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Join Date: Sep 2004
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GT3 crank on 3.2/3.4

Folks

Wondering if anyone is running a gt3 crank on their 3.2/3.4 and can detail what changes were made to accommodate the crank? I'm in touch with William Knight (great guy) but keen to hear from anyone else whose done this. Guessing most people won't change the crank isolation so difficult to gauge benefits of the crank change on its own?

Cheers peeps
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Shirish
1987 Carrera, Granite Green
Old 03-24-2016, 03:59 PM
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Talk to Henry from Supertec. I bet he has done a few, and he might have a super crank to sell you too.
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Old 03-24-2016, 06:54 PM
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Happens all the time. There are a few different flavors of GT3 cranks, and dependent on stroke, there are different challenges. Especially with the latest GT3 cranks and their very long strokes, there are some clearance issues around the oil pump. Generally, you have to run custom rods and pistons, but the increased displacement is a pretty nice benefit.
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Old 03-25-2016, 08:01 AM
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Thanks chaps - yes I've just read about the stroke differences between early and late cranks. The 2003 on cranks are reasonably priced, especially if I can sell on the 3.2 crank and rods.
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Old 03-25-2016, 09:55 AM
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Just bear in mind that an increase in stroke will necessitate a decrease in compression height of the piston and or rod length. All easy to calculate and the results in stroke are really good compared to just making the bore larger.

regards
Old 03-25-2016, 05:15 PM
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Hello - I thought the GT3 crank only fit in the 3.6 engine and not the 3.2. I could be wrong on this.

Are there super cranks that feature increased stroke that would fin in the 3.2 case?

Regards,
Steve
Old 03-28-2016, 01:53 PM
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Yes the GT3 will fit in a 3.2 and yes you could have a 3.2 stroked.
Old 03-28-2016, 03:21 PM
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Quote:
Originally Posted by racing97 View Post
Just bear in mind that an increase in stroke will necessitate a decrease in compression height of the piston and or rod length. All easy to calculate and the results in stroke are really good compared to just making the bore larger.

regards
Thanks - was speaking to William Knight last night, he concurred that stroke is king, and if he can source at the price he's been quoted i may consider a 4.0 crank. May require a new oil pump, definitely new rods, but with my 98mm mahles should make a healthy torque laden 3.7.
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Old 04-21-2016, 09:56 PM
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I'm not a professional engine builder but I've been searching details to build this configuration in an SC case using a GT3 crank (76.4mm stroke) and 98.0mm bore. While a slightly different starting point from a 3.2 the configuration to get the end result is fairly the same.

As others pointed out the longer stroke has to be accounted for in the build to ensure proper piston deck height, clearances, and parts alignment during assembly. I have not found where anyone has documented building this engine or which dimensions to alter for the best overall results.

Following what Porsche did when the 3.0L was replaced by the 3.2L they accounted for 4.0mm increase in stroke (+2mm each direction up/down) by moving the piston wrist pin higher by 1.2mm and shortening the connecting rod by 0.8mm. One possible downside was it also lowered the Rod Length to Stroke ratio (RLSR) from 1.82 (SC) to 1.71 (Carrera).

Taking a similiar theoretical approach to fit a GT3 crank in a 3.0/3.2 case this is (in simplified form) what I'm considering...

Connecting Rods:
Using a rod with SC dimensions could provide several benefits. 1) the big end is properly sized for the GT3 journals 2) the length 127.8mm (SC) vs 127.0mm (Carrera) helps maintain RLSR ~1.67 3) the smaller piston pin diameter 22.0mm (SC) vs 23.0mm (Carrera) could save an extra .5mm to move piston pin higher.

Custom Pistons:
Given the 127.8mm connecting rod length, the piston pin location would need to be moved 3mm higher in comparison to stock (SC) pin location. I spoke to someone at JE and they believed it was possible with a custom piston order but I couldnt say it was definitive since I haven't requested an order.

Perhaps one of the professional builders on this site has actually built this configuration and can comment if this is the best approach to maintain required assy dimensions or not...

Last edited by White911SC; 05-07-2016 at 11:16 AM..
Old 05-07-2016, 11:11 AM
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Yes I built one with a 3.2 case 76.4 X 98 with a 22 pin and a 130 ti rod from a Cup Car, CP piston with
1.113" piston compression height and a 106 L/C cam. The Cam is important to keep the top center timing out of the pockets.

