Engine Development - Crankshaft

V12-Engine
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I’ve been head down bum up for a while now and haven’t updated the list with progress.

Inlet flow development stalled when I needed to start simulating the valve timing and combustion event. I needed to be confident in my piston design.

However, I have been trying to see if I can use the Chev LS7 titanium con rods in my engine.

These are very nice rods, very well designed and are available cheap. For some reason, the fad in the US is to take a brand new motor, rip out the titanium rods and put in forged steel (because they are “stronger”) and then (I assume) supercharge the motor to increase output. The result is that there is a market for unwanted titanium rods. No one wants them because they are considered “weak”. That may be the case for an 800hp V8, but it is not the case for an 800hp V12.

My crank is already machined for the 2.1" big ends to make use of forged Chev rods. However, the LS7 rods are wider and require more journal length (or need to be machined down and then plasma coated again - ouch)

So … I have a fully counter-weighted crankshaft design developed to make use of the LS7 titanium rods.

Simulations are showing substantially reduced centre bearing stress with a fully counter-weighted design. Even with the full counter weights, the total crank weight is lower than standard and inertia is reduced substantially.

However, I have a couple of issues that I am still working though.

  1. The bank offset is 0.75". 19,05mm. The LS7 rods are 24mm wide. Thus the con rod offset (bank to bank) is 24mm. Because the bore offset (bank to bank) is 19mm there is 5mm mismatch or 2.5mm of “off centre” happening at the small end of each piston pin. Doesn’t sound like much, but is scary to look at and feels wrong. From what I can see, the factory standard engine has an offset also. (Rods are 0.84" wide (21.34mm) meaning there is an offset of 0.09" or 2.3mm). In fact, I think one banks rods are dead centre in the pin, and the other bank is offset by the full 2.3mm. (But I could be wrong there)

But the crunch is that I need to be very very careful with my block measurements and crank design to make sure that when fitted up (and thrust washers in place) that the con rods are in an acceptable position and there is clearance at the piston pin. And I need to run some calcs to see what impact the little end offset will have.

  1. Total deck height on my block is marginal. The block has been skimmed too many times. And I am trying to see if I can get away with very high lift valves (15mm - 16mm). I am hoping to maintain at least 78mm stroke. This requires big valve pockets in the pistons which has a negative impact on compression ratio. So I am juggling a few things to see how it pans out.

Once I have a piston crown design that I think will work, I want to try to see if I can simulate the compression stroke to predict swirl and tumble, and see if there aren’t some small details that can be added to promote that. Even through there are large valve pockets, I am trying to encourage the mixture into a bowl around the spark plug.

None of this costs me anything but time. But if it does all come together and make sense, then I need to take a deep breath and see if I commit cash to see if it all works.

1 Like
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Mark many V type engines run with the small end of rod offset, one that i have had some experience with is the GM 3.8L turbo Buick V6.

i never was satisfied with around 5MM offset, seems the piston would try to cock in the bore?

BUT , those engines have reached up into the 1000HP range , with 30/40 psi boost pressures. stroked to 4.3L

anyway i had some custom made forged steel rods made with the mod on the big end to get it centerd and a custom Steel center counter weighted crankshaft, 15yrs a back.

plenty of other mods also, bigger turbo/intercooler, custom ECU ,exhaust mods, because that engine has a following parts were available, so costs were moderate.

engine ran great plenty of power, owner was happy!

the GM titainium LS7 rods have been known to fail more than one time, GM had them redesigned and not heard much waht happened, altho heard that some times on the bottom end were the rods come close to each other Galling from contact to each other! donno.

for piston crown design, i like the type that TWR ended up with on there final engines , someone has some for sale ,dont know where, i think in UK,(of course). maybe Grp44 design.

44 used a much bigger capacity engine ,rules did not limit there CCs, they were the 1st to use many Chevy/GM parts in there V12s!

i dont understand your worry about block deck hite, you would design piston pin hite to match your CALCULATED measurements!

to get F/A mixture into a piston bowl,(this is a big mod), go Direct chamber injection, that would be a 1st for sure!! ??

OK so many mods that could work, but to much money involved!

make a custom aluminum BILLET block with banks even, and use tounge and fork big ends, and a Titainium crankshaft, and liquid Mercury to balance it!

HEY never said easy! over and out!

Ron

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this piston design for swirl, may have some merit,donno.

not my idea ,an older guy has passed,Daytona FL. he drew it up around 1975!

Scan_20170420.jpg1170×850 100 KB

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I agree. It just looks wrong! It must create a weird pressure differential across the lubricant boundary in the bush. But obviously it must work, I guess.

The titanium on titanium rubbing does create galling. So they are supposed to be plasma sprayed with a coating to prevent galling on their thrust faces. I assumed they had got it right in such a high tech engine. The rods I have seem to show that only some areas are sprayed. Perhaps there was a quality issue - no, surely not! LOL.