Building a 3.6 from a 3.7 CMW mess.
 

We are doing our CMW 3.7 Stroker engine over, since the crank snapped two seasons ago.
We have a 2010 997 GT3 street crank, part number 996.102.021.94
inbound, and need the specs on the stock 997 GT3 Street Rods, Part number 996.103.012.89.

We need Big end bearing bore, and Big end width for custom 131.1mm R+R Rods, LN Engineering order, which is pending these numbers.

Our engine will run the 76.4 crank, 131.1 rods and 100mm pistons, just over 3.6 I believe, minus the torque of the stroker, but more race reliable..

The original build was 77.5 stroke (offset grind on a stock 3.0 shaft) 130mm Cunningham Rods and 100mm Custom JE 12.5/1 pistons. Engine was orginally built by PAul Schwarz at Cyntex in West Chester PA, and ran very strong and hard until we had out mishap..

Any specs that can be provided, or a place to search myself, would be great. We have found very little on the GT3 specs via the WW Web..

TIA

Andrew

Hi Andrew,

GT3 journals are the same as SC

Journal dia in 53mm

Journal width is 22mm

Ah,, SC rods WILL run on a GT3 shaft.. Thanks

Your welcome Andrew.

GT3 Crank is in house. Amazing that a new 997 GT3 Cranks is less expensive than a used 993 crank.. Even with the added expense of a new pulley, this is a no-brainer... Now, Carrillos are next, estimate 498 grams, so we will spin into the 9's again.. $2.2K not too shabby.. Need to see what R+R weights are before we order anything..

Thanks for all the info and help..

I will document ths build, and post details..

You will have to shorten the snout by approx 4mm and redrill the pin for the pully.

IMHO, the recommended RPM for a 2 valve 3.6 is still 7500 regardless of crank choice. The piston weight and rods are too heavy to safely spin above this without running in to harmful harmonics. Beyond this rpm and you are on significantly diminished time regardless of your rod choice. There is not enough room to make a stronger crank to deal with the high piston weights.

Weights I have recorded for a 130mm Carrillo are around 540-560 grams. A Ti 130mm GT3 rod is about 480 grams

GT3's have very light pistons(less than a 2 liter piston), along with a light rod and pin give it a higher rpm limit. The decreased weight keeps Harmonics and rod bearing loads within reasonable limits.

Aaron,
Understood.. We tear down after 45 hours on each of our race engines, the 2.9 revs to 7400 on the Motec. We had the 3.7 limited to 7600 during race days, but dyno time we pulled to 9K with no issues. Granted, I agree that this eats away at engine time between repair numbers.

We are leaning toward either Carrillo or Cunningham Ti rods @ 490 grams..

the plan is to have the Motec set to a 7800 limit, keeping the inners happy..

Thanks for all the feedback, it helps keep one honest..

Andrew

Sorry for the misunderstanding :) When 9K and 3.6 are in the same sentence the following sentence usually involves writing big checks.

Nevertheless! Check the snout of your crank for the dimensional differences if you did not know already.

Hope your weekend is going well!

Working with Clewett on their GT3 friendly Pulley.. I am finishing up the Rod specifics before Carrillo gets the spec sheet for an accurate quote.

I should have been more detailed with the 9k Revs statement.. I like the grunt,but not the rebuilds, so 7600-7700 it shall remain. Unless I land a big paying Sponsor, than 3.8 will get transplanted...

Ahh.. nice to dream..

sounds like a great motor, can I ask where you got your GT3 crank?

I am building a 964 case, 3.8 motor and looking to use a GT3 crank but was just told they are currently hard to find?

thanks
C

Dealer item.. If you are looking for a Motorsports crank, that is gonna be hard.. Mine is a GT3 Street car crank. There are differences, motorsports does a piece inspection on every crank, all are weighed to be the same,and each gets mag'd and xrayed, from what I was told by a Grand Am Crew chief..
Street cranks are lot inspected like every 500 or so... Still a great deal, 1600ish from the dealer, compair that to a 964 crank, used.,.

our local dealer said he could not find one in the country? I read that PP may have them so I will try there. Thanks

C

there were 3 incountry, US, as of last week..

Here is the 2010 997 GT3 Crank we are using..
http://forums.pelicanparts.com/uploa...1294948451.jpg

Please help, I tried to buy one again yesterday and I was told by 3 different dealerships that there are zero in country and I need to get from Germany.
TIA

I noticed that the crank does not have the current trend in oiling such as cross-drill and groved mains.

regards

Well, it is the current Street GT3 shaft.. Sure you didn't see a Porshce Motorsports shaft?

