924Board.org

[ideola]

I am slammed today, but will call try to call the crank guys on Thursday. I solicited some add'l opinions from some of my other trusted car geeks last night and they also felt that some hardening would be required.

Also, yesterday I found some more alternatives for piston slugs that are quite affordable, so if we can find a way to use these rods and address the crank hardening issue, I think we might have a winner of a solution.
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erstwhile owner of just about every 924 variant ever made

[daniel]

In the old days, we used to get cranks hard chromed then polished. This was done to recover a crank that had been ground down too far, i.e. passed the case hardening. Cant remember what this cost or how much trouble it took, but with the advent of the internet and been able to source lower cost steel cranks from US and China, we all stoped doing it.

All this been said, my understanding is that grinding a crank through the case hardening does not make the crank more likley to break (except for the fact that there is less material ofcourse, but this can be a blessing in disguise) the removal of the case hardening reduces journal life and makes the journal more prone to scratching and pitting. Not a huge issue with a well built clean race engine that does low miles, could be a concern with a road car though.
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

[ideola]

Update from the crank guy:

Offset grinding of the journals: estimated @ $200
Optional nitriding of the crank: estimated @ $170
{EDIT}Obviously, the crank work should be sourced locally to avoid shipping costs, and should be doable by most reputable crank servicing outfits...{/EDIT}

He said he would recommend doing the nitriding, but it wouldn't be absolutely necessary, especially given the low horsepower / low RPM characteristics of our motors. If it were me, I'd definitely do the nitriding.

The big question now is which rod to use. I have two alternatives with different big end widths and bores. I am going to take a crank in for him to inspect, and we'll settle on which rod will be best for our desired approach. Once we know that, I can finalize piston selection. The big issue is that the two different rods use different wrist pin sizes, so I have to figure out which rod to use before I can finalize the range of available pistons. The beauty is, either rod has at least 10-12 different piston configurations that should work.

Unfortunately, I can't get a crank to him until first week in March because I will be traveling starting tomorrow thru the 27th.
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erstwhile owner of just about every 924 variant ever made

[Fifty50Plus]

Good news on the grind. +1 on the nitriding. Price is great.
Chuck
r.e. travel...intl or local?
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car

[ideola]

I have done some additional research on the possibilities of using a 931 head for an NA application. I have a whole range of piston setups that would work, but unfortunately, the biggest challenge is the exhaust.

I measured the exhaust ports today. The NA ports are ~32mm, and the 931 ports are ~38mm. The bolt spacing is too far apart to be able to "slot" an NA manifold or an NA header, not to mention the whopping 6mm of difference in diameter. Point being, I have not been able to come up with an easy way to modify an NA exhaust for a 931 head that would not result in massive restriction in flow, negating much of the benefit of the 931 head.

I inquired about modifying the existing aftermarket NA headers, but it would entail nearly $2000 of up front cost to develop the jig and the first header. Having to install a custom exhaust header would negate any savings available by going with the "cheap-a$$" stroker. So unless someone has a brilliant idea about how to inexpensively solve the exhaust issue, it looks like 931 head on an NA build is a no go.

All that said, we can still come up with a stroker set up for both applications...but it will definitely make the NA solution an interference motor. No way to avoid that without getting custom pistons with valve reliefs.

Planning to stop by the crank shop tomorrow or Wednesday to further investigate the crank end of things, and hopefully arrive at a final decision with respect to the rod dimensions required.
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erstwhile owner of just about every 924 variant ever made

[!tom]

[tyfighter123]

Dan, what are the main differences between the N/A and turbo heads? I know the turbo head flows better but is it still better than a N/A head with the big valve job?
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Porsche 924 1977 N/A
Mustang GT/CS 2007
Porsche 924S 1987 (parts car)(cut up and recycled)
Porsche 911S 1976
Porsche 931 1980
Porsche 931 1980 (parts car)

[!tom]

Lots of threads on this; do a search.

Big thing is the shape of the ports. The air flow in the 924 head goes through something like a 100 degree turn, while the 931 head is more like a 90 degree turn. This is more to put a picture in your head than the actual difference.

Also, the 924 head has no combustion chamber (it's completely flat), whereas the 931 head does have a combustion chamber.

Plugs are on opposite sides too.
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78 924 NA
5-lug

[ideola]

Yeah, the turbo head has a better flow profile, which you can see by looking at the pix in this thread. The 931 ports are larger to begin with, have a better entry angle, and have less radical radiuses than the stock NA head. A big valve head overcomes some of that, and the one you are having my guy prep for you will be ported to make maximum advantage of the bigger valves...but the restriction on the exhaust port is significant, much moreso than I previously realized until I measured today.

I was surprised there was 6mm diameter difference. As a percentage, that is a whopping 30% difference in surface area (~804 sq mm vs. ~1134 sq mm), to say nothing of volume difference. There was a reason the D-Prod guys were running 931 heads, but they had race budgets and were undoubtedly making their own exhaust manifolds.

The other benefit of the 931 head, I believe, is that it is not a heron style head, so I suspect that it provides better quench and combustion characteristics. Everything I've read indicates that the shape of the 931 combustion chamber is pretty well conceived (dish in the head, dish in the piston, quench ring on the piston circumference) for promoting proper combustion and flame travel. The pistons I've identified are for late model cars, and exhibit the exact same design characteristics as our stock 931 slugs, the only difference being the compression height and pin diameter.

