OHC in 4.0?

Before you flame me, understand I am just asking a couple of theoretical questions out of general curiosity.

I understand why a v8 would have pushrods. That way you can minimize the number of camshafts you need. By why use pushrods in an inline engine? Really, doesn't using pushrods just add rotating mass, complexity, and cost? While we are on the topic of head design, why not use a crossflow head? Or more than two valves per cylinder? None of these design concepts are new; they were around long before the 4.0 (or AMC's first I6 in '58 for that matter). With an inline engine couldn't you make an engine that incorporates all of these design elements while using only one camshaft?

Don't get me wrong, I know that an OHC, crossflow head, and multiple valves only really make a difference in a high RPM engine. But why did AMC/Jeep/ChyCo make a less efficient than they had to?

I am sure the engineers that designed this engine did so for a good reason... I just don't know what it is.

I am done now, flame away.

The 4.0L is a very old engine-- however its one of my favorite, and most reliable.. and it makes a lot of awesome power.

But-- people thinking like you do, is why they are getting rid of it.. its been around forever.. !!

You've got to understand that Jeep/AMC engineers designed the 4.0 to be a tough truck engine that was designed for longevity and reliability in mind.

Trucks engines are (or at least used to be) engines that are happy to idle or chug along at low to medium rpms, with an occasional sprint to 5000 rpms or so for highway passing.

Nowadays, except for diesels, nearly all gasoline engines have a gutless low end and the mid range/top end doesn't show up until well above 3000-4000 rpms. This is why I believe that many more people are buying diesel trucks is because they like the torquey nature of them.

Granted, modern gas engines really scream when they are "on the cam" but just don't have the solid feeling a torquey engine like the 4.0 has.

I've driven a Ford 4.0 SOHC V6 and that thing flies above 3000 rpm, but its hopelessly weak below that. It's almost as if several cylinders are magically turned on above 3000 rpm.

Driving a Jeep 4.0 is a totally different experience.

I too like the 4.0L, even though it is based on very old technology. I can't help but think that with some of the modern design elements I mentioned it could have continued production and met current standards.

But there is the question... why was it designed the way it was? Is a pushrod engine somehow cheaper to produce? Is it inherrantly more reliable? What am I missing?

Couldn't you tune an OHC engine to have low end torque?

Pushrod engines are cheaper to make (and put together) and have less things that can go wrong with them.

Yes, you can tune an OHC engine to have lots low end torque. But why would you spend more for an OHC engine when a pushrod engine can do the same thing?

It costs more to tune an OHC to be torquey than to tune a pushrod engine to be torquey. Just like it costs more to make a pushrod engine a high rpm screamer as opposed to an OHC.

It has to do with the intended purpose.

dennisgrimm said:

I too like the 4.0L, even though it is based on very old technology. I can't help but think that with some of the modern design elements I mentioned it could have continued production and met current standards.

But there is the question... why was it designed the way it was? Is a pushrod engine somehow cheaper to produce? Is it inherrantly more reliable? What am I missing?

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Back when the AMC I6 was designed, pushrod was pretty much the only way. There were OHC motors (and AMC's Tornado I6 in early Waggys and J Trucks was OHC) but they were rare, and the tech wasn't quite where the era's pushrods were.

Personally I think the pushrod vs OHC debate is stupid. GM is making crazy stupid numbers and decent mileage out of the pushrod LSx series, proving that it isn't outdated technology.

Pushrods are cheaper and easier to put together for an inline engine? Hmmm, I wouldn't have figured. I am not arguing with you, I really don't know any better. I just assumed that an OHC would be easier because you get rid of the pushrods, rocker, and the associated passageways you need in the block and head.

How about a crossflow head? Wouldn't that have been about the same cost to produces and get us some performance gains?

In the end, it is all water under the bridge. The 4.0 is what it is. I am just trying to figure out how it got that way.

a few issues that the designers faced, one is engine height, an OHC set-up in an XJ would put the valve cover squarely through the hood with the extra few inches on top of the head. a few inches longer to incorparate the cam chain/belt, water pump and other accessory drives which would put the front of the motor in the same spot as the radiator. the challenges to design the OHC were know at the time the xj was developed, but they were designing around the 2.5 and the 2.8 v-6. the 4.0 was installed about the time Chrysler was coming into the picture and i would not think they wanted to redsign the engine compartment to accomidate a "new" OHC engine, when the 4.2 design was easy to redisign and easy to fit with the new-at-the-time Multi-port injection.
putting the cam on the top is the easy part, providing ample oil supply and allowing either adjustment. or hydraulic lifters, ample oil return, and a reliable cam drive and tensioning system is where the majority of the work is involved. you would think in modern engineering time that this stuff is easy, but with failures still common, such as the Chrysler 2.7 it is not as easy as it seems.

