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4.0 head change dyno results

4.0 head change dyno results

tinman

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Idaho Falls
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81 CJ7, I6, T5, D300, 4.2 block with a 4.0 head, Cherokee MPFI
Just got back from the dyno, and while not exactly a straight swap, it's pretty close.

First pull was back in April with a stock CA AMC 258 i6 / 4.2l engine, but with all smog stuff gone. It did have the 2100 carb and HEI for the test, but everything else was stock.

166.1 ft-lb@ about 2250 RPM's and 88.1 HP@ about 3300 RPM's.

Today's test had an '84 block, bored .030 over, '95 7120 head(rebuilt, but stock), and a comp 252 duration cam. Same carb and HEI, but I did replace the muffler with a new magnaflow one.

224.3 ft-lb@ 2500 RPM's and 140.3 HP@ 3500 RPM's.


So, a gain of 58 torque and almost 52 HP with mostly just the head. I'm not sure how much the cam, the boring, and the muffler added, but I was pretty happy with the results!

Now I need to get off my rear and start a build thread I guess:). I still plan on doing some sort of FI sometime, just not sure when, or what kind.
 
Do you have the dyno sheet? I would love to see it.
 
The description for the red line should read 81 CJ7 with the AMC 258 i6 / 4.2l , instead of 82 and a 4.0.

Looks like everything moved up about 200 rpm's. Is that primarily due to the cam?

Jeep dyno.webp
 
Thanks for the info.

Mine is close to your build. I sure can tell the difference on and off road but really couldn't guess what difference it was in HP.
 
The cam could have a lot to do with the HP/rpm range.
 
The headswap is probably the best upgrade you can do for the AMC 258 i6 / 4.2l .
When I did mine, I went with a 262 duration cam and flat pistons. A Clifford intake and 500cfm Edelbrock. A healthy P&P on the head and I dynoed at just under 300hp. It was rather impressive.

A few people I know have gone to the full extreme of milling the head for larger chevy valves and jumping to a 272 duration cam with results well over 300hp.

My only issue with my set up was offroad performance lacked with the carburetor I was running. Some tunning and re jetting would have probably helped it out but I was having to much fun at the stoplight with the ricers :D
 
Did you have problems with a fast idle? I notice your charts start at a higher RPM after the head and cam swap. The last person I remember having a dyno after a head swap gained about 30 HP as I remember, I thought that was well worth the effort.:cool:
 
The headswap is probably the best upgrade you can do for the AMC 258 i6 / 4.2l .
When I did mine, I went with a 262 duration cam and flat pistons. A Clifford intake and 500cfm Edelbrock. A healthy P&P on the head and I dynoed at just under 300hp. It was rather impressive.

A few people I know have gone to the full extreme of milling the head for larger chevy valves and jumping to a 272 duration cam with results well over 300hp.

My only issue with my set up was offroad performance lacked with the carburetor I was running. Some tunning and re jetting would have probably helped it out but I was having to much fun at the stoplight with the ricers :D
What kind of torque #'s did you see and at what rpm's? Also what octane gas did you have to run? What kind of compression where you running?:confused:
 
Did you have problems with a fast idle? I notice your charts start at a higher RPM after the head and cam swap. The last person I remember having a dyno after a head swap gained about 30 HP as I remember, I thought that was well worth the effort.:cool:


You know, I'm not sure why that starts at a different RPM. I'll have to go in and ask him, but I never had a problem with the idle, so who knows.

And yes, my test was done on a chassis dyno.
 
You know, I'm not sure why that starts at a different RPM. I'll have to go in and ask him, but I never had a problem with the idle, so who knows.

And yes, my test was done on a chassis dyno.
I wasn't questioning your print out, it is far closer to what I would expect to see, then the 300PH stated by anther poster.:)
Cjim7 just what are the specs of your build, what components did you have to use to get that hp, I know it's possible to get that out of a 4.0L stroker but your looking at changing the displacement closer to 4.8L or 5.0L. I think the compression and all would require high octane fuel or race fuel. I'm looking at building a motor. I"m not sure if I want to build a 4.0L stroker or just build a AMC 304 . I know theres no replacement for displacement, but I think for what I want 230-250 hp would be plenty. I"m looking more at torque then just plain hp. Just would like to get good power and mpg, and the best bang for the buck.:)
 
Last edited:
HP is expensive when building a motor. I went on Comp Cams cam selector entered all my specs for my AMC 258 i6 / 4.2l build they estimated mine at 225 hp 289.9 foot Lbs of torque. I havnt been able to back those number up yet because of unforeseen personal problems. But when I get the motor broke in with a few oil changes I will dyno it on a chasis dyno.
 
Yes, this was gross hp at the flywheel. I installed flat pistons after a .040 overbore along with the 262 Comp cam boosted my compression ration to 11:1. I left the valvetrain stock except for the tighter valve springs recommended by Comp Cams. I was able to run 87 octane which is still a huge mystery to my...everyone I know running 10:1 or higher experienced pinging with anything but premium fuel. I was just lucky I guess.
I P&P'd the 4.0 head and used an XJ header with the Clifford aluminum intake. Kept the exhaust free-flowing with a glasspack. I completely de-smogged the engine which gives back a lot of robbed raw power. Mid-80's california smogging was archaic at the best even under perfect working condition.
The 500cfm Edelbrock was a perfect combination with my engine build.
Most of the AMC 258 i6 / 4.2l 's powerband lies at between 2000 and 2500rpm, this stayed true to form with my engine

All this was a healthy contributing factor to my T-5 being replaced with a T-18 . I couldnt keep the T-5 together anymore.

As great as this build was, my 401 turned out to be a masterpiece as far as keeping something user-friendly and yet exploiting the optimal power output while still staying on a budget.
AMC V8's are capable of huge power, the architectural block strength and design leads way to upgrades many engines cant handle mechanically without further upgrades. The biggest drawback is a poor oiling system which is easily solved with the addition of a cheap diverter.

...But this is about inline 6's. Im a huge fan of them and was extremelly happy with my AMC 258 i6 / 4.2l hybrid. I would DEFINATELY do another one.
 
The headswap is probably the best upgrade you can do for the AMC 258 i6 / 4.2l .
When I did mine, I went with a 262 duration cam and flat pistons. A Clifford intake and 500cfm Edelbrock. A healthy P&P on the head and I dynoed at just under 300hp. It was rather impressive.

A few people I know have gone to the full extreme of milling the head for larger chevy valves and jumping to a 272 duration cam with results well over 300hp.

My only issue with my set up was offroad performance lacked with the carburetor I was running. Some tunning and re jetting would have probably helped it out but I was having to much fun at the stoplight with the ricers :D
Heh aheh Heh! Aint it fun tho?
 

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