CJ I6 question

For someone with a stock CJ using the 4.2l I-6 with the T-5 tranny and
stock tires...

What is the engine RPM at any speed (say 55 MPH) in 4th? In 5th?

What size, actually, are the stock tires? (In metric values please -
ie, p215 75R25 etc).

Thanks

We need to know your Differential ratios. Your T-5 will be direct,
one to one in fourth gear. And your tire will be 27.7 inches in
diameter for the other number to plug in at:
http://www.mysporttrac.net/calulator.htm
God Bless America, =DFill O|||||||O
http://www.billhughes.com/

L.W.(Bill) Hughes III wrote:
> We need to know your Differential ratios. Your T-5 will be direct,
> one to one in fourth gear. And your tire will be 27.7 inches in
> diameter for the other number to plug in at:
> http://www.mysporttrac.net/calulator.htm
> God Bless America, ßill O|||||||O
> http://www.billhughes.com/
>

True I forgot to ask for axle ratio. But, I'm not looking to put my
info into a calculator. I want to know what RPM the engine was
*designed* to run at while driving at a particular speed.

As an aside, I'd also like to see the factory torque/horsepower curve
for a stock 258. Which reminds me, I should have specified the 258 in
the original post.

What year? Many older Jeep engines are designed to run efficiently
around three grand. Choked SMOG engines down around two. Of course,
that it will vary with carburation size in Cubic Feet per Minute,
Camshaft cross flow duration, compression ratio, and headers
God Bless America, =DFill O|||||||O
http://www.billhughes.com/

I have a 258, T-5 with 3.31 gears turning 33x9.5" BFG muds (no metric
equivalent for the bigger off road sizes that I know) and at 65 mph in
4th I spin 2300 rpm. If I use 5th it drops to 1750 and my gas mileage
drops radically as well.

That 2300 rpm seems to be a nice 'sweet spot' for my engine with the
best mileage (23 mpg) and tons of punch if needed. It will hold it up
any highway hill I have hit.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

Clap Trap wrote:
>
> For someone with a stock CJ using the 4.2l I-6 with the T-5 tranny and
> stock tires...
>
> What is the engine RPM at any speed (say 55 MPH) in 4th? In 5th?
>
> What size, actually, are the stock tires? (In metric values please -
> ie, p215 75R25 etc).
>
> Thanks

Mike Romain wrote:
> I have a 258, T-5 with 3.31 gears turning 33x9.5" BFG muds (no metric
> equivalent for the bigger off road sizes that I know) and at 65 mph in
> 4th I spin 2300 rpm. If I use 5th it drops to 1750 and my gas mileage
> drops radically as well.
>
> That 2300 rpm seems to be a nice 'sweet spot' for my engine with the
> best mileage (23 mpg) and tons of punch if needed. It will hold it up
> any highway hill I have hit.
>

Thanks Mike. Mine has the 258, T-5, and 31x10.50s on it, also with 3.31
gears. Mine runs about the same as yours. But, mileage stinks in
either. RPMs in 4th are too high, 5th too low. I am getting ready for
another round of upgrades and was thinking about regearing the axles
since I really don't need bigger tires or a lift.

Any idea what the factory RPM was supposed to be in 5th at 55 or 60?

Factory gearing was typically the 3.07 axle ratio which will give you
about 1700 rpm at 60 mph in 5th. A real DOG!!!!

I'm with Mike, around 2300 is a good spot on the 258 engine. The 4
banger runs best around 2800 to 3000.

Clap Trap wrote:

> Mike Romain wrote:
>
>> I have a 258, T-5 with 3.31 gears turning 33x9.5" BFG muds (no metric
>> equivalent for the bigger off road sizes that I know) and at 65 mph in
>> 4th I spin 2300 rpm. If I use 5th it drops to 1750 and my gas mileage
>> drops radically as well.
>> That 2300 rpm seems to be a nice 'sweet spot' for my engine with the
>> best mileage (23 mpg) and tons of punch if needed. It will hold it up
>> any highway hill I have hit.
>>
>
> Thanks Mike. Mine has the 258, T-5, and 31x10.50s on it, also with 3.31
> gears. Mine runs about the same as yours. But, mileage stinks in
> either. RPMs in 4th are too high, 5th too low. I am getting ready for
> another round of upgrades and was thinking about regearing the axles
> since I really don't need bigger tires or a lift.
>
> Any idea what the factory RPM was supposed to be in 5th at 55 or 60?
>

> I get a sweet 23 mpg on the highway fully loaded running at 65 mph. If
Impressive! I'm lucky to get 17 unloaded, downhill, with a tailwind.
Mostly, I get about 14.

> Accel SuperCoil for a hot spark with the engine set up manually. I
> killed the computer.
No computer on mine either. I also did a TFI conversion and noticed an
improvement. I don't have to worry about emissions testing so all that
is gone as well.

> Manually the 258 can be tuned up to run sweet and just purr at idle,
> though a bit of a lean rumble at idle is better for the emissions
> sniffer.
Mine is all done manually (except for the electric choke on the Weber).
It runs great. Plenty of power, unJeeplike smooth idle. But mileage
just stinks. And it smells like it is running a bit rich. But I've
leaned it as far as the carb will go and it still smells rich.


Your tagline says you run 33s on your glass bodied Jeep. How much lift
does it have? Did it have 33s and that same lift pre-glass?

Clap Trap wrote:
>
> > I get a sweet 23 mpg on the highway fully loaded running at 65 mph. If
> Impressive! I'm lucky to get 17 unloaded, downhill, with a tailwind.
> Mostly, I get about 14.
>
> > Accel SuperCoil for a hot spark with the engine set up manually. I
> > killed the computer.
> No computer on mine either. I also did a TFI conversion and noticed an
> improvement. I don't have to worry about emissions testing so all that
> is gone as well.
>
> > Manually the 258 can be tuned up to run sweet and just purr at idle,
> > though a bit of a lean rumble at idle is better for the emissions
> > sniffer.
> Mine is all done manually (except for the electric choke on the Weber).
> It runs great. Plenty of power, unJeeplike smooth idle. But mileage
> just stinks. And it smells like it is running a bit rich. But I've
> leaned it as far as the carb will go and it still smells rich.
>
> Your tagline says you run 33s on your glass bodied Jeep. How much lift
> does it have? Did it have 33s and that same lift pre-glass?

The first thing I would check on yours is the accuracy of the
speedometer. For some reason my speedometer was about 7 mph off with
31" tires and became exactly right when I put the 33's on???

I think someone needlessly changed the speedo gear when they installed
the 31's not knowing the stock gear was right for them or the t-case is
used with a different gear, who knows. This is GPS verified speed.

Meanwhile, the 7 mph difference affects the 'perceived' gas mileage big
time.

I had 31's on with the stock suspension and steel body. I put YJ
shackles on for a 1/2" lift and got a 2.5" spring lift. I have tons of
clearance.

You could have issues with that Weber carb running rich. It is too big
for the engine by rights so can have too rich a jet in it stock if you
aren't running headers and a high flow exhaust. You might need to rejet
it.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's