Thread: SBC in Jeep, some tune/gear/MPG questions

Tweet

I bought a 90 Wrangler with a busted 305 in it a few years back and have been trying to get things right ever since. It has a 305 Autozone reman motor (so the story goes) that I got out of a friends 84 camaro.

This year I put a new QJ on it - the old one that came with it was a real frankenstein. The new QJ (summit's house brand) really made a HUGE difference in the way it runs at part throttle.

Here's my issue - on the back roads or anything below a steady 60mph hiway type ride the engine runs fine and gas mileage is around 17. However, on the hiway, particulary up hill (and it's hilly here) you have to put the pedal nearly to the floor to maintain speed and MPG drops...to the point you can watch the guage drop - I mean 5mpg territory.

It's not a daily driver, but I can't take it anyplace that involves a hiway under this condition (it seemed a tad better on the old carb, but that carb had lousy throttle response, ran lean (pinging under load), malfunctioning choke, and overall got 10-11mpg).

A person at a 4th of july party says chaning the gears is what I need to do.

Here's what I got:
305 (stock 84 camaro more or less specs), HEI w/ MSD internals, 195 t-stat, new QJ with stock jets/rods, cast iron log manifolds but true dual exhaust, 700R4, 4.11 gear w/ 31" tall mudder tires.

It will idle in OD at 30-35mph but at 60mph revs begin to approach 3000 rpm.

So I'm thinking that he's right - the revs are past the peak torque on the hiway. I plan to replace the mudders with stock (29" tall) tires and I think that will only make things worse.

Anyone have any thoughts? Is it a tuning thing on the carb or truly a gear issue?

---------------------------------------- reponse (can't seem to post to this thread...)
TV cable is good and properly adjusted.
lockup is good (you can feel it and revs drop a bit) and all four gears engage. I can manually d/s to third (has a gated B&M shifter on it) so I know all 4 gears and OD/lockup are working.
mech and vac advance both working as designed. double checked this last week.
no cats...so I doubt the exhaust is plugged. One muffler is blown (rust) and the other is a year old.
Hmmm...i'll have to see what's up.

Engine temp does rise (or did with a 180 t-stat) when I hit hiway cruising speed. With a 195 in there it essentially does not rise above normal cruise. (I put the higher t-stat back in after the new carb fixed the pinging issue)

Now the PO didn't have the TC lockup hooked up. I went a simple way - when the trans shifts to 4th a pin in the trans connector goes to ground, so I used this to lockup the converter. I have a bypass switch so I can verify it's locking up, and when you slow and it shifts back to 3rd it unlocks the converter. There is a trans cooler as well.

The tach is new aftermarket (the old one was oem for a 4cyl) so it should be accurate.
Roadspeed at 1000 ish RPM is right (30-35mph). I'll check 2000 rpm to see if it's 60mph..it's close as I recall.

A plugged exhaust best describes the issue - had a converter plug up on my last jeep and it was very much like this. I guess when I put the muffler on i'll check and see what I can find. I guess the rusty muffler could be collapsed in on itself...

Any clue what the symptoms would be if one side of true duals was plugged? Same as a plugged single exhaust?
How to 'test' the tranny for slippage at highway speeds? I'll lay a patch if you jump on it from a standing start and I've had 1-2 shift chirps from it as well.

I doubt it`s a gear issue ....sounds tune related/engine related.
Those gears are a good match for a 700r4 and that combo was installed behind many 305 smallblocks.
Sounds like you need to do some engine testing ,check the valves,compression test,leakdown test,sounds rpm related ,such as bad advance on distributor,clogged exaust,bad cam.....check for blowby also.

I really doubt its gear related.

Originally Posted by DennyW

700R4, 4.11 gear w/ 31" tall mudder tires.
but at 60mph revs begin to approach 3000 rpm.

It should be this: 3000 RPM 96.17 MPH

Sounds like a Trans problem to me. Maybe it's not locking up into overdrive at 60 mph ???

I agree,when I had my '65 chev C-10 with 4.10 gears and 31X10.50's and 700R4 70 MPH was around 2400 RPM.

I'll agree with the other guys. You did reconnect the TV Valve cable right?? Did you set it properly? You can do some major trans damage if it's not connected or adjusted improperly! It can eat a trans quick!

If you just run the numbers a 31" tire with a 1:1 transmission output gives you 2674 rpm at 60 mph. Assuming a 1.1OD ratio that would drop to 2430 rpm. Your stated rpms are way off, and you should have lots better power on the hills. One question - Did you correct your speedometer gear when you went to 31's? Can you check your speed with a GPS to confirm actual speed?

Backing off to 29" tires is the wrong thing to do, for looks and off-road performance if nothing else, but it will also kick your rpm higher. I'm running 33's on my TJ with 3.73:1 gears, manual tranny.

Something seems off there in his setup......

He should still not have to floor it to do 70 in 3rd gear ,maybe the torque converter is shot ?

His RPMs are High and it has no power under load -- Sounds Like clutches slipping

but at 60mph revs begin to approach 3000 rpm.



I'll check 2000 rpm to see if it's 60mph..it's close as I recall.


??????? Which is it????????????