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Old 02-16-2009, 12:09 PM   #1
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Question Highway speeds - who can run them?

All,

I'm starting work again on our '87. It has trouble staying at 65mph and drops down to 50 mph on hills. It has 235x75x15" tires and 3.54/3.55 gears.

I've removed all of the emissions stuff, Nuttered it, replaced the entire exhaust system and did a Team Rush upgrade. Also tried running an MC2100 but it was running too rich so I took it off and put the old Carter back on and rebuilt it. I live above 4500 feet and set the timing to 8 BTDC. Anything higher and it pings like crazy. It starts fine and will rev to 4000 (but I don't like doing that because it sounds horrible). It will toodle around town all day long without breaking a sweat. But shift it into 5th and blah, it just has no power. I'd run it in 4th but doing 80 in 4th gear, well the RPM's are just way to high.

Anyway, the highway speeds out here are 75mph but everyone does 80. Who here can run 75 - 80mph in their carb'd Jeep that has limited modifications (i.e no 4.0 head)?

Thanks,
Shawn

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Old 02-16-2009, 01:12 PM   #2
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Rear gearing is probably what your looking at. Your could try this link--> Gear Ratio Calculator I'm thinking with the size tires you have 4.10's should do the job for you.

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Old 02-16-2009, 01:23 PM   #3
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Running 4.10's at 75mph I'd be turning 2826 rpm's.
Stock tires were 225x75x15 and w/3.55's and those tires I would be turning 2498 rpm's.

With 235x75x15 tires and 3.55's I'm running 2447 rpm's.

It's funny how such a small increase in tire height (we are talking .59" difference) can have such a huge effect on the highway with these Jeeps. If I put 31" tires on it I'd never be able to get it into 4th.
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Old 02-16-2009, 01:32 PM   #4
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Sounds about right for what you are running. Most of us have to DS even with lower gears when it comes to hills. Just not a lot there in 5th gear and that is with a TJ.
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Old 02-16-2009, 01:48 PM   #5
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Yup I am running 4:10's right now with a 2.5L and 32x11.50x15's .... which mean 70 is a far away land in 5th gear. Semi's pass me going up hills. Planning on 4:56's or even 4:88's in the real near future...
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Old 02-16-2009, 10:42 PM   #6
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my 2.5 will hit 70 on the flats but on hills it slows down really fast
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Old 02-17-2009, 08:43 AM   #7
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My 2.5L is stock running on 225x75x15 max is 80 on flats with a hill if I put it to the floor i'll lose 4 or 5 mph
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Old 02-17-2009, 09:12 AM   #8
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5th gear is a dream away if I want any type of gas mileage. My 2.5 stock running 32's I get 65 at about 3100 rpm and stay pretty steady on the hills as well. But if it's flat i can get 5th gear and get 70-75 mph.
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Old 02-17-2009, 02:20 PM   #9
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Yesterday I decided to play around with the timing.
I usually use a light but this time I used a vacuum gauge.
I adjusted the timing to get the highest idle vacuum as possible then dropped it down 1 in.hg. I locked it down then checked it with the light. It was at 17 BTDC. I'm above 5000 feet and the stock setting was 16 BTDC for that altitude so not bad. My idle vacuum was 16 in.hg. Again not bad.

So I took it out for a spin. Yes, there was pinging but it was inconsistent. However, what was interesting was that it had waaaaaaaay more power and I was able to hold 70+mph on hills without any pinging and it wasn't struggling. The only time it would ping was if I held it above 2500 rpm for just a few seconds while in gear (1st, 2nd, 3rd or 4th) OR if it was under load in 3rd or 4th. But if it was under load in 5th it would not ping.

I came home and backed it down to 15 BTDC. The pinging was still there. I tried 12 BTDC and the same thing. So I backed it down to 8 BTDC where I had it originally and now I can't hold 70mph but there is no pinging. It just has no power and it runs hotter.

I really do think I have a bad timing chain or distributor or a combination of both OR there is a ton of carbon on top of the pistons. I've seafoamed it and let it suck down a quart of water (very carefully I might add) but there hasn't been any change.

It has done the same exact thing with an MC2100 and the old Carter BBD so I'm sure it's not a carb issue.

I've got a new timing chain on order. Hopefully I'll have some time in the coming weeks to change it out. I'll get this old 258 running better if it kills me.
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Old 02-17-2009, 07:09 PM   #10
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What exactly are you trying to accomplish? If it is getting rid of your ping problem then timing, better quality of gas, downshifting a bit sooner or up shifting a bit later might help. If you are wanting to get freeway speeds with those gears and tires it ain't gonna happen by messing with the timing. It's a gearing issue.
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Old 02-17-2009, 07:34 PM   #11
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Yesterday I got a ticket for doing 77 in a 55. I guess my 4cyl on 31s does alright.
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Old 02-17-2009, 09:43 PM   #12
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lol thats good

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