I don't care why or how. I love the 4.0 in my jeep. It may not be a screamer but it does just fine for about 90% of my driving tendancies

Try looking up the I6-230 "Tornado" engine that Kaiser/Jeep built in the early 1960's. Inline six, overhead camshaft, and one cam lobe operated both valves per cylinder! Very much ahead of its time, which is why it's still not around. It was, however, used in the J-truck of the time and some of the military production trucks as well. I think it was around 1960-1963.

The problem with OHC is that most people are going to drive the damn thing with a belt - which, y'ask me, is a mistake. If you've got to take an engine down to get to a belt, use a chain. Belts are for pants...

I'd like to see a revival of the 4.0L using a roller chain (or a double roller) with a cross-draft head and OHC - but it's probably not going to happen (use the high-nickel alloy and thick castings that AMC liked, and we're on to something. Forge the crank, and you've got a damn near bulletproof engine...)

There is more going on in the engineering, than just moving the cam to the top. Look at a rocker arm and notice the pivot is off center. The cam doesn't have to lift as far to open the valve an equal distance. Besides with a top cam, where are you going to put the lifter? Too much lift on a cam, makes it hard to get the push rod to follow the curve of the cam, you need more valve spring to get the job done. Which equals more wear. Solid lifters are notoriously temperature sensitive, noisy and need periodic adjustments. One the negative side the I-6 valve train does waste some horses.
There are more than a few engineering worries moving the cam to the top.
One thing that always impressed me was the size of the mains in the 4.2 and 4.0. compared to the mains in say the Ford I-6 5.0 liter variety, the 4.0 are larger. Almost like Jeep planed on increasing the cubes or horsepower someday.
If you do it right the first time, you really don't have to do it again.
I had a 66 J truck (M-715) with the 230 OHC, valve seals kept leaking even on a relatively new motor, plugs fouled continuously. It would push the 3/4 ton along at 85 all day long though, for what it was it ran pretty well. One I'm still kicking myself in the rear for selling.

5-90 said:

Try looking up the I6-230 "Tornado" engine that Kaiser/Jeep built in the early 1960's. Inline six, overhead camshaft, and one cam lobe operated both valves per cylinder! Very much ahead of its time, which is why it's still not around. It was, however, used in the J-truck of the time and some of the military production trucks as well. I think it was around 1960-1963.

The problem with OHC is that most people are going to drive the damn thing with a belt - which, y'ask me, is a mistake. If you've got to take an engine down to get to a belt, use a chain. Belts are for pants...

I'd like to see a revival of the 4.0L using a roller chain (or a double roller) with a cross-draft head and OHC - but it's probably not going to happen (use the high-nickel alloy and thick castings that AMC liked, and we're on to something. Forge the crank, and you've got a damn near bulletproof engine...)

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The Tornado was an ingenious engine all right, ahead of its time perhaps, and certainly over the head of its manufacturer. I doubt Kaiser could have made an electric motor without it somehow burning oil, and the Tornado goes down in history as another ingenious idea that didn't quite work, along with the aluminum F-85, the Vega and the Crosley Cobra.

A new Jeep OHC could be really sweet, but I don't think you could base it on the Tornado, which was killed not only by material shortcomings (the edible camshaft, for example), but serious design faults, including a long thin block with mounts built into the front plate, which warped, leaked and promoted odd cylinder wear. The double-duty camshaft, in addition to requiring a compromise in cam profile to run both valves at once, ran rockers, which were very hard to lubricate. Mercedes and Honda can make a rocker OHC work. Kaiser couldn't.

I think you'd have to start from scratch, with a slant-6. Imagine a 4.0 on a slant, with chain driven OHC on an iron head, hydraulic bucket tappets. Yeah, dream on. We need a 21st century equivalent of Frontenac, to build aftermarket heads for XJ's.

