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Confused on Radiator Caps

Old 03-01-07, 07:36 AM
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Confused on Radiator Caps

I posted on another forum for those that saw it, after 26 views there and no responses I'll assume no one knows.

I just got a Ron Davis radiator. Here's my problem, I can't figure out what radiator caps to use.

AWR sent me the Stant 10372 (21-25lbs) for an open cooling system (no recovery bottle). But wait, doesn't the Ron Davis radiator setup for an overflow? Maybe I am confused on terminology.

I have the S4 motor - which has a radiator cap on top of the thermostat housing - so I have 2 radiator caps in my cooling system. I found the Stant cap 10362 (22-24lbs) that looks like it would work, but then with this rating technically the cap on the thermostat may release first rather than the radiator. I wouldn't want this as the overflow is on the radiator.


What's the correct radiator caps (Stant or otherwise) for an S4 motor with Ron Davis (or similar) radiator?

Here are Stant Racing radiator caps: http://www.stant.com/brochure.cfm?br...ocation_id=179

BTW, I had the heater port and 2 sensor ports welded shut, no potential problems there in the future.
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Old 03-01-07, 08:45 AM
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There should only be one pressure cap in the system and it should be placed on the neck where the overflow bottle is attached. If no overflow provision then it doesn't matter where the pressure cap is placed but there should only be one.

Any other necks should not have pressure caps, they should be sealed. If other necks have unused nipples on them for overflow these should be sealed as well; preferably welded shut. You can easily create a plain sealed cap from a pressure cap.

How much pressure to run in the cooling system is up to other factors. The higher the pressure the hotter the coolant can become without boiling. The downside is that increased pressure causes greater stress to the hoses etc so they should be replaced more often.
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Old 03-01-07, 09:41 AM
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Thanks for the input, makes good sense.

Since this is 100% race car, I don't mind the pressure. AWR recommends the 21-25lb cap. The overflow nipple is on the radiator - so I guess that's where the pressure cap should go.

The s4 thermostat housing that has a cap also has a sensor port for the factory fan - that sensor port is already closed up.

I thought about permanently closing the cap area on the thermostat housing but that's where the system should be filled from - as it is the high point.

So, I think from what you said (which makes good logical sense), the pressure cap on the thermostat housing should be converted to a sealed cap. This sounds like the right idea.

Question , how do I convert a pressure cap to a sealed cap? I have a cap on my desk right now and am looking at it. It's not obvious to me at the moment.

Another question, the cap from AWR is for an "open cooling system" which according to Stant site means "no recovery bottle". A "closed cooling system" is for a "recovery bottle". Maybe I have terminiology wrong and a recovery bottle is not the same thing as an overflow bottle?
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Old 03-01-07, 10:25 AM
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Originally Posted by SCCAITS
how do I convert a pressure cap to a sealed cap?
For a sealed cap you're only concerned with the cap sealing tightly to the top of the neck. Any pressure cap with a rubber gasket that will seal tightly to the top of the neck will work. The pressure assembly won't vent anything because with the vent nipple welded over there's no where for it to go. You can remove the pressure assembly if desired so you're left with a plain cap with a rubber gasket at the top.


Originally Posted by SCCAITS
Maybe I have terminiology wrong and a recovery bottle is not the same thing as an overflow bottle?
Technically a recovery bottle and an overflow bottle are different but the names are often used interchangably.

An overflow bottle holds anything (in this case coolant) that is pushed from the system but it doesn't return that coolant back to the system; it stays in the bottle. The overflow merely prevents spills onto the track and could be as simple as an empty beverage container with a hose stuck in the top.

A recovery bottle returns coolant back to the system when it cools. In order to do this the hose is attached to the bottom of the bottle so that as the coolant cools and its volume shrinks this hose will suck the coolant out of the bottle and back into the system. If the hose were merely placed into the top like an overflow is the coolant wouldn't return because the hose would merely suck air.
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Old 03-01-07, 10:27 AM
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Originally Posted by SCCAITS
Maybe I have terminiology wrong and a recovery bottle is not the same thing as an overflow bottle?
They can=but don't have to- be the same thing.
Really, the only difference would be minor...the "recovery" bottle would have provision for the delivery tube to be always submerged so as the system cooled down it would suck just coolant back into the radiator.
An overflow bottle ("catch can") is only concerned with keeping coolant from blowing onto the pavement (track surface?) and the coolant is emptied periodically instead of recycling back to the rad.

