Running without Balance Shafts

[EGSMachine]

i am rebuilding my 84 engine, and was wondering ifi really need the balance shafts. is there a certain RPM where the engine is incredibly rough? can i do it with a crank mounted harmonic balancer? because the ballance is important for maximum hp., but i would like to lose the danger and the weight of the shafts and thier belt.

i was planning on racing/autoX, and some streetrodding, so driveability is important. However, i think that the weight of the shafts, both static and rotational, is worse than a single crank mounted harmonic balancer. any of you racers that have run without the shafts, i am very interested in your input about engine performance along these lines.
Thanks,
e




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84-9445m
Porsche, "Overtly Masculine Feel".

93-325is 5m;
driven daily.

[eligunn]

I think the balance shafts do most of their work at lower rpms, so if you keep it reved up it would help. But I have no direct experience with this.

[Obin Robinson]

the Mitsubishi Starion IMSA cars ran without balance shafts in the engine.

it would vibrate about as much as a 924 engine... which is more than you're used to, but not that bad.

obin

[Ehab]

My balance shaft was oil-drenched since I had the car three years ago. The mechanic fixed it and resealed it while changing the belts last month ... the engine became more stable during revving, but no noticeable change at idle. The only difference is that the engine now doesn't tilt to one side when revved up ... because of the now functional blance shaft belt

[wolfrpi]

I drove my 944S about 7k without knowing the balance shaft belt was toast - I thought the motor itself was just rough and that the timing was incorrect.

Once revved, the motor was ok (like on the highway). I'm telling you though, the car will vibrate itself apart in terms of motor mounts, cat converter and muffler (which were quick to become toast). Also, all of the heat shielding started to rattle. It will be much worse than a 924 because the pistons are much bigger. The inherent unbalance in a 4 cylinder only gets worse as you increase the displacement. This is not true with a motor such as the boxer six in the 911. My advice if you are going to drive the car daily is not even to consider leaving the balance shafts out. Trust me on this one!

Erick

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Erick

86 951

http://www.pelicanparts.com/cgi-bin/pmpre/pm.cgi?action=display&login=aka_951

[Mrmerlin]

[QUOTE]Originally posted by EGSMachine:
Hi since your in the engine you might notice that there are dedicated oil outlets for the shafts removing them might also affect your oil pressure.
Also the shafts do remove the secondary imbalance forces and this will let the engine live a longer life, as there are other components that will shake themselves apart from the vibes, IE water pump, oil pump, alternator, fuel injectors, and exhaust system, clutch, and transmission..........
In any case I wouldnt try this modification, the engine and driveline will thank you for putting on the balance shaft belt and keeping it correctly tensioned..........Stan

[RealRideRacing]

Originally posted by EGSMachine:
i was planning on racing/autoX, and some streetrodding, so driveability is important. However, i think that the weight of the shafts, both static and rotational, is worse than a single crank mounted harmonic balancer. any of you racers that have run without the shafts, i am very interested in your input about engine performance along these lines.
Thanks,
e

We campaigned a 944na in POC Clubracing for 2 seasons on the same engine. It had been balanced and ran without balance shafts. Once beyond idle, there is no discernable difference without the balance shafts. The balance shafts really are there to prevent the engine from vibrating the rest of the car loose AT IDLE and lower RPMs. Nor did removing the balance shafts create any significant gains in power.
But for your purposes, I would keep running the balance shafts. If the car is to be a street driven car, you really should run them. The difference between a Race engine and a Street engine is that race cars are generally driven with the throttle wide open almost always where street cars are rarely wide open. Even in AutoX applications, you spend a lot of time at idle while waiting your turn to run.
Remember, just because it works on the track doesn't mean it'll work on the street.

Regards,
David
PENNYWEB/REALRIDE.COM RACING
http://www.RealRide.com

[EGSMachine]

thanks, guys. i did some research also on using the crank maounted harmonic damper as well. it looks like there is going to be more trouble than its worth. as usual.
thanks again.
e

[jnkyrd944]

I ran my NA 944 without the balance shaft for almost a month, and it would struggle to rev to 4800rpm. with the balance shaft it pulls strong all the way to the redline.

[Lawrence Coppari]

Friend of mine used to have a Porsche NA race car with no balance shafts and 3 liter engine (I think). The engine would vibrate a lot and loosen things up.

[lm6y]

Everything I read points to the fact that even the factory raced the car with functional. They even make mention that a driver pulled in the pits with what he thought was a blown engine. It ended up that the balance shaft belt slipped ONE tooth. He said it was nearly undrivable.

[drew1]

Like others, I been watching theb post, because this has been a question of mine. I will say that it takes no measureable torque to turn the balance shafts with the belt disconnected, therefore the only thing they would rob is the aceleration of the belt.

drew1

[EGSMachine]

drew1,
the weight i am concerned about is the staic mass nad the rotational mass of the balancers. the rotational mas can be almost 10 times as much as the static mass. my thinking is that one crank mounted harmonic balancer is lighter and just as capable as the 2 shaft arrangement.

[MAS]

From Michael Cotton's 924/944 book, page 58:

"Tests showed that the balancer shaft system absorbed three or four horsepower at maximum engine speed, an entirely acceptable figure for an engine so efficent otherwise."

-MAS

[drew1]

EGS you're right that rotational mass can be 10 times as high as static mass. I'm talking about flywheel effect or rotational mass, too. The outside diameters are relatively small compared to other rotating parts. I think how far out from center the center of the mass is, is either squared or cubed to find flywheel effect.

I was giving this some thought a while back while doing a belt change on a 944. Thats when I determined that it takes a small amount of torque to turn the shafts. I did not disassemble & measure shafts so I could calculate the flywheel effect. A cuple of days latter, I looked into crankshaft mounted Fluidamper. They do not make a 944 or universal model that you can bore & key.

I'm surprised that there is a 3 to 4 HP gain from no shafts (per MAS). Considering how much a 3 to 4 HP gain costs $$$, it might be worth more study. I did not think that even accerating that no shafts would have the same affect as 3 to 4 HP.

drew1

[MAS]

"I'm surprised that there is a 3 to 4 HP gain from no shafts (per MAS). Considering how much a 3 to 4 HP gain costs $$$, it might be worth more study. I did not think that even accerating that no shafts would have the same affect as 3 to 4 HP"

It might not be so straightforward a calculation as it may seem. I'm no engineer, but I think that an engine that is less well-balanced (and consequently vibrating more) will sacrifice horsepower too. After all, it takes (and robs) power to make the engine vibrate more than it has to.

-MAS