I have a stock 67 OTS - 4.2 (9:1) and 4 speed. When cruising at 70
mph the tach reads 2500 rpms. Previous listers have said that their
rpms are from 2600-2750 at 70 mph with rebuilt rear ends. Assuming
my speedo and tach are accurate, is 2500 at 70 mph better than a
higher rpm? How can I find out what my rear end ratio is?
Thanks,
Steve–
steven davis
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from steven davis sent Sun 19 Mar 2006:
Steven
There are two numbers stamped on the back of your differential like
43 13. The first is the number of teeth on the ring gear, the
second is the number of teeth on the pinion. Divide the larger
number by the smaller one and you’ll have your ratio.I would guess
you have a 3.07 ratio, which would be 43 X 14.
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc
Denison, TX, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from steven davis sent Sun 19 Mar 2006:
Steven,
Since you have a '67, I’m assuming you have the S1 tach, with
the cam-driven tach-generator, right? If so, by far the more
likely explanation is your tach-generator is shot, and the tach is
reading way low. This is the typical failure mode for this tach.
Needless to say, this is really dangerous, if you frequent red-
line, as 5500RPM indicated will be far higher actual. My own was
reading almost 20% low when I retired it years ago. Nearly all US
E-types got the 3.54 axle, for which 60MPH is, IIRC, 2960 RPM, so
70MPH should be up around 3450 RPM. I’d go buy or borrow a decent
shop tach, and check the calibration of your dash tach. I bet you
find it’s reading considerably low.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Ray Livingston sent Sun 19 Mar 2006:
Ray
Those of us poor suckers with 4.2s redline at 5000RPM, which is why
you guys with 3.8s can woop our ass on top end.
Joel–
The original message included these comments:
Needless to say, this is really dangerous, if you frequent red-
line, as 5500RPM indicated will be far higher actual. My own was
–
ex jag, '66 E-type S1 4.2, '56 XK140dhc
Denison, TX, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from ex jag sent Mon 20 Mar 2006:
Joel,
What is the difference between the 3.8 & 4.2 that changed the
redline? In 1988 I updated my mossbox to a syncro box. Part of
the update required changing the flywheel and starter. I
immediately noticed the 4.2 flywheel was MUCH heavier than the
3.8. So, I took them to my machine shop and said ‘‘make this one
like that one’’!!! They did. My 3.8 ran, and still does, like a
thoroughbred.
I am currently building a 66 ots and am installing an aluminum
flywheel with the SFI dampener. I am hoping that this will allow
this engine to be everything my 3.8 was and more.
If there is something I am overlooking, I would appreciate input.
Thanks, John–
66 ots 1E12025
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from John DeLay sent Tue 21 Mar 2006:
The mass of the flywheel will not affect ultimate rev capability -
just the speed at which the engine reaches that limit. The 4.2 and
3.8 engines are different in other ways that limit the
desirability/feasibility of a high rev limit on the bigger engine.
There was a recent thread on this last week(?)–
Peter Crespin 66 2+2 ‘E’
Buxton, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Peter Crespin sent Tue 21 Mar 2006:
Peter - your sugar coating the answer.
John - the heavier pistons of the 4.2 will ultimately break the
connecting rods if overrevved repeatedly. Rebuild/resize the big &
small ends of the rods, use ARP rod bolts. If you can find late
model XJ rods (wide blade) with out the small oil hole halfway up
the rod, use them. That’s where they tend to break.
Or as Wiggy advises, just spend a grand on 6 Carrilo’s and
fugetaboutit.
5 grand Dave–
1969 BRG OTS
Skaneateles, NY, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from David Ahlers sent Tue 21 Mar 2006:
HOLY MACARONI !
Ratio for the rear end and HEAVY PISTONS???
Using your round numbers:
You have about 28 mph per thousand RPM
Looking at your ‘‘Speed in gears’’ chart shows
2780rpm at 70 mph for a 3:07…
I don’t think other Dana Ratios are close enough
for confusion (2:88 or 3:23)–
The original message included these comments:
John - the heavier pistons of the 4.2 will ultimately break the
In reply to a message from John DeLay sent Tue 21 Mar 2006:
John
There have been numerous discussions regarding the ‘‘smoothness’’ of
running between the 3.8 and 4.2. There have been theories that
because the 4.2 does not have all its cylinders symmetrical with
the combustion chambers it runs rougher than the 3.8. Even though
the volume of each combustion chamber is the same, the shape is
somewhat different. This is due to the dome of the piston not being
in the center of the chamber on four of the six cylinders. This
could cause the air-fuel mixture to ‘‘swirl’’ differently in those
cylinders resulting in a different coefficient of energy produced
by combustion. At higher RPM, say over 5000, a harmonic vibration
occurs causing undo stress on the snout of the crankshaft. This can
be overcome, however, with the installation of a high performance
harmonic balancer. The $500 investment will give you that extra
500RPM without damaging your engine. Is it worth it? That’s up to
you, and your driving habits.
