Mike said, “My car also had a 2.88 rear, not a 2.87.” Yeah, I know - every
reference I ever saw written to either the “standard” Sailsbury or Dana was
2.88, but when I dropped the rear cage, the diff cover plate had two metal
tags attched: one said “limited slip” and the other said “2.87”.
Jim Johnson
XJ-S, V12, Lucas CEI ignition, TH400, Dana 2.87
Annapolis, Maryland
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Mike said, “My car also had a 2.88 rear, not a 2.87.” Yeah, I know - every
reference I ever saw written to either the “standard” Sailsbury or Dana was
2.88, but when I dropped the rear cage, the diff cover plate had two metal
tags attched: one said “limited slip” and the other said “2.87”.
That’s .4% difference. Wouldn’t that almost be the difference within
tolerances? Are people that picky with figures. I’ve seen the 5.3L stated as
a “326” or “328,” but not much difference-especially when you consider that
every engine is unique. Dad was saying that you can build 2 engines exactly
the same and still get different results. Heck, I’ve heard of it being
necessary to hand-fit pistons on a Jeep Cherokee engine simply because the
factory mills get “dx” smaller in diameter as the cylinders (on the same
engine) are sequentially bored out. Some guy was told that he needed to
hand fit the pistons, ignored it and the part ordered didn’t work because of
the diamter distances.
Just seems that splitting hairs on quoted figures is not useful (considering
tolerances).
The practical implimentation of tolerances is how tighter tolerances on
balancing of an engine allows the red line limit to be extended. That is
why a Jaguar has a higher RPM than most small block Chevys.
And motorcycle engine RPMs ususally exceed most cars (only 2-3 pistons to
balance cost effectively).
The other case of this is timing. I’m not recommending it on a Jaguar, but a
person can in practice slowly (on a small block chevy) advance the timing at
the rpm limit. When it starts to knock, back it off. Better HP, better gas
milage, but every engine is different and doing that sort of thing takes
experince. There are tons of warnings about not to do it. I could see this:
What happens when the engine is cold or higher altitude-or just the right
combination of negative effect on the efficiencies? When the springs wear
down? So subsequently Jaguar puts the timing curve at a same limit for the
worst of all possible circumstances with the worst of all possible machine
tolerances and you get factory timing curves-you need a margin or error to
cover your butt. Dad (with about 30 years mechanical experince) on the
other hand, ignored these warnings and set the timing on the Jaguar more
advanced. He got 18 mpg in town and 22 mpg on the highway (US stats). I
don’t know if he drove it on a cold day at 6000 rpm climbing up into Lake
Tahoe, pulling a trailer but I can pretty much conclude that he ran it like
a bat out of hell. If only I had one of the Jags with a knock sensor!
Just some thoughts on useful application of tolerances.
Rob
// please trim quoted text to context only
Hill Robert J 1stLt AFRL/DELS wrote:
Mike said, “My car also had a 2.88 rear, not a 2.87.” Yeah, I know -
every reference I ever saw written to either the “standard” Sailsbury
or Dana was 2.88, but when I dropped the rear cage, the diff cover
plate had two metal tags attched: one said “limited slip” and the
other said “2.87”.That’s .4% difference. Wouldn’t that almost be the difference within
tolerances?
The ratio of a final drive unit is not subject to “tolerances”. It’s
one number of teeth divided by another number of teeth. What
probably happened here is someone rounded it to the nearest two
decimal points, while someone else merely dropped the digits beyond
two decimal points.
– Kirbert
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Actually, I think what’s happening here is that Rob’s car has a Dana diff.
The Salisbury diffs use 17 tooth pinions and 49 tooth rings, which is a
ratio of 2.882352941:1.
The gear counts differ on the Dana diffs, though I can find no reference and
Dick Maury, our diff expert, can’t recall the tooth counts either. According
to Dick, the Dana units really do have a 2.87:1 ratio.
Rob, does your car have a Dana diff? If you don’t know the differences, see
this link:
http://www.coventrywest.com/diffpics.htm
Best regards,
Gregory Wells 800-331-2193 x103
Coventry West, Inc. - Atlanta, GA www.coventrywest.com
New, Rebuilt, and Used Jaguar & Land-Rover Parts
Kirbert wrote:
Hill Robert J 1stLt AFRL/DELS wrote:
Mike said, “My car also had a 2.88 rear, not a 2.87.” Yeah, I know -
every reference I ever saw written to either the “standard” Salisbury
or Dana was 2.88, but when I dropped the rear cage, the diff cover
plate had two metal tags attached: one said “limited slip” and the
other said “2.87”.The ratio of a final drive unit is not subject to “tolerances”. It’s
one number of teeth divided by another number of teeth. What
probably happened here is someone rounded it to the nearest two
decimal points, while someone else merely dropped the digits beyond
two decimal points.
// please trim quoted text to context only
Mike said, “My car also had a 2.88 rear, not a 2.87.” Yeah, I know - every
reference I ever saw written to either the “standard” Sailsbury or Dana was
2.88, but when I dropped the rear cage, the diff cover plate had two metal
tags attched: one said “limited slip” and the other said “2.87”.That’s .4% difference. Wouldn’t that almost be the difference within
tolerances?
Just some thoughts on useful application of tolerances.
Rob
Tolerances are not applicable, since this is the ratio of the number of teeth on a ring gear divided by the number of teeth on a pinion gear. While there are plenty of tolerances associated with the proper set-up of a differential, the number of teeth is a fixed integer.
There are two possibilities here.
Mike
// please trim quoted text to context only----- Original Message -----
From: “Hill Robert J 1stLt AFRL/DELS” robert.hill@kirtland.af.mil
Greg, I inadvertently started this accidential thread when Mike pointed out
(from another thread) that he had a 2.88 diff ratio (most probably a
Salisbury) rather than the “2.87” ratio in my Dana differential from my
signature line. Rob was following up on my reply. Like I said to him, the
Dana had two tags under the diff cover bolts: 1) “limited slip” and 2)
“2.87:1”.
Jim Johnson
XJ-S, V12, Lucas CEI ignition, TH400, Dana 2.87
Annapolis, Maryland
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