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GT2EvoGuy 11-26-2009 09:09 AM

8000rpm bottom end rebuild - what do I need to do?
I want some qualified advice on what I will need to build my 930 bottom end to withstand 8000rpm bursts. Im thinking about having my local builder dig into the engine this winter but before I plunk down any money I really want to get advice from some fellow forum members who are successfully spinning their motors this high.

The engine right now is a 3.4L stroker 930 engine laying down 585whp at 1.2 bar with Motec EFI. The car is still street driven currently, but I ultimately intend to retire it from street service and run it in PCA racing. Id like to build the motor to a higher spec in 2 stages. First stage would be to bulletproof the bottom end and have the heads reworked a bit while retaining my 3.6 plastic intake setup and streetable GT2 Evo cams. The second stage would be to go to ITBs with much larger ports in the head and custom racing cams. In this second stage Id be looking to rev the motor to 8k.

Here are the specs of the bottom end currently:
3.3L 930 case stroked to 3.4L via a 964 crank
Stock rods shot peened and prepped with ARP bolts
964 oil pump
Stock 3.3 P&Cs
Shuffle pinned

This setup with my current heads, cams, intake have been fine for a 7000 redline but obviously above 7000 things have to be changed considerably.

My builder (Auto Associates) is recommending:

*Have the 964 crank race prepped with oiling and other mods (does anyone know what must be done specifically to live at high revs?)
*Motorsport bearings - maybe the ones sold by Smart Racing?
*Carillo rods (is there a specific rod length I should go with?)
*Custom 98mm CP pistons 7.5:1 - the motor will be boosted up to 1.4bar
*Have the OEM cylinders bored and re-Nicked to accept the larger pistons
*Upgrade from 964 to GT3R oil pump

Are there any other things that should be considered re the bottom end of the motor? Looking to get this done right the second time around and be done with it once and for all. Wasted a lot of money by having to go into it a second time :(

onboost 11-26-2009 09:42 AM

Hmmmm, not sure how much more you want or really need. Sounds like a nice motor already though you may find that you'll enjoy it better with a cam change. The GT2 EVO cams tend to make power up top similar to the old Gruppe-B grind.

You have a quality/qualified builder in Auto Associates as I've dealt with them over the years and they tend to be both straight forward and knowledgeable. Their work is very tidy and clean. Some of the best prepped cars in the Northeast come out of that shop.

My only thought is you may want a race prepped GT3 crank and Titanium rod$.. but hey, what the hell do I know?

Good luck and keep us posted!


GT2EvoGuy 11-26-2009 10:09 AM

Yes they are the best shop I have ever dealt with in terms of their knowledge and the quality of the work they do. Im just trying to understand for myself what needs to be done and why.

They did bring up the possibility of going to a GT3R crank - same stroke as the 964 but Im guessing its the best choice because its a factory motorsport crank to begin with. They said the 964 crank could be brought up to the task but will a GT3R crank still be the best choice? Im assuming the crank mods would run me $1k+ and a good used 964 crank is worth some money so maybe net net its just better to source a GT3R crank? How much do those go for new/used?

TI rods would be nice but are they strictly necessary?

JFairman 11-26-2009 10:10 AM

Sounds like you know what you're doing.
If you don't already have or use fully finned 3.3 cylinders with all that horsepower than I think you'll need them for the motor to last a long time.

Post over on the engine rebuilding forum if you havn't already, lots of experienced and talented folks there.

9dreizig 11-26-2009 10:56 AM

wow I was just looking at your garage.. all that under a 1977 ?? what does that beast weigh?? Just curious but what is the weak link at 8000 rpm and what's the HP going to be ?

GT2EvoGuy 11-26-2009 11:26 AM

Hi 9Dreizig,

Yes, building the car off an early chassis for less weight was my primary goal. The car scales at 2200lbs now with back half caged, doorbars and half tank of gas. For full race use I would extend the cage forward and replace the steel doors with carbon to get it down to maybe 2100. If I had to do it over again I would have started with a late 60s 912 chassis like my shop did with their 935 replica.

