Can I relate this thread to the fact that some people put carbs on the 258 with too much CFM ratings....if you use one of those CFM calculators online it looks like the 258 with a 'max' rpm of 4000 comes out to < 300 CFM required of the carb. Any comments on this.
FSM states that peak HP is at 3500, peak torque at 2000 for my 75. It will easily rev to over 4000, I've done it once.
Why would you want to run your engine past the peak HP #? Unless you wanted to be putting out less HP and tempt fate, all at the same time.
Don't get me wrong here, I've got a nice set of inframe long tube headers, DUI, and I've done a little work with the intake system. You build a 258 for torque, not HP. They've got long cranks and blocks, why whip it for no reason? You guys are aware that there are two basic 258 crank setups as manufactured, are you not? The old ones are generally regarded as much stronger. I don't remember off the top of my head when they went to the weaker set up, all I know is that I've got an early.
when you are climbing a muddy hill you need all the rpm you can get hp and torque may go down but usually with 4.88 gears up a muddy hill big hill those extra rpms get you up there. it is not like i am always going at 4500 i have it at redline so i can shift at the top of the speed on the wheels. if i am wrong then i will change but i so far thats how it works for me.
i agree with you 100% like i said i am not always taking it to the top. i am fully locked front and back. have 36 tsl tires and 4.88.
sometimes there is need to just let it go and keep it close to it very little times but for me it is good to know when i need to throw third or second depending on where i am at. the higher i am on the rpm the higher next gear is to not loose momentum but this are extreme hill climbs the rest is mud bogging and i dont go past 3500 most of the time.
the v8 will come soon i just need to get the most of this engine cause it was well built.
thanks for the info. i rarely hit over 3600 but i was just making sure that i wasnt overrevving it if i did hit 4k. i am looking at cams for my I6 and i see some that say their power range goes up to 4500 rpm. i want something that can build some power and more torque and i would like it to have a mild lope to it also. any recomendations?
Last edited by 00bluedak; 02-21-2009 at 05:00 PM..
Camwise I stayed stock so far for idle quality, if I need the lope I take out an 8. Like crusty said, you are building a tractor motor for torque. If you want to hit 5K go the V8 route and you'll be way happier.
i dont want to hit 5k. im just looking for ways to improve my already strong 258. if i wanted a v8, i would have swapped in one when i pulled out the original 4 banger and replaced it with a 258. call me a purist, but i love my I6. i built it and swapped it in by myself when i was 17. its no longer a daily driver like it was then so i would like to step it up a bit.
If jeeps were meant to have V-8's, willys would have installed them hee hee! I'm a jeep purist as well, if jeep saw fit to install it, then work with it ha ha. I love the jeep I-6 engines, there awesome and make a jeep a jeep. The great thing about the 258 is that it makes peck hp and torque at low rpms. Depending on cam and other mods, you could have power drop off at a higher rpm. I'm set at 3,800 rpms with a holly 4 barrel, offy intake, comp cam, header and MSD. Anything over that and the power starts to drop off. Gears won't change where you make peck power, just how fast you reach it. Once I install a 4.0 head and rollor rocker it should rev out a little farther with better flow. I'll have to use my butt dyno and play with it a little to see where I'm at. Just let your engine talk to you, it will let you know your RPM range.
1987 wrangler, 258 I-6, 5-speed.
2007 liberty sport 4X4, automatic
2009 ford ranger 4x4 FX4 OFF/ROAD, 5-speed
The most common cam "upgrade" for the 258 is to go with a mild "RV cam".
RV cams are mild, but they do increase your torque down low where you will use it off road. They don't do much for HP though.
Your 6 will never lope like a V8, simple fact. Over camming an engine that flows 300 cu ft wide open at most will lead to decreased performance and reliability. Same with carbs, lots of companies out there are dependent on hyping their hot rod parts, with no regard for if they actually work with a low revving engine like the 258 or not. They need to sell product to stay in business. It is very common to see threads posted by poor devils who bought into the whole catalogue bolt on increased power thing, and then wind up with a host of problems that they paid big money to create for themselves. Very sad.
The 258 is what it is, and it is one of the best, if not the best engines designed for it's intended purpose ever made. You can spend a lot of money, and get not much benefit, if any, by thinking that the marketing folks who write the catalogs, and the shills who help sell there wares in the magazines know more about engine design than the engineers who designed probably the best offroad powerplant for a light weight vehicle in the world.
If you want some "lope" stick a set of good headers on the thing. I've had many folks on the trail tell me that my 6 with headers sounds like a V8. It doesn't, it sounds like a 6 with headers. Yeah, it has a nice little "lope" but it is running the stock cam.
If money is burning a hole in your pocket, put it where it will make a noticeable improvement in performance. Gearing being foremost, that will eat $$ at a prodigious rate, but you will get real off road performance benefit. Do an ignition upgrade, take care of all the little stuff that affects reliability, re- wire, make sure your cooling system is bulletproof, stuff like that. A mild RV grind cam if you must. That is the stuff that counts when you are out in the sticks. True though, stuff like that won't give you much in the way of bragging rights in the mall parking lot.
If you want a hot rod, sell the jeep to someone who wants it for it's intended purpose. Then go build a hot rod.
Always keep in mind that "upgrades" if not carefully thought out, always cause a cascade of other "upgrades" to solve the unanticipated problems caused by the "upgrade" before. A very slippery slope.
How did I come to such a strong stance on the issue? Because when I was younger, I got sucked in, seduced. Only to find that in the end, the engineers who designed and built the vehicle really did know more than I did, and more than the advertising writers and magazine shills. I left a trail of screwed up "upgraded" classic vehicles in the wake of my ignorance. I started messing with cars in High School in the early 60's, and it took me a long time to truly understand the basics. Strangely, the more I began to truly understand why things were the way they were on a given vehicle from the factory, the less likely I was to do my own "upgrades". Has nothing to do with money, either.
Just be careful, go slowly, understand the ramifications of any mods before you do anything. Don't be swayed into doing a mod because it is popular at the time. Don't be a sheep, think for yourself.