ 10.6:1 compression made 163Hp. What's sad is the 2.7L has larger valves, and also doesn't have a knock sensor. I guess engine size doesn't matter in Porsches.

Back to 968 Turbo's: From the June issue of Excellence, Dwain Dement of Vision Motersports stated: "We had Carrillo custom make a set of shorter rods to lower the compression ratio from the factory 11.2:1 to 9.5:1." says Dement. Factory Mahle 968 forged pistons wer used, though they were handpicked to assure that they were as similar in size as possible. "The factory Mahle pistons are forged and, in my experience, are far superior to anything available on the aftermarket."

Now if I bored my block out to accept 968 pistons the reduction in compression height from using them would result in a compression ratio of about 7.4:1 CR with a 2.5L crank. And the only way it would make any power would be to pump in a *LOT* of boost. Anyways I think this hole (as in hole in my pocket) exersize, while very interesting would not be whorth while.

[ronniebaldino]

o.k. now I am lost, you started off saying you wanted to build a 2.7 L 16v and now you are talking boost???? I think you will end up putting one hell of a hole in your pocket. Now I am going to pull a page out of the Pelican textbook and say if you ever start down the road to actually doing this - back up sell your nice 2.7 and buy a 951 and call huntley racing for the rest. They have more than enough proof that 4 valve heads are not needed unless you want over 450hp. Save your money and end up with something you know will end up starting at the end.

I say enjoy the 2.7 but like I said before if you think that the ROW 2.5 was 163 then think about how much power is hiding somewhere in your motor?????

[Warwood]

This has been a very convoluted posting. The thing is I currently own two 944's. The wrecked one is an '85.2 and it has a 2.5L 8V. I was playing with the idea of boring out the block to take 104mm pistons from a 968, then slapping a 944S head on it. But it doesn't appear that that is a viable option. To make it work you would need to a. bore & sleave the block and b. have custom rods made. Then install the 16V head, with all of the other stuff that goes with it. Sure it's do-able but not that easy.

My other car is an '89 944, and nope I am not messing with it. I like the way my '89 2.7L works as it is. But (and this is a BIG but) if circumstances warranted me to do a rebuild of that 2.7 block I would consider rebuilding it as a 3.0L or just out right replacing it with a 968 engine.

951's are cool but I have never been impressed with their lack of torque off boost. I think a S2 or 968 is a better car for the money.

[ronniebaldino]

I agree I think the S2 is the finest example of the line.

And now I understand the confussion.