High Revving 4.0L?

1985xjlaredo · May 31, 2009

I am needing to redo the head no my 88 renix. The engine has about 100,000 miles on it and I know the bottom end is in good shape. I want to be able to raise the redline up to 7500 rpm. Is this possible? I thought that I could get at least another 1000 out of it if I switched to roller rockers and mopar valve springs. Does anyone make solid lifters for the 4.0L. Would I really gain anything from building a nice head and sticking it on the stock compression?

Sherman_thetank · May 31, 2009

I remember this got covered awhile back, as i recall it is at about 7k-7500 the camshaft become multiple pieces, and turns into shrapnel. There is a guy that races with the 4.0L and he gets just a few runs on his multiple thousand dollar engine at the 7500 area.
As for your head, as in any engine the head is were the power is, regardless of compression.

Goatman · May 31, 2009

You'll run out of power at the higher RPM. The cam isn't ground for power at those higher RPM's, so you'd need a different cam along with the other valve train parts.

Why more RPM? Isn't overall power what you want?

What's wrong with your head? They don't go bad at 100k unless the motor's been hurt, and if the motor's been hurt how do ou know the bottom end is good?

NW-ZJ-SCOTT · May 31, 2009

Your wanting Higher HP. throw in a Built 13b and rev out to 10k RPM all you want..

the 4.0l is not known for HP, its a torque monstor. stroke it, and enjoy the power.

Milford Cubicle II · May 31, 2009

From what I've heard 6k aint no big thang for stock push rods with new valve springs and of course the right cam but you should build the whole motor for H/P, otherwise it's not gunna be nearly as effective (i.e. open up the valves, intake, exhaust, etc.). But 6k is about as high as I would ever go for that type of 4.0. And yes, you have plenty to gain with a well built head on stock compression. Also, I'm assuming this is a street application, correct?

1985xjlaredo · Jun 1, 2009

Well i was figuring on getting a new cam that would be ground for high end HP.
This is for a trail application, not really like a rock crawling trail but like muddy trails and big open fields we play in. I really just want it to not feel like the valves are floating when i get around the 5,000 rpm mark. I have an 85 that I had a 3.2L 60 degree v6 built for it and it has a 3 speed auto in it. It doesn't really have a lot of low end grunt past 1200 rpm, but once I hit 3000 to 7000 rpm makes some good power. And it doesn't even give a hint of the valves screaming. I dont even need alot more power just to be able to keep the rpms high and the tires spinning.

Slip Kid · Jun 1, 2009

High revving inline 6 with HP in mind.... S54B32 Peak HP at 7900, stock. Peak torque at 4900, right about where my 4.0 starts floating the valves. Hopefully (for other drivers) I will never get ahold of an E46 M3. But I want one...
Revving high is rather interesting coming from a Jeep, kinda fun, but it gets old sometimes. I did not like traveling down the highway at 3500 RPM. Tho my M44B19 did not have quite the sack of the S54. And it hit the limiter at 6500...

cal · Jun 1, 2009

The problem you'll run into is the 4.0's cam is only supported by 4 bearings, so at higher RPM's the harmonics get to be really nasty. At 7,000 RPM's the lifters would litterally be bouncing off of the cam.

IslanderOffRoad · Jun 1, 2009

cal said:
The problem you'll run into is the 4.0's cam is only supported by 4 bearings, so at higher RPM's the harmonics get to be really nasty. At 7,000 RPM's the lifters would litterally be bouncing off of the cam.

As I recall Cal once said that approaching 5k rpms the cam bounces around like an epileptic badger

Slip Kid · Jun 1, 2009

At near 5k mine stops revving higher and sounds funny, I always thought it was the valves failing to open/close all the way, or "floating". But, I suppose it could be camshaft badgerizing....

cal · Jun 1, 2009

IF, and only IF, I were going to attempt this.

Comp Cams 239

http://www.compperformancegroupstor...roduct_Code=68-239-4&Category_Code=A6CAMSx4X4

Comp Pro Magnum lifters (they dont compress/inflate as much)

http://www.compperformancegroupstor...C&Product_Code=867-12&Category_Code=A6Lifters

Comp Cams 926-12 springs withi appropriate hardware

http://www.compperformancegroupstor...=CC&Product_Code=926-12&Category_Code=A6Valve

I'd do a mopar performance 1.6 ratio roller rocker (or yella terra).

You'll need upwards of 10:1 compression, quench in the .045" range, a VERY well balanced bottom end, heavy porting and polishing with larger intake and exhaust valves (heavily deshrouded).

You won't have any real power under about 2,000 RPM's at all (like, NONE) and it will still suck over 6k until it shakes itself apart.

Milford Cubicle II · Jun 2, 2009

Just switch to adjustable roller rockers so you can run a solid lifter and a mechanical cam and as long as you use good springs/retainers/etc you should be good to go... for 6k. You could probably go to 7-7.5k but it probably wouldn't last very long and most likely wouldn't have much more peak HP than the same set up at 6k. Even at 6k though you should at least get the bottom end completely rebalanced. But 6k is completely doable and even worth it if you just wanna keep some serious wheel speed through some mud.

High Revving 4.0L?

1985xjlaredo

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Sherman_thetank

Keep it pinned!

Goatman

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NW-ZJ-SCOTT

TEXAS-JT-SCOTT

Milford Cubicle II

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1985xjlaredo

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Slip Kid

That'll buff right out

cal

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IslanderOffRoad

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Slip Kid

That'll buff right out

cal

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Milford Cubicle II

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