Here is how you do it
1. find the deck of your case
2. length of rod
3. 1/2 stroke
4. length of barrel
5. figure 1 mm of deck clearance so subtract that
6. the remaining dimension is going to be piston compression height which is center line of piston pin to piston periphery.
7. give the dimension to CP contact Jerry Roche he will reassure youand he knows what to do.
Regards
Old 05-07-2016, 03:41 PM
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Old 05-13-2016, 02:14 PM
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Old 05-13-2016, 07:35 PM
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Just wanted to see what kind of safety factor with ARP rod bolts. Rod bolt stress for above come's out to about 126 Kpsi assuming the 9800 lbs is per bolt. ARP 2000 are supposed to be good for 220 Kpsi.
Old 05-14-2016, 05:48 AM
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This was a Ti rod with ARP bolt

Regards
Old 05-15-2016, 04:42 PM
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Quote:
Originally Posted by racing97 View Post
Yes I built one with a 3.2 case 76.4 X 98 with a 22 pin and a 130 ti rod from a Cup Car, CP piston with
1.113" piston compression height and a 106 L/C cam. The Cam is important to keep the top center timing out of the pockets.

Here is how you do it
1. find the deck of your case
2. length of rod
3. 1/2 stroke
4. length of barrel
5. figure 1 mm of deck clearance so subtract that
6. the remaining dimension is going to be piston compression height which is center line of piston pin to piston periphery.
7. give the dimension to CP contact Jerry Roche he will reassure youand he knows what to do.
Regards
Given what you've shown in calculations I've assumed you were able to assemble this build without need of custom spacers for the cylinders and chain boxes to accomodate the stroke...

I really like that you used a 130mm rod for this build. For this rod length (etc...) your build required a piston pin compression height of 1.113" (28.27mm). Did this Compression height require the wrist pin bore to be so high as it encroaches into the oil ring groove on the piston?

If so, do you just use oil ring support rails and move on, and/or any concerns using the support rails on our engines?
Old 05-19-2016, 04:13 AM
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I don't believe it required any spacers and I know it did not require oil ring supports.
It all hinged on the length of the particular barrels I used. The wild card is the cam I shoved a (for a Porsche) farily racey cam and it required a particular Lobe Center to clear the Valve Pockets so I knew what I wanted to run as far as a cam was concerend but waited on the Pistons and missed by a few degrees so the Intake pocket need to come down by 1mm and that put the material thickness of the Piston Dome at the bare min.

regards
Old 05-19-2016, 06:48 AM
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Quote:
Originally Posted by racing97 View Post
The wild card is the cam I shoved a (for a Porsche) farily racey cam and it required a particular Lobe Center to clear the Valve Pockets so I knew what I wanted to run as far as a cam was concerend but waited on the Pistons and missed by a few degrees so the Intake pocket need to come down by 1mm and that put the material thickness of the Piston Dome at the bare min.

regards
Great input on your build experience with this crank...

You got me wondering about the attention to the cam specs...I would think the same timing/clearance issues would occur if used a similarly aggressive cam profile with a stock 3.2 Carrera crank, assuming same heads, deck height, etc.

Or....you really did mean the valve timing/clearance issues become even more critical with the longer stroke of the GT3 crank?
Old 05-20-2016, 07:47 AM
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This task doesn't involve off the self Porsche Pistons at least not with the 130mm rod and
the tight compression height and the type of Forging used don't leave alot of material under the Piston valve pockets so you can't just bury the valves by fly cutting the pockets for clearance, you need to kind of know what kind of valve drop you have. This is determined by the Top timing (how far the valves are off the seat at TDC overlap at your proper timing) now you can tell the Piston guys what this is but they sometimes don't pay attention that will possibley force you to change the lift of one of the cam lobes at TDC hence the widening of the L/C angle. Wider has less lift Narrower has more. These engines aren't deadly responsive to L/C so you aren't sacraficing any performance.
The only reason I mentioned it is without the change in length you can find a suitable Pistons that allready are clearnced this will be a new application and I did not want to leave you without covering this possiblity. I am sure if you mention it to the Piston guys they will at least be aware and leave you some recourse in case you have some fitment issues.
Hope I didn't make to confusing

Regards
Old 05-20-2016, 08:33 AM
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racing97,

Ok, makes sense now....designing custom pistons to accommodate both the longer 130mm rods and longer stroke in combination doesn't leave much extra material for clearancing deep valve pockets. Hence careful planning in cam specs. Thanks!
Old 05-20-2016, 09:05 AM
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Correct
Old 05-20-2016, 09:47 AM
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