What's the most HP for a 4.0?

[seanyb505]

Imagine the sound of a 4.x revving to 10k..lol

 

[Dr. Dyno]

Can you get the cam/crank balanced to increase the rpm limit, and by how much if any?

There's little you can do about the cam 'cause the problem is that it's only supported by 4 bearings. You'd have to modify the block to add extra support to the cam and reduce the amount of flex at high rpm. The other fix is a gear drive timing set but Mopar Performance stopped making those a while back.
As far as the crank goes, your best chance of making that survive very high rpm would be to lighten it, knife-edge the counterweights, balance it, and nitride harden the journals. The other alternative is to have a forged steel crank custom ground.

Imagine the sound of a 4.x revving to 10k..lol

Nobody's ever managed to get a straight six to rev to 10,000rpm so I'd say the only sound you'd hear is kaboom and the sickening sound of the engine being reduced to shrapnel.

 

[MaXJohnson]

Nobody's ever managed to get a straight six to rev to 10,000rpm so I'd say the only sound you'd hear is kaboom and the sickening sound of the engine being reduced to shrapnel.

Honda cbx-1000

 

[Beej]

Max has a point, but the CBX I-6 as smooth and seductive as it was, was notoriously unreliable and really only revved to 9500. 10000 is past the redline. Maybe I'm splitting hairs, but oh well...

 

[j99xj]

Unreliable Honda? I've never heard that before.

All my Honda's were/are great little machines if they have their oil changes and occasional adjustments. (valve tappet clearance, contact points)

 

[seanyb505]

Nobody's ever managed to get a straight six to rev to 10,000rpm...

Thats why you would have to imagine it

 

[BBeach]

There are some guys that get there bmw m3's up to 9k rpm (doubt its stock because stock redline is a tad over 8000rpm) and its an inline 6?

 

[Dr. Dyno]

There are some guys that get there bmw m3's up to 9k rpm (doubt its stock because stock redline is a tad over 8000rpm) and its an inline 6?

It's a smallish capacity I-6 (3.2L) with an all-forged bottom end. That's how it can survive 8000rpm.
The Honda CBX-1000 engine has small cylinders and with a tiny stroke, the piston speed is reduced so it too can survive very high revs.
The bottom line is that a big capacity I-6 by nature is a torquer, not a revver, simply because it has bigger heavier pistons, a longer stroke, and thicker heavier rods than a V8 of the same displacement.

 

[BBeach]

It's a smallish capacity I-6 (3.2L) with an all-forged bottom end. That's how it can survive 8000rpm.
The Honda CBX-1000 engine has small cylinders and with a tiny stroke, the piston speed is reduced so it too can survive very high revs.
The bottom line is that a big capacity I-6 by nature is a torquer, not a revver, simply because it has bigger heavier pistons, a longer stroke, and thicker heavier rods than a V8 of the same displacement.

I was just responding to the fact that you said
Originally Posted by Dr. Dyno
Nobody's ever managed to get a straight six to rev to 10,000rpm...

I wasnt trying to say that a Jeep inline 6 would ever do that, just an inline 6 in general.​

​

 

[nnvaxj]

I don't know about tons of power but Honda ran an inline six cylinder 250cc
motorcyle built for racing. 60bhp@18,000rpm, Honda rc-165

 

[JJacobs]

I don't know about that Dino- it's not like the Jeep 4.0 is a diesel, the rods weigh no more than a v-8 rod. Pistons for a 6, bore size considering, will weigh the same as for an 8. I checked the specs on the first 4.0 v-8 that came up, the 290 hp Rover motor used in the 97 Jag XJ8 and it has a paltry .6mm less stroke.

 

[gradon]

So using the Hypertech Programmer to bump the 96's redline to 5800 isn't a good idea?

 

[j99xj]

We really need to find an XJ owner with a 4.0 who is planning a V8 swap so we can ask them to completely thrash the engine (their old 4.0 with rev limit removed) to see what it is truely capable of.

(not me, if anyone is wondering)

Or perhaps someone could go pull a junkyard 4.0 and hook it up to dyno and give 'er all she's got. (and video tape it so we can see what fails first)

We can argue all day about what is the weak link on the 4.0 when high revs are demanded, but noone really knows for sure until we sacrifice an engine to truly find out.

 

[JJacobs]

Did you miss my post? I did that a couple years ago. 6400-6500 wavering. No oil, broke two rods. The alternator bearings failed as soon as it passed 5k rpm.

 

[j99xj]

No oil.

 

[Flash]

No oil was this on purposely or and mistake.......


Wouldn't a harmonic balancer tuned for the higher RPM fix the crank braking probem..........

If the cam has a bad harmonic too..........couldn't where be one build for that too?

Purely hypothetical here but.............

Could balancing a crank, or for that matter, a cam, help to achieve a safe, Higher RPM?

Flash.

 

[boise49ers]

I'm thinking more like 700BHP/1000NM +, attained through clever forced induction with sequential twin-turbos and serious intercooling. The limits, as I can see it, lies within the outdated valvetrain.

One must not forget that there are countless BMW, Toyota and Nissan I6-engines (similar factory size and output as the 4.0) that push upwards of 1000 horses.

How would you fit all the forced induction equipment under the hood of an XJ. Not a lot of room to work with ?

 

[92DripCherokee]

Does anyone know the highest compression ratio a modified AMC inline 6 engine has ever sustained?

 

[MudDawg]

Balancing won't help...the problems have to do with how long the crank and cam are, number of bearings...one harmonic is caused by "twist" of the parts and the other harmonic is a "whip" problem...it's probly best to runt the 4.x in an rpm range where the problem doesn't exist.....if you want to shove the rods out the pan.....all you need are big enough nitrous jets....the amount of power you can build would be limited only by mechanical strenght of the parts.

Here's a high revver form the past....Honda RC149....125cc 5cyl...not a typo...5cyl ....usable power thru 21,500 rpm....talk about making your ears bleed! It is almost impossible to describe how tiny the parts are...DOHC with all gear cam drive, 4 valve and a 7 speed gearbox.

 

[j99xj]

This has probably been brought up before but what if someone used an AMC 199 crank in a 4.0 block?

This would be the smallest stroke (3 inch) with the biggest bore (3.88 inch) combo which should offer the best overall horsepower. This is exactly what Honda has been doing for years building oversquare motors. This would give 3.5 liters.

Naturally this would reduce low rpm torque but I believe that to be irrelevant when you can easily put lower gears in the axles.