Technical Documents

Keeping Your Engine Cool When All Others Are Boiling Theirs

By Anthony Parle

Have you ever had the not so enjoyable pleasure of boiling your car every time you push the revs up or drive it hard, you watch the temp needle rise?  You have been doing well in the race but you had to back it down or you are going to cook your engine  You have read all the different articles on keeping you engine cool and walked away more confused then ever.  Asked your self the big question what is truth and what is fiction?  Which ideas belong in the real world and which one belong in the tooth fair world? What ideas are backed by fact and what are historical artifacts from the stone ages? Questions like

Cooling tests

Should I give an electric pump a go, will it save power?
Is my coolant flowing to fast and do I need it to slow down?
Should I drill the impeller blades?
Why put a top tank in for air, do I need it?
Is a down flow radiator better then a cross flow?
What happens if I run with out a thermostat, should I?
Does my engine block cause too much restriction in the cooling system at high revs?
In the case of Subaru’s, does the left bank really run hotter than the right?
If I increase the radiator cap pressure will that solve the problem?
Why does my Ford have wire in the bottom suction pipe, that’s right to stop it collapsing dummy?  Is that okay?

Well in the next couple of articles I going to answer all these question and separate the “Dream time from the bull time”.  I have spent a year running tests on Subaru cooling systems with temperature gauges, pressure gauges, air flow gauges, water flow gauges & vacuum gauges mounted in every conceivable spot of a Subaru engine that people know about and a few that they done.  I have run tests on “Cross flow”, “Down Flow” and “Multi Pass” radiators. I have cut, drilled, milled, welded, bled and got burnt all in a effort to answer the big question “Why the hell does my engine keep boiling when the revs go up”.  I have made friends, lost friends and been abused all in a effort to get to the bottom of the black art of cooling a car engine.

Okay lets get rocking

I want to answer some of these big questions up front then I will back up my answers with the research to support the statement.  My reason for doing this is that the solution needs to be clear so people don’t have to read the whole article to find the answer or get to the end and be more confused.  Or think I am on drugs and don’t have a clue what I am talking about.  So here we go.

The best radiator is a cross flow with the cap on the same side as the water pump inlet.  If the bottom hose has wire in it you have a problem the pipe is to small to keep the water pump happy at high revs.  Even if it doesn’t have wire you could still have a problem with the pipe to small.  The thermostat needs to be at the top of the radiator not the bottom on the water pump cover.  In most cases you need to find a bigger water pump inlet pipe.  Your bottom pipe out of the radiator needs to be in the best place to get maximum flow from the tank.  If it’s a bottom tank radiator it needs to be in the centre or on a side tank slightly up.  The pipe needs to be as large as possible to help flow to the pump.  Electric water pumps in most cases are a waste of time and don’t have the kind of flow needed to transfer the heat generated by the engine.  They also only save power because they pump less coolant they are not more efficient at the same flow rate of your existing pump. Nearly forgot changing the radiator cap won’t help and may blow your heater core due to the increased pressure, it just masks the main problem.

I can hear the screams now, where do I get off making such statements and have such confidence that I am right, you could take my word for, but I would be disappointed if you didn’t demand my proof, we are dealing in facts here and not fairytales.

And_ _  here we go.  A couple of years back people around the world started to put manual gearboxes in Subaru SVX’s, for those people that never heard of these cars they were a executive sports car built by Subaru from 92 to 96.  They were only available in auto because at the time Subaru did not have a manual gearbox capable of handling the high hp of the engine.  The engine is 3.3L NA (naturally aspirated), they are known for being as tough as a preverbal brick building.  They ended up in planes, buggies, drag cars and more then a few WRX’s.  Some were even fitted with twin turbo’s by the drag guys.  This was all good “except they overheated” when the revs went up the engines started to overheat.

In an attempt to solve this problem I started my search and found that a lot of high performance Subaru engines were having overheating problems, even rally cars in the UK were modifying the top cross pipe in a effort to beat the heating.  My internet research led me to two conclusions, first that Subaru weren’t the only company to have issues with cooling and the second, best of all, there were a lot of people out there that must be on drugs or best friends with a tooth fairy because they didn’t have a clue what they were writing about, there was no logic or science to back up the statement.