Joel–
The original message included these comments:
What is the difference between the 3.8 & 4.2 that changed the
redline? In 1988 I updated my mossbox to a syncro box. Part of
I am currently building a 66 ots and am installing an aluminum
flywheel with the SFI dampener. I am hoping that this will allow
this engine to be everything my 3.8 was and more.
If there is something I am overlooking, I would appreciate input.
Thanks, John
–
ex jag, '66 E-type S1 4.2, '56 XK140dhc
Denison, TX, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from ex jag sent Tue 21 Mar 2006:
Hey guys,
Sorry to get off track on this subject!! I will research the forum.
After I posted my question, I thought about the weight of the
larger pistons. I have a set of XJ6 rods and at this point
standard AE pistons, so that should get me in the ball park.
Joel, very interesting about the head/cylinder matching. I
remember hearing about that some time back but not really seeing
the possible performance issue. I don’t really plan on RACING my
car, but like all of us, I want it to be the best it can be.
Since I have not assembled my engine yet, this is the time to GET
IT RIGHT.
Thanks for the insight, I will be watching and searching for more!!
In reply to a message from ex jag sent Tue 21 Mar 2006:
Joel,
I have always been under the impression (not sure why…) that
the rev limit on the 4.2 was a result of torsional vibration in the
crank, as a result of the increased displacement, and offset
bores. I assume the rod journals are also offset (i.e. not evenyl
spaced), which will make for some more interesting torsional modes
on the crank.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Ray Livingston sent Wed 22 Mar 2006:
Ray
It’s possible, but since torsional vibration is caused by the
change in speed of a rotating shaft in one rotation, it occurs in
all engines to some degree. Any variance between the cylinders will
cause it, but the harmonic balancer compensates for it. Could it be
then that the 4.2 engine is more susceptible to TV because of the
reasons you listed? Excellent theory!!! Which would explain why a
better performing harmonic balancer would keep excessive TV from
breaking the crank. You would think that when the 4.2 was
developed, this shortcoming would have been solved with a better
engineered harmonic balancer. I guess it was cheaper and easier to
just change the red line on the tach.
Joel–
The original message included these comments:
I have always been under the impression (not sure why....) that
the rev limit on the 4.2 was a result of torsional vibration in the
crank, as a result of the increased displacement, and offset
bores. I assume the rod journals are also offset (i.e. not evenyl
spaced), which will make for some more interesting torsional modes
on the crank.
–
ex jag, '66 E-type S1 4.2, '56 XK140dhc
Denison, TX, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from ex jag sent Thu 23 Mar 2006:
Joel,
Yes, all crankshafts are subjected to torsional vibration,
but the frequencies of those vibrations, and how they add and
subtract from each other will be quite different for different
designs. Think about what happens if you hang a weight from a
spring, pull the weight down slightly, then let go. The weight
will pull back up, above its rest position, then go down below the
rest position, etc., over and over. It will bounce up and down
with decreasing amplitude until it eventually stops, due primarily
to internal frictional losses in the spring. If you plot the
amount by which the spring is extended or compressed as a function
of time, it will look like a decaying sinusoid. Essentially the
same thing happens with the crankshaft each time a cylinder fires.
The crank twists in response to the power strokes of each cylinder,
and after each cylinder fires the crank continues twisting back and
forth with descreasing amplitude. Again, if you plot the amount of
twist as a function of time, the twisting will look like a decaying
sinusoid. In general, when each cylinder fires, the crank will
still be twisting from the force of previous cylinders firing. All
those decaying sinusoids add together. At some locations along the
crank, and at some times, the sinusoids will add together,
increasing the amplitude of the twist. At other places, and other
times, they will counteract each other, and the amplitude will be
reduced. Changing the cylinder offsets will change how these
sinusoids align, and hence how they add and subtract. The changes
to the crank to accomodate the offset cylinders will also change
the natural frequency of the crank, and how the vibrations
propogate through it.
Because and engine is a complex system, there will also be
multiple frequencies present in the vibrations, in addition to
their harmonics, so the overal vibration will have very complex
spectrum. The harmonic damper can really only effectively dampen a
fairly narrow range of frequencies, and will be less efficient at
other frequencies. And, it only acts on one end of the
crankshaft. So, no damper can completely counteract all of the
torsional forces, it can only attempt to dampen the worst of them.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–