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flywheel to balancer weight proportion- inline 6

General engine tech -- Drag Racing to Circle Track

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lc-gtr-1969
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flywheel to balancer weight proportion- inline 6

Post by lc-gtr-1969 » Tue Aug 06, 2019 2:30 am

Hi all,

I am currently designing and building a fairly high hp to cc inline 6 engine... it should come close to 1.8hp per cube and will be revving to about 7500rpm. With the right induction/exhaust, similar setups have made over 2hp per cube with these engines and the head I am using. With this kind of level of tune in an inline 6, I am seeking advice for the flywheel/balancers weights as a method of controlling harmonics.

I am planning on running an ATI balancer but as there is not currently a hub available, it will run a custom steel hub adapter. The engine is a holden (GM) 202ci. Similar to Chev inline 6.

These engines are known for issues with harmonics. I have recently killed the bottom end from harmonics. On my previous bottom end, the old balancer fretted out on the keyway- the cam gear also fretted on the keyway and caused cam timing to be retarded. I was running an 8kg flywheel and 4kg Romac balancer when this happened and was not long after installing the 8kg flywheel (I had previously run a 4.5kg aluminium flywheel). I presume the heavier flywheel shifted harmonics towards the front of the engine?

So with my new engine, I would like to avoid these issues with harmonics- the ATI balancer I have been recommended is a 3 ring 6inch unit aluminium shell that weighs around 2.8kg. The custom hub to suit is steel and weighs 2.68. The combined weight is just under 5.5kg (just over 12 pounds).

My question is, is it best to run a flywheel that is of similar weight to the balancer (say 6kg/ 13 pounds? give or take) in order to help keep the harmonics in the centre of the crank- ie to keep the engine from shaking flywheels and from shaking balancer/fretting keyways etc? Or am I barking up the wrong tree here?

I have taken other typical measures to help dampen harmonics and stabilise the bottom end (half grout fill, 10mm crank girdle, light forged reciprocating parts/rods and pistons, slightly longer rod ratio for lest thrust force on bores etc)

Any advice much appreciated...
Fumbling around in the shed...

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Re: flywheel to balancer weight proportion- inline 6

Post by dwilliams » Tue Aug 06, 2019 11:08 pm

About a century ago Frederick Lanchester did some pioneering work on crankshaft torsional vibration. He concluded that as much flywheel weight as possible should be on the front of the crank.

Unfortunately his papers were published by the Royal Society, which limits access to their archives, and I've only seen summaries, not all that detailed.

Considering how many Detroit production engines went from no balancer at all to ten-pound monsters by the end, I suspect Mr. Lanchester was correct, but I've never seen anything on how to calculate what the weight might be. Lanchester put 100% of the flywheel weight on the front of the crank for his own engine design, so maybe "as much as you can get" would be a workable first approximation.

Note Lanchester's crankshafts were probably conventional for their day, that is "bent bar of steel" with no counterweights at all, so they had a lot more couple across the mains than counterweighted cranks have.

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Re: flywheel to balancer weight proportion- inline 6

Post by blown265 » Tue Aug 06, 2019 11:29 pm

G'day LC

My experience with dampner/snout failures, cam gear fretting, keyway damage, and flywheel/crank interface issues have been the same as yours- terrible!! (I6 265ci)

These were solved in two ways:
1. Spending as little time as possible in the known rpm damage zone (in this case 6200 to 6400rpm) by either short shifting or running through/past as quick as possible.
2. Significant upgrades to bolt hardware/locating dowels/ interference fit, and machining/assembly processes.

I currently use an all steel Romac Balancer and a lightened flywheel assembly, so their respective weights are close. I agree that the closer they are, the better. Low mass dampners made the situations worse.

Regards
Paul

lc-gtr-1969
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Re: flywheel to balancer weight proportion- inline 6

Post by lc-gtr-1969 » Wed Aug 07, 2019 5:06 am

Thanks for both replies... this is an area that is hard to find solid data on, as most common V8s do not have the same level of harmonics issues as inline 6s (at least to my knowledge), hence a lot of my readings are based around V engine configurations, and whilst the principles are the same, the length of an inline 6 crank would mean that the crank would be ringing pretty bad at the balancer (if its under-engineered, i'm guessing?)...
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Re: flywheel to balancer weight proportion- inline 6

Post by Circlotron » Wed Aug 07, 2019 8:28 am

Having had seven or eight of them, I think the most brain dead feature of the standard Holden red motor harmonic balancer is the fact that the fan belt goes around the outer ring. When the crank and *hopefully* the balancer resonances coincide, at that rpm the outer ring of the balancer should be moving exactly the opposite direction the crank happens to be twisting, and a lot further given their relative masses and moments of inertia. A belt around it is going to restrain those rotational oscillations right when you want them! Probably the bean counters won out over the engineers.

