DOHC 4v - Stock forged Cobra crank or aftermarket?

3437 Views 11 Replies 6 Participants Last post by Collinsautomotive, Jul 20, 2009

cogged cobra

Discussion Starter · Jul 17, 2009

I'm just trying to get a better idea of when its necessary to upgrade from the stock Cobra crank?

I run a cog drive Procharger now, its x2 keyed, with an ATI dampner, and the everything was balanced precisely. But I'm starting to see more and more guys snapping off the snouts of the Cobra cranks, mostly guys running the big twin screw blowers - Whipple and KB.

Starting to think that the cog really beats up the motor and I am really putting the entire motor on the line everytime I'm at the track with the car. If the crank broke and the chains came off and I lost timing, the entire motor could end up junk.

I'm contemplating a move to a big, single turbo 88mm+, and I just want to figure out if a move to the new Boss 5.0 block is necessary (Teksid now), and if I need to upgrade to a stronger forged crank, or possibly a billet piece.

I would probably turn out the #'s I make now with 5 less psi thru an 88, and it will be much easier on parts.

I know Joey Bridge goes high 7's with a stock Cobra crank and a Teksid block, so it seems like there may be no need to re-invent the wheel here - just have it assembled by a pro, and go race.

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Collinsautomotive

· #4 · Jul 18, 2009

the compression won't hurt the spool up won't help much either. It'll be a benefit but you'll get more from cam timming.

The rods should be ok upto 8000 rpm or so. The piston is really,really light.

Stock crank. Good shot peening, turn the journals down 10-20 thousandths to put a better fillet in it get it stress relieved and then get it nitrided and micro polished.

cogged cobra said:
Good info, thanks. The 47-88 definitely seems to be the big single of choice and would provide enough flow to get me where I'd like to be.

Shortblock is 9.2:1 right now and I'm hoping it'll be in good health at the end of this year, so I may just use the same motor. Although, I'm sure some flat-tops and 10.0:1 would really help spool the 47-88.

Can you share what compression you guys run?

Also, would my Oliver billets be ok in a turbo setup? Is an aluminum rod better suited to lower the rotational weight?

Any special crank prep at that power level?

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Collinsautomotive

· #6 · Jul 18, 2009

whoever gave you this info was entirely full of shit. I would not build a high powered stroker 4.6. Not with a 3.750 arm. If your willing to offest grind the factory crank pins to a 3.630 or so stroke you can build a good piston but thats still really marginal. Stick with a stock stroke. as for getting the turbo up. Cam timming and intake manifolding as well as a proper turbine wheel,housing and wastegate arrangement can make that work.

298 snake said:
u would have to go with a bigger cid to spool that turbo i was goin to put a gt 47-88 on my 298 cid cobra manley i beam rods mahle extreme duty forged pistons 8:8 to 1 and i was told i wouldnt make shit at low rpm but once u get up to about 7000 to 8000 rpm the motor will finally come alive just my 2 cents

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Collinsautomotive

· #12 · Jul 20, 2009

stock pumps and stock style pumps get maringal at 8000+ rpm. I would be worried about thrust bearing issues trying to break the pump gears. when the thrust bearing fials to carry the load it galls up the pump gears and front cover plate.

So good converter pressure control is a must. I would say to keep the converter pressure down under 100psi and you should be ok.

cogged cobra said:
Its easier to package a large single over twins.

I think the 47-88 is the perfect single for drag 4.6. Getting the car to leave hard is all in the turbo setup, cam selection, tune, and converter. I dont really care if the car leaves a bit soft as that will help with traction when the track is marginal. if it can 60' in the 1.3's and trap close to 170, I'll be happy, lol.

Gonna stick with the stock stroke for sure to help keep piston speed/oiling under control when the RPM's get up there.

I currently run the billet Melling pump. Are you guys having success with the conventional pumps at this level, or switching to external belt driven pumps?

Any recommendations on places that can build a set of some nice stainless forward facing headers?

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