----- Original Message -----
From: "Mark Haynes" <haynes386@netzero.net>
> I have a question for those of you who have experience with testing an
> engine's limits (hopefully I can avoid finding the upper limits for
> mine).
> How do you establish an upper rev limit for an engine? and what
> factors determine where you set your rev limiter?
First, it does no good to rev an engine higher than it produces horsepower,
so you need to figure where power is made and where it falls off. Usually
you need the engine on a dyno to find this out. The advantge of most
aftermarket cams for small block Chevys is that the manufacturer gives a
power range for the cam. This is a good start. I don't know if you have
info like that.
How much oil pressure and volume do you have? If you dont' have enough to
float everything in oil, then that is your limit. Basically, you can spin
as fast as you want if you have sufficient oil pressure to keep everything
floating. Touch metal once and it is all over. But spinning alone is not
all.
Where do your valves float? How much spring pressure do you have? This
determines how fast you can spin until undesireable stuff happens on top.
The best that happens is that it just disrupts the fuel/air mixture flow and
the bad is it lets a valve hang a bit too long and piston slaps it. The
worse is that it lets the head of a valve snap off and you can guess the
rest.
Balance is critical. Find someone you fully understands balancing and for
high revving engines. With the 9000 limit on the 302s, we even balanced the
valve train, rockers, retainers, valves, etc. Don't forget timing gears.
Anything that moves can cause a mismatch and that could create a harmonic
that you don't want.
Rod stretch happens with speed. The faster you throw that piston up, the
harder it is to pull it back down on that intake stroke. Measure all rods
with precision before running and on your first teardown, check to see what
happened. If one stretches too much, a picton can slap a valve or the head
and then it is history. Of course, downshifting to use engine braking also
causes stretch, so don't.
> Case in point is this...I have a '62 Austin XSP motor (built for
> Formula Junior). It was the factory"cheater motor" for the 1100cc limit.
> It's' built on the 948 base-crank has 1 3/4" journals, and 3.00"stroke,
> it's cross-drilled and has flow restrictors in the oil passages. To
> achieve the 1100cc's, the factory (Eddie Maher & co.) bored these motors
> to 2.65"-giving a displacement of 1096cc. It's got the 1100
> full-floating rods, but they're not lightened, and an AEA649 cam, and
I hope I'm misreading this, but it sounds like you have oil restrictors in
the pasages to the crank. I hope not, although I know nothing about these
engines.
With the SBC, we ran solid lifters and oil restrictors in the lifter galleys
so that oil was concentrated on the lower end. In most SBC, we ran .0025"
clearance and 60 psi with a high volume pump. With the 302, we were running
8500 and so .004" clearance with 85 psi from a high volume pump. With full
floating rods, you need a lot of oil to float the pins and you don't do that
by restricting crank oil.
> all of the go-fast period parts. When I build it, How do I establish how
> high to rev it?(and not blow it). I can't find anyone who used to race
> them who remembers what they used for an upper limit. I know that the
> 1275s have a tendancy to snap cranks,and I really don't want to do this
> either.
> I'm planning on putting it in my '62 Sprite, I know the 1275s will
> eat me up,but $#%@ happens
>
> TIA, Mark Haynes
> RMVR
Put it together, run it 500 rpm over stock, take it apart. If anything
shows anything bad from above, then that _IS_ your limit. If not, then go
up another 500, run it, then take it apart. Check again for the bad stuff.
Once you find bad stuff, back up 500 and that is the top.
Don't reuse old bearings. You pull them out to check them, replace them.
This is all general stuff, your mileage may vary.
|