924Board.org

[XLR8]

If I lived in the US, perhaps. Alas shipping to Australia may prove prohibitive.

I have a block with a scored bore, so have read this entire thread with interest to build my own stroker in a similar manner.

Did you start it? I would love to hear that!
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'81 Porsche 924 NA
'89 Eunos Roadster NA6
'06 Landcruiser HDJ100

[Mike9311]

Me too!

Technically its close to being able to start
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

[Beartooth]

I don't know why, but I've found myself very interested in the question of what would be the best way to refresh and upgrade the bottom end of the to-for. I actually independently stumbled across the Mitsubishi rod idea by plugging some dimensions into Summit Racing's website, so to find this thread tells me that's the best of several options (none of them cheap or ideal).

The ultimate solution would be a billet crankshaft - that would maybe even allow you to machine the block for a different main bearing combination. But I'm thinking unless you've got money to burn but don't really care that much about hitting big power numbers, this doesn't make much sense. The trouble is, you could spend $5k on the bottom end alone (and probably $10k by the time you've got the motor finished) and end up with power levels that are mediocre compared to any number of potential engine swap candidates. An engine swap isn't a no-brainer, but at some point you've got to decide between more power potential and keeping the original engine.

With that in mind, I'm liking the idea of the Mitsu rod and turned down rod journal, but I don't see a good option in an off-the-shelf piston. The trouble is, at least for a serious build, you're stuck with a ton of compression because you wouldn't want to run a big deck clearance and lose quench and detonation resistance with that. In short, more deck clearance can be self-defeating, because a tighter clearance has a "quench" effect that reduces the potential for detonation. 1mm is an aggressive, but generally safe clearance (the closer to zero, the better for det resistance, but obviously you can't run less than zero, plus you have to allow for piston expansion). Bear in mind, that takes the head gasket into account, so we're talking about bringing the piston just shy of even with the deck. With off-the-shelf pistons and a 931 head, you could actually get in the range for an aggressive NA motor if you decked the block (would be around 3mm with the Wiseco pistons ideola used; hopefully the timing issue wouldn't be too bad with an adjustable sprocket and possibly shorter timing belt). One other outside-the-box idea would be to enlarge the combustion chamber in the head. The amount you'd need to expand it by (roughly doubling the volume) seems excessive to my mind, but if that's more practical than I'm thinking, then the off-the-shelf piston might become a better option.

Here's what I'd do today if I had money to burn: I'd start by collecting the bearings I need and possibly sourcing a good crankshaft if I couldn't find undersize bearings (seems that part could take a while). Then I'd have the crank ground to the Mitsu rod; also, it sounds like offset grinding is a no-brainer, but don't get excited because it's only another 25cc... Then, where my approach would differ, is I'd have a custom piston made so that I could avoid too much compression, but keep some quench. Anyway, to continue fleshing this out, let's say you could get the crank done for $500 (that could vary a lot). Next, the rods are around $450, or $1200 for top-of-the-line Carrillo rods. Custom pistons would come in at $1200 (hopefully those aren't getting silly expensive). Bearing should be around $250, but could go a lot higher if you were stuck scrounging off ebay and other enthusiasts' "some day" collection... throw in a gasket set ($50), $300 on block prep (again, could vary) and you've got $2700 in the short block. Not great, but a good investment, especially considering you'd have a much lighter rotating assembly, and good as new (maybe better) oil control and ring seal.

So, I'm mostly thinking out loud here, but to my mind, the one piece that needs fleshing out is a custom piston. Maybe some day I'll have the money to throw at making a set with enough dish for a 931-friendly compression ratio (I think in the 8.5-9:1 range; anyone wanting to push big boost would probably want lower). One other option may be a longer rod.The trouble is, only the early 4G63 engines had the rod with the wider big end, an I can't confirm there's an off-the-shelf rod with the wide end, outside of the stock (150mm) length. It could be the 150mm rod is about the right height for a dished piston anyway though. I'm coming up with around 39mm compression height with that rod and zero deck clearance; most newer engines use shorter pistons - just over 30mm seems to be pretty common - but with just a slight dish compared to what you'd need for a 931.

Finally, by way of closing, it seems this theoretical piston might work for both the 931 and NA engines. I'm calculating you'd want around 35cc of dish. With a near-zero deck height on an NA head, you'd get just short of 12:1. On a 931, you'd have around 9:1. That's definitely too much for NA on the street, but if you had a 35cc dished piston with no valve reliefs, then cut valve reliefs to make for non-interference with an NA head, you'd probably add 5-10cc, dropping you into a reasonable range. Like I said, I'd also cut reliefs for the 931, but they shouldn't have to be as deep. Anyway, hopefully this is some food for thought. To my mind, a custom piston solves what would be a ton of issues (at least for turbos), and it'd be nice if some day you could go to JE or Diamond and order that piston without having to go through the full custom piston process.
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1980 931 diamond in the rough

[motormouth]

Resurrecting an old thread.... Does anyone have the specs on the pistons used for this build with the offset regrind approach?

Many moons ago, I ordered rods and had the crank work done. I never ordered pistons as I'd moved on to other car projects. I am thinking about finally finishing a 924 build and using this setup. Looks like Dan's CR tool was taken offline also.

[Mike9311]

[Mike9311]

actually, it had 87mm and looks like right up to the deck




_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

[motormouth]

Thanks Mike, more questions:

I already had the crank done and ordered rods. I found pistons, but never ordered, and lost the part number for those.

If I am looking through my notes correctly, it was offset ground and the rods are stock 4G63, so they have 21mm wristpins. Looks like the Wisecos you posted also have 21mm pins.

Did you offset grind the crank? 5.906" center to center for con rods? What did your CR end up being?

[Mike9311]

[Jschwartz]

First track test, Thursday at BFR. Car ran great! I have Nankang 200tw tires.

https://youtube.com/shorts/J1fRTziC9AM?si=5LhHvXX9uBF81ezu
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JS

[Cedric]