Lets get the pros involved here...

I was reading a thread by Erik about raising the compression on his 4.0. I just know there a few of us here who could give difinitive answers here. What is the stock comp. ratio. And to what comp. can we raise it to and still be able to run 91 octain. I am aware of the over heating issue with many XJs and with more comp. and more HP this could be leading down a path of diminished returns...so What do the experts here think?

To raise the compression you're talking about installing new pistons, which means a motor tear down. If you're going to rebuild the motor, there are better options than just raising the compression ratio.

I don't really understand your question. :dunno:

Sure it couldn't be done with head work alone? The raising of the comp.? ....what is the stock comp. ratio? Can a run up to 9:1 work with the in-line six? How are the main bearings? Are they prone to failer?
This isn't something the average joe would do to build performance....I personaly like the 4.0 almost stock. But there is no question that comp.= hp. I know that torque could be lost which is not at all ideal....but who knows...that was the point of the thread....

You simplify too much.

Compression increase alone offers a limited improvement in power, when you understand the balance of comression and fuel octane. A lower compression with lower octane can perform just as well, as the higher compression and octane, if the cam timing and air management are unchanged.

Low octane fuel burns fast, to the point of detonation with high compresson, if the timing is not corrected (retarded). Combine this high compression and a stock cam and chamber finish, and you risk compression ignition detonation (Diesling).

Does the high octane fuel needed to correct these problems have more BTU's (heat/power content) that the low octane? No.

You can dump the same quantity of high octane fuel with high compression, and benefit little from the effort.

What high compression can allow for is longer cam timing and more free flowing intake and exhaust systems (to pump more combustion air into the cylinders each engine revolution) without sacrificing low rpm power. This "equivalent to stock low rpm compression" is the result of higher static compression combined with a lower effective compression due to the longer duration cam timing bleeding off airflow efficiency at low rpm.

Yes, you can mill the head to raise compression, but it will not achieve much unless you change cam timing as well (and fuel delivery, intake & exhaust porting, and other power factors).

The experience of most modern era (87- present) AMC six builders is you will risk detonation and possibly lose power (after correcting timing advance) if you do not change the stock cam when raising compression. The stock 4.0L engines are well balanced for cam timing & compression (they need to be for emissions).

of course I was over simplfying. I just wanted to start an intelligent ring of banter. Kinda hoping that someone who has built up the verable 4.0 this way would interject. We know that we can enhance the fuel flow without too much difficulty. and clearing out the exhaust flow can also be had ...but does someone make an appropriate cam? I am still not satisfied with this thread!!!! Someone come to its rescue!

The subject cannot possibly be covered adequately in one thread. The Strokers' group on Yahoo has been debating and discussing innumerable permutations and combinations of ways to build/modify the 4.0L engine (and the stroked variations ... plural) for several years now, and the topic has not reached saturation.

In general: Stock compression ratio is, I believe, 8.8:1. Stock compression ratio on the 2.5L is 9.2:1. Both engines share the same pistons and the same bearings, so the bearings will certainly live under that increase in compression.

The 2.5L has higher compression by virtue of the pistons coming closer to the top of the block. I don't know if this is done with rod length or a lower deck height ... or a combination of both. What I do know is that, with the SAME camshaft specs, the 2.5L does not have the same broad, flat torque curve that the 4.0L has. The 2.5L torque curve is comparatively "peaky."

The Strokers group has a list of cams that people have used -- with moderate but varying success, depending on the cam -- to bleed off the excess compression that results when doing a "budget" stroker (4.2L crank and rods w/ 4.0L stock pistons, which puts the pistons too high in the bore and raises compression too high). Crane seems to be the preferred manufacturer, but I don't recall the specific cams discussed. In general, you want a cam with more overlap than stock.

Jump this, not to undermine your thread, but my general thought is this. The stock 4.0 works really well and is well balanced, easy to maintain and reasonably efficient. In my experience building engines (stock and not stock, not professionally by any means) you have to look at the entire engine as a whole and how it works together (intake through to exhaust) and if you are going to the trouble of building a new 4.0 then you have to match all the mods to get the most out of the engine. In my opinion for a 4x4 I would really prefer as much power and torque as possible but with as low of compression as possible. In other words I would prefer to get my power from stroke and bore than from compression because I have found that this way of providing power is more effecient and reliable. Again this is just my opinon and experience.

Michael

Thanks to all. One thing I have found (as far as engine building goes) is any area where opinions outweigh pistons, nobody has got it right!! I was unaware that an endless debate had already started on another site. Leave me not waste any more bandwidth!!

2xtreme said:

In my opinion for a 4x4 I would really prefer as much power and torque as possible but with as low of compression as possible. In other words I would prefer to get my power from stroke and bore than from compression because I have found that this way of providing power is more effecient and reliable.

Click to expand...

I agree. This reminds me of a saying that was around when I was racing -- probably still going around, but I'm no longer in the fray: "The only substitute for cubic inches is cubic money."

Or my favorite..."Speed costs.....how fast do you want to spend?"

Ok as far as cams go sinch your hungry for an answer. I have found that in a mild non stroked 4.0 the clifford 4.0efi cam works realy well and has a nice big power band. I put the whole clifford valve train minus the rockers in my first XJ and loved it that with a header and goo exaust and minor tuning realy make it a rocket and got great milage aswell I'm sure ther are other cams on the market that are very closeluy match to itso i'm not saying clifford is king but just that they seem to have found a set up. Now i did thimotor in 96 so things have channged a bit and I haven't had time or the need to build another one YET

Red, you still run this engine? Or has it made it into someone elses garage?

that is to say have you sold that XJ?