I have torn down several GT3 RSR and Brumos DP motors that PMNA supplied. I have never seen a cross drilled crank or boat tail work on any of these engines. They never do it and these motors run 9600 rpm and sometimes higher.

http://forums.pelicanparts.com/uploa...1305531473.jpg

That has been my experience as well. It seems like the oiling issues are self imposed

regards

GT3s have almost 50% reduction in rod a piston weights over it's aircooled bretheren. Spin a 3.0 liter to 8500 rpm and take a look at what the rod bearing will look like at 25 hrs and report back...
Apples and oranges fellas.

I've no contributions to this thread but >9000 RPM is insane! Would love to hear that.

I contacted Sunset and Suncoast -- there are no GT3 cranks to be had anywhere -- including Germany.

I was told I could order -- but there was no expected date when they would be available.

M

Tippy,
Go to an ALMS (IMSA) race and listen to the flying Lizards motor. You will see the light and hear the sound.

As to piston / rod weight

GT3 RSR piston 428g
JE 11.5:1 piston 464
GT3 RSR 137mm rod and bolts 369g
stock 3.0L rod 4xx?

It's not as different as you might think. The water boxes and other castings reinforce the motor and case a lot on the GT3 and basically, everything stays much more geometrically and thermally stable. That's and the superior valve train design are the big difference.

Nice try,
You are not counting pins/ rings on the JE piston. The GEN1 piston with pin and rings is 490 grams and that is not the lighter later pistons
A 3.6 piston with pin is dancing around 625-630 grams.
A 3 liter rod is 680 grams without bearings. a 3.6 rod is similar in weight.

The GT3 crank will produce significant harmonics in an aircooled engine above 7000 rpms whereas the GT3 reciprocating mass does not exhibit that particuar problem. Another reason why the 3.8RSR engines did not go higher than 7500 without significant problems.

2x's the weight is a bit of an overstatement but 1.5 times is very close to true.

G Force
 

I did not re-run the calculations for the longer stroke of the 3.6 but just for reference on a 70.4 stroke crank a 90 gram reduction in weight of a piston / pin / rod assembly assembly at 8000 is a 631 pound reduction in load at the crank pin. The longer stroke would make this number larger, only 90 grams....

Stay tuned,

PFM

OK, From what I can gather, it seems my new R+R Racing Rods, 131mm will be using a standard SC bearing, now, I am hearing two different schools of thought on this. Our Crank is a 2010 GT3 Street crank, and has the SAME journals as the SC, and our rods are custom and also are EXACT to the SC on the BE dia. So, one would think that the SC bearings are a go. I know my friend and builder (Paul Schwarz from Cyntex and Bodymotion Fame) wants GT3 rod clearaces, so, first, what are those specs, and second am I correct on the SC bearing thought?

TIA

Andrew

Aaron,
We are running Custom JE pistons (From CMW Stroke) 100mm, with a new set of R+R rods (131mm) on the gt3 crank, in a Euro SC case (all mods have been done). We have a new Clewett GT3 custom pulley as well. Now, our old CMW based engine ran into the high 8's all day long, and we want to get this new setup into low 8's on occasion, but will live around 7600 as a redline. We are sending the entire spinning assembly to our Machinist to be balanced. From what you have said earlier in this thread, we are asking for trouble with this GT3 crank?

Thanks for the explanation..

Andrew

Hi Andrew,

My statements above are to clarify that the GT-3 crank is no better than a 964 crank in regards to strength. The engine will not immediately blow above 7500 rpm or even 8000 rpm, they just do not like running up there for extended periods of time.

If you plan to use a 7600 rpm redline you will be fine. Anything above that you should monitor very carefully.

The 100 JE pistons use 100mm forging dies so the weight is as reasonable as they can be. I would definitely have them remove material from under the crown or better yet run the Forged Side Relief piston (FSR) to get the weight down and a set of Ti wristpins wouldnt hurt if you have the budget. You may be able to shave 50 grams from the standard JE package. Short spurts up to 8 may be tollerable but I could not recommend it.

Aaron,

My mentor, and friend, Paul Schwarz, agrees, and I have the 7600 pill ready once the engine is assembled. We are running an old CMW 2.9 now for the season, and will post the build of the 3.6 once we start.

I look forward to seeing it.

Enjoy the rest of your season!

An SC case will accept a GT3 crank? I know the early GT3s used the 930 case but I thought the newer ones were different.

Interesting. I just installed GT3 rod bearings onto my SC crank for my SC motor. Why do you want the SC bearings?

Yes, the crank should drop in. The new GT3 case reinforced the weakspot on the #7 main web and removed the provisons for oil return tubes and other air cooled related castings. The cranks have a few changes to them but retain most of the original aircooled dimensions that are important to us.