Anyway, not talking about viscous dampers here, but the ones with a rubber ring between the hub and the outer mass. The combination of the elasticity of the rubber and the mass of the ring will resonate at a certain frequency, but I have never ever seen that frequency quoted. Or the sharpness or "Q" of that resonance. I imagine it should be at the same as the installed crank resonant frequency. Just chucking any old balancer is totally hit and miss. There may even be a case for attaching several balancers, one behind the other, tuned to the fundamental crank resonance and the third and fifth harmonic. Not sure about the even ones. Again, not talking about viscous dampers here. And also, totally not my speciality.

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Re: flywheel to balancer weight proportion- inline 6

Post by dwilliams » Wed Aug 07, 2019 9:23 am

David Vizard did a test a few years back with some kind of sensor to detect vibration at the harmonic balancer, so he could see the crank flex in real time.

If you can find out what kind of resolution he was using, you could probably cobble something together for a couple hundred bucks. Sensors and data acquisition equipment are ridiculously cheap, at least compared to twenty years ago.

Then you wouldn't have to work with mystery formulas or guesswork; you'd *know*. And I bet it would cost less than one prepped crankshaft...

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Re: flywheel to balancer weight proportion- inline 6

Post by BillK » Wed Aug 07, 2019 9:32 am

Just wondering what the application is going to be ? Drag race or circle track / road race ? I would think that would make a difference too.

What does ATI suggest ? I know they offer different "durometer" o-rings for different applications.
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Re: flywheel to balancer weight proportion- inline 6

Post by Juho_ » Wed Aug 07, 2019 1:37 pm

Could the crankshaft mass, journal diameter and crank length (-> bore spacing) have more effect than flywheel and/or damper weights/diameters?

Some straight six engines have horrible crank harmonics, that cause problems with oil pump gear's nut (BMW M54B30 atleast, fix is to use Ati super damper and lock the nut properly) or might even break the crank or ruin bearings because of crank flex.
But oddly engines with quite heavy cranks with larger journals haven't got the same problems, and can be revved to ridiculous numbers.
Atleast that's what I've noticed.
E.g Volvo B6304 crank weights almost 30kg and has 65/50mm journals, stroke length 90mm.
BMW's cranks are somwhere in 22-24kg if I remember correctly and smaller journal diameters.

I've read about Nissan L28 engines built for impressive rpm and hp numbers.
They use very lightweight flywheels - rule was the lighter flywheel, the better it is for crank.

But I have read the opposite - the flywheel could be too light, but compared to damper.
I don't have enough experience to say anything about that.. just like I said, I've only read about them.

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Re: flywheel to balancer weight proportion- inline 6

Post by lc-gtr-1969 » Wed Aug 07, 2019 8:30 pm

BillK wrote:
Wed Aug 07, 2019 9:32 am
Just wondering what the application is going to be ? Drag race or circle track / road race ? I would think that would make a difference too.

What does ATI suggest ? I know they offer different "durometer" o-rings for different applications.
Cheers.. ATI suggest a 7inch, 3 ring damper, 6.25lb using the steel hub adapter... They have suggested that the flywheel weight does not matter as such, but rather just ensure the balancer is big enough (enough rings, ie 3 ring, and larger diameter in 7 inch rather than 6 inch).

My understanding of ATI's position is that the harmonics are always sent towards the front of the engine (due to flywheel weight), so just run a balancer that can sufficiently absorb those harmonics, and it will be ok. I can see 100% understand this point but the accumulative flywheel and balancer weight is going to be quite heavy, which could hurt the responsiveness of the engine- but I am happy to do this if it helps the engine survive. Also, I suspect if harmonics are not controlled, ie if using a smaller / lighter balancer and flywheel combo, the engine may be responsive but those harmonics would have to hurt HP in my mind...

