Stroker Cooling Systems and Upgrades

[gonridnu]

Being that this is a stroker engine website it seems fitting we have a thread that details cooling system function and many of the standard upgrades that are available.

Factory cooling systems are designed for the horsepower rating of the stock vehicle plus a margin for safe operation under anticipated conditions. As we increase the horsepower, so too is the heat loss associated with the combustion process increased. Without the benefit of cooling system upgrades, the additional heat transfer required by the increase in horsepower infringes on the designed safety margin of the stock cooling system. The system is taxed under more adverse operating conditions, such as slow speed off road operation (who of us is guilty of that?), high outside ambient temperatures such as those experienced in the desert Southwest, operation in mountainess terrain, towing, or even highway operation at elevated RPM as the result of low gearing. Under these conditions the buffer engineered into the cooling system can easily be overcome resulting in an overheating condition.

Jeep 4.0’s employ two different cooling systems that are comprised of most of the same components. The early Renix era models 87-90 utilized a “surge tank” system whereas the later models 91 and up employed a “overflow and recovery” style system. Aside from this one distinction the two systems are essentially the same with regards to the other components in the system setting aside model year changes to specific parts. In essence though, they function and operate in a similar manner and adhere to the same basic principles of cooling.

The cooling system on 91+ Cherokees is a standard recovery system. A spring loaded cap on the radiator holds pressure at a given rate for the cooling system. When the coolant pressure within the system overcomes the cap rating the coolant is forced into the recovery bottle. As engine temperature drops (or more accurately cooling system pressure) a siphon is created by the resulting vacuum and pulls coolant from the recovery tank. The sheer number of Cherokees that used this system and the interchangeability of parts makes changing over to this system a very easy and popular swap for Renix owners. Although some would disagree, I did not list it as an “upgrade” in the sense that it increases the system's cooling capacity.

The early Renix cooling system has been the source of much frustration and discussion on Cherokee sites. I also happen to be the proud owner of one and have experienced some “incidents”. This is what I know.

The “surge tank system” used on the Renix era jeeps is nothing new. It has been employed for decades and decades by various manufacturers. Many 60’s era Fords had a copper surge tank mounted to the front of the engine and surge tanks go back decades before then. Currently, there is a resurgence in this type of cooling system. It became quite popular in the 80’s and 90’s as engine positioning as well as body design placed radiators below the engine coolant level. It is currently used by GM in everything from Corvettes to trucks, BMW, Mercedes, and Honda just to name a few and is found on most big rigs. It can be considered a true “closed system” and many claim it affords some benefits over a “recovery system”.

Given the number of Renix cooling system issues there is undoubtedly someone thinking to themselves “What benefits? Those things are junk.” And he would be correct! The plastic surge tank, it’s cap, and the fact that it hits the hood on so many Jeeps including mine, is one monumental pile of junk, but not so the basic system design.

When working properly a surge system will actually hold more coolant available to cool the system. A surge style system continually purges air from the entire cooling system including the radiator and holds an additional small amount of coolant in the surge tank which is also being circulated. In a recovery system the additional coolant available from a recovery bottle can only be accessed by the system when the engine cools down below the temperature the cooling system released pressure at. It is also being reported on various sites that Dexcool is more stable in a surge system as it is not continuously exposed to oxidation caused by fresh air. Some even claim this is the reason GM has made the switch to surge systems. Another benefit, albeit short lived, is that because a surge tank is the highest point in the cooling system it will continue to release coolant to a pinhole leak regardless of height in the system. In a recovery system a pinhole leak high in the system will suck air before it pulls fluid from the recovery tank.

Aside from a broken stock radiator, ALL of my cooling issues have been the result of a crappy surge tank. I’ve gone through 4 in the 15 years I’ve had the Jeep and even used to carry a spare. They always break at the worst time and place possible, or at least that’s when you find out about it. I’ve cracked ‘em and I’ve had the cap quit sealing and it’s always the same. The temperature begins to elevate and you hear the hiss when you shut it off.

RENIX UPGRADE … So before we go any further, if you are going to retain the stock Renix cooling system do something about the surge tank. First inspect it for cracks, particularly around the cap area, and replace as necessary. If you are on a budget try the Napa part #703-1396 Volvo cap. It is a lower profile design is said not to hit the hood which reduces the stresses put on the bottle. It is also a 22lb cap which on the surface is appealing as it will increase boil over temp, although one might also be equally concerned the design pressure for the system is not as high and could result in ruptured parts, particularly the POS plastic tank.

