Renix MJ + 5 MPH wind or inline = can't hold 70 MPH

[895XJ]

Seeing as my MJ is a gutless turd I'm looking into a few options. With no wind it can cruise in 5th no problem. I would like opinions on what everyone thinks I should do since I'm sure this isn't an uncommon issue.

 

1. Regear, currently on stock 3.07s. I have 3.55 axles to put in but I'm concerned that with 31s it'll put me right back into the same spot I'm currently in.

 

2. Injectors, four holes. I did these on my 95 and they seemed to help. Not sure how the Renix would respond to them, input?

 

3. HO head, intake, throttle body and exhaust swap. To the best of my understanding, the TB and intake is the choking point on a renix. Can anyone offer input on the results of this swap?

 

3. Full HO harness with 7120 motor, currently on the bottom of my list since I've done a good amount of work to my current harness and motor, would hate to through that all out but if this is what needs to be done, I'll do it. The results of this are obvious to me but I really do want to exhaust all resources before tackling this one.

 

So far I've done a 90 ECM swap, plugs, cap, rotor, wires, timing chain and gears. Good compression on all cylinders.

 

 

 

 

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[Strokermjcomanche]

Sell the 3:55 axles and go 4:10

[NorthWoods]

Gears is what you need.  3.07 with 31's is not in your favor. You are lacking the power in 5th, given changing gears will increase your rpm's at said 70mph, you probably need higher rpm's to be further into the torque and hp range that will allow for you to be in 5th at 70. I'd be surprised if you were turning more then 1800 or 1900 rpm. 3.55's would get you 200-300 rpm's. With 4.10's you'd be more in the range of another 600 rpm's. Putting you in the 2500 rpm range, which won't do anything good for fuel economy in a 4 liter but you should have plenty of power to cruise in 5th, while being well under 3000rpm.

[rokinn]

I just did the Volvo injector upgrade yesterday on my 90 renix.  Took it for a short spin (less than a mile) afterward and was very duly impressed with the change in performance.  Of course I'm running 215/75/15 on a 3.07 but happy with result non the less.

[Marine1Texas]

I think if you put a 4.10 with 31's your be very happy. Either find your self a 4.10 axle or you can put a bigger ring gear on that 3.55 you have. With 3.55 you will not have to upgrade your carrier, however if you try with the 3.07 you will have to upgrade everything in the axle but the shafts. If you do a regear take your time to do it right. Or find your self someone who can do it right.

 

The problem I see is 31's even on a more powerful motor is still going to be in your way on a 3.07. If you upgrade to the 3.55 you will be at around the same rpms again with the 31's however with more to turn you loose a bit of power. If you do the 4.10 with everything you have now, you will raise your rpms at 70, which will give you more power. When I did my 3.31 to 4.10 on the 31's I could go from 70 to 80 easily even going up hill on renix non HO.

 

I think the larger throttle body with the larger injectors are a good upgrade, but you are still going to run in to the power not being there at the low in.  Right now 31's your be around 1850 rpms and with the 3.55 your be at 2130 rpms at 70. however if you do the 4.10 your rpms with be at 2460 rpms at 70. So your be in higher torque to HP curve.

 

that my 2cents!

[Crash]

Gears are definitely necessary. 3.55's would help in comparison, but not ideal for 31's. If they are plug n pray, do it for now.

 

I have 32's and 3.73's in my 4.0L/manual MJ. It does decent on the highway and depending on 'grade' I lose a couple mph unless I downshift. But when in 4th I can easily hold 70+ without WOT. I'm sure that the engine is probably tired-ish but never lets me down.

 

I do want more gear. But it works for now. I'm thinking 4.56's as I may go taller

[Hippie66]

I have 3.07 rear axle and 31's. 70 is no problem. The Volvo injectors did help.

[DirtyComanche]

The reality is this isn't a 300+HP truck.

 

I'd throw those 3.55 axles in.  If you can find a set of 4.10 axles, sure, that might make sense, but not if it's a lot of money.  If you're thinking about a rear axle swap/upgrade this is the time to do it.  I don't know what size tires you have, but most likely going from them to 31s (which probably measure 30.5") will still be an improvement when going to 3.55 from 3.07 gears.

 

Otherwise get used to downshifting to 4th when needed.  Contrary to the oft repeated belief on many Jeep related forums, the transmission in a Jeep has multiple gears with the intention that in high load situations it will allow you to downshift and extract more power from the engine, and then allow you to again upshift when the loading is decreased.  I know, the banhammer should swing and I become an unperson for dare saying such, but it must be said.

 

The upgraded (Volvo) injectors may make a small difference, but IMHO you're more likely to see other improvements (easier starting, cleaner idle, slight fuel economy increase) rather than any butt-dyno certified power increase.  They're worth it regardless and I meant to order another set of them, thanks for reminding me.

 

Swapping to a bigger throttle body will help.  I don't know about the intake.  My XJ has a HO intake and TB on it, and I ported the TB, but the HO intake really doesn't line up that well on the Renix head so it's hard to say what the gains are there.  I wound up doing a minimalist port match job on the intake, but without a dyno or flowbench you're just guessing at if you helped anything or not, and the end result left me less than warm and fuzzy feeling about the amount of gasket contact around the intake.  There is some options if retaining the Renix intake, this guy did a vortec 4.3L throttle body swap: http://www.cherokeetalk.com/forum/f64/throttle-body-swap-renix-70mm-6236/

 

A proper exhaust with a decent flowing muffler is worth a few ponies, especially if whatever is on there is half plugged.

 

Another thing is the actual airbox doesn't have a big enough opening in it to flow nicely.  Personally I'm a fan of cone filters and tossing the whole airbox in the trash, but some people prefer to keep it.  Just make sure you run a quality filter either way, no oiled K&N type crap (unless you don't care that much about your engine, I admit there is an Edelbrock one on my XJ, but it's on the list of things to be replaced).

[johnj92131]

I have to think this engine is simply not running very well.  70 mph in top gear should not be a problem on a level road.  My AW4 equipped truck with stock 3.55 gears and 235/75 x 15 tires would do much better than 85 mph on the freeways.  But when the plugs were shot and other things were wrong, I had performance problems.  Once the problems were fixed, the truck had no problems power wise - even towing another Comanche over 7000 ft mountain passes. Yes, it was slow.  But it did it and had no problem maintaining sensible highway speed towing the other truck on a level road.

 

Check the basics first:  new plugs, new plug wires, distributor cap rotor, etc.  Then read your plugs and see what they tell you.  Simple things like that first.  You also don't say what size tires you are running now - I "think" you are saying you want to run 31s - not that you are running 31s now.

[mvusse]

I had an XJ, 4.0 stick shift. On 235/75R15 tires (just a tad larger than stock) it would cruise 75mph in 5th with ease and throttle to spare.

 

3.55 would help, but should not be needed. Your engine must not be running the way it should.

[Minuit]

My '89 barely can stay at 75 up hills in 5th and it's got 225/70/15s, but its engine is extremely tired. The 3.07s are definitely not helping, but I'd definitely make sure the engine is running right. How is the power at slower speeds? I'd be checking for an exhaust restriction. When the '91 coughed up its cat it would struggle at any speed, but especially up any kind of hill. The exhaust also sounded very, very wrong.

[FrankTheDog]

I have 31's with 3.07's on a 92.

I just don't use 5th gear as much.

Come to any kind of hill on the highway and it get's shifted down.

If not going any bigger than 31's 3.73's are a good ratio.

[cruiser54]

It's likely all in the gearing as others have stated.

 

The throttle body is certainly not a choking point on a renix. It will flow enough for me to point my speedo needle straight down. But, I'm on 235s and 3.55 gears. 

 

I do advocate using the 60mm Renix throttle body from www.strokedjeep.com, but save that for later. 

[WyoCherokee]

All of the above. 3.07's and 31's is what is hurting you the most. i have 3.55's, 31's, 60mm TB, cowl intake, injectors, exhaust and the ax-15. It will hold 75 mph in a 30mph headwind in 5th gear, but with foot to the floor and struggling, so idrop it to 4th and cruise at 3k rpms. If you have the axles, swap them in, and do the rest as you can.