In essence, the recovery bottle is a functioning part of the cooling system and the overflow bottle is an addendum for safety or aesthetic sake only.
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Old 03-01-07, 10:41 AM
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Thanks for the responses, I was over thinking this. It all makes sense now!
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Old 03-01-07, 05:56 PM
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I had something similar to what you have now with my ITS car except with the S5 pump. With the pressure cap on the radiator as the fill point the only downside of the setup was purging air when filling it. Having the fill point at the neck would of made things a lot easier.

That said there is no reason, IMO, to worry about placing the overflow vent at the highest point. Everything worked fine for me venting from the radiator. In fact some of the stock S4s and all the S5s had the vent at the radiator with a simple fill cap on the neck for the S4s. There are vented and unvented S4 neck assemblies that are available from Mazda.

As far as a recovery bottle the return does not need to run out the bottom. I may be wrong but that's the way I'm reading what Damon has written. To make an effective recovery system the end of the overflow tube simply has to be constantly immersed in liquid so that when the coolant cools and contracts it can draw an effective siphon and pull the coolant back into the system. Many, many stock systems have the vent line running into the top of the bottle, through the bottle, and the end of the hose very near the bottom. The tank can be higher or lower than the system. It works off siphon.
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Old 03-01-07, 07:27 PM
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Originally Posted by C. Ludwig
That said there is no reason, IMO, to worry about placing the overflow vent at the highest point.
Agreed. The overflow may be placed at whichever neck is most convenient.

Originally Posted by C. Ludwig
To make an effective recovery system the end of the overflow tube simply has to be constantly immersed in liquid
Agreed. As long as the end of the hose is always immersed all is well.

Originally Posted by C. Ludwig
To The tank can be higher or lower than the system. It works off siphon.
True.
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Old 03-01-07, 08:55 PM
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I don't know much about about this stuff but this is what Mazda Motorsports says:
Pressure
"Increasing the cooling system pressure by changing the cap will raise the boiling point of the fluid and will also keep the fluid from being expelled into the overflow tank. We recommend use of a radiator cap with a pressure rating no higher than 17-18 lbs. "

FWIW I run a Ron Davis as well and use a 17 cap and never have had a cooling problem. Oil temp get a little high at times but water stays at 180-190.
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Old 03-02-07, 05:09 AM
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Originally Posted by cpa7man
I don't know much about about this stuff but this is what Mazda Motorsports says:
Pressure
"Increasing the cooling system pressure by changing the cap will raise the boiling point of the fluid and will also keep the fluid from being expelled into the overflow tank. We recommend use of a radiator cap with a pressure rating no higher than 17-18 lbs. "

FWIW I run a Ron Davis as well and use a 17 cap and never have had a cooling problem. Oil temp get a little high at times but water stays at 180-190.

That's another good point. IMO your going to damage seals long before the 20+ psi cap comes into play. If you're consistently running over 210* you have problems a radiator cap isn't going to solve. No need in stressing the system with even higher pressure.
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Old 03-02-07, 06:57 AM
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AWR sold me the 21-25lb cap. I'm going to assume they've tested this out and know from experience. Tony seems to know what he's doing - www.awrracing.com I've also got Autometer gauges and a warning light, a new 160 degree thermostat and new hoses. The sensor and heater ports on the radiator are welded shut, no concerns there. I will be running distilled water and water wetter. Add to this a tested and proven custom splitter and some ducting. This should be the ultimate setup. I guess my only potential concern would be the heater port on the motor, which I know was addressed recently on this forum or another. I have mine closed off with a new piece of heater hose, hose clamped on with a bronze barb fitting and bronze cap. I have read it should be tapped out instead.

