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Max RPM of Jeep Engine

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Plechtan
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Max RPM of Jeep Engine

Post by Plechtan » September 26th, 2008, 5:56 am

I was trying to get some actual information of someone who has a Jeep engine that will rev into the 5,500-6.500 rpm range. I am looking to build a land speed record engine (probably be good in a desert racer too!) I understand the special cam, solid lifters, special rods, etc. I was concerned about the crank, valve float, etc

Any input would be appricated

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Re: Max RPM of Jeep Engine

Post by seanyb505 » September 26th, 2008, 6:44 am

They get bad cam or crank harmonics around 6k i thought.
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Re: Max RPM of Jeep Engine

Post by Plechtan » September 26th, 2008, 7:26 am

Is this just somthing you heard, or do you have actual experience on tis subject? Crank problems could probably be handled witha a fluid damper, and the cam issue maybe could be addressed by a timing belt drive, although i do not know anybody who makes one.

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Re: Max RPM of Jeep Engine

Post by yuppiexj » September 26th, 2008, 7:45 am

5-90 over on NAXJA has noted that there is a 3rd order harmonic around 6500 but going to 8000 on a spin balanced bottom end can and has been done.

the harmonic needs to be "hurried through"

Now I'm seeking a MA formula for calculating 3rd order harmonics.
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Re: Max RPM of Jeep Engine

Post by Cheromaniac » September 26th, 2008, 7:50 am

seanyb505 wrote:They get bad cam or crank harmonics around 6k i thought.
The 4.0 (or stroker) gets a bad cam harmonic at 5600-5700rpm that can either break the timing chain or throw the distributor out of the block. The only known cure (a timing gear drive) doesn't exist anymore unless you have one custom made.
The crank harmonic occurs around 6300-6500rpm and that can be minimized by balancing the bottom end.
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Re: Max RPM of Jeep Engine

Post by John » September 26th, 2008, 7:59 am

There is a vicious cam harmonic at starting around 5600rpm capable of breaking dual roller timing chains, spitting dizzys out of the motor mount, The problem centers around the length and support of the camshaft. It is a situation that has been dealt with. Some ran gear drives instead of chains and ran through the harmonic quickly, various damping methods have been used, Hesco feels the design of the aluminum block they are working on addresses that issue. Search Barney Navarro AMC I6 for info on a 650 HP Indy car. Oiling system upgrade will be necessary.
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Re: Max RPM of Jeep Engine

Post by Plechtan » September 26th, 2008, 9:24 am

Found a good link to the Navaro motor, it was only 188cid. http://wps.com/AMC/Navarro-turbo-motor/ Not much on the cam though.

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Re: Max RPM of Jeep Engine

Post by Plechtan » September 26th, 2008, 11:03 am

Did anybody ever try and drive the cam from both ends?

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Re: Max RPM of Jeep Engine

Post by yuppiexj » September 26th, 2008, 5:40 pm

Plechtan wrote:Found a good link to the Navaro motor, it was only 188cid. http://wps.com/AMC/Navarro-turbo-motor/ Not much on the cam though.

Pete
here's the specs
http://wps.com/AMC/Navarro-turbo-motor/ ... -specs.txt
Last edited by yuppiexj on September 26th, 2008, 6:30 pm, edited 1 time in total.
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Re: Max RPM of Jeep Engine

Post by John » September 26th, 2008, 5:53 pm

Barney sleeved the 199 ci I6 down to meet the displacement requirements for turbo motors. AMC advertised the camshaft as a reground factory cam.

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Re: Max RPM of Jeep Engine

Post by Alex22 » September 27th, 2008, 8:14 pm

If you want a cam that is designed for higher RPM's, I know that Charles at Camcraft.com has a few cam designs for high rpm jeep engines. He has one that hill climbers like to use (6000rpm) and one that is in a heavily modded 258 drag race engine turning 6500rpm, the customer has never broken the cam.
As for balancing the bottom end. A 6 cylinder crankshaft doesn't need any bob weights when it is being balanced, which would mean that it doesn't matter how much the pistons and rods weigh, as long as they are all equal with weight matched ends and pistons. That would mean that the same harmonics that exist in a stroker would also exist in an equally built 4.0.
If you're concerned with the amount of oil getting up to the rod journal then use upper bearings in the block side and in the main cap. If the tangs don't line up then you can cut a new grove in the block for the tang to locate in. Cutting the crank like that will make it weaker and cost you a lot more money than the few minutes with the cut off wheel. Shimming the pressure relief spring will help as well.
Barney's camshaft looks like it is a flat tappet design. If you look at this picture you can see the lobes are worn slightly on the edge due to the concave shape of the lifter face and the sloped cam lobe face. If the cam was made for roller lifters the faces of the lobes would be parallel to the bearing surfaces and there would be a slight wear pattern down the center of the lobe.
One good thing to keep in mind is that you're looking at 40 year old technology and theories. The engine oils today are much better than what they had back in the day so you don't need as much of it now. When I asked about the grooves in the main journals of Barney's crank at work the guys said that it was an old school trick and said that using the double set of upper bearings will do the same thing.

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Re: Max RPM of Jeep Engine

Post by Plechtan » September 29th, 2008, 6:33 am

I know that Charles at Camcraft.com has a few cam designs for high rpm jeep engines
I Went to the CamCraft.com website, nothing at all about cams (go figure!) are you sure about the webaddress?

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Re: Max RPM of Jeep Engine

Post by yuppiexj » September 29th, 2008, 6:49 am

TurboTom wrote:i will eat my words later if need be.
TurboTom wrote: Not sure of your rules...but you need to start with an engine that works best for the rules and cheat from there!
Proud owner of many stroker parts, that have not yet spontaneously assembled themselves.

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Re: Max RPM of Jeep Engine

Post by Alex22 » September 29th, 2008, 7:06 pm

yuppiexj wrote:http://www.camcraftcams.com/ perhaps?

Yep, Thats the one.
He is in the processes of updating the website so not everything is listed.

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Re: Max RPM of Jeep Engine

Post by Plechtan » October 6th, 2008, 11:10 am

Been looking around, found this in a article on the history of the jeep engine in JP magazine
1999
This is the last model year for improvements to the 4.0L, which include new direct-mount accessory drive bosses, a thrust washer added to the camshaft, a new chain oiling system, and larger-diameter casting between lobes (to stiffen the camshaft in "bending" mode). Also, ribs are added to the rocker pedestals and holes are tapped for the new coil rail system in the cylinder head.
So maybe the new engine does not have the cam harmonic problem. Does anybody have any experience?

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