Myles Posted April 25, 2005 Share Posted April 25, 2005 What pressure does the k-series run the cooling system? I've had a look at the cap on my expansion-tank, but it doesn't have a clear pressure rating stamped on it. I've also tried searching BC, but I haven't found anything useful yet... Project Scope-Creep is live... Alcester Racing 7's Equipe - 🙆🏻™ Alcester-Racing-Sevens.com Link to comment Share on other sites More sharing options...
sjmmarsh Posted April 25, 2005 Share Posted April 25, 2005 Myles A quick Google found this: here. It says it opens at 0.9 - 1.0 bar. Steve Link to comment Share on other sites More sharing options...
Myles Posted April 26, 2005 Author Share Posted April 26, 2005 Thanks - I did a lot of lengthy Googling - best I came up with was a general site saying they range from .9ish to 1.7ish depending on make and model... ☹️ Project Scope-Creep is live... Alcester Racing 7's Equipe - 🙆🏻™ Alcester-Racing-Sevens.com Link to comment Share on other sites More sharing options...
Jason Plato Posted April 26, 2005 Share Posted April 26, 2005 why whats the problem Myles ? The caps are very unreliable Link to comment Share on other sites More sharing options...
Myles Posted April 26, 2005 Author Share Posted April 26, 2005 No problem. I changed my 'stat and fitted an oversize Pacet fan this w/e - sourced from Steve @ S-V-C. In e-mail conversation, he asked what pressure the system was running at on the basis (as I understand it) that: The higher the pressure, the higher the temp the coolant should be during normal running conditions - you don't want the fan cutting in too soon. My new running-temp seems to be around 72 degC rising to mid-high 70s in traffic. The fan seems (so far, on limited testing) to be able to maintain the temp at around the 76ish deg C mark. Project Scope-Creep is live... Alcester Racing 7's Equipe - 🙆🏻™ Alcester-Racing-Sevens.com Link to comment Share on other sites More sharing options...
oilyhands Posted April 26, 2005 Share Posted April 26, 2005 AFAIK they generally run at around 13PSI maximum. Oily Link to comment Share on other sites More sharing options...
elise_s1 Posted April 27, 2005 Share Posted April 27, 2005 Hi Myles, quote: -------------------------------------------------------------------------------- The higher the pressure, the higher the temp the coolant should be during normal running conditions - you don't want the fan cutting in too soon. -------------------------------------------------------------------------------- I find it hard to understand what is behind this.... unless you are running close to the coolant boiling point (in which case the higher pressure will allow a higher temp without boiling) I cannot see the link between pressure and normal running temp... could anybody enlighten me? thanks Carlo Link to comment Share on other sites More sharing options...
MorganMan Posted April 27, 2005 Share Posted April 27, 2005 I agree, the higher pressurisation will raise the ultimate boiling point but will not effect the temp at which the fan cuts in (unless you change the sender) and therefore the normal running temp. Link to comment Share on other sites More sharing options...
Peter Carmichael Posted April 27, 2005 Share Posted April 27, 2005 Good point Carlo. The original phrasing seems to be putting the horse before the cart. Bit of an essay, this one, but once I started it seemed to keep on coming... The pressure cap is a safety release. The system is designed to run at *some* pressure. This pressure should never in normal use reach the pressure cap pressure. When the pressure cap blows off, you no longer have a sealed system; coolant, vapour and air are lost, but at least the system is self-regulating. If you start with a small air gap in the expansion tank, you will get higher pressure in your system than if you start with a large air gap. If you run hotter, you will get more pressure. The temperature and the air gap are the things you control. The resultant pressure just falls in consequence. If you release pressure from a hot system and then reclose it, the newly closed hot system has no pressure in it and if you continue to run the engine, you will potentially suffer localised overheating effects. So what about the desirability of any particular running temperature. In the end it comes down to keeping the hot bits below any number of criteria of critical maximum temperature. These criteria include... ...minimising the chance of detonation ...minimising hot surface erosion of the cylinder head where it mates with the gasket fire ring ...keeping the aluminium alloy of the exhaust port below critical temperatures where softening occurs ...keeping the coolant mixture chemically stable so it maintains its anti-corrosive properties The critical temperature is not necessarily the boiling point of the coolant. Even if the coolant forms micro-bubbles, the heat transfer from the hot surfaces of the engine will be much reduced, leading to localised overheