jimoshel Posted November 8, 2009 Share Posted November 8, 2009 One thing to remember when swapping over the HO head for the Renix: The ports on the HO are a different height and to maximize the swap you need to also swap both manifolds, or do a port, and polish, job. And check gasket alignment, They're notorious for a sloppy fit. On the gear ratio the way the Big trucks do it is first find the engines torgue peak. Say 3200 RPM. Then take the speed the truck will be travelling, 90 MPH,,er,,I mean 75 MPH. Now you select the GR or TS that will let the engine achieve it's power peak in it's highest gear. However you don't see too many 18 wheeler's rock climbing, but the basics are the same. Example. I have 2 '89 MJ's One is a 2.5 with 5speed and 4.11. The other is a 4.0, 5speed and 3.73. I notice no difference between the two on take off or doing the hwy at 75. I'm sure with a load it would make a diffrence. My 2 cents worth. Link to comment Share on other sites More sharing options...
Geonovast Posted November 8, 2009 Share Posted November 8, 2009 Just so you know... The AX-15 and AX-5 aren't geared the same. As for take off.. 4.0 w/AX-15. 3.73 x 3.83 = 14.286 2.5 w/AX-5. 4.10 x 3.93 = 16.113 On the highway.. 4.0 w/AX-15. 3.73 x 0.79 = 2.947 2.5 w/AX-5 4.10 x 0.85 = 3.485 All 4.0s with a 5 speed came with 3.07 gearing, unless it's been swapped in or specified to be different from the factory. Link to comment Share on other sites More sharing options...
jimoshel Posted November 8, 2009 Author Share Posted November 8, 2009 I pulled the cover's and counted the teeth. They're 3.73's altho I wud prefer the 3.07. Link to comment Share on other sites More sharing options...
Geonovast Posted November 8, 2009 Share Posted November 8, 2009 I pulled the cover's and counted the teeth. They're 3.73's altho I wud prefer the 3.07. You must've never driven on 3.07s. How big are your tires? 155/65R13? Even the smallest factory tires are a dog on 3.07. Link to comment Share on other sites More sharing options...
Eagle Posted November 8, 2009 Share Posted November 8, 2009 I pulled the cover's and counted the teeth. They're 3.73's altho I wud prefer the 3.07. No you wouldn't. You may think so, but trust me ... you wouldn't. With a 5-speed and stock tires, 60 MPH cruise in 5th gear is somewhere around 1650 to 1700 RPM (the OD ratio is different for the three possible trannies). That's WAY below the torque peak of the engine. Even with the 5-speed, they should have used 3.54s or 3.73s from the factory. Link to comment Share on other sites More sharing options...
jimoshel Posted November 8, 2009 Author Share Posted November 8, 2009 You are right. Just wasn't thinking.Again. Link to comment Share on other sites More sharing options...
mvusse Posted November 8, 2009 Share Posted November 8, 2009 My XJ has 3.07 gears. It will burn rubber in 1st, then squeal for 2 or 3 seconds going into 2nd, again for about a second into 3rd, chirp into 4th, finally into 5th it keeps traction, but I'm going about 90 when I hit 5th. Pete mentioned getting better tires when I first mentioned this. It will do this with 3 different sets of tires, all 235/75R15. Some generic "Traction XTC", Wintermark snow tires and Armstrong Coronet SX/A. 3.55, 3.73 or 4.10 with stock size tires would scare the begeezus out of me. I thought the MJ with an AW4/3.55 had torque with the stock tires, but compared to the AX15/3.07 XJ it's a piece of poo. Also have no clue what the top speed it, but I've had it over 100 making a quadruple pass. Link to comment Share on other sites More sharing options...
Eagle Posted November 9, 2009 Share Posted November 9, 2009 My XJ has 3.07 gears. It will burn rubber in 1st, then squeal for 2 or 3 seconds going into 2nd, again for about a second into 3rd, chirp into 4th, finally into 5th it keeps traction, but I'm going about 90 when I hit 5th. Your '94 XJ has the HO engine, which for that year had a torque peak of 3,950 RPM. Most MJs are Renix era, not HO, and the torque peak for the Renix engines was 2,000 RPM for 1987 and 2,400 RPM for 1988 - 1990. I don't have any way to post up my full spread sheet, but let's look at a cross-section. The following numbers are based on 225/75R15 tires, 5th gear, 70 MPH: 3.07 ==> 1883 RPM 3.54 ==> 2171 RPM 3.73 ==> 2287 RPM 4.10 ==> 2514 RPM 4.56 ==> 2796 RPM 4.88 ==> 2992 RPM Considering that in pre-overdrive transmission days we used to cruise basically the same engine at 3,000 RPM on the highway all the time (3000 RPM was 72 MPH), even 4.88 gears is not "excessive." 4.10 gears puts you basically right on the torque peak at 70 MPH, 3.73s puts you on the torque peak at about 75 MPH. My '88 is currently running stock tires and 3.73 gears, and I like it. Link to comment Share on other sites More sharing options...
mvusse Posted November 9, 2009 Share Posted November 9, 2009 The torque peak being that high explains why I get better mileage at 75 (21mpg) than I do at 60 (19mpg), despite the higher wind resistance.My Comanche, however, when it was still stock got it's best mileage doing 60 (24mpg, despite automatic transmission). Around town the XJ gets 17mpg, though, where the MJ used to get about 15. I'm guessing the transmission makes the difference here. Link to comment Share on other sites More sharing options...
Pete M Posted November 9, 2009 Share Posted November 9, 2009 squealing a tire peg-leg style is actually pretty easy with a manual trans (all you need is to shock-load it with the clutch pedal to break that initial traction) and isn't generally a good indicator that you have enough gear. Especially so with pickups. Link to comment Share on other sites More sharing options...
mvusse Posted November 9, 2009 Share Posted November 9, 2009 Even when I take off easy from a stop sign to make sure I don't break loose, and THEN get on it it squeals. And it's not a pickup. That's in my Cherokee. Link to comment Share on other sites More sharing options...
Pete M Posted November 9, 2009 Share Posted November 9, 2009 whoops. XJ, MJ, we all know I can't tell the difference. :rotf: still, I never had that problem with the BFGs in my MJ when it was 2wd with an empty bed. Something different about your Jeep. not sure what it is. :dunno: Link to comment Share on other sites More sharing options...
mvusse Posted November 9, 2009 Share Posted November 9, 2009 Had a chance to get it on a chassis dyno for $50 a couple weeks back. Didn't have $50 at the time :( Link to comment Share on other sites More sharing options...
Oizarod115 Posted November 9, 2009 Share Posted November 9, 2009 Had a chance to get it on a chassis dyno for $50 a couple weeks back. Didn't have $50 at the time :( a higher gear seems to break tires loose easier in my experience, because once you get it spinning it will spin REALLY fast in that gear. my truck also has 3.07s and a 5spd, and while it will do EXACTLY what you describe... it is WOEFULLY underpowered even with a 225-70r15 on it. the truck NEEDS lower gears to move, not to spin tires Link to comment Share on other sites More sharing options...
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