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More power out of a 258

9K views 34 replies 16 participants last post by  slomo19er 
#1 ·
Ok I feel as though my cj is lacking in power it's got a 258 in it with a weber no emissions sh** and a tuned duel output header. It has to work hard to get to 65 and god forbid I have to slow for traffic because getting back to that speed takes forever... I think maybe the issue is gearing I have 32 inch tires and plan to go to 33's but I have 2.73 gears would rehearing to say a 3.54 or 4.10 be even slower?
 
#2 ·
i was having the same trouble w/ my 78 cj w/ 258(brand new to) and i switched from 3.55's to 4.10's and man what a difference!! i couldnt believe it instead of losing speed on a slight incline i for once gain speed and hang with typical traffic haha! im running a 31 inch tire to...33's and 4.10's would be a good combo in my opinion and would be a big change for you also.
 
#5 ·
Dylan, I think there are several issues here.
1st - you should have problems getting to 65 mph with 2.73s and 32" tires with a good running 258, My was tired and would do that all day long with 2.73s and 32" tires.
2nd - Bigger gears (numerically higher) will give you more accelleration but less top end (your speed at red line, which you shouldn't ever see in a CJ). You didn't say wether it was a 4 spd or a 5 spd. If you got over drive then bigger gears are possible. I just had a thought, you weren't tring to hit 65 in 5th gears were you? If so that's the problem. Don't use 5th gear, ever. 4.10s are kinda steep for 32/33" tires without a 5th gear.
Provide more info and you will get better recommendations.
 
#6 ·
I have God awful 2.73s and 33 inch swampers but I can nab 65 mph pretty darn quick. I think you need to look into your advance curve. If you still have the stock distributor, check this thread out. Matt did a great job of getting his stock distributor right. http://www.jeepforum.com/forum/f8/help-advance-curve-1238634
 
#10 ·
I agree with Swatson. You need to get the 258 running correctly first. When you do the gears and tires will be tough on your clutch. Then move to 3.50s - 3.70s. I don't know about the different carrier, usually the break is 4.10 and down. Remember the cost is times 2. The AMC 20 is a decent rear if you get 1 piece axles, another $300, R&P $200 each, Master re-build kit $150 each, Aussi-locker $400 and lots of beer for your friends $100.

A much easier way would be to find complete front and rear axles with the correct ratio, hard to do but not impossible.
 
#9 ·
I feel your pain with 2.73 gears and 32" tires. I did that for years in my 82. I switched to 3.73's about 14 years ago and am very happy with it. My wife had an 84 with 3.54's and a 258 5 speed running 33's and it was fine. Great on gas, but again it was a 5 speed. I think the 3.73's are a good compromise for 33's used on and off road.

You'll need to buy a new carrier for the rear amc20 for sure and the front Dana30 will need one only if you go numerically higher than the 3.54's, otherwise your current one will work. And then of course gears and bearings for both. Plus now's the time to do a locker if you want, it'll save the cost of a carrier in the rear.
 
#15 ·
How will I know when the 258 is running correctly? It feels as though it is running fine... I just have no acceleration I can nearly keep speed uphill never mind accelerate
This ^^^ is how you know that it isn't running correctly ;).

If you currently have the distributor from the feedback BBD era installed on an aftermarket carb or nutter bypassed BBD engine, your ignition timing is way off of optimal and will definitely keep you from making the power that it is able to.

I'm a little surprised at how many people overlook this important fact and buy new carbs, big distributor caps, fancy ignition parts, etc. to try to get more power when the ignition curve itself is often times the main culprit.
 
#16 ·
Well with 3.54's you'll just need a new R&P and a master install kit for the front. But if you go with a locker then that will be extra. The rear will need a new carrier, R&P, and a master install kit. To put a locker here you could nix the carrier because the locker will replace that.

There are many articles to explain the differences in lockers, just do a quick search. I would say a mechanical is more reliable, just because if you lose an air line your SOL! There are good arguments for both. Personally I'm putting a Detroit TruTrac in my Dana 30 front. An air locker will need an air pump, typically mounted under the hood. Check out ARB's website.
 