Lots of gauges

I started my research by fitting myroad car with three temp gauges, radiator return coolant temp, left and right heads by removing the plug into the back of the head against the fire wall.  Then on a 38c degree day I conducted a test, I was driving straight on flat level ground in 6th gear at 110kph measure the engine temp, then changed to 5th, 4th and 3rd while keeping speed the same.  The aim was to raise rev without changing load on the engine to see what effect increased coolant flow would have on the engine temperature.  What I discovered was that as the revs went up the engine temp shot up and not by a little bit but on a increasing scale. It was so bad that in 3rd gear the test lasted about 30 seconds before the engine boiled.  As you can image I had my tail between my legs because I had just boiled my engine (possibly stuffed it) and learnt nothing.  My new research career was over.  At anything above 5k continuous the engine was on a path to become a barbeque for a beer ad.

After a couple of weeks I decided that I need to learn more. Too dumb to quit I decided to inflict more pain on my dearly loved car.  With great reservations I drilled holes in the water pump just in front of the impeller and mounted a vacuum gauge, installed pressure gauges on the outlet of pump before the engine block, outlet of block and before the radiator.  It was a hot day – 40c – had the car parked, took a deep breath and did a new set of tests.  Revved up the car while recording the readings of the gauges every 1,000 rpm.  We again had to stop because at 6k it was good and boiling.  What we noticed was that the vacuum gauge on the inlet to the water pump showed increasing vacuum until all of a sudden it showed zero vacuum.  This occurred at around 6k and the engine would boil.  The test at this point had to be terminated as the engine temp was heading past 130C and fast.  The other pressure gauges stated to drop at 6k.  I posted this data on the Subaru SVX forum and it created a lot of discussion as to why it happened.

My good friend Harvey was the one to hit the nail on the head he suggested that the coolant was boiling before the water pump and then the water pump was losing prime as a result no coolant flow.  For those not to bright and wrote that garbage on the net about cooling.  “No pump no coolant flow means boiling engine”.  Okay so we did a bit of math.  The vacuum gauge read -5psi before it all turned bad and went to zero when you work this out water boils below Zero at a lower temperature there for the calc says that at 88C the coolant was going to boil going into the pump.  All right I can hear everyone say but what about the cap holding pressure 10psi?  You are right (only to a degree, I will go into it later) the boiling point has moved to 98C and your engine is going to boil its little head off.  If you rev the engine more and the suction goes lower the temp at which your engine will boil will drop.  For example I tested a special radiator and it reached a suction of -9psi at the pump in which case before cap pressure it would boil at 78c inlet temp to pump.

Engine on a stand

At this point I figured my poor car needed to be spared any further abuse from its unloving owner (if it had of been a pet the RSPCA would have lockned me away by now), so I built a test bench with a motor mounted on a stand and a electric motor driving the water pump with a clear pipe nstalled to be able to watch the coolant flow through the pipe and look for air.  Filled with water as it was clear and a standard radiator from a SVX (pinched the one from my wife’s car, was not a good idea, “did not feel the love”).  Away I went looking to see what the clear pipes showed.  Due to the circuit being cool I had to remove the thermostat.  Installed onto this test bench were a bunch of gauges, as the revs went up the vacuum increased on the suction to the pump and there was nothing that could be done stop the air getting in the circuit.  Below 3k the water was clear, the faster you went the worst it got and at 6k you could have had a milk shake there was so much air.  Also this air was cause back pressure in the cooling loop to the point that air was being blown out the overflow pipe taking with it liquid.

As far as I could see there was not a thing I could do to sort it out, was the block just to restrictive, the radiator the problem or the pump just too good?  The only way to find the answer was to break the cooling system do to smaller test units.  As a result I mounted a Subaru water pump on its own belt driven by electric motor with no radiator just a clear looping pipe to check for air.  To my surprise I noticed that the air was better but still there above 4.5k rpm.  At this point in a state of frustration I figure its time I got locked away as a nutter, done heaps of work seen lots, achieved nothing and getting nowhere.  This job was going to turn a man to drink.  However there was a clue, the vacuum gauge on the test unit was still showing -5psi (this did not make sense) and in my looped pipe the water level was rising as the revs increased.