The other option is an aluminum surge tank offered on the net by Mac’s Radiator. They are a direct fit aluminum replacement for the hated plastic piece and come in natural or diamond plate finish. They come with a traditional 16 lb radiator cap and since they are aluminum you can do anything you want with it including welding in a bung for an under hood mechanical temp gage. They are currently $150ish delivered and address the problem in a sanitary and permanent fashion. Although it is not necesarily needed, this tank has a nipple under the pressure cap that gives you the ability to use a traditional recovery tank as well. I will post pics of the install I did recently.

RENIX owners should also use a later model thermostat housing with a petcock installed to ease the coolant burping process.

It is important to view improvements as components in a greater system. As with all system upgrades any singular improvement may have limited impact. It is the combined performance of these components that will determine the ultimate success of the system upgrade as a whole. Thanks to an abundant supply of O.E.M. parts and an aftermarket that loves us, the basic parts of the cooling system on our Jeeps are easily upgradeable.

The following upgrades can be performed to all years of XJ’s with equal effectiveness and are presented in no particular order.

UPGRADE #1 …. Tranny Cooler

If your automatic equipped XJ did not come from the factory with an externally mounted transmission cooler I have good news. A lot of them did and they are a dime a dozen at the bone yard. They are basically plug and play in that if you bag the hoses and brackets specific to the cooler, they will fit your Jeep with your existing transmission to radiator hoses. Any decrease in transmission temperature will result in a gain in cooling buffer.

The other option involving the cooler is to remove it entirely from the radiator and use an external cooler larger than the factory unit. If the remote cooler is mounted in the radiator air stream there will be an increase in ambient temperature at the radiator and a an increase in airflow restriction that may or may not offset the gains realized by removing it from the integral radiator cooler. Optimum placement of a remote tranny cooler would be out of the radiator air stream and aided by an electric fan.

UPGRADE #2 …. Electric Fan

The factory six blade auxilary electric fans found on 87-94 model years were improved upon 2x’s. It is reported that some vehicles did not come with them but I have never seen one. If that is indeed your case get one. If you have a 6 blade you can get a better one. I believe the first upgrade was to a modern 8 blade curved blade design in ’95-’96 model years. This fan features a GM WeatherPak connector and is a direct plug in to earlier vehicles.

The second upgrade affects model years 97-2001 at which point the factory went to a 10 blade fan again utilizing the curved blade fan design that has become the industry standard. This fan, while readily available from the aftermarket (currently as low as $60 bucks delivered on Ebay & Amazon), employs a different electrical connector than earlier vehicles.

The 96ish ones have been hard to find in good shape. If the cars at the bone yard aren’t already wrecked in the front end then some bone head at the bone yard screws up the fan getting something else. I haven’t had a shot at a fresh one so I ordered new 97-01 unit a couple days ago and plan to cut and paste a Weather Pak end back on it … here’s hoping to the wires being long enough. The mounting points are said to be the same.

Another popular modification to the cooling system involving electric fans is to replace the clutch fan with another electric fan from a Ford Taurus or other suitable performance electric fan. Another option is to replace both factory fans with three electric fans. I have no personal experience with installation or operation of all electric fan upgrades in Jeeps but I have with lots of other vehicles and let’s just leave it at “It can wok but I’m not a big fan” and move on. Bear in mind with any of the electric fan combinations, be they stock or all electric can be wired in such a way as to provide a wide range of manually and/or thermostatically controlled operation.

UPGRADE #3 …. Clutch Fan

First let me start by stating clutch fans are very efficient at what they do. For starters, they move a lot of air when they are engaged.. In fact, at all but the lowest speed and RPM they move a generous amount of air when compared to electric fans of similar size. While this ability to do so comes at a parasitic loss to engine power, the fact that they only work when they sense increasing temperatures and idle when they don’t makes them somewhat more intelligent than a simple hub mounted fan, flex or solid. So you could say I’m a clutch fan, fan.

There are several write ups on the web regarding an upgrade to the fan clutch from a 97-98 ZJ(?) without Max-Cooling. XJ lore has it that the stock unit spins 60-70% of pully speed when locked up while the larger ZJ upgrade spins at 70-90% of pulley speed. Given the size difference in the two units there is no reason to doubt it. Within every thread regarding this upgrade there is at least one eyewitness testifying to the fact “it sounds like an airplane”. If you don’t mind the sound when it’s engaged this one’s a no brainer and It doesn’t even matter what the ZJ production years are cause brand spankin’ new ones are available as Napa part #272310 for around $40 bucks.

There is a thread elsewhere that makes mention of certain years having trouble with certain 3 core radiators as the ZJ fan clutch is reportedly +.250” end to end compared to that of an XJ. Reporting only my observation, my RENIX era XJ with a 3 core copper brass radiator of unknown brand has plenty of room.

