how much crankshaft twist? 1

[jstein]

We race a roots supercharged motor. After dynoing we noticed a small amount of timing(1 degree)made a huge difference in power. Now with the ignition signal triggered from the front of the crank and the load at the back of the crank and a firing order of 1-8-4-3-6-5-7-2, what cylinders would receive the greatest amount of twist and about how much? With individual cylinder timing there should be some power there(?) How is this twist related to the rpm? Thanks

 

[Metalguy]

I don't think crank twist is anywhere near constant like that. You have to look at the firing order. At some point on the power stroke of each cyl. the twist is at max. When cyl. 1 fires, it's throw is already advance wrt the rest from cyl.2.

Also, the longer the distance between twist-input and twist-resistance the more the twist. Think of how much more you have to rotate the handle of a breaker-bar when you have a long extension vs. having the socket right on the breaker. Note-I've actually had people (Densans, not Mensans) tell me you lose torque with long extensions!

So, since the vast majority of "twist-resistance" is at the rear, throw 7/8 doesn't ever get adv. or ret. by much at all, but 1/2 could easily get adv., and also adv. the cam and spark for the next cyl. to fire, no.8

What I'd like to hear from Jstein is how the small timing change caused his big power loss-was it from being retarded or advanced?

I used to run a Mallory (I think) ignition trigger that used a light beam and a plastic rotor for timing. There were small slots in the OD of the rotor, which was ~2.5-3" dia. That big a dia. gave very even timing-spacing, but it could be easily modified to compensate for individual cyls. such as retarding no. 8, where I think the biggest variation would be.

But I agree the HP diff. would be tiny if even measureable.

 

[evelrod]

Just a thought. Any effort to individualise timing using a stock type dist., Mallory or other, will ultimately be a waste of time. BTDT! Too much gear/chain deflection/stretch (some of our little engines operate reliably at ~9500rpm).
The Electromotive system (and others I presume) can easily be converted to multiple pickups (four in our case) using single coils per cylinder. I am not very competent on how to modify/adjust the modern ECU's to do this same job but, I am told that "You can do anything with a PC these days, Dad!"

Rod

 

[jstein]

Metalguy, with the timing retarded 1 degree we lost 100ftlb 0f torque. This is on a 2100 hp motor. This is with a state-of-the-art MSD mag. The dyno also showed smaller "holes" of 20-60ftlbs as the rpm increased. Now if the mag is always under acceleration on the dyno then all the "slop" should be taken up. Perhaps the mag is actually accelerating and decelerating because of the twisting and releasing of various components. Yes I realize the there are lots of other factors that may explain the "holes" but it would be nice to make the timing method as efficent as possible before dealing with the other variables.

 

[Metalguy]

First, I am amazed that a 1 deg. retard can make such a big difference. Sounds like you're running a BBC, and getting the same HP on meth. as we used to get in our blown fuel AA/FC 427 BBC way back in 1971!

I don't know what a "state of the art" mag. is now-we ran a Cirello, with no advancer, period. Started and ran with 50 deg. lead, IIRC. But it sounds like it doesn't vary the timing for crank-twist, and you can probably pick up a little torque/HP if you can figure out a way to trigger it differently. The dyno will tell you if your changes are working. Cyl. 8 is the most affected in a front-drive setup, which means just about all BB and SBC's. If you can set up cyl. peak pressure or actual press. vs. time instrumentation, you'll see the results without the dyno. I think the next cyl. to be most affected is no. 6, following the power strokes of 4 and 3 on that throw just ahead of 6, but it won't be as retarded as no.8 because of the shorter distance involved.

Note that none of the above includes anything on crank harmonics/vibrations-I really have no idea how they affect timing, but when your pistons/rods are REALLY trying to turn that crank, it will twist elastically for sure. And it doesn't matter what kind of steel the crank is or how it's heat treated either!

 

[Tmoose]

I'm imagining the relative position of each rod throw relative to a crank mounted ignition pick-up (front or center or rear) to vary with torque (throttle position) AND how close the crank speed is to one of its torsional resonant frequencies. When a crank is in the throes of a full boing resonance (even snubbed by an effective torsional damper) it is winding and unwinding in one shape or another, and some throws might be advanced OR retarded relative to a reference based on pure (instantaneous?)torque. Then a few hundred rpm change might sneak way down on "amplification factor curve" and reduce the amplitude of twist by 90%. Sounds tough to track.

Around 1968 FORD felt they could improve the perceived smoothness of their 302 V8 by changing the firing order.
I'm thinking the crank wind-up must have been significant at some street type rpm ( 3000 ?).

I'd fully expect A little time on the dyno with a timing light checking cylinder-to-cylinder timing variations and general timing scatter at various speeds and loads might be rather revealing. If a high speed timing light and a finely degreed hub or flywheel (not damper outer ring, which is supposed to dance, relatively, at resonance) could not make the changes obvious, I would concentrate on other areas, figuring the much harder to measure variations in valve timing events might be more harmful.

Then again, after the recent excess tempeature problems we've had at work with splash lube gearboxes it would be easy to convince me to start thinking looking at ways to "minimally lubricate" transmissions and rear ends as a way to preserve the horsepower the engine already makes.

 

[GregLocock]

Oh, one other twist related story - camshaft twist upsets the valve timing as you go back along the engine. BTDT.



