Jim bailey - 928 International

Some clarification for the casual reader ..... The short skirts are necessary for clearance to the crankshaft counterweights when the piston is all the way down. Since the deck height top of the block is fixed and limited by the head when you add stroke all the increased piston movement must be moved down the cylinder. Which is why you need short pistons with the wrist pin as high as possible to run a rod of reasonable length otherwise the total amount of the stroke (not the radius) must be taken up by a shorter rod.

Tony

951 pistons and a stroker crank yeild appx 1.65...

heres a good read...http://www.victorylibrary.com/mopar/rod-tech-c.htm

and another..ive got so much stuff bookmarked over the years...its good to back and read it now and then.
http://www.hotrod.com/howto/69883/

BC

That could work.

Jim bailey - 928 International

You might want to rotate the crank 180 degrees to show what happens when the piston is all the way down and the crank shaft counterweights are up.

Tony

Dremel.

928SS

hacksaw, and lots of race gas

slate blue

Tony I think you jave the deck height incorrect, I always thought the deck height was 232 mm not 245.5 mm I think it actually ends up at about 1.56 to 1. Anybody else know if this is the case?

Cheers Greg

P.S check out the bits for sale suitable for a 4 valve stroker

BC

I've been looking at that for a while.

Tony

I just added it all up?

1/2 crank throw + rod length + compression height + deck clearance if any.
the graphic prettty much sums it up?

crankshaft center of rotation to top of block where head sits

928SS

ah, that friggin deck height thing!! keep plugging along greg!!! glad you didn't give up!!!

fwiw, a few notes from my build - the problem w/deck ht is that they are all different due to clean up and casting issues - and the 968 piston while pretty close, isn't quite designed for this app... so on mine w/a 968 profile piston and 5.85" rods, the slugs were .025" out of the hole, due to a .010" resurfacing to fix a "minor" bore nick (long story)... at least .015" would've had to be machined off (more or less), so rather than raise comp any more and run the risk of ruining the pistons, I just opted for a new set of pistons w/a bit more cc to drop comp. and to get a nice 0.0" deck ht. (new comp ht was: 1.395", which left .320" to the 1st ring land (decent dist for NA/NOS stuff)

the other issue is clearances - iirc, block/cyl castings at the bottom of the bores are the 1st thing a long piston skirt will hit, not the crank... ie w/a 5.85" rod, the 968's won't clear w/out trimming the skirts... the pistons I used also had a custom skirt length (shorter than the 968) in order to clear the block castings(.500" from the bottom of the pin casting cutout), otherwise it's grinder time for the skirts too... not as much fun as grinding w/skirts of the female persuasion, IMO

also, the counterweights need to be clearanced for the small rod ends, and some rods require the block(for the rod bolts) to be notched too (my carrillos w/the 7/16" bolts needed it, but olivers/pauters don't)...

so for me, going w/nicom and je's really made everything super easy and clean, cause I could just use a lightweight, custom billet/forged piston that fit w/out all the chopping and grinding.. it also allowed me to add an oiling hole in the pin, use a .927" (cheby) pin hole (easy fit w/the rod) and was able to hog out the dish as much as I wanted so I could use our lovely 91oct street crap w/out fear (although it does seem to "appreciate" a little 111oct VP if daddy is feeling flush w/cash and has a swollen foot, hehehe) (JE job number 459735)

hope this helps

as always - great pics tony!!!

slate blue

Thanks for all that info Rob, I meant to get back to you earlier just been very busy. Great info, it is really a worry when you order pistons and you are not totally sure of your deck height due to various differences, such as previous machining and who knows what Porsche original tolerances were?

I cant believe you guys get that horible octane fuel either, why is that?

Good to know that the 4 mm shorter my skirts are will probably be doing me a favour.

Is you car running now Rob, what is it like if it is? Is it what you thought it would be? Any 1/4 mile times? Have we ever had a stroker post a 1/4 mile time? Even a 0 to 60 or 100?

