dave1123

Well, there is that! LOL!

third coast

Gmac - thanks for posting this. I like your approach. Also having an engineering backgound I am interested in taking a data/fact based approach. I am considering replacing the radiator and water pump on my red ’91. I have not had a chance to drive it extensively since acquiring it to fully evaluate its cooling system capability but I expect most of the cooling system parts are original. This Jeep may see time in high altitude highway and off road trail conditions that seem to push XJ cooling systems to the limit, and then some. I don’t want to put new parts in that are not as good as the OE parts I now have.

Some thoughts:

I have found other comments about the Spectra radiator transmission cooler proximity to the radiator outlet and concern this is a significant restriction. Your cutaway photo is revealing. There really is a lot of area around the cooler at the radiator exit fitting. (It would be interesting to see a similar cutaway of a standard duty and/or HD OE radiator) I expect the Spectra designers knew what they were doing or just did a good job copying the OE design. From what I see I doubt the trans cooler to radiator outlet interface is of major significance when you look at pressure drops across all components of the cooling system. For the most efficient heat transfer to the transmission cooler (heat exchanger) you want turbulent flow at a relatively high velocity across the length of heat exchanger surface. Laminar (straight line) flow that occurs at lower velocities and with unrestricted flow paths develops an insulating like boundary layer at the heat exchanger surface. If the transmission heat exchanger was further from the end tank and radiator exit I expect it would not be as effective, especially at lower cooling system flow rates. And if the trans cooler was removed the radiator might actually become less effective as there likely would be more of an unbalanced flow through each of the radiator tubes to the exit. Those in line with the exit would get disproportionately more flow, shortchanging those at the ends. For those without some background in flow physics and heat transfer I can see that this probably does not seem right.

Some things to think about:

Lifts: at highway speeds it’s likely that high lifts upset the intended airflow as it splits between what goes over the hood, what goes through the radiator and what goes under the vehicle.

* Altitude: water boils at a lower temperature as altitude increases. At 8,000 ft water boils at 198 degrees compared to 212 degrees at sea level. Cooling systems would be slightly more prone to boiling at altitude compared to sea level.

* Missing front fender liners: at highway speed this likely changes airflow for the worse in the engine bay

* The effect of the OE engine splash shield on engine cooling: that rubber like shield that goes from the lower radiator support to part way back under the engine oil pan often falls apart or is damaged and removed when repairs or modifications are made. That shield may help direct more air along the sides of the engine and intake and exhaust manifolds instead of being pulled down under the vehicle. As of last year you could still get an OE splash shield from Chrysler for later model XJ’s. I can’t find anything for my ’91.

* The design of the 4.0 engine with intake and exhaust manifolds on the same side of the head and thermally closely coupled does not help with cooling under high load conditions when exhaust temperatures increase. Would insulating the exhaust side be of any real help? I am not sure I believe in any practical benefit of exhaust side coatings for an XJ application.

* Roof racks, front brush guards or other equipment, heavy payload, large tires along with improper gearing: requires more power to move the Jeep through the air at highway speeds and thus the engine generates more heat

* Engine condition: An engine that is running too lean will result in higher temperatures. Eliminating possibility of a vacuum leak could be helpful. An engine that is worn will not be as efficient, generating more heat in bearings and with higher combustion gas blow-by. A bad head gasket can introduce hot combustion gas into the cooling system, increasing temperature and pressure.

I have yet to find any conclusive data on a superior radiator choice for an XJ. It should be something that fits without modifications. Lacking good evidence for something else my preference would be the Chrysler OE Heavy Duty radiator that was an option with towing and possibly off road packages. It seems that it is no longer available.

Morat

Fascinating thread - I look forward to hearing some data driven discussion on how to keep an XJ cool

However, I don't think that altitude will play a part in cooling efficiency due to reduced boiling temp of water as it's a pressurised system governed by the coolant cap.

BlueRidgeMark

I doubt that airflow in the engine bay is a significant factor. There has been a lot of discussion over the years about the heat in the engine bay, and many run hood vents to reduce this. Some just use spacers on the hinge mounts to raise the back edge about an inch. I've even seen a custom made hood with very nice vents built in. In reading many threads on this, the consensus seems to be that it does not really make any difference in cooling the engine. It just makes it easier to work in the engine bay right after driving (e.g., checking oil, trans fluid at a refueling stop).

IIRC, it has been shown to be helpful with the infamous heat soak issue, but I'm not sure that directly translates to cooling the engine. See my point above about airflow.




Hmmm. I think you are right, but it would certainly increase the drama of a sudden release of pressure.


Good discussion, gentlemen!

BlueRidgeMark

This is a hotly debated topic. I agree with you on this. Dump some of the heat BEFORE it gets into your coolant. If you are concerned about transmission heat, add one AFTER the radiator as well.

IJM

That's my conclusion as well after doing a bunch of research. I bought the last Mopar HD radiator off Rock Auto a few weeks ago, so I'm hoping it works well. IIRC, the core is 1/4" thicker than the standard factory radiator.

That is correct, though the pressure cap only works relative to atmospheric pressure, not absolute pressure, so you'd still get a reduction in boiling point at higher elevations once the system got up to 16 psi or so.

EatonXJ

Thanks for cutting that one open. I'm glad I went with the one I did.

third coast

Any mounting issues with the Mishimoto?

EatonXJ

I had the shop put it in when they did motor. They had one bracket left over that I took off the factory radiator that they said they didn't need. Feels good and secure to me though. I read one review that said his hood latch damage his. Mine comes real close but doesn't hit it but it's something to watch out for. I'm still planing on getting their electric fan upgraded eventually.
Here is an install video.
Anyone know what that part that is at 4:53? I took my whole front end apart and I didn't have that.

third coast

Regarding the torn open inlet tank, you might check out your engine mounts. I wonder if excessive engine movement had been tugging on the radiator inlet fitting for some time, fatiguing the tank. Damage is close to that fitting. Bad engine mounts are also prone to sending the mechanical fan into the radiator, especially if a thicker radiator and different fan is used that decreases clearances. Transmission cross member spacers (drop kit) to help with a big lift will also tend to push the rear of the engine/transmission down and the front up, possibly stressing the top radiator hose and radiator fitting. Maybe just go ahead and replace the engine mounts anyway. It might be a little easier with the radiator and related out of the way? Let me know if you do that and it is easier.

third coast

You may have convinced me to spend the extra for the Mishimoto high performance radiator, MMRAD-XJ-89. A little concerned about clearance to the mechanical fan. I don't plan on updating to the Grand Cherokee fan however. Open to other recommendations.
As a side note my son, who now owns the white '91 XJ, had it out west recently. It has a new radiator from one of the major auto parts stores. It ran reasonable temperatures on the highway but tended to run hot at high altitude when going up extended grades with 100 degree F ambient temperatures. (manual transmission, 31" tires, properly regeared)

OldTires

That looks like the skinny, lower half of the mechanical fan shroud. The angle makes it look like something else.

EatonXJ

Yeah that's what I thought it was but mine doesn't have one. Guess that's why I've always thought the shrouds on these were half a**ed.

Morat

Ahh, OK - that makes sense!

Spencer_P

I'm enjoying this, although I can tell it's gonna cost me.

third coast someone mentioned they'd be reproducing the splash shield for 87-93 here.