Cooling testing

Went away sat under the tree, sucked up to my wife (after the radiator was returned) by taking here for a drive to the Birsdsville races and drove across the Simpson desert in the SVX sports car (I was running a stress test on our marriage).

When I returned I examined the water pump cover and found that while it looks large the internal size is about 27mm.  At 6k this the engine is pumping 200L per min and you are trying to suck that volume of coolant through a 27mm pipe.  It just wasn’t going to happen.  The next step was to build a new pump thermostat cover of larger size so I tried 45mm.  Assembled this all again with 50mm clear pipe and gave the stand alone pump a go again.  What I found now was after the small pockets of trapped air escaped from the block the system settled down.  At 6k the water flow through the pump was so good you couldn’t tell at all if the pump was running or not.  Success was with in sight.

Modified PWR radiator

Feeling the taste of success getting closer I install the modified pump cover and the 50mm pipe on the motor with out the radiator and ran the unit.  All was good, no air at 6k the liquid was clear it was as if I did not have the engine attached.  Now I felt I was on the home run so I bolted on a brand new PWR radiator that I had built specially for me.  It had 45mm top and bottom pipes, I turned it on and was shocked at 5k and up you could see the air forming in clear inlet pipe to the pump.  The vacuum gauge while lower was still reading -3psi.  The air was coming from the bottom of the radiator out let pipe. The next step was to move the radiator to the centre of the radiator bottom tank which I did.  The next lot of trials proved that at 6k with a 50mm outlet pipe in the bottom tank the problem was solved.

The answer was clearly that you can’t suck the kind of flows (200L per min) through a 27mm pump cover and that’s without the thermostat install.  A couple of guys in the USA, Tom and Dan had come up with the idea of putting the thermostat inline on the top radiator hose so as to improve flow into the pump but still be able to have a thermostat in the system.  This has proved a real winner.

The big question is does it work, well a number of cars are now racing around the world and are not overheating and don’t even look like they will.  My friend in Australia Matt has been rallying a SVX that has always overheated.  He changed the bottom pump cover, took the thermostat out and moved the bottom radiator pipe to the centre and now the car runs to cool.  My friend Adam in the USA has a WRX with a SVX engine in it and he has changed the bottom pipes as suggested and he also installed a top pipe thermostat.  He wanted to find out what would happen so he put the car on a dyno trying to overheat it but it never happened the thing ran as cool as can be.

Lets talk top tank radiator versus cross flow radiators, I don’t think I need to cover all the work I did to figure out the answer as to which is best.  It is quit easy to explain the reason for using cross flow.  On top tank radiator the coolant is forced through the radiator under pressure from the pump but what everyone is no aware is that under high flow that pressure can be 9 psi or higher depending on the flow characteristic of the core.  If the cap is set for 10psi then you are going to blow coolant at 102c and you need to think back to what I said about the boiling temperature of the inflow into the pump.  If the cap is only effective to raise the pressure by 1 psi then at a suction of 7psi the engine will boil at the pump 85c.  Think about that you can boil the car and the cooling fans are not even on.  Due to the fact the pump is a centrifugal when you do slow down the rev the pump won’t start pumping.  It will not restart till the rev drop to idle.

Water Pump SVX Differential Pressure

Compare this to a cross flow radiator with the cap on the suction side.  If it’s a 10psi cap then you get the full 10psi on the pump less the vacuum which means you boiling point into the water pump will be over 100c.  That’s a big difference also if the system needs more coolant the radiator will suck it through the cap not blow it out.

As I get to the end of this article I need to defend my honour and get in before all the people out there call for my blood and demand that I be burnt at the stake for not taking on board one important factor.  They will scream “BUT IT’S A CLOSED LOOP” pressure coming out of the pump ends up on the inlet so there can’t be a vacuum, you must be on drugs.  Well the bad news is it is a closed loop but no pressure turns up at the inlet of the pump.  Just think if the pump was capable of boosting the pressure by 30 psi and 10 psi turned up at the inlet then the next loop would mean the pump would boast by 40psi.  It just doesn’t happen it would be a type of perpetual motion and we all know that isn’t possible.

Depending on how this article is enjoyed we might discuss why high coolant flows are best next time.  May you engines run sweet and cool.

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