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Unread 09-25-2006, 06:47 AM   #31
Sweeney
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2006 LJ Wrangler 
 
Join Date: Aug 2006
Location: Spencer, MA
Posts: 1,307
Quote:
Originally Posted by 300bhp/ton
I haven't read all the posts so I'm not sure whats been said.

Essentially configuration of the engine probably has little to do with power deployment, for the most part anyhow.

Now the 3.8 is a modern engine and thus has all the traits of modern engines. Thi good and bad. Generally modern engines are efficent, economical and reliable. However they generally are revvy little suckers.

Older engines like the 4.0 are longer stroke and in typcially American fashion are good on low end grunt. This means they are very responsive low down. But generally the torque PEAKS early so HP isn't great.

To understand this you need to know what HP is. Well its mearly an expression of torque at speed.

HP = torque (lb ft) x rpm / 5252

So you can make less torque at higher rpms but make more HP.

In fact making torque at high rpms is the key to high HP applications, such as modern 4 cylinder engines. Or more extreme sports bikes and Formula 1 engines. An F1 engine is capable of 800bhp from only 3.0 litres, but it needs to rev past 17,000rpm to achieve this.

Now the new engine is a OHC engine and I would guess it has been designed to take advantage of this valve train setup, this means short stroke which in turn means higher revs. So PEAK numbers and highway performance should be superb, although you will probably have to drive a it bit hard to make the most of it.

The old 4.0 was a push rod OHV engine, now generally speaking these are long stroke engines. This means the don't like revs but will make more power down low. Personally I prefer such a setup as it means you don't have to drive the engine as hard to make the power. The down side is the engine is being less efficenct in terms of specific output and also maximum HP is potentially limited.

For off roading I honestly believe the 4.0 is a better powerplant. You want lots of power/torque and engine breaking from idle. The newer V6 will not be able to deliver this. Gearing can help but not overcome. This is why diesels are often great off road, they operate in low rpms and can make massive lb ft.

A nice example is I have a 2.5 turbo diesel Discovery (111bhp), my brother recently had a 3.5 litre OHV V8 Land Rover (137bhp). Off road these are both great and in low 1st will crawl along at idle no problem and on steep rouge inclines there is plenty of power to keep it going without risk of stalling.

My mother however has a Vauxhall Frontera, a modern 4x4 which weighs less than either of the Land Rovers. It has a moder DOHC 2.2 litre 16v 4 cylinder engine and makes 136bhp. So more power than the Discovery and pretty close to the V8. However as it is a modern engine its all revs. And off road it is pretty cutless. To get up some slopes you have to ride the clutch this is because it will stall due to lack of grunt low down. As long as you are above 3000-3500rpm it's fine, but driving in such a manor off raod is not only potentially damaging but dangerous also.

However due to the incresed displacement of the new V6 I'm sure it won't be as bad, but the inherant traits of its design will still exist.

For high speed use or racing the V6 is so much better than the old 4.0, however for low end grunt and relaxed driving I'd plum for the old timer each and every time.
You have a few essential things wrong;

The 3.8 is an OHV pushrod engine.
It has a longer (86.9mm) stroke than the 4.0 at 86.7mm.
No one has posted the actual torque curves to compare the two engines below their torque peaks. Torque in the 1500-3000 rpm range would be nice to compare.

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Unread 09-25-2006, 08:19 AM   #32
300bhp/ton
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1995 XJ Cherokee 
 
Join Date: Mar 2006
Location: England
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my bad, just assumed the engine was the same as in the current Cherokee (UK model, think its liberty or something in US).

Difficult to find much info really the Jeep website SUCKS! it really does. No proper pics limited info, plenty of pointless waffle though.

Anyhow if this new unit it an OHV and a long stroke then it stands a chance of being very nice, although the new Jeep does appear to weigh more than previous versions. But hay that's modern car design for you.

Still the theory of HP, torque and rpms is still relative.
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Unread 09-25-2006, 08:33 AM   #33
Sweeney
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2006 LJ Wrangler 
 
Join Date: Aug 2006
Location: Spencer, MA
Posts: 1,307
Quote:
Originally Posted by 300bhp/ton
Still the theory of HP, torque and rpms is still relative.
Yes, you cannot actually measure Hp unless you have an actual horse (of average strength)

We do lots of dyno testing for our turbo business and even those numbers can be skewed and/or vary without changes to the engine or it's tune... just different dyno runs.

I still want to see those power curves... I wonder if one of the current JK owners would mind strapping their Jeep on a chassis dyno???
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