Cheers

Greg Locock

 

[evelrod]

Me,too, Greg. Thus the so called "scatter patern" camshaft profile that I am using. Mine is ground up by Kent but, other grinders use the same idea, just not to the same degree I suppose. Most engine designs are not as bad as mine.

Rod

 

[Pancholin]

Metalguy.
I would think that driving a blower off the front of the crank would yeild less twist because the power is extracted from both ends of the crank.
I do agree however that the crank is not so rigid as I once thought. I once installed a 390 crank in a 370 Caddy, and there was about a sixteenth inch clearance between the rod bolts and the pan flange of the block. Once the deck height piston problem was solved I started it up to hear a real clatter in the bottom end. Pulled the pan and could see clearly where the bolts were bumping the block, but they still cleared nicely when turned by hand.

Thought for today (Not original but works for many things)

"When theory and measured results don't agree, invent a new theory."

Regards and Happy Holidays.
Pancholin

 

[Metalguy]

Agree. Even having a heavy damper on the front would lessen the effect-one of the reasons it's there.

I really have no idea about the number of degrees of twist the crank would have-wouldn't expect it to be more than a few tho. But the orig. post noted a large drop in HP for a 1 deg. retard (all cylinders of course), so there should be a little "free" HP to be gained-and a racer wants every pony he can round up.

Merry Christmas to all!

 

[Azmio]

Hi Buddy,
The torsional vibration of a forged crankshaft can cause variation of up to 0.2 degree. Trust me, if we allow the crankshaft to vibrate up to 1 degree, the crankshaft will self destruct. The t.v. is at its worst at the crank web longest to the flywheel.
Anyway, you should check on how many teeth the trigger wheel has. The lesser it has, the bigger the variation may be.

AO

 

[GregLocock]

No, my figure is MEASURED, so I won't trust you. I did make a mistake tho, our rig is calibrated in degrees peak to peak. So at resonance we see around one degree peak to peak, 0.5 degrees peak, or 0.35 degrees RMS

Cheers

Greg Locock

 

[idano]

I can see one degree difference of lead making a 5% difference in TQ or HP....especially with forced induction.

youve got to understand that a V motor with crank wind up and cam wind up (especially with heavy valve springs) is like a big rubber band.....especiallly at the first and second harmonic rpms. Now add to that spark scatter from the cam to distributor gear, the distributor shaft, vertical movement of the distributor shaft , and fore and aft movement of the camshaft.

What to do to help the situation....

1) live with it..as you have plenty of horsepower and you dont get on and off the throttle as its sounds like this is a drag race motor..... or...

2) try to minimize losses and maximize the power that can be gained thereby. This is all accepted practice. Take for example the BMC "A" motor cited above. The skew gear distributor drive was fine for tractors and MG Midgets....but terrible for performance....thats why a crank trigger system is used on these motors in race trim (rules allowing). Crank whip is so bad with the three main bearing design, that to run the motor sucessfully for prolonged periods of time at higher rpm (7 to 8.5K) you have to run LOTS of rod side clearance and crank end play to avoid pinching rod journals.

Back to your blower Chevy. A crank trigger system with the Electramotive box will allow you to be selective on firing each cylinder. You will definitely pick up some HP and the motor will be happier. NASCAR cup motors use the MSD distriibutor with adjustable firing capabilities also. Try a manual timing light when the motor is on the dyno or running in the car.....follow the bouncing timing mark. I am very surprised that you could even read the lead to + - one degree.

Good luck..

 

[Azmio]

Greg,
Interesting! How did you measure the difference? I know that some people measured the rotation at both ends. Is this similar to how you measured yours?
Anyway, I am also curious on how many teeth the trigger has?

AO

 

[patprimmer]

idano

At over 2000 HP, he is running at least alky, possibly some nitro methane. I can't see where he says what fuel

For alky, and certainly for nitro methane, he will need a high amperage long duration spark, that to date has not been obtainable with computerised electronic engine management systems, unless you call MSD magnetos electronic.

I would love access to a true high amperage and duration maggy with crank trigger and long spark dwell time.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dieselsmoke]

When Chevrolet brought out its new LS1 aluminum small-block engine, the company claimed less crankshaft stress due to a revised firing order, compared to the outgoing cast-iron small-block engine. Can anyone comment on the reasons why the revised firing order reduced crankshaft stress?

 

[Azmio]

Dieselsmoke,
It may be possible to reduce the crankshaft stress due to a revised firing order by firing the cylinders as close as possible from one to another.
Torsional vibration is at its worse if the engine is first fired at the 8th cylinder and followed up by the first cylinder. Imagine how much twist the crankshaft will experience.
On the other hand, it will be great if we can fire the V8 engine in series. Perhaps I should put this in my wishlist.

AO

 

[Kamikaze]

In reading some of the answers it seems some have forgotten that the cam is angled slightly in it,s bearings to load it towards the rear of engine on Chevs, this was designed to save wear on the timing chain, but as the chain stretchs timing changes, when all is well try a trick we use to individual time using a larger gap on the plug to retard the timing slightly, then check with a good light as to see what effect this has.

 

[2ofdem]

Top fuel engine tuners allow for crankshaft twist.

 

[JWell]

I hear that the new Cummins "600" engine has a crankshaft twist sensor.