[Torbjorn968]

Working (or rather my friend mostly is, not good with spanners) on a 968 race engine. The idea is to do all "bang for the buck stuff" with the bottom end. About to replace the con rods and came by the concept of longer conrods, shorter cylinder. We have a 550 hp 924 with 928 engine in the the endurance racing series I am a part of, that team runs a 10 mm longer conrod. Claims of increased torque are substantial. I have understood that this is standard procedure at Cosworth as an example.



Any opionon about this?



Not to expenive: 1800 Australian dollars for a complete piston/rod set (or 800 for the rods) from Pure Performance Motorsports. Slighly more in dollars.



//TL

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[RS Barn]

TL

I've seen longer rods and shorter piston setup on 968 3.2 motor. My engine guy built an engine for someone in AUS. it was fairly low compression to run on street gas. It made 335 engine HP with Webers at 7200

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[Torbjorn968]

335 hp is rather impressive. Would you attribute part of this to the long conrods?



//TL

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[Cloud9...68]

I've read a little about this engine. Yes, the power output is very impressive, but it required extensive internal mods, including sleeving of the bores, iirc. Probably a highly modified crank, too. Definitely not a bolt-on upgrade.

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[RS Barn]

The engine has very thin sleves to keep bores straight and maintain ring seal. It also has special pistons (super light). Even with longer rods there is less rotating mass than stock. So this motor with rev more quickly.

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[jfrahm]

Interesting. How far up can you move the piston pin? I did some math once on using honda-sized big ends and offset grinding for more stroke and was not wild about the resulting rod ratio. With the stock location of the oil control rings you have maybe 1mm. I suppose the ring package can be relocated, and you could go with smaller diameter small ends too and get 1 mm there. Some engines use 22mm pins and make pretty good power, the 944 uses 24mm I believe.



If you went with aftermarket rods and pistons, offset grinding the crank would be the next logical step.



-Joel.

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[RS Barn]

Joel,

You have some insight to this package. One of the very special features is the ring package. My engine guy is in with Nascar suppliers. The two engines he built for me don't use any oil. I mean zero.

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[Cloud9...68]

"[color="#1C2837"][size="2"]offset grinding the crank would be the next logical step."[/size][/color]

[size="2"][color="#1c2837"]

[/color][/size]There seems to be some difference of opinion on this. Some people, primarily in racing applications, do lighten their cranks, but I've heard other knowledgeable people say that's a bad idea because the inherent imbalance of the large displacement 4-banger "needs" some weight in the crank (and the flywheel) to keep from shaking itself apart (which sounds like a significant exaggeration to me).

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[RS Barn]

I won't go into too much detail but cutting weight off 968 counterweights isn't an option with my engine builder. Reducing weight in other ways isn't out of the question though.

Pete

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[savvas968]

are you still having the balancer shafts in place or are not operational?

Wouldnt these counter any vibrations caused by lighter AND fully balanced

crank?

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[RS Barn]

Balance shafts are removed and oil galleys plugged

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[Torbjorn968]

Any quantifiable opinion on long conrods.



//TL

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[RS Barn]

They need to be part of a well thought out package.

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[Cloud9...68]

"[color="#1C2837"][size="2"]They need to be part of a well thought out package"[/size][/color]

[color="#1C2837"] [/color]

[color="#1C2837"][size="2"]Well said. From what I've heard, that seems to apply to any strategy that involves any appreciable alteration of the rotating mass of this oversized inline four.[/size][/color]

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[jfrahm]

[quote name='Cloud9...68' timestamp='1306201495' post='110122']

"[color="#1C2837"][size="2"]offset grinding the crank would be the next logical step."[/size][/color]

[size="2"][color="#1c2837"]

[/color][/size]There seems to be some difference of opinion on this. Some people, primarily in racing applications, do lighten their cranks, but I've heard other knowledgeable people say that's a bad idea because the inherent imbalance of the large displacement 4-banger "needs" some weight in the crank (and the flywheel) to keep from shaking itself apart (which sounds like a significant exaggeration to me).

[/quote]





Offset grinding would be to lengthen stroke, not to save weight. If you take a 54mm rod journal down to 50mm by offset grinding, you move the center of that journal 2mm farther out and add 2mm to the stroke. To keep the piston from popping out of the block like a prairie dog, you have to move the piston pin up, and/or go with a shorter rod.



With a 108mm bore and a 90mm stroke you get a 3.3L motor.



You can get 108mm chebby pistons with the pin located up in the oil ring groove.



Cheers,

-Joel.

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[flash]

yup - then you can use those cool gapless rings (Total Seal) they have been using for 30 years and really get some power

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[Cloud9...68]

Joel,



Thanks for the explanation - learn something new every day (at least one would hope!). Although, to be nit-picky, I suppose it would slightly lighten the crank, though not by a whole lot, and as you point out, lightening wouldn't be the intent of this procedure.

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[jfrahm]

Total Seals, I reckon you could use them if you like but I am indifferent to them. They would have to not be chrome if you were not sleeving. If you were not sleeving, you'd have to deposit a gliding metal or other coating onto the piston skirts.

I'd be more concerned about adding variables with the Total Seals, getting them gapped and run-in is not as straightforward as typical rings and you'd be blazing that trail.

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