Larry,
The 80% value comes from the ratios in the transmission and is really
different for each shift, and depending on what transmission/gears you have.
I have an article that covers this on the TigersUnited Website that has more
of the details, but taking an example from that article, if you shift at
6,400 rpm, going from 1st to second you drop to 4,662 rpm, 2nd to 3d you
drop to 4,885 rpm, and 3d to 4th you drop to 4,961 rpm; and that's with a
close ratio box. The average rpms through each of the gears is around 5,600
rpm, so this would be optimized for an engine that gets its peak HP at about
this rpm. Actually, since the HP curve falls off faster on the high side of
the peak, it's probably optimized for somewhat higher peak HP point; maybe
even 6,000 rpm, depending on how quickly it drops off. You have your own
motor's dyno curve, so take a look at it and guestimate where you would need
to shift to get the most area under the curve between the shift points. I
know this is not totally precise because you really need to use a
time-weighted curve for example, but you can probably get as close as you
need to just eyeballing it. Compare this method with what, for example,
someone who shall remain nameless, but who probably represents a lot of
racer types, do. Have Dave Drahle build you a motor that gets 425 HP @ 7,200
rpm, and then shift it at 7,000 rpm because you're afraid it's going to
break. This is nonsense. A motor that puts out a mere 375@6,500 would
undoubtedly beat it every time if you shifted at 7,000 rpm because it's
average HP output would be greater than the 425@7,000 motor.
BTW, in a lazy evening you can replace your valve springs with some stiffer
ones that will keep your valves working to 6,500 rpm. A good set of double
or triple springs will require taking the heads off and machining the spring
seats, but a set of stiffer single springs would work just fine at 6,500.
TTFN,
Bob
-----Original Message-----
From: owner-tigers@autox.team.net [mailto:owner-tigers@autox.team.net]On
Behalf Of Larry Paulick
Sent: Wednesday, February 21, 2001 5:58 AM
To: Bob Palmer
Cc: cvaught@hawaii.rr.com; tigers@autox.team.net
Subject: Re: E-Mail me off the list
Bob, interesting point on the shift points. I understand having the
revs in the sweet spot, but how did the 80% come about.
If I have the stock motor and cam from the 90 5.0L, and its max hp is at
5200 rpm, the the shift point is 6500 rpm. That is quit a jump, and
with stock valve train, mainly springs, I am sure that valve float would
become a problem.
Larry
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