930 3.3 crank options

jpnovak · 05:51 AM

It seems that a 3.3 in a 78 930 has lost a rod bearing. Still working on the investigation to understand what happened. Let's assume the crank is done.

This is a strong running street car that may see an occasional track day. Currently running CIS with most of the usual bolt on mods until you max out the fuel system. The new version will probably be EFI with cams and a compression bump. This is all undecided.

What are the options to build the bottom end? It will be a given to cross drill the crank and perform some oil mods for longevity.

Find a replacement 74.4mm stroke crank?

Stroke it with a 76.4mm (964) crank?

Grind the rod journals to smaller size and run custom rods? Would this get a custom stroke at the same time using an offset grind?

turbobrat930 · 04-28-2014, 07:01 PM

I use custom Pauter rods, with JE pistons... My crank is offset ground and uses a 2" chevy Nascar Clevlette (sp?) bearings and the crank has been stroked to 78.8mm.. Armundo at CCR (Custon Crankshaft Repair) in CA. JB Racing did the work... Crank also has the center oil mod done.... (I spun #2 and #5 rod bearings).. Wastegate line came off, too much boost, and stretched the stock rod bolts... it was all over but the $$ signs...

jpnovak · 04-28-2014, 07:28 PM

Did you gain torque with that much stroke increase? I assume that you have good rod length ratios with the offset grind.

How does it drive? This will probably not be an unlimited budget build. Was the machine work and matching custom rod worth it from a cost perspective?

AlfonsoR · 04-28-2014, 07:36 PM

Other options are GT3 crank and of course Henry's SuperTec crank. What's our budget here?

sjf911 · sjf911's Garage

Isn't the '78 crank already a 74 mm stroke?

jpnovak · 04-29-2014, 05:48 AM

Yes. I was thinking about 964 cranks and typed the wrong numbers. Edited.

BURN-BROS · 04-29-2014, 06:18 AM

Hi Jamie,

the 3.2 crank is the stoutest crank of the bunch.

depending on the severity of damage to the journal, you can regrind it to 53mm (SC) and run stock 3.0 liter rods w/ ARP rod bolts and a custom piston.

Best bang for the buck and you get 10mm rod bolts with that setup.

jpnovak · 04-29-2014, 06:58 AM

Thanks Aaron.

What is a good source for the crank work? I like the stronger rod bolts for sure.

Do you offset the piston pin position by half the difference in rod length? Is that a JE order? We were thinking of targeting 8:1 compression on the updated build just to improve the around town performance at low rpm.

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by BURN-BROS

Hi Jamie,

the 3.2 crank is the stoutest crank of the bunch.

depending on the severity of damage to the journal, you can regrind it to 53mm (SC) and run stock 3.0 liter rods w/ ARP rod bolts and a custom piston.

Best bang for the buck and you get 10mm rod bolts with that setup.

This is a great solution but be sure to clearance the oil pump or run a 3.6 pump. With SC rods there is an interference with a stock pre 3.6 pump.

You can even use your stock pistons with only a small amount of machine work.
Trim .030" off the the perimeter deck of the piston and install a 23 mm wrist pin bushing in the rod. This modification gives you better rod to stroke ration and slightly higher compression.

jpnovak · 04-29-2014, 07:43 AM

Thank you Henry. That is a great idea. I would guess the trim on the pistons is to clearance the top edge near a squish zone and to maintain a decent deckheight at the head cylinder interface. Almost like a Chamfer on the outside edge of the heads when I did my 3.2SS and increased from 95 to 98mm - but on the piston crown.

The oil pump clearance should not be a big deal. I assume its the top edge near#2 and #5 that need the trim. Is there enough meat there? is a 1-1.5mm gap sufficient?

I like the idea of modifying the stock pistons from a cost perspective. We can not make that decision until the engine is torn down and we inspect how much shrapnel circulated and what was destroyed.

Is there enough thickness in a stock 3.3 piston to put valve pockets? We tried a GT2-EVO cam on a different 3.3 build and it would not clear the stock pistons. This engine currently has SC cam grind but more cam is almost always a good thing. Besides, we might as well take advantage of the extended rpm range given the larger rod bolts and better rod/stroke ratio. Combined with some oil mods and we are good to go.