ptuomov

Anybody knows what they were thinking when switching from the S4 crankshaft design to GTS crankshaft design? Were they trying to keep the bearing loads the same while increasing the stroke from 78.9mm to 85.9?

S4 crankshaft:


GTS crankshaft:


(Both photos by Dennis Kao).

Quadcammer

I'll take the fully counterweighted piece every time.

Vilhuer

People were complaining especially GT engines run rough. Adding two more counterweights was response to that.

Nose was changed to make it stronger.

GregBBRD

And that was only an idle issue.

Throwing on a bunch of extra weight onto the crank made the engine slower to rev. This wasn't an issue with the GTS, since it was, by then, an uber luxury car. Sure not very good for a "performance" engine.

ptuomov

But you still tend to see both designs survive, wonder why that is? (By the way, I am not sure abut the terminology, but I think both cranks are "fully counterweighted" or "full-weight counterweighted." If I am wrong about this, please correct me.)

Interesting info.

There's actually two changes that I can see. The first is as you note the extra two counterweights. The second is that the counterweights are clocked at the same angle per end in the S4 crank, whereas the GTS crank counterweights are clocked to be 180 degrees from each crank pin.

I think you see both variations in modern cranks along the first issue. I don't think you see that many modern cranks with all three cws per end clocked in the same angle. I could be wrong, my survey is limited to surfing the web.

It's kind of interesting and sometimes counterintuitive.

I thought that as far as the counterweight design goes, heavier cranks spin faster. The rod journal, rod, piston, etc. are given. The balance is determined by weight times radius. The polar moment of inertia is determined by weight times radius^2. So a heavier design accelerates faster, holding everything else constant.

In this case, not everything else is held constant, of course. The S4 crank has the cws further away from the center main journal than the GTS crank. I think that the purpose of dual plane V8 cws is to cancel the first order imbalance of the crank. If I've understood this correctly, further away the cws are from the center main journal, smaller they can be. So because of this difference the S4 style crank can have both a lower weight and lower polar moment of inertia than the GTS style crank. I've been told that the downside of moving the cws further away from the center main is higher bearing loads -- otherwise if there were no downside why not just externally balance all crankshafts?

I don't think it was just for idle though. At idle, the firing pulses matter more than balance, I think. If it were just the idle, they could have just made the flywheel heavier. Sounds like a much cheaper solution than those extra cws.

Rick Carter

IIRC there are two stroker race cars that were close to identical one with 6 counterweights and one with eight. The one with eight had more horsepower, the one with 6 accelerated more quickly.

ptuomov

The car accelerated quicker with the gear on or the engine accelerated quicker without load?

IcemanG17

it had a lower dyno #, but pulled harder out of corners when both cars had the same gearing...HOWEVER the car that pulled harder is slightly lighter too by about 100lbs

Rick Carter

What he said,I didn't know there was that much weight difference.

ptuomov

So can someone explain to me like to six year old how the middle two counterweights are any more useful than my love handles, given that they are so close to the center main? Do they reduce the crank flexing and therefore bearing loads? Or what do they do?

IcemanG17

From what I understand......cranks-rods-pistons are all balanced as an assembly....having MORE counterweights does NOT necessarily mean its heavier...it just means that each rod-piston is balanced individually...vs as an assembly like the 6 weight design.....

This makes for a more ideal balancing of the rotating assembly.....for smoother operation and the ability to spin higher RPM.....however for the RPM a 6 weight crank 928 engine was designed, its fine....its not until the higher HP-RPM ranges that the vibrations become detrimental to the engine

For example in the mid 90's when Bob Devore was designing the 1st 928 strokers he built cranks with 6 counterweights (since thats what Porsche used)...but after breaking a couple cranks and blocks, he went to the 8 weight design.....

If the 8 weight crank is heavier, then sure it will spin slower.....but if its built to the same weight...why not do it

ptuomov

Holding other aspects of the cw design constant, mainly the radius, putting counterweights closer to the center main necessarily means that the counterweights in aggregate have to be heavier.

Why? Why isn't cancelling the first order imbalance enough?

What is the evidence that the S4 crankshaft has any practically relevant rpm limit from the cw design or bearing loads? The oiling issues seem to surface thousands of rpms earlier.

I doubt any of the stroker cranks, 6 or 8 cw, have the same size counterweights in middle as in the ends. I can't imagine an optimization approach that would give same size counterweights in the middle right next to the center main as in the ends. As long as we are talking about dual plane V8 cranks, of course. Yet it looks like the GTS cws in the middle are about the same size as other cws.

For the acceleration, it is my impression that it's important how much heavy metal is used in the cw design. More heavy metal added, faster the acceleration. The other important thing is how far the cws are from the center main, further away they are, faster the acceleration.

Rick Carter

Contemporary Crankshaft Design
THIS IS A MODIFIED VERSION OF AN ARTICLE
BY Jack Kane WHICH APPEARED IN ISSUE 033 of
RACE ENGINE TECHNOLOGY MAGAZINE

ptuomov

I've read that article a couple of times. Frustrated about the figures 9 and 10 missing...

In any case, the article shows a picture of crankshaft with 8 cws. So they help with stiffness, and thereby with bearing loads. However, the center cws at least appear to be much smaller than other cws. This is in contrast with the GTS crank cws that appear to be about as large as most other cws and only slightly smaller than the end cws.



Not sure that the photo is of a modern, expensive crank though. I can't see heavy metal anywhere, and if one cares about polar moment of inertia, then one should have heaviest material available in those cws.

mark kibort

thats hogwash! HP will tell all from the dyno, BUT, in neutral, there could be a slight diffrence. an indication of that , might be a 1st gear dyno run, but that is hardly done, ever. more hp will yeild more hp in even neutral for 99% of the times you test it. my 6.5 liter will rev, in neutral, much faster or at least the same, as my S4 crank with only 30 more HP.

no load, out of gear, possibly, but very unlikely, and very hard to prove. either way , a non issue. in 1st gear, the greater hp will always be an advantage, at all speeds of the car. Think about it, would you bet, in 1st gear, that an S4 crank, would ever produce more power at any rpm or vehicle speed?