intake ?

[Rescue Diver]

Regarding carb CFMs for a AMC 258 i6 / 4.2l , I found the following on Forums.Off-Road.com and this point is very well made.

"the maximum flow is dependent on the displacement of the engine, and max rpm the engine will be subject to. So calculations aside, the max flow (CFM) the AMC 258 i6 / 4.2l cid engine can handle at 3300rpm, 100% efficiency is approx 226cfm."

Since no engine is 100% efficient, the real CFM requirement is much lower. A carb in the neighborhood of 300 CFM is all you need for an engine that is built to redline at around 4000 RPMs.

Ford didn't put 287 and 300 CFM carbs on their V8 302 engines in mustangs and trucks just for the fun of it, and they weren't too worried about gas mileage back in the 1960's. Even sticking a bigger 4 barrel carb on one of their stock V8's that came with a 300 CFM 2 barrel wouldn't give you much more HP without making a lot of other modifications like cam, heads and exhaust while your at it.

 

[dalefan88]

Regarding carb CFMs for a AMC 258 i6 / 4.2l , I found the following on Forums.Off-Road.com and this point is very well made.

"the maximum flow is dependent on the displacement of the engine, and max rpm the engine will be subject to. So calculations aside, the max flow (CFM) the AMC 258 i6 / 4.2l cid engine can handle at 3300rpm, 100% efficiency is approx 226cfm."

Since no engine is 100% efficient, the real CFM requirement is much lower. A carb in the neighborhood of 300 CFM is all you need for an engine that is built to redline at around 4000 RPMs.

Ford didn't put 287 and 300 CFM carbs on their V8 302 engines in mustangs and trucks just for the fun of it, and they weren't too worried about gas mileage back in the 1960's. Even sticking a bigger 4 barrel carb on one of their stock V8's that came with a 300 CFM 2 barrel wouldn't give you much more HP without making a lot of other modifications like cam, heads and exhaust while your at it.

mines 287 cfm so thats close.

 

[dalefan88]

so what would be the best all around intake for a AMC 258 i6 / 4.2l . single or duel plane. i drive around town alot, some highway when i take it to work, mudd, and trails.

 

[Rescue Diver]

mines 287 cfm so thats close.

That's just about perfect!

 

[fzsk4p]

I wasn't planning to go into all the other aspects of a cam, and for this thread, the topic is about a stock AMC 258 i6 / 4.2l with a stock cam that's not made for higher RPMs. Throwing a 500 CFM carb on a stock AMC 258 i6 / 4.2l is over kill. Sure it'll run fine on the carb's primaries, but when you put your foot on the floor, you're throwing money down the drain on gas, and bogging your engine down. That deep throaty sound coming from under the hood while acceleration stays flat is a good indicator of this. Please show me ANY industry doccuments that say to put a 500 CFM carb on a stock AMC 258 i6 / 4.2l ? I'd love to see that!

This is why the most common carb swap on stock or near stock AMC 258 i6 / 4.2l engines it the 287 CFM 1.08 venturi Motorcraft 2100, and not the 351 CFM 1.21" venturi model.

But you bought the cam issue up.

Let`s look at a basic installation, mine. I run the Hesco 274/.470 cam. A cam that is not normally recommended for fuel injection because of it`s poor vacuum at idle. The MAP sensor doesn't like poor vacuum. Plus the HP/TQ peaks are high in the RPM range.

Hesco offers that cam in several lobe center-lines. By choosing a 112* C/L I was able to draw enough vacuum at idle to keep the MAP sensor happy. Advancing the cam 4 degrees in relation to the crank shifted the HP/TQ down a bit. Making what would have normaly been an overly aggressive camshaft work well in my application.

On carbs.

You make a good point. My reply is the quality of the fuel metering.

The engine will only pull as much as it can breathe in a NA application.

Just because a carb can flow 500CFM does not mean it will in a given circumstance. Nor does it mean it will perform poorly if it only flows a fraction of it`s rated flow.