UPGRADE #4 …. Radiator

An improved radiator is logically the first upgrade considered by many and for good reason. This single upgrade will often provide the majority of make up margin required by the larger stroker power plant and the results can be such, depending on conditions, that no other cooling system modification is required. It is also the single most expensive upgrade.

I’d prefer to skip the discussion of stock plastic tank aluminum radiators altogether, but here goes. They break and I don’t like them. It really doesn’t matter if a two core factory style aluminum one cools near as good, as good, or better than a 3 row copper brass cause plastic tank radiators break. Every vehicle I’ve ever had the misfortune of owning when the radiator came apart had plastic tanks. It’s made worse when you take them off road, which I do, so I’ve had that misfortune 3 different times in 3 different vehicles in nifty places from Nowhere Nevada to Death Valley.

That being said, you essentially have some radiator choices from the aftermarket. They range in price based on materials and quality of construction. A quality 3 row copper brass can be had for about the same price as an inexpensive two row aluminum radiator. As brands come an go and different companies enter the market place products change so it’s up to you to do your homework. Just be aware there are differences in quality between a Chinese radiator with glued on tanks and a Ron Davis part other than the price.

Upgrade #5 …. Water Pump

There are 3 popular brands I’m aware of offering high flow pumps, those being FlowKooler, GMB, and Hesco. The FowKooler and GMB High Volume pump are essentially the same. They utilize an age old hot rodder’s trick by attaching a cavitation plate to the stamped steel impeller. Does it work? I think so in certain applications and conditions and to a certain degree. Anyone with data is welcome to post it here.

The Hesco pump stands alone not only in price but in quality as well. The 5 blade stamped steel impeller is replaced by a CNC’d 8 vane impeller similar to other performance water pump manufacturer’s designs. They are available for a variety of Jeep applications and Hesco claims 10 -20% improved cooling efficiency over stamped steel impeller pumps.

UPGRADE #6 …. Thermostat and Housing

This is probably the easiest and least expensive upgrade. As the money rule applies in hot rodding it won’t yield a bunch, but every little bit helps.

There are several examples and pictures of “ported” thermostat housings on this site as well as others. Hesco also has them available made from brand new castings. The casting is such that even if you have never ground on anything before, with an air grinder, a carbide, a few hard rolls, and one beer you can increase the flow potential of the water neck. Every little bit helps right? I guess.

Do yourself a favor and get a quality Robertshaw high flow thermostat or equivalent. There is no point grinding on the water neck and then putting an El China Mart thermostat in that may or may not flow as much. In the unlikely event your quality thermostat does not have a hole in the outer sheet metal, drill an 1/8” bleed hole a ¼” or so from the edge and position it at 12:00 on installation. I have seen it mentioned where it is suggested to drill two opposing holes in order to prevent bidirectional lock but I honestly have no clue if it is necessary or better, but one hole has been proven to work on a variety of applications over the years

One other area I’m gonna avoid like the plague is the discussion of lower temperature thermostats. In any engine power and emissions are relative to combustion chamber temperature. In addition, computer controlled vehicles are designed to operate within a given temperature range and many do not operate in closed loop until they reach operating temperature. If the cooling system is correct there should be no need to give up horsepower by lowering the engine’s operating temperature assuming it even works. Ultimately, if you are going to overheat with a 195 thermostat you are also going to overheat on the 180* that's been open longer so it seems like a kinda silly discussion to me but such is life.

UPGRADE #7 …. Water Wetter.

You’ve done all this stuff and your supercharged 5.0 liter XJ still overheats going 110 down I-15 from Vegas to L.A.? Water Wetter just works. It might not cover that scenario but I’ve used it in a bunch of racing and street car applications and it helps. It won’t completely solve an inadequate cooling system under every condition but it can make a noticeable difference controlling boil over. And no, it is not “cooling system in a can” for your ailing cooling system.


UPGRADE #8 …. Under Hood Temperatures

While not technically part of the cooling systems efficiency and ability to transfer combustion heat, under hood temperature and engine bay airflow can be improved upon. Ceramic coatings on exhaust systems are a proven method by which to reduce under hood temperature as is engine bay ventilation. By installing aftermarket or Le Baron hood vents or with the installation of a Reflexxion cowl hood hot engine bay air has additional exit points and reduced pressure at speeds above approximately 35 MPH.

