Dennis-
You ask about putting TR6 pistons in your Spit...
First things first...
Measure the piston pin to top distance.
Find somebody else on the net (or where ever) to measure a piston as the one
I measured came from a junk motor that someone had rebuilt at some time prior
to it coming to me, however, the deck height looked about right before I tore
it down.
Let me know what the hepolite number is. (Don't know why it would say 7.7) ?
Are the pistons flat top or dome ?
The GT6 piston (same bore size as TR6) has a thicker distance from pin to top
of piston.
Yes Hepolite pistons are decent units, used them in many a race motor
Venolia pistons are a forged piston from So Cal, have used them for many
serious race motors.
When you buy the "good" pistons, get five and balance them as this way you
can fix a broken motor without having to wait for another piston and then
having to balance it when it arrives.
Now we get to the tough stuff...
Try to visualize this as it gets complicated.
By putting pistons in "backwards" many things happen. First lets define
backwards. The pin hole is not dead center under the piston, but off to one
side by a small amount.The reasons for doing this are complex, but the 2
major ones are to: keep the flame front from "hammering" the top of the
piston, and to keep the internal forces in check between the crank and rods.
There are many other considerations, including available piston/valve
clearances in respect to cam timing, head design, and induction/exhaust, but
we will ignore these for now.
When an offset piston is installed correctly, the angle formed between the
rod and the crank at the time of flame front propogation is figured. When
this is correct, the flame front hits the piston top as the piston nears tdc
and the flame front "pushes" the piston down the bore just as the piston goes
"over the top" of its stroke. When the piston is installed "backwards", the
angle between the rod and crank is altered. The flame front now meets the top
of the piston a little before it has reached the top of its stroke and as a
result the BMEP (brake mean effective pressure) is waaay higher. This results
in higher "effective" compression as the mixture is expanding and the piston
is still trying to "squeeze" it. This also results in higher loads placed on
the rod and crank levers. At low rpm's this can result in serious
detonation, but at very high rpm's it will produce more power because of the
time lag for the flame front to travel to the top of the piston.
If you are going to street drive this motor, I would re-think what you are
about to do, however, if this is indeed to be a hot rod motor, then go for
it.
I have a hard time explaining things on a keyboard, so if this is unclear, I
apologize. Where are you ? Perhaps we can talk on the phone if you have
further questions.
Nick in Nor Cal
I think I will send this to the TR list in hopes that someone will be able to
measure the distance from the top of the wrist pin hole to the top of the
piston on either a TR6 or GT6 motor as this will help figure out what you
have...how about it guys ?
|