Not to mention flow is measured in inches of vacuum. There is no industry standard for flow. So to say a MC flows 270 CFM without including at what vacuum rating is baseless in comparison to anything other then an MC.

As is Edlebrocks claim of 500 CFM. Measured of the MC scale it might flow an entirely different amount.

The Edlebrock is a vacuum secondary controlled carb. Meaning that the secondary's will only flow (CFM) what the engine will handle. The trick is to mate the CFM to proper fuel metering.

I suspect the Edlebrocks venturi(s) design is a little more advanced then the MC`s. In addition the Edlebrock offers a variety of tuning possibilities that the MC does not.

If you recall, when I installed an 1403 on a AMC 258 i6 / 4.2l . I changed a few things in the metering circuits. Jetting was changed. As was the accel pump shot. What was not mentioned are the other adjustments that were made in order to make this application run well.

Edlebrock offers a detailed tuning manual and a plethora of parts to modify each carb to a specific application.

When the carb was baselined it ran just as you said RD, poorly. When it was done. It ran very well on both the primary and secondary circuits. From Idle to 5200 RPM.

Did it pull 500 CFM ? I doubt it. My guess is about 300 +/-. But what it did pull was was a good fuel mixture in relation to the CFM.

The quailty of the fuel atomazation is what makes the differance

 

[Rescue Diver]

so what would be the best all around intake for a AMC 258 i6 / 4.2l . single or duel plane. i drive around town alot, some highway when i take it to work, mudd, and trails.

If you're not wanting to make a lot of other changes along with it to add horsepower, I'd stick with a stock intake and adapter.

 

[fzsk4p]

If you're not wanting to make a lot of other changes along with it to add horsepower, I'd stick with a stock intake and adapter.

I disagree completely.

If the aftermarket intake only balanced the flow between each cylinder better if would be an improvement. Which I suspect is it`s biggest advantage,

 

[Rescue Diver]

I disagree completely.

If the aftermarket intake only balanced the flow between each cylinder better if would be an improvement. Which I suspect is it`s biggest advantage,

I haven't read anything anywhere that says stock AMC 258 i6 / 4.2l 2 barrel intakes have any problem with flow balance. I'd be interested to learn more about this. Do you have a link?

The question is, would the minor performance gain, if there even is any, would be worth the $300 investment to him?

Here's a good link for info on the most common carb swaps that Jeep owners are doing, and the benefits of each.

Carter BBD Carburetor, Jeep 258, Weber, Howell EFI

 

[Rescue Diver]

Can anyone give us a link to any site that has dyno information or even how much horse power gain that Offy or Clifford claim a stock AMC 258 i6 / 4.2l will have with one of their intakes. I looked at both of their sites and they only say things like big horse power increase, but they don't give any numbers?

I wish the Jeep magazines would do their job and try this stuff out on a dyno. All the car magazines do this stuff regularly.

 

[fzsk4p]

I haven't read anything anywhere that says stock AMC 258 i6 / 4.2l 2 barrel intakes have any problem with flow balance. I'd be interested to learn more about this. Do you have a link?

The question is, would the minor performance gain, if there even is any, would be worth the $300 investment to him?

Here's a good link for info on the most common carb swaps that Jeep owners are doing, and the benefits of each.

Carter BBD Carburetor, Jeep 258, Weber, Howell EFI

This is one of the better links that I have seen.
4.0L 99+ intake manifold swap with REAL DYNO NUMBERS! - MallCrawlin.com Forums

Is it worth 300 bucks ? Only each individual can answer that question.

Personally...I`d do a complete 4.0 head and EFI system before spending 300 on an intake/carb.

 

[dalefan88]

ok you two calm down lol. i think ill do some more checking be fore i buy one.

 

[Rescue Diver]

This is one of the better links that I have seen.
4.0L 99+ intake manifold swap with REAL DYNO NUMBERS! - MallCrawlin.com Forums

Is it worth 300 bucks ? Only each individual can answer that question.

Personally...I`d do a complete 4.0 head and EFI system before spending 300 on an intake/carb.