It’s not really an upgrade, but while we’re on the subject of air, it should be noted that both under hood temperature and the cooling system’s ability to transfer heat are dependant on a good supply of outside air. Cherokees have a relatively small grille area so please don’t be that guy with 6 KC’s mounted to the winch bumper he had special ordered with the hi lift jack and shovel mount sporting an empty desert water bag from his grill just for style points.

Other than broken surge tanks, I have never had a serious cooling issue. I replaced my stock radiator with a 3 row copper brass unit 15 years ago when the plastic tanks started leaking. At the time the swap to a 91 and later cooling system was just a thought in my head but I had no way of confirming it could be done so down the merry surge tank path I went. With the new radiator installed I recognized a few degrees lower operating temps under normal operation around home. When on trips we could be more aggressive on long steep inclines than before.

My old pump was rough so I put the Hesco pump on at the same time I installed the stroker and have no before and after data. I can only say we did not have a cooling issue with the stroker all summer climbing the mountains around here which is where we usually see problems. There was one instance where we had to take a rest and let it cool down at the end of a long steep off road hill climb, but that was also the exact moment we realized the last in a long line of surge tanks had cracked and was hissing, so the system was operating under reduced pressure but it still recovered.

My 97+ electric fan gets here tomorrow and Napa has the fan clutch in stock. I’ll post pics when I get ‘em installed. After the radiator, it is assumed the combined improvement of both fan’s CFM under load is my second biggest upgrade in cooling capacity. Summer is not far off and I’m anxious to see how stable the coolant temperature stays under a variety of conditions with the increased headroom in the system provided by all the upgrades listed here.

Unfortunately I installed parts without regard for collecting good data which is sorely lacking on many of the threads you will find on the internet, this one included. These are all upgrades that have been around longer than I’ve known about them and I didn’t invent them. They should all be considered standard performance upgrades that would be applicable and effective on most vehicles. The point was to create a list of reasonably easy modifications that should produce increases in the available thermal capacity of the cooling system to accommodate the increased power output of a stroker engine, and then put them all in one place. And just maybe in the process save some other guy some time and trouble….

[GoMopar440]

Great post and very worthy of a sticky IMHO.

FWIW, I'll throw in my two cents here as I've been dealing with this issue a lot. I've done most of these upgrades you mentioned on my 97 XJ already, and it still runs moderately warmer than I'd like on warm days and under heavy loads (going up hills, into heavy headwinds, etc). One possible factor I didn't see mentioned above is obstructions in front of the radiator. Winches and fog/driving lights being the main offenders. I'm looking at something like a JCR bumper that mounts the winch a few inches lower than the TnT bumper I have now. At this point, I'm just trying to get any other ideas to fix my temp issues before the warmer weather gets back here.

My TnT bumper with everything in front of the grill opening:


A JCR prerunner type bumper for comparison:


The plan is to mount the winch in the JCR bumper but relocate the control box behind the bumper then add a remote plug with a cover for winch control. The lights would be replaced with rectangular ones and relocated to the inside of the side wings of the bumper (under the factory blinker area). That should free up a lot of frontal area.

If that doesn't solve the temp issues, I'll look into a full aluminum radiator (no plastic tanks) after that.

[SilverXJ]

A few people run two stock late model e-fans instead of the 3 aftermarket or additional Taurus fan. I'm going to give the second stock fan a try. Two stock fans should pull as much CFM as the 3 aftermarket fans. I'm debating getting a used/OEM from a dealer or going for some other non-oe supplier though.

I've used the flowkooler style pump, but it didn't work out for me. It would over heat on the highway. Initially I had the stock rad and when I installed the flowkooler pump I went with a highflow t-stat housing and a Robert Shaw t-stat. I eventually replaced the stock radiator with a CSF brass radiator, but that didn't help for the highway over heating issue. Replacing the flowkooler with a stock pump fixed the over heating issue. I didn't do any testing using restrictors, but the theory that the coolant was flowing too fast to make effective heat transfer to the radiator. I would like to try the Hesco pump when I get a chance, though, to see if it makes any difference.

Also, do you have a source for Robert Shaw t-stats that have them in stock? I've contacted a few places and couldn't find one stocked.

[GoMopar440]

I got a 180* Robert Shaw high flow T-stat from Summit not too long ago. It was kinda hard to find on their website since they didn't advertise it as a "Robert Shaw". Lemme go look for the part #...

EDIT: Found it. It's advertised as a Flowkooler in the title, but if you read the description, it's a Robert Shaw piece. Link-> http://www.summitracing.com/parts/BRA-330-180/

It say's they'll ship out by tomorrow so I guess they have them in stock.

[Cheromaniac]

Great post and very worthy of a sticky IMHO.