Cheers Greg

928SS

I do miss the days of cheap 95leaded... at 30% more than some parts of the country w/higher octane stuff, it's just the price for keeping our cities green in the land of milk and honey (or someone's pockets full of green)

probably best to measure, then just order what you need... you could just modify the JE job I noted for whatever CH you want and be done w/it... iirc, the je's only weighed in at 432g ea, and w/std pins and the carrillo rods were still 3500g less rotating mass than OEM stuff, w/bigger slugs!! pauter also makes a real nice thin profile rod at any length for $1200... could just use your slugs and order a nice custom rod and be done w/it too... lots of options

old gal is running quite well, idles nicely and behaves in traffic - yet pulls like a beast on command... of course, any form of street racing is illegal, but I've had no problem w/"passing slower traffic" - it constantly surprises all involved... the link in my sig has specs, pegleg 1/4 mi times, dyno stuff, etc... only thing I'm changing for sure is the pegleg for an LSD unit, maybe some sharktuning whenever it's handy - and tires of course that should knock a few tenths off next time around at the local drag strip...

was kinda thinking bigger devek L2 headers (1.75") and a bigger bore TB would give it some more top end for dyno-bragging but I've been watching my tach during "normal" driving and it's really never much above 5K unless I"m "in a pinch"... and having the low end grunt sure is nice around town, freeway ramps, spirited canyon runs, etc (kinda like a harley vs 2 stroke - the wide powerband just pulls from any gear)... so I'm not too sure if its worth the effort just for 20-30 top end hp, and retuning, etc... might just slap on a 100 shot of NOS and be done w/it, for those brief moments when enough isn't enough (and the dyno bragging game)

so based on that (anectodal admitedly), I"m not so sure you should worry too much about rod length (6.1" vs 5.85") for torque - at 440 rw ft lbs @ 2200rpm w/the 5.85" rods, it's got more tq than most 928 boostards and more HP than some too (w/out all the fuss ) but most importantly, it's plenty for melting the street - even w/ the slushbox - and the same specs on andersons/fans track beasts have survived season after season of high rpm thrashing... so IMO, it's a safe spec for street/track use...

on the other hand, as you know - heads (cams/valves/springs/porting), headers, and intakes really make all the diff between an anderson/devek style track screamer or a well behaved, smog legal street beast, IMHO got some real trade offs in those depts

slate blue

Hi Rob the link to your page is great, very informative. The car is great too, a very nice piece of work. I got to say I was suprised at the hp figure. I would have expected 400 rwhp. I note what you said about headers manifold etc, are they MSDS headers? Have you done any porting to the manifold? I know when we ported my manifold, we really cleaned it up quite a lot.

Also since your GT cams have been ground and you have 968 valves I would have thought the top end breathing would have been better. Is the cam timing advanced, giving such a big torque figure so early in the curve and then not much later which of course then drops your hp figure. The flappy working? Maybe it is the gearbox sucking up that power? Or maybe it is as you said, you might be in need of the shark tuner.

Anyway well done, you project is on the road and driving, that is more than many of us can say on this board.

928SS

thx! it's a constant tweaking game, IMO... my same initial thoughts on HP too... but the est. chp is a heftty 430hp... the devek L2 headers are same as msds 1 5/8"... cam timing is spot on std spec. IIRC, mark a. and others have btdt w/intake clean up and got zero hp gains on their strokers, so the intake is probably fine... I suspect it's just a funciton of the AT, throttle body, mild porting, and headers... AF is around 12.5 pretty consistantly from 3k to redline... but the killer torque is a hoot, albeit more like a tractor/twinscrew than a high reving beast... flappy is working fine- you can see it on the dyno chart... NOS is probably the easy "fix", but I'll wait till I ck the tbelt tension/timing, do the LSD, and shark tune before I swap out any more stuff... spent enough time in shops - kinda fun just to go around melting tires/scaring the crap out of friends and family....

fwiw, devek will swap out the smaller headers for me if I want and the bigger TB isn't a big deal either, so it's just a matter of deciding if I want to give up any low end torque for top end HP or just add NOS and have it all

in any event, the 5.85" rods sure arent' an issue limiting masssive torque

slate blue

Hi Rob I agree, the throttle body has got to be an issue, they use a 90 mm throttle on the LS2 and LS7 engines and that would seem to be a lot better than 78 mm which I believe the S4 one is. Did you get any headflow figures from your port job? I ask that because you say mild porting, I take it that you only opened it up for the bigger valves without then hogging out the bowls?

I know you also mentioned the LSD, is traction an issue when you posted your 1/4 mile time? What would it take for you to get down to the high 12s? That is where I think your beast deserves to be with all the work you have put into it. That puts you in some elite company and your reliable to boot.

Thanks for all the info and encouragement, the rod to stroke ratio is not what is currently stopping me, it is funds now, but I need all the encouragement I can get, especially as I'm now off tp my garage to change the front crank seal on my 85.

Cheers Greg