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  • CFM rating?

    I was reading about the Weber 400CFM conversion kit for my 4.2L I-6. I noticed it said that it is suggested for modified engines. Do I have to have a modified engine for this? Also, what would be the highest CFM rating a stock 4.2L could handle? Thanks!

  • #2
    first you have to know your volumetric effeciency. then go to THIS LINK

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    • #3
      Nope, you don't have to have a modified engine for it... aside from losing most of the emissions garbage (a plus in my book).

      I assume you're talking about the 38/38 DGAS webber carb?
      That's what i'm running with a totally stock 4.2, runs like a champ. Plus if i ever do add some mods i know the carb will be up to it.

      The 38/38 supposedly gets worse mpg... but i doubt it's by much unless you have a VERY heavy foot. May/probably will need to lean out the passenger side idle needle to get a decent mixture, but that seems to be a problem with all aftermarket carbs, something about the design of the intake manifold.

      Don't know the max cfm for a 4.2, but that would depend more on carb designs i would imagine... and depend on how well you wanted it to run. It would probably spit and sputter and stuff just by squirting fuel right into the head with spray bottles?

      Good luck.

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      • #4
        Get the 38/38. Much better carb for the 258 than the 32/36. The 32/36 is big enough, but because of its progressive design, you are always into the second barrel, which can cause hesitation when you crack that second barrel open. I’ve had both and I like the 38/38 mucho better. Both are great off-road carbs once you lower the float a little.

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        • #5
          What does the 38/38 32/36 mean anyways. I built an AMC 304 Eldelbrock Performer Package, yet they call for a 600cfm, I am thinking about getting the 500cfm for the little V8. What ya think?

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          • #6
            How do I find out what my volumetric efficency is?

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            • #7
              Originally posted by cj-7
              How do I find out what my volumetric efficency is?
              follow the link about 4 posts up

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              • #8
                Get the 38/38 - you won't notice the mileage difference (as stated if any) and it will provide you with plenty of umph that that 32/36 doesn't. Modified engine is not required for the 38 - not sure why most suppliers push that (probably a legality thing).

                Top-End Performance is a great source of info and helpful if you get stuck.

                Color me gone...
                Roll Bar Bound

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                • #9
                  Questions from stuff on this thread:
                  How come the adds in the catalogs say Weber is for off-road only?
                  (I'd like to use it on the road)
                  Do 33's rub much with no lift?

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                  • #10
                    are you joking about the 33" rub question?

                    it's marketing with the weber. the best off road carb is the holley truck avenger. hands down winner. the only thing better than a truck avenger is FI.

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                    • #11
                      That was actually directed toward Wally- his ID says he has 33 tsl's with no lift.
                      Before the hack-attack, there was a great thread on here regarding the truck avenger, even about the newest version- a 470. Someone mentioned it was either available now or very soon. It seems that would be the perfect match for the 258, but obviously, no one has it yet. That is combination that I really want to hear about.

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                      • #12
                        sup craig, i like the new name...i am 100% with craig on the truck avenger being the best off-road carb out there and only ting better is fuel injection...and as for 33's rubbing...depends on how much he likes the sawzall, i love it so i had no problems with the 33's rubbing at full stuff...when i went to 35's i had to do a 2" body and i still rub a good amount if i try to punch em into the fenders. once the SOA goes on i wont have anyproblems with rub, but till i get about 1700 bucks i aint even gonna start on the SOA...once that is done i am think 37's might look decent. and you best bet is to go with a weber...or if you can do without your baby for a couple months, wait till the 470 becomes available then go with a 4 barrel intake and a truck avenger 470! good luck on it and glad to see everyone back on

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                        • #13
                          Originally posted by 84MSRB
                          Questions from stuff on this thread:
                          How come the adds in the catalogs say Weber is for off-road only?
                          (I'd like to use it on the road)
                          Do 33's rub much with no lift?

                          Weber isn't 'street legal' because it gets rid of the computer, and about 16 out of 20 emission devices on a later cj7, and other reasons...
                          It is 'possible' to run it on the streets, but it may cause trouble for you when it comes time for emissions... and if you life in Cali forget about it.

                          33's rub a whole lot with no lift... actually 31's rub a lot with no lift. BUT, if you don't mind cutting all 4 fenders up quite a bit, it doesn't rub much at all. The front tires still rub the leaf springs at full turn (but it would do that too with a lift kit if the steering geometry didn't get all screwed up with a lift).

                          Edit: At some point this may cause an argument about lifts screwing up steering geometry... and yes of course i realize it's theoretically possible for you to have a lifted CJ7 with out screwed up steering.... but i haven't seen one yet.
                          Last edited by 86cj_wally; 02-20-2005, 07:11 PM.

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                          • #14
                            Originally posted by 86cj_wally
                            .............doesn't rub much at all. The front tires still rub the leaf springs at full turn (but it would do that too with a lift kit if the steering geometry didn't get all screwed up with a lift).

                            Edit: At some point this may cause an argument about lifts screwing up steering geometry... and yes of course i realize it's theoretically possible for you to have a lifted CJ7 with out screwed up steering.... but i haven't seen one yet.
                            Hey, total agreement. When you lift a Jeep it does mess with the steering. I have proved this. However you can get it to a tolerable level with the right parts and tools.

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                            • #15
                              Back to the CFM issue- I went to the link listed previously. I have a tach, but I'm not too sure how accurate it is in the first place, and I have no idea what the red-line value should be for a 258. Right now, speedo cable is broken, but cruising at highway speed is around 2000 RPm, per the tach. If the default value for VE is left at 83% on the calculator, redline value has to be around 7000 to get the CFM value close to 470. Manipulating the numbers can obtain the desired value of 470 pretty close- but how close is close enough?

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