Agree 100%.
The OEM 2-row radiator works fine for me even though I don't like the plastic side tanks but if you've added more weight and you're carrying extra gear, you'll need more cooling capacity than the OEM radiator has to offer.
I have a Robertshaw 180* t'stat and I find that the lower cylinder head temp. during the cooler months of the hot desert climate that I live in, when ambient temp. is less than 90*F, allows me to run 87 octane fuel without pinging. If you live in a cooler climate the 195* t'stat is a better choice.
I've been using a Flowkooler water pump for many years and I've found that it keeps the coolant temp. lower for a longer period compared to the OEM pump when driving the Jeep up long inclines. The Hesco pump is probably even better.
I also tried porting my t'stat housing but it didn't make the slightest bit of difference because the main restriction to coolant flow is the t'stat itself.
Finally, I've been using dual electric fans for nearly 12 years. My set-up is a 14" aftermarket fan mounted in the stock shroud on the passenger side, and a '97-'01 OEM 10-blade fan on the driver's side. It works well even when it's 120*F outside, but I think a dual OEM 10-blade fan set-up might perform better. Those fans are excellent.

[gonridnu]

Here are some pics of the Mac's Radiator surge tank for RENIX. It's a 15 minute install.

[gonridnu]

I bought this Olympic bumper long before there was such a wide selection available. It's not the best thing out there but it does drop the winch down out of the incoming air stream reasonably well. It has two holes for lights out near the edge of the radiator openingl that I think about from time to time. The Jeep has an H4 headlight conversion with 45w low beam 100w high beams in it now that light up the night, but perhaps if it cools really well I'll put a couple smaller narrow beams on the bumper.

[yuppiexj]

Here are some pics of the Mac's Radiator surge tank for RENIX. It's a 15 minute install.

That was a cleaner install than this one http://www.summitracing.com/parts/MOR-63650
I had to remove the shelf the OEM sat on and zap screwed it to the firewall

[gonridnu]

This is a RENIX with a later model thermostat housing that has a petcock installed to help burp the system. The crossmember that is part of my long arm kit has some drop to it so this petcock is located at the highest point of the cooling system on the engine. I had no problem burping my stroker and even checked it by pulling the temp sender which thankfully I will never need to do again. I burped it again a day or two later just to check and it's been good to go ever since.

[gonridnu]

Hesco impeller and standard impeller...(photofrom Hesco website)

[gonridnu]

FlowKooler Impeller design.

[Cjbeer]

I wish i could figure out cooling issue. I have replaced the radiator with a 2 core alum.. hesco Water pump and t stat housing new hoses cleaned out heater core. have newer electric fan and new clutch fan. once it hits 100 plus out here in Phoenix AZ my temp spikes driving down the freeway with the AC on. with it off it runs perfect right around 210-220. sitting in city traffic the AC is on ice cold and temp is right around 215 with it 100 plus outside. i do have a four real steel bumper on the front with a winch. i have taken the winch off and no change i have taken the winch and the hood off, no change. so i put it all back on and used a spacer to space up the back of the hood still no different. I am currently working on something to duck all the air into the radiator because if you look there is a lot of room for air to move around the sides of the radiator.

96 xj
6 inch lift
33 bfgs
4:56 gears

[604rail_king]

cjbeer, maybe you are experiencing what Silver described a few posts up...

[gonridnu]

The few times I've seen overheating at cruise in muscle cars it has been a collapsed lower hose that had too short of or no spring at all in the hose. There's a pretty good vacuum on the inlet side of the pump, enough that it can partially or even fully collapse the hose.

[gonridnu]

Late Fan on the left, early on the right. It was very easy to reuse the GM weather pack connector with a mounting post from the early fan. It sounds crude but I did not have the tool to get the new style connector off the new fan's wiring so I broke it with a ball peen hammer (are ya happy now Silver...LOL). It is very brittle and took about 2 wacks to shatter into a pile of pieces. To preserve wire length, I cut the terminals off where they crimped to the wire and left the crimp on the insulator which came off when I used my strippers. I lost a total of about 1/4" - 3/8" of wire. I crimped some male weather pack terminals on and installed the fan in the Jeep to check polarity. Once I established correct fan rotation I installed the terminals in the connector from the old fan and voila .... looks like it came from the factory! Total install about an hour.

I use GM WeatherPak connectors for almost everything I wire and had the tools and terminals. If you do not have the correct crimpers take the fan to a shop that does and have them do it for a few bucks.

I should mention I had to massage my tranny line for fan clearance and grind a minimal amount on the very upper driver corner of the fan shroud for radiator clearance. I believe the latter to be due to the 3 core copper brass design.