The design of the stock 931 pistons provide good quench, and the dishes in the head and piston crown promote good combustion while being more detonation resistant than say a flat-top or domed piston design. Porsche engineers obviously knew what they were doing.

My research indicates a high compression NA engine would benefit significantly from this kind of combustion chamber...it might not be as advanced as today's technology allows; but within our constraints, the 931 head with the pistons I've identified should work really well on high compression NA applications, just as they would on a forced induction application. The pistons I'm hoping to use are from platforms that essentially use the same combustion chamber design for both NA and turbo applications, and simply vary the compression height and piston dish to achieve the desired CR. Point being, they didn't use different combustion chamber designs (as with the 924) for the NA and turbo'd applications.

If I could solve the exhaust header problem of the 931 head, I could create an NA combo using a long rod, +6mm of stroke, and an ideal combustion chamber with dished pistons almost exactly like the stock 931 slugs, achieving CR anywhere 9:1 up to 11:1 (or greater). Add in the availability of Cometic gaskets in a variety of thicknesses, and I can create a setup that is tuned exactly to a given customer's requirements.

If we could get 25 guys to commit to an NA build, we could get headers made at a reasonable cost (<$350 per header), but probably 80% of my customers doing custom big dollar mods are 931 guys, not NA guys. I doubt I could get 25 commitments.

I can still create a good NA setup with most of the above characteristics, but we'll be stuck using the NA head, and it will definitely make the NA an interference design. As I said earlier, there is no way to avoid interference when increasing the stroke and using pistons with no valve reliefs. It is conceivable that the pistons could be machined for valve reliefs to make them non-interference, but I won't know that until I have a set in my hands and can evaluate how much material is available on the piston crown.
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erstwhile owner of just about every 924 variant ever made

[motormouth]

So I probably have no idea what I am talking about, but just for the sake of conversation; what if you used a turbo exhaust manifold as well as a turbo head on an NA application? Then instead of having the exhaust go through the turbo, make up a custom exhaust from the manifold back. You wouldn't need to make up a custom header/manifold, instead only a custom exhaust to go from the manifold back.

[leadfoot]

Here's my experience in fitting an N.A exhaust manifold to 931 head...

The ports of the 931 head has a lip for the fire rings to sit against, it's about 2mm, not necessary for the na manifold and these can be blended out when porting the head

the bolt holes need to be offset ground to accommodate the spacing,

the head is mismatched to the 931 manifold gaskets by about a further 1-2mm

the manifold is cast iron and a bitch to grind
(sorry just had to have a winge there ever since I got some in my eye I've been pissed with cast iron, nothing better than wasting a sunday sitting in the doctors office getting filings pull out of your eye)
the lesson is to wear a full face mask, I had on the extra large goggles on with the square edges that hug around your face and still got hit.

So if you offset port the N.A manifold you can get a fairly good match, theres plenty of metal on the manifold to work with but it takes a while to do as the metals hard as rock, an air grinder with a heavy duty carbide burr is the way to go, leave the dremel in the drawer for this one or you'll be wasting your time...

just for another 6 hrs work you'd also want to port match the manifold to downpipe and downpipe to final exhaust section.
stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Khal]

[ideola]

RE: turbo manifold for an NA application, that is an absolute non-starter right out of the gate. As Khal mentioned, there are not many uncracked examples available; the log-style manifold would be a horrendously inefficient design for an NA application; and you'd still have the problem of blocking off the j-pipe port + fabricating a completely custom downpipe. Not at all an elegant or cost-effective solution, and the inefficiencies would probably rob any power gains from the head.

RE: NA manifold, after second look tonight, I can see where it could be made to work after comparing it side-by-side to the turbo manifold. While grinding the stock NA cast iron manifold would be an option, my assumption was that anyone interested in doing these mods might also want to install a high performance header vs. the stock manifold. There would not be enough material on a thin-wall steel header do the necessary grinding...I'd be willing to bet that the outer diameter of the header is actually less than the inner diameter of the 931 port. Even with the stock manifold, the restriction of going from 38mm down to 32mm diameter can not possibly be a good thing in such a short distance.

As I'm mulling my own NA build, I think I'd rather deal with the short-comings of the NA head and run a high performance off-the-shelf header than to introduce an absolute kludge at the exhaust port. I was really hoping to find a breakthrough for using the 931 head, so I'm somewhat disappointed that the choice appears to be a custom (i.e. costly) fabricated header, or the major compromise of using the NA manifold.

That said, I'm wondering if it might be possible to make a manifold adapter? Kind of like the intake adapters used for carb conversions? I'll have to check with Vaughan to see if there's enough room between the header and shock tower to accommodate a 6mm-12mm thick adapter plate.
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erstwhile owner of just about every 924 variant ever made

[motormouth]

I thought this was a problem only on the series 1 cars... (or maybe it was series 2, I forget) point being a good turbo manifold would be easier to find than a nonexistent NA manifold for a turbo head....
but like I said, what do I know....

[ideola]

No, the 931 manifolds are all subject to cracking. While the S2 manifolds did have reinforcement that made them less crack prone, they are still susceptible, and uncracked ones are VERY difficult to find.

But that's missing the point. Even if they were plentiful and cheap, the 931 log-style manifold is NOT a good starting point for an exhaust header on a normally aspirated application.
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erstwhile owner of just about every 924 variant ever made