[895XJ]

My '89 barely can stay at 75 up hills in 5th and it's got 225/70/15s, but its engine is extremely tired. The 3.07s are definitely not helping, but I'd definitely make sure the engine is running right. How is the power at slower speeds? I'd be checking for an exhaust restriction. When the '91 coughed up its cat it would struggle at any speed, but especially up any kind of hill. The exhaust also sounded very, very wrong.

It's straight piped but has plenty of power 2-4. First gear had so much power it destroyed itself!

[895XJ]

To everyone with a 5 speed and 4:10s, what is your RPM at 75? 

[mikekaz1]

http://www.grimmjeeper.com/gears.html

 

here's a cool site for calculating any tire/tranny/gearing combo you want.

[FrankTheDog]

To everyone with a 5 speed and 4:10s, what is your RPM at 75?

What size tire?

[895XJ]

31s preferably

 

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[Marine1Texas]

at 75mph in 5th I run 2670 rpms, and in 4th at 75mph 3380 rpms. That is ax15 with 4.10 and bf Goodrich 31's.

 

I think the 4.10 with 31's highway speed is at the sweet spot for power range too....

 

Before I had the 31's with 4.10, had the 235/75's and 5th at 75mph was 2825 rpms, and 4th was way to high for my likes at 3575 at 75.

 

When you plus size tires your loosing rpms and power ranges go's down. to get the power range backup that's why you regear. as you plus size gets higher and higher you need to compensate for more mass turning as well. I think 3.73 with 31's is about the same as power band as the 3.07 with 225/75's. However a 4.10 bumps your rpms up a bit higher and your engine has to do less work because it is producing more power.

[895XJ]

What do you average for highway mileage ?

 

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[Marine1Texas]

at 75 I get 19 mpg, at 70 I get 20.5 and at 65 I get 22 mpg that is 5 speed 4.10's  with 31's and a tonneau.  When I am pulling a trailer that is 7200 lbs fully loaded I only drive 65 mph max, because of my own safety reason's I get 17mpg. I also run no lift stock 4wd height that is on my 87 mj 4.0.

[WyoCherokee]

  When I am pulling a trailer that is 7200 lbs fully loaded I only drive 65 mph max, because of my own safety reason's I get 17mpg.

:???: :bs:

[johnj92131]

at 75 I get 19 mpg, at 70 I get 20.5 and at 65 I get 22 mpg that is 5 speed 4.10's  with 31's and a tonneau.  When I am pulling a trailer that is 7200 lbs fully loaded I only drive 65 mph max, because of my own safety reason's I get 17mpg. I also run no lift stock 4wd height that is on my 87 mj 4.0.

 

Here is my towing MPG experience.

 

Last September I towed an 86 MJ on a Uhaul tow dolly with my 91 Comanche 4.0, AW4, stock 3.55 gears, 225/75 x 15 tires from Victoria, B.C. to Michigan. 

 

I averaged 14 to 15 mpg for the trip towing.  The 86 MJ and the tow dolly likely weighted about 4000 lbs.  Compared to my trip home with nothing in tow - it cost me 5 to 6 mpg over "normal".  And I was in a hurry to get home - so my speed was up there at 70+ (or more when I could).

 

Here is a link to my posts last September:  http://comancheclub.com/topic/51141-86-diesel-mj-on-its-way-to-new-home/?hl=johnj92131

 

In June/July 2016, I took the Comanche from San Diego to Boston and back to San Diego.  My best MPG was 26+ when I kept the speed to 60 mph.  65 mph would net me 23-24.  Higher speeds dropped the MPG a bit more to the 19-21 range.

[Marine1Texas]

That shows you that wider and larger tires take more gas to turn. This is why plus sizing from stock 225/75 of 28.3 inch's tall to 31 inch's tires is a good reason to regear from 3.07 to at least a 3.55 on a manual or better. I think even 3.73 is a great option but 4.10 don't let the hills slow you down at all.

I picked the 4.10 over 3.73 because the gas mileage was not going to be that much worse, it would make hitting hills way easier and when your in 4wd low it really just rolls over everything with out any effort. Before we chipped the back ring tooth it was 3.31 and it was good on gas and pulling hills with 235/75's. I tried 31's on it with the 3.31 gears I and my employees did not like it much. So I switched back. Now we have the 4.10 and 31's it is even more peppy then with the 235/75's and 3.31.