It's going to the track 3/17, I can't wait. It's been through a substantial overhaul since it last ran on 1/21. Time will tell. I expect to keep my temps under 200. This car overheated everytime it's been out with the stock cooling system. Nothing but problems.
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Old 03-02-07, 07:07 AM
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Originally Posted by SCCAITS
I will be running distilled water and water wetter. Add to this a tested and proven custom splitter and some ducting. This should be the ultimate setup. I guess my only potential concern would be the heater port on the motor, which I know was addressed recently on this forum or another. I have mine closed off with a new piece of heater hose, hose clamped on with a bronze barb fitting and bronze cap. I have read it should be tapped out instead.
Don't be concerned about the heater port. Mine was that way for a long time before I tapped it and put a plug in there. I run a little bit of anti freeze as well. but it's mostly d. watter and water wetter.
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Old 03-02-07, 12:17 PM
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For a track only car I'd recommend ditching the thermostat and plugging the bypass port in the water pump. Mazda has always recommended setting up the pump in this way. The thermostat and especially the bypass port are failure points that can be easily eliminated.
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Old 03-02-07, 01:19 PM
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Originally Posted by C. Ludwig
For a track only car I'd recommend ditching the thermostat and plugging the bypass port in the water pump. Mazda has always recommended setting up the pump in this way. The thermostat and especially the bypass port are failure points that can be easily eliminated.

Funny you say that as I had a thermostat fail when I had the car at SCCA school. I had one of those miserable school experiences, day 2 the unknown deteriorated foam in the fuel cell clogged the fuel pump which led it to finally fail early in the day. Rule #1, buy a complete car your first time, don't build it or finish someone elses project.

I ditched the thermostat and had nothing but overheating problems after that. I was told the thermostat slows down the rate at which the water flows giving it time to actually cool down while in the radiator. Without a thermostat the water moved too quickly through the system, not allowing it to cool. I bought one from the dealer, before install found out it was probably a 190 and not to use it. A winning RX7 ITS driver on the east coast told me to get the 160, so I did.

Either I got a different opinion OR plugging the bypass port in the water pump accomplishes the same thing by slowing down the rate at which the system moves. Comments?
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Old 03-02-07, 02:04 PM
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Originally Posted by SCCAITS
Funny you say that as I had a thermostat fail when I had the car at SCCA school. I had one of those miserable school experiences, day 2 the unknown deteriorated foam in the fuel cell clogged the fuel pump which led it to finally fail early in the day. Rule #1, buy a complete car your first time, don't build it or finish someone elses project.

I ditched the thermostat and had nothing but overheating problems after that. I was told the thermostat slows down the rate at which the water flows giving it time to actually cool down while in the radiator. Without a thermostat the water moved too quickly through the system, not allowing it to cool. I bought one from the dealer, before install found out it was probably a 190 and not to use it. A winning RX7 ITS driver on the east coast told me to get the 160, so I did.

Either I got a different opinion OR plugging the bypass port in the water pump accomplishes the same thing by slowing down the rate at which the system moves. Comments?


The "slowing the flow of coolant" addage is bunk. Old wives tale.

The reason you had overheating problems when removing the thermostat all together is because you didn't plug the bypass port under the thermostat. That port opens when the engine is cold allowing coolant to recirculate between the outlet and inlet of the pump bypassing the radiator. Helps shorten the time of warmup. When the thermostat is removed and that port is not plugged the coolant is allowed to short circuit the cooling system and the engine will run hot.
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Old 03-02-07, 03:18 PM
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In addition to plugging the bypass port, I believe in running a restrictor in the cooling system where the thermostat used to be.

I believe it is important to keep the pressure in the engine high, in order to reduce the chance of localized boiling at local hot sports. After the coolant passes the restrictor, the pressure drops and it then moves on into the radiator. The radiator does not need to be at high pressure like the engine needs.
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Old 03-02-07, 03:42 PM
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Originally Posted by speedturn
In addition to plugging the bypass port, I believe in running a restrictor in the cooling system where the thermostat used to be.

I believe it is important to keep the pressure in the engine high, in order to reduce the chance of localized boiling at local hot sports. After the coolant passes the restrictor, the pressure drops and it then moves on into the radiator. The radiator does not need to be at high pressure like the engine needs.

Good point. Guess I need to back off the "bunk" comment. I still think slowing the flow isn't the right description. Your pressure comment makes more sense.