#17 ·
swatson454 said:
This ^^^ is how you know that it isn't running correctly ;).

If you currently have the distributor from the feedback BBD era installed on an aftermarket carb or nutter bypassed BBD engine, your ignition timing is way off of optimal and will definitely keep you from making the power that it is able to.

I'm a little surprised at how many people overlook this important fact and buy new carbs, big distributor caps, fancy ignition parts, etc. to try to get more power when the ignition curve itself is often times the main culprit.
I'm not disagreeing that the computerized distributor isn't one of the worse distributor to run but timing isn't the end all of jeep power problems.

I'm running the same stock comp dist and even with the vacuum disconnected I had no problems going up to 85 with 33's 3.54 and a tiny 1 bbl YF.

A few years ago I ran a stock hp curve against hp needed against hp available at different rpm. It had quite a few holes in it when plugging in different ratios. Worse case was the t5. In fact, there were negative hp available. That means the jeep probably will slow down in 5 th gear.

So I don't think that fixing the timing curve is going to fix his problem. With all the possibilities of vacuum leaks, poor spark, wrong mixture. I would look at those first. Along with static timing. I'd worry about the curve last.

Just my opinion.

Sent from my iPhone using JeepForum
 
#21 ·
I'm running a 258, MSD ignition and long tube headers with a 2150 carb 3:73 gears and 33's. I have no issue with pick or accelerating on on-ramps. (I live in FL and can't account for any real hills). My dustributor is the stock 258 distributor. I've only done a the big cap swap onto it. I am having a slight running rich issue but I was able to drop from a 51 to 48 on the jets. Also my mpg was garbage but I found out I need to vent my tank and bowl and along with the smaller jets I should be up a bunch.

I turn 2300 Rpms at 80 and 1500 at 55 in OD.
 
#27 ·
Sorry.. Was at work binge reading JF posts and didn't realize how old it was. Yup.. 2300.. She kinda like the highway. Also has OD so that helps a bunch.
 
#28 ·
Don't you just love it when people show up on these forums ask questions, get answers and then disappear. Makes me think I wasted my breath.
 
#30 ·
All the time.. Did it work or not!!

Not sure if this is meaningful, but I will add as a FWIW. I was having lunch a few days before the charlotte NASCAR race with the gentleman who owns the full size pick up that holds the lands speed record at the salt flats. The truck is obviously far from stock running two hundred sixty something MPH. What I found interesting is that his tires are 31" tall and they run a 2.75 rear end. Similar sizes and ratios that we commonly see as wanting of more acceleration or top end. Now, this pick-up is pushing over 2000HP with a twin turbo NASCAR Race engine and it runs at 9000 RPM, but I thought it ironic that the gearing is the same.

FWIW.
Yes they run a lower numeric gear and 31" tires but they also have a few miles to get to speed. Most I think get a pull start to get moving. I'm sure they also run a trans with some crazy or gears in it. Figuring at a .5 over drive the tire is spinning 2x as fast as a 1.0 drive gear it will pick up a ton of speed.
 
#29 ·
Not sure if this is meaningful, but I will add as a FWIW. I was having lunch a few days before the charlotte NASCAR race with the gentleman who owns the full size pick up that holds the lands speed record at the salt flats. The truck is obviously far from stock running two hundred sixty something MPH. What I found interesting is that his tires are 31" tall and they run a 2.75 rear end. Similar sizes and ratios that we commonly see as wanting of more acceleration or top end. Now, this pick-up is pushing over 2000HP with a twin turbo NASCAR Race engine and it runs at 9000 RPM, but I thought it ironic that the gearing is the same.

FWIW.
 
#32 ·
do the math- 3:73 lowest you should go with a 258 and 32 inch tires. (same carrier you have but if you go to a 4:10 new carrier is added to the cost)

Also if you have never changed out a set of gears you may want to have that done by a shop. Rebuild the entire axel all seals and bearings while you rthere or you will waste money in the long run.

my 2 cents
 
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