That link is for 1999 and newer 4.0 with direct port fuel injection. I was hoping to see dyno numbers for Offenhouser and Clifford intakes with no other changes including the carb, versus the stock AMC 258 i6 / 4.2l intake.

Every guy and his brother with an old small block camaro back in the 80's would throw an Edelbrock Performer intake on their 350 expecting it to run 13's in the quarter. If they took it to ths strip they were always disapointed to find they were still running 16's or maybe high 15's if they were lucky. A performer intake over a stock cast iron spread bore 4 barrel intake added on avaerage about 5 hp which is a drop in the bucket on a 200 hp motor.

I'm sure your Edelbrock carb and aftermarket 4 barrel intake works absolutely great running on the primaries over your old worn out BBD or whatever you had. The million dollar question is, how do you know that the intake added any of that performance at all? Could the improvement be due only to having a brand new well functioning carb? How do you know that running a brand new proper sized 300 or so CFM carb on your stock intake wouldn't run as good if not better?

The bottom line is, what do most CJ owners want from their engine? Usually thay want a reliable smooth solid running engine with enough pep to pass someone on the highway if need be, that won't stutter or stall on steep inclines. They want and engine that get's decent gas mileage, and that makes lots of torque at really low RPMs. After all, it's a Jeep, not a hot rod. All of those things can be had with a stock intake, and $50 for a junk yard Motorcraft 2100, a rebuild kit, and a couple cans of carb cleaner. To each his own, but I'd much rather put that extra $550 it would cost to buy the Offenhouser intake and Edelbrock carb into a winch, a locker, or a new top for my CJ.

I kind of agree with you on the fuel injection idea. I've read a little about making a do it yourself fuel injection swap. There are a million 91-99 Cherokees in the junk yards with all the parts you would need to install a factory direct port fuel injection system for under $200. The only thing holding me back is I want to keep my Jeep as simple as possible, and you can't get much more simple than a stock mechanical fuel pump feeding a non computer controled carburetor.

 

[fzsk4p]

Too bad we don`t live closer together. It would be fun yakking over a couple cold ones on the subject.

 

[Rescue Diver]

Too bad we don`t live closer together. It would be fun yakking over a couple cold ones on the subject.

Absolutely!

On that note, I think I'll grab a cold one and head down to the garage to rebuild my MC 2100 300CFM carb that I just bought from PetesCJ on this forum!

 

[Cheepjeep85]

pissing match resolved, good job on agreeing to disagree.

 

[CJ]

pissing match resolved, good job on agreeing to disagree.

Almost thought I may have needed to say something there for a minute! Whew!!!

Nice thread!

 

[dalefan88]

pissing match resolved, good job on agreeing to disagree.

ya i was just asking what would be better with my mc2100 lol. i think they both had a beer and cooled off now lol

 

[dalefan88]

Almost thought I may have needed to say something there for a minute! Whew!!!

Nice thread!

thanks for keeping open for me cj lol. i just wanted to know which intake was better lol.

now which is better. single or duel plain. not looking for 7,000rpm something around up to 4,500rpm range. for driving around, mudd, and trails. thanks

 

[CJ]

thanks for keeping open for me cj lol. i just wanted to know which intake was better lol.

now which is better. single or duel plain. not looking for 7,000rpm something around up to 4,500rpm range. for driving around, mudd, and trails. thanks

Dual is always the go for lower end torque and HP. Single's are for higher RPM. When I dragged I ran a single. Sucked below 3000. The Victor 2 I used for a while (single) never even woke up until 4750 or so.

NASCAR runs Victor Jrs (mostly), that should tell you something....

 

[dalefan88]

Dual is always the go for lower end torque and HP. Single's are for higher RPM. When I dragged I ran a single. Sucked below 3000. The Victor 2 I used for a while (single) never even woke up until 4750 or so.

NASCAR runs Victor Jrs (mostly), that should tell you something....

so would it be alright to run that duel plane one in the pic i posted with a 2 bl carb?