The motor right now is making 585whp and my goal with the higher spec build would be 700whp+. To get there with a 3.4L motor its going to need to be able to make power above 7000.

BURN-BROS 11-26-2009 11:28 AM

What I would build per your requirements:

R&R billet H-beam rods from LN engineering with custom aged bolts(SC/dimensions)
The new pistons from JE that are box-in-box construction(much lighter)(alter comp height for the longer rod)
Sell the 964 crank and buy a GT3 crank and mod #4 for oiling.
Turbo or GT3 oil pump

Shuffle pin your case! I have seen a great deal of cracks in the case on high rpm high output turbos.

964 cranks are poo...

A better, more reliable build would be:

R&R billet H-beam rods from LN engineering with custom aged bolts(SC/dimensions)
SC crank modded on #4
The new pistons from JE that are box-in-box construction(much lighter)(22mm wristpin)
GT-3 oil pump
Shuffle pinned case

Less rod ratio issues for the rpm
better ring land support @ the piston
Less overall money as SC cranks are much cheaper than the 3.6 stuff.

98mm would be max with 3.2 barrels, 100 or 102 with a set of Nickies from LN would be possible.

GT2EvoGuy 11-26-2009 11:49 AM

What are the differences between the GT3 and 964 crank? What center to center rod length would work best with a GT3 crank for 8000rpm?

BURN-BROS 11-26-2009 12:24 PM

GT3 cranks are fully counterweighted,are 76.4 stroke, have an SC rod journal dia. and width.
Longest rod available is 130mm. They are made by Pankl and are Titanium. This would be about the best you could do.

Running a 130mm rod would require the piston to only have 1 compression ring and a small oil control ring(if a ring that thin is available in that dia).

Crankcase vacuum is necessary with this set-up so your options are limited running that rod with that stroke. So you would have to run a 127mm rod or 127.8 max.

GT2EvoGuy 11-26-2009 12:35 PM

Sounds like I should go with the gt3 crank and a 127mm rod. I will have to discuss the TI vs steel rod issue with my builder. Besides the cost I wonder how that affects the racing hours between teardowns.

BURN-BROS 11-26-2009 01:08 PM


Originally Posted by GT2EvoGuy (Post 5033674)
Besides the cost I wonder how that affects the racing hours between teardowns.

Bearing load at the rod will be much higher without the lighter pistons/pin/rod package

Pankl has made shorter rods for the GT3, but they are Extremely expensive IF they are even available.

Why run so much stroke/rpm on a turbo? If possible I would opt for a bigger bore and shorter stroke to get some longevity and lower some of the cost.

On 3.8 turbos that have seen high rpms and high boost experience cracks at the main webs. They form at the thin sides of the case thru bolt holes and run from the centerline of the case all the way up to the head stud bosses. #7 main on the #123 side has experienced cracks as well. Longevity is definitely not a term I would use with this particular build.

I'm not trying to scare you. I am trying to hint that there are better alternatives. Besides, if you are short on displacement, why not crank up the boost a bit?

GT2EvoGuy 11-27-2009 08:11 AM

Thanks Aaron for your insightful advice.

I hear what you are saying about going with a shorter stroke and bigger bore but I kind of like the idea of having that longer stroke to help spool the turbo and get me off the corners a bit faster. My engine runs a large single GT4094r.

Maybe my thoughts about where the engine would peak on HP are way off. If I go with ITBs, huge ports and wilder cams the horsepower would peak probably well short of 8000 anyway so really no need to rev so high.

I was just doing some simple math on the weight reduction needed to safely rev higher. Right now my redline is set to 6800 with the stock prepped rods (ARP bolts) and OEM pistons. From the Powerhaus website they say the stock piston assembly on a 3.3 Turbo weighs 622 grams (including rings and wristpins). If I set my redline target short of 8000 - say 7700, the goal becomes much more doable. If I take 7700/6800 and then square the resut you get 1.282. If I then divide the stock piston assembly weight by 1.282 that comes out to 485 grams.

If I were to go with those new JE pistons you suggested (those FSR pistons look very trick) and then factor in some savings on forged rods a 7700rpm redline is probably doable right?