From the Mazdaspeed site:

"Water Pump
Water Pump/Thermostat
If you are using a stock (cast-iron) water pump, we recommend "gutting" the stock thermostat, leaving just the thermostat casing. Because some "restriction" is helpful, generally removing the thermostat is not as effective as using a gutted thermostat or restrictor. It is also important to plug the thermostat housing's water bypass. This can be done very easily by tapping the hole (1/2" pipe tap) and installing a plug. "



Though this describes the 12A cast iron pump the design is similar to the later 13B pumps. I believe they are making the distinction between the stock bypass style pump and the old "race" non-bypass style pumps.

For SCCAITS, in addition to what you have done already you can add the S5 pump and either it's pulley or the comp pulley to have the truely ultimate setup. The S5 pump is supposed to be a better design (no evidence on my part, just what I've read/heard) and the S5 pulley is larger than the S4. This slows the pulley at high RPM to ward off cavitation. The comp pulley is larger yet.

With the S5 pump, comp pulley, gutted thermostat, comp rad, comp oil cooler setup on an ITS car we recently did some dyno work with we litterally struggled to keep the coolant and oil temps UP to workable temps on the dyno! It was a cold day outside and ambient temps in the dyno facility were in the 50* range. Still I was amazed that we had to continually cycle the single cooling fan we were using and had to load the car and warm it up prior to each pull. And just to brag a little we netted 174 whp with the stock ECU on a Mustang dyno that day.

Last edited by C. Ludwig; 03-02-07 at 03:51 PM.
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Old 03-02-07, 05:57 PM
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Great information and thank you very much. I'm half way there, I recently got and installed a dual belt underdrive main and alternator pulley from Rotary Performance.

What a waste on new thermostats, I'll go ahead and gut one of them. I assume the S5 waterpump will fit the S4 motor no problems. If so, I'll add the pump to my never ending to do list.

Now i just have to figure out how to plug the bypass port. I did a search in the 2nd Gen section with nothing. Anyone have a link or quick tip? Is a tap my best and only option? If so I can buy a tap (I'm still building my tool collection), but then how about keeping the shavings out of the block? Alternatives?

The car is still at the paint shop so I can't exactly pull it apart for another few days.
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Old 03-02-07, 06:04 PM
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Originally Posted by cpa7man
Don't be concerned about the heater port. Mine was that way for a long time before I tapped it and put a plug in there. I run a little bit of anti freeze as well. but it's mostly d. watter and water wetter.
Let me take my comment back......I was refering to the water port on the side of the motor. Never mind.

You second gen guys have all the fun
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Old 03-02-07, 06:06 PM
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The S5 pump isn't a bolt on for the S4 engine. It hits the S4 front cover. I've used them by shaving a bit off the pump and just a tad off the front cover. I'd guess the pump itself isn't that big of a deal but who really knows? The pulleys are an easy cheap swap so why not? The comp pulley is only available in single sheave so it won't work with your pulleys.

To plug the by pass port just tap it and install the plug with some pipe dope. It's very easy. I like to use the internal wrenching plugs. Much easier to stick and allen wrench down in the pump to install it. They'll also sit flush in the spot they go which is nice. Earl's makes the plugs. If you can't find them locally Summit, Pegasus, etc. sell them.

I posted a pic of the bypass port either on here or on it.com. It's easy to spot. Pull the thermostat out and look straight down. There is a hole about 1/2" in diameter that the thermostat sits on. The bottom piece of the thermostat actually covers the hole you want to plug.
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Old 03-02-07, 06:19 PM
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So no S5 pump, works for me. I'll move forward with the plugging on the port and gutting the thermostat. Again, thanks for all the help.


Originally Posted by cpa7man
Let me take my comment back......I was refering to the water port on the side of the motor. Never mind.

You second gen guys have all the fun

The one on the side of the motor is what I was referring to and thought you meant, you had it right. That one I have sealed with new heater hose, bronze barb and bronze cap and hose clamps. The "weak link" in my cooling system is probably this area. It should be tapped I guess. The heater port on the Ron Davis radiator (why they put it there I don't know) I had welded shut by a local shop.
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