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OT: But seemed appropriate: If anyone's interested, I'll find an old article towards the reasoning behind why in the hell half a 928 motor, and one of it's heads don't mingle well.
Ten Nightmare Cars You Can't Resist
Some cars are a dream to buy because of their beauty and a nightmare because of their…
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Ten Nightmare Cars You Can't Resist
10.) Porsche 944 Turbo S
Suggested by: Dravs
Why It's a Nightmare: On average I see about four 944s daily and originally, there were 163,192 944s produced in total, meaning that chances of owning one are high. Good luck finding one though because only 40% of all 944s are still on the road today. It's even harder to find a decent Turbo S variant, so prepare to shell out a good amount of cash. Why wouldn't one want a Porsche that weighs less than 3000 lbs? The problem is that owners loved their cars too, so a lot of examples have high mileage, are falling apart, or just don't run. On top of that, expect to remove your left arm and leg because that's what maintenance costs. But the Turbo S's 247 HP and damn good looks make all the pain and drained bank accounts worth it.
The piston Design changes the CR between the 944turbo and the 944NA engine. The ONLY difference in the 8v head is the ceramic port for the turbo head.
Please find that article. I would like to prove it wrong as well.
The piston Design changes the CR between the 944turbo and the 944NA engine. The ONLY difference in the 8v head is the ceramic port for the turbo head.
Please find that article. I would like to prove it wrong as well.
For one thing the ceramic coating (Liner) and lower CR. 944 NA motors either have 9.5 -1 and 10.2 -1 vs the turbo head with 8.0 -1.
Lowering the CR and restricting the internal volume = loss of power on a non forced induction motor. Feel a tad silly posting this, but saying they're the same forced me to respond.
Originally Posted by George D
I'm sure I'll hear, "It's the piston, not the head that changes the CR." Well, this is definitely part of the equation. If these engines were easy to make power, we'd all have 400whp reliable cars like the EVO folks with just a flash and exhaust.
no, they're more or less identical heads with the exception of the ceramic exhaust port on the 951 head.
valve sizes and chamber size are the same NA vs 951.
the compression difference is entirely in the pistons.
some NA pistons have a dish about the size of a quarter, these are the 10.6 or 10.2 pistons, some have a larger dish maybe 2" or more across, these are 9.5 cr...the turbo pistons have a big dish in the middle, across the whole piston lengthwise, that make 8.0cr.
Humboldtgrin, based on your photos, it appears the intake ports are completely different from any 944 variant? Do the water cooling passages line up to the 2.5L block? If they don't, do you have a 3L block to compare the head to? If it lines up to the 3L block, this looks like you could potentially retain the factory S2 pistons but replace the rods with RARST turbo rods and have an incredibly cheap turbo build. Very exciting!
Humboldtgrin, based on your photos, it appears the intake ports are completely different from any 944 variant? Do the water cooling passages line up to the 2.5L block? If they don't, do you have a 3L block to compare the head to? If it lines up to the 3L block, this looks like you could potentially retain the factory S2 pistons but replace the rods with RARST turbo rods and have an incredibly cheap turbo build. Very exciting!
It is the same as the S head but with an open combustion chambers. Also a little shorter valves. The cam sets in the same spot height wise. I found intake runners for sale and am getting them to mock up an intake manifold. This head is for a 100mm block and will bolt right up to the 944 2.5 block. I have to measure the cc of this head still. Been busy. This may very well be the holly grail of the 2.5 turbo engines. The turbo s engine can be replaced with a m44/40block and Pistons and this 85,86 928s head. We will see about the CR when all the math is done. It could be some simple mods and bolt on parts if all goes well aside from the intake. And any DOHC turbo needs a fabricated intake anyways so why not use this head if it works better?
If it fits the 2.5 block then you could just use 944S pistons and mill 1mm off the top like Racerboy did with his car. That, coupled with the larger open chamber should drop CR down near 9:1 which is very safe for the 16V design. You're right about the intake but a simple flange could be designed and water cut to match the bolt pattern of the head but the intake ports on a 951 manifold. A machine shop should be able to mill off the 951 intake flange and weld on the new flange.