BURN-BROS 11-27-2009 09:36 AM

On a 3.8 993 RSR I set the redline @ 7500 with the aftermarket rods and custom aged bolts for reliability(it actually saw a few overrevs of over 8000 rpm without failure). I personally would choose a cam that would peak at approx 7300. With a lighter reciprocating package I could increase that rpm limit. That engine ran a GT3 crank and SC dimensions for the R&R A-beam rods.

The above build will give you pretty good longevity.

smokintr6 11-27-2009 10:44 AM

Out of curiosity, how much effect on required overall ignition advance (if any) do you guys see with relation to longer rods, and lower piston piston acceleration rates? Any more or less tendancy for detonation?

BURN-BROS 11-27-2009 11:50 AM


Originally Posted by smokintr6 (Post 5034599)
Out of curiosity, how much effect on required overall ignition advance (if any) do you guys see with relation to longer rods, and lower piston piston acceleration rates? Any more or less tendancy for detonation?

The theory is that longer rods increase detonation. In reality, it has not been an issue. I have not looked into timing changes corresponding with rod length, but my guess is it would be very small.

beepbeep 11-27-2009 01:43 PM

Why such stratospheric revs? It all gets very expensive above 7500 RPM. If you need more power, you could try following combination:

6.5:1 C/R
Light JE pistons
H-beam rods
Cams that peak ~7000 RPM
GT3 crank/pump
Bigger turbo, more boost
Headwork,optimal headers

Yes, you would loose some off-boost toque but as long as you can keep it on boil, it will be fast.

Rod forces go up with square of RPM, so those last 500 RPM's demand very tricky parts. If power is needed, I would rather compensate with more boost and lowered C/R (to keep effective compression within reasonable levels).

It's usually TDC piston turn that loads the rods most, not the boost.

911st 11-28-2009 10:16 AM

Great, great info!

I would think that a turbo and cam that supports HP to 8K would not be as friendly when it comes to the lower rpm drive-ability.

Further, if the gears are not also then matched to the new power band the gain might be less than the bump in HP might suggest.

Such gears then might then be uncomfortably close for street driving with shift points every couple hundered rpm and / or require a very steep first gear to work.

Just a thought.

911st 11-28-2009 10:28 AM


Originally Posted by beepbeep (Post 5034822)
It's usually TDC piston turn that loads the rods most, not the boost.

I wonder why this is?

My best guess would have to be that the rod design is typically at it's weakest point at that angle with the bolts seeing its highest stress of 100% of the centrifical fource at TDC?


GT2EvoGuy 11-28-2009 12:38 PM

I've thought about the low compression ratio high boost approach but I don't think that will work so well in conjunction with a big single turbo. It would be a very tricky car to drive even on the track I'd think. Aarons advice has me rethinking the approach I will take. A little less revs and a cam that peaks at say 7300. Even for a race only engine that seems like the way to go

Lukesportsman 11-30-2009 07:36 PM

I think you'll likely be happier with Aarons ideas also. I'd really be concerned with the rpm spread and peak rpms without consideration for your tranny ratios. If your peak power goes up in rpm there is always some give at the bottom of your power curve also. You will likely find areas of the track where your waiting on boost later than before. I like the bump in compression idea, but the larger cam may negate this. Just keep in mind that the stock gear spread is for a rather mild broad torque curve. Yours is not mild and likely a narrower band of power than OEM. The farther from this, the more disharmony that will exist.

Just another thought:
Extreme heads
Custom Cam
L&N cylinders and JE
What is your exhaust situation?

I'm thinking about increasing efficiency both VE and heat dissapation. This would result in better compression both static and dynamic. Better surface area and thermal stability. More torque both off boost and throttle tip in. Power under curve is going to be easier to drive on RC....peak is for top end pulls. At 2200lbs, your still 300lbs over the tube chassis cars and are you willing to spend another 5+grand on tranny and give up all streatbility?

Saying this, I've raced for years but your ahead of me on actually putting down power with my turbo build up. Just my .02 for thought and devil's advocate. Not a disagreement to your plan, but an alternative thought pattern. 2.1 turbos ran well on a track!

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