The application is street/drag.
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Re: flywheel to balancer weight proportion- inline 6

Post by Circlotron » Wed Aug 07, 2019 8:58 pm

Which crank are you using? The later, fully counter weighted blue/black one or the early partially counter weighted red one?

lc-gtr-1969
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Re: flywheel to balancer weight proportion- inline 6

Post by lc-gtr-1969 » Wed Aug 07, 2019 9:41 pm

Circlotron wrote:
Wed Aug 07, 2019 8:58 pm
Which crank are you using? The later, fully counter weighted blue/black one or the early partially counter weighted red one?
I am using the full counterweight crank (from a blue engine). I have not lightened it in any way, just machined it to suit Honda H beams (5.63 rods rather than the standard 5.25 rods).
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Re: flywheel to balancer weight proportion- inline 6

Post by SupStk » Sat Aug 10, 2019 1:10 pm

I’m curious if the engine you mentioned in the first post (2hp/ci) has the same issues? Was just thinking you may acquire a short-cut if someone had covered the same ground successfully. Other than that, all I can say is “good luck”!
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Re: flywheel to balancer weight proportion- inline 6

Post by peejay » Sat Aug 10, 2019 2:27 pm

Juho_ wrote:
Wed Aug 07, 2019 1:37 pm
But oddly engines with quite heavy cranks with larger journals haven't got the same problems, and can be revved to ridiculous numbers.
Atleast that's what I've noticed.
E.g Volvo B6304 crank weights almost 30kg and has 65/50mm journals, stroke length 90mm.
BMW's cranks are somwhere in 22-24kg if I remember correctly and smaller journal diameters.
Audi five cylinder engines except for a few rare models generally all had the same 86.4mm stroke as contemporary VW four cylinder engines, but Audis had a crank snout driven oil pump, and a really bogus harmonic damper setup that bolted the damper to the sintered crank sprocket instead of pressing onto the crank snout. Both are known failure items in high RPM/high boost

Dahlback Racing used to destroke their high power builds, I assume with the short stroke crank that Audi made to get their fives below 2 liters for the Italian market. I've long suspected that the reason wasn't "more RPM" (you can get a lot of RPM from an 86.4 crank) but rather "less harmonics", since there would be more journal overlap.

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Re: flywheel to balancer weight proportion- inline 6

Post by Kevin Johnson » Sat Aug 10, 2019 2:46 pm

Aside: Remember that the principal advantage of accessing documents through the NASA site versus the mirror NACA sites is that OCR scanning was used. If you press F3 a window should appear where you can spell out search terms. This is quicker than reading hundreds of pages of texts.

https://ntrs.nasa.gov/archive/nasa/casi ... 094744.pdf was the first hit. Lürenbaum was cited by Taylor with regards to crankshaft design. I purchased a copy of the 1937 German source document and can post photos. Of course, it is in German. NACA arranged to have many foreign documents translated which is what you see in this pdf.

Here is a possible search; you can modify the search parameters:

https://ntrs.nasa.gov/search.jsp?N=0&Nt ... ction|NACA

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Re: flywheel to balancer weight proportion- inline 6

Post by chevyfreak » Sun Aug 11, 2019 2:19 am

peejay wrote:
Sat Aug 10, 2019 2:27 pm
but rather "less harmonics", since there would be more journal overlap.
To add to your quote , with the shorter stroke,.
I have seen this. Well sort off. :lol:
I use many chevy inline sixes,(my friend as well) mainly 250 ci but i have some 230 units.
The 250 is counterweighted at every cyl and the 230 with shorter stroke only has 4 counterweights (2 in the center and one ea at ends of crank) but when rebuilding them all my 230 cranks cleans up with a polish only and dampers still good where the longer stroke 250 cranks almost always needs to be cut down on journals and dampers need to be replaced. Dampers breaks bonding and/or pushes out rubber,
This is still in stock form so no high rpm, max at 5000, but can still see that there is alot more wear on damper. The damper can just be too old and fails but mostly on the longer stroke cranks.
I also noticed that the journals also shows more wear on the 250 cranks, dont know why, maybe harmonics effecting it or just the leverage of it. But something does it. At least not destructive. Just more to fix when rebuilding.

But i did know a guy that build a 250 for racing, he actually used an 8inch damper and had to retorque the flywheel bolts periodically. They kept coming loose with the high rpm useage.

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