Sorry...I didn't realize this is about the 928 4 valve heads. The reason is the 928 4V heads have 36mm valves. One of the mods for these heads is to install 968 valves. I recall the reason these heads weren't used for turbo applications was the 951 sodium valves were a fitment issue.
I was stuck on the 928 16 Valve (32) heads installed on our motors. Technodyne in Phoenix looked into this, and tabled the idea. Could have been Powerhaus. We're going back over 20 years. The S2 8 Valve heads are easier to add larger intake valves, and use factory 951 sodium exhaust valves.
Do you have to run sodium exhaust valves, no. But I'd not go that route.
Sorry for my complete misunderstanding. There wasn't much R & D on the 4v 928 heads used on the 951 because there were much better options, even twenty years ago.
I'm genuinely curious why no vendor or person with machinery has taken the step of making a simple adapter plate between the 951 intake manifold and the 16v head bolt pattern.
When I had an "S" head and a 951 head side by side, and overlaid gaskets and stuff, it looks like it'd work no problem. One bolt hole appears to even line up, maybe that one should get a long stud installed in the head to align the whole assembly.
It wouldn't be ideal with the 951 runners versus the 16v port size but it'd work and be cheap...
The answer to that question is very simple. COST/DEMAND.
7.18 CR!!! Awesome if it's true!!! I think it's realy close!!!
I've been playing around with measuring the 85,86 928 4v head CC and came up with 59cc each time for the CC area. And a turbo piston is 33cc. And this is what I cam up with...
And I didn't know the deck Hight so I just put down 1mm. So CR is most likely higher. Maybe a half a point which I think is still great!
This one may be more accurate. I searched more numbers. What is the cc of the turbo piston? 25.7 or 33? If it's 33 I just dropped below 7:1CR not good. Would have to buy Wiseco 25.7cc 8:1 turbo Pistons.
May be more accurate.
Update just measured about 27cc for the turbo piston so it's close. May be 7.2:1
7.2 CR off boost...stumped - why? My daughter's Sonata 2.0 with 275hp and close to 18psi at 1750RPM with a 9.5 CR, and she can run 87 octane, 91 gets you a few more ponies.
My 3.1 motor is around 9.0 CR. During tuning on a Mustang type "load" dyno with 100 octane pump fuel there were no knocks at 28psi, then turned it down to 25 to get a reliable/repeatable dyno chart line to see where the turbine opens the WG, and AF ratios. Tuning with 91 pump, 18psi was easily consistent. Got very few knocks at 21psi using 91 pump, much less than my last 330whp 350tq 2.5 951 at 18psi.
I'll shut up here, but seeing such low CR goals reminds me of the early - mid 80's 930 days. Light switches - laggy bitches.
Last edited by George D; 08-25-2015 at 01:12 AM.
Reason: To no longer be part of the discussion.
7.2 CR off boost...stumped - why? My daughter's Sonata 2.0 with 275hp and close to 18psi at 1750RPM with a 9.5 CR, and she can run 87 octane, 91 gets you a few more ponies.
My 3.1 motor is around 9.0 CR. During tuning on a Mustang type "load" dyno with 100 octane pump fuel there were no knocks at 28psi, then turned it down to 25 to get a reliable/repeatable dyno chart line to see where the turbine opens the WG, and AF ratios. Tuning with 91 pump, 18psi was easily consistent. Got very few knocks at 21psi using 91 pump, much less than my last 330whp 350tq 2.5 951 at 18psi.
I'll shut up here, but seeing such low CR goals reminds me of the early - mid 80's 930 days. Light switches - laggy bitches.
Think 25 psi on 91 octane. And four valve per cylinder. Low Cr will only help me get high psi out of my turbo. It's ok, you just don't see what I do. It much more complicated to deceibe all the workings, when I see it perfectly. It a track car that not going to see much low RPM's.
08-21-2015, 08:53 PM
