Hi,
I have another data point regarding engine rebuilds.
This one is about balancing.
Everyone says "balance the engine".
This is good advice.
Why? According to sources that dyno'd a lot of these motors way back when,
some motors were better than others (balance wise) coming from the
factory. When they (the "tuners") would do initial runs on stock motors,
one of the things they would measure was crankshaft "flex". They would do
this at the flywheel and run the motor through the range - beyond the
dashboard orange and red lines. Like up to 6000 and more.
At any rate, the word is that in some motors the crank flex measured at
the outer edge of the flywheel could exceed 3/8" (I seem to recall a
number of .600" was mentioned, but I can't find a reference.) The bottom
line - this sort of crank flex would lead to premature crankshaft failure
due to stress cracks in the rear crank web or at the rear main journal.
To fix this, the tuners would "spin balance" the crank. Done correctly,
this will reduce the amplitude at the harmonic frequency that the crank
sees. For example, there is a first harmonic at 6300 RPM's and while you
can't tune that out you can reduce the amplitude. Then gear the car to
avoid long stretches at 6300 RPMs. (:-0) - sheesh, that over 120 MPH in
4th. Not too many TR6's see that.
To "do it correctly", you need to do the following:
1. measure your pistons, wrist pins etc. get them all within an gram or
two.
2. calculate the "bob weight".
3. spin balance with bob weights in place.
Now, the next step varies, depending on how you intend to use the car. If
you intend to compete, for instance, there may be rules that limit how you
can actually do the balancing. In SCCA, for instance, stock and Street
Prepared rules (Solo II) and possibly IT (road racing) require that you
have to _add_ weight.
Now, if you can use the "remove weight option" for your intended use, your
balancer merely needs to drill holes or grind some metal from your crank.
Done.
If you have to add weight - this is ex$pen$ive - you need to tell the
balancer to use "mallory metal" to obtain the dynamic balance.
The benefit of the balancing is smoother operation and less pronounced
harmonics at the upper revs.
Note that there are likey two possible crank types, early and late. The
early cranks are supposedly better for very high revving motors, but they
seem to be pretty hard to find. To wit, with 5 core motors, I only have
one early crank. You should only find early cranks in motors with numbers
lower than CC/CP50000.
4. balance the rods "end to end".
Last point - if you intend to spin the motor to above 6000 on a regular
basis (for a road racer, for instance), you should seriously consider
getting the crank nitrided as this will increase the strength by 50% over
the stock crank. I'm not talking "plasma nitriding", I'm talking a nitride
"bath". This might be hard to get done, as the process involves toxic
materials.
The TR6 crank is fairly strong (the stock crank is forged!), and the motor
has good initial balance (in-line sixes are supposedly "naturally
balanced"), but spending the money to get the motor balanced properly for
the intended use is money well spent.
regards,
rml
TR6's - CF14111U Driver, CR1871UO Racer
------------------------------------------------------------------------------
Bob Lang Room N42-140Q | This space for rent
Consultant MIT unix-vms-help |
Voice:617-253-7438 FAX: 617-258-9535 |
------------------------------------------------------------------------------
|