Note to 8Mud: there was a special spring-loaded valve seal kit for that engine, that worked a treat, especially with new valve guides (you had to get the right guides, with a milled top). They didn't even try to seal the stems directly on the guides, but had what looked like a big faucet washer held down by a spring on to the top of the guide. When I rebuilt my 64 many long years ago, I put those in and that's about the only thing on it that didn't leak. It ran for a few thousand miles absolutely flawlessly, and then began to burn oil again, but what went wrong wasn't the valve guides, but the oil control rings. It seems that the design was prone to bell-mouthed cylinder walls, as well as to block warping, and it just gobbled up even the special super-duper oil rings I put in. When I sold it, it still had 140+ pounds in all 6 holes, but burned a quart of straight 50-weight oil every 40 or 50 miles.

flht1997 said:

a few issues that the designers faced, one is engine height, an OHC set-up in an XJ would put the valve cover squarely through the hood with the extra few inches on top of the head.

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Here's a good example of the above statement:
http://www.autozone.com/images/cds/jpg/small/0900823d801c88de.jpg

This is a four cyl engine I had in a '76 Audi, but you get the idea when you see the amount of space needed above the intake/exhaust ports, compared to the 4.0 Jeep engine.
Easy motor to take apart and work on, strange valve adjusting with the "discs"...

scca28 said:

Here's a good example of the above statement:
http://www.autozone.com/images/cds/jpg/small/0900823d801c88de.jpg

This is a four cyl engine I had in a '76 Audi, but you get the idea when you see the amount of space needed above the intake/exhaust ports, compared to the 4.0 Jeep engine.
Easy motor to take apart and work on, strange valve adjusting with the "discs"...

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"Discs" - you mean 'shims' (that's exactly what they are.)

The overall height of the OHC can be reduced slightly, but not by much. About all that can be really done is to shorten the valve stems and go with a "direct action" (cam lobes direct on the valve stems/lash adjusters/bucket followers,) but even that can be a bit iffy. It's easier to adjust things when you have rocker arms with adjustable pivot balls, or you just have to make room for lash adjusters (or bucket followers with replaceable shims, or ...)

That's the main reason that the pushrod valvetrain has been around so long - no need to adjust it, and you can cover most engines with a small variety of standard components (if the designers want to, anyhow.) And, it's easier to adjust for head/block decking, since all you need to do is install slightly shorter pushrods (just try decking an OHC. I dare you...)

5-90 said:

And, it's easier to adjust for head/block decking, since all you need to do is install slightly shorter pushrods (just try decking an OHC. I dare you...)

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i had to deck the head in my 280z, had to buy cam tower shims (that were a PIA to find) and shim the cam towers up the same height that i decked the head. i prefer OHV to OHC in cars any day of the week. sooo much easier to work on as far as stuff like that

Wikipedia has a good article on straight six engines in general including the AMC 6.

http://en.wikipedia.org/wiki/Straight_6

5-90 said:

"Discs" - you mean 'shims' (that's exactly what they are.)

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Yeh, they are shims, but they call them disks. Must be a European thing....While doing a rebuild, you're supposed to have an assortment of them handy to adjust each valve as you go along.

SCCA28: That cylinder head looks very familiar. If you hadn't written what it was from I was going to guess a solid lifter vw head. Incidentally, the only other vehicle (aside from my jeep) I have spent any amount of time working on.

5-90: From working on vw's I have seen both solid lifter and hydraulic lifter heads. They don't look a lot different size wise to me. The hydraulic lifters don't (read: shouldn't) need any maintenance.

Everyone: So if OHC is so tall... how has BMW managed to squeeze OHC inline 6's as large as 3.2L into their little European sedans? My internet research doesn't tell me if BMW mounts their I6's on an angle, but I didn't think they did. Don't tell me that they have a short stroke engine. Their S54B32 engine has an 87mm bore and a 91mm stroke. This is a longer stroke than our 4.0L (98.4mm bore / 86.7mm stroke)!!

I know I am comparing apples to oranges here and the engine I mentioned probably cost more than a brand new XJ, but BMW is the next biggest manufacturer of I6's (that I can think of).


***don't chew my a$$ off, I just wish my XJ had a little more technology. by technology I don't mean computers, I mean basic engine design****

dennisgrimm said:

I just wish my XJ had a little more technology. by technology I don't mean computers, I mean basic engine design****

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Technology costs money.

The Jeep Cherokee was a budget 4x4 with a budget price. It isn't good business sense to put a highly developed engine into a vehicle your selling to the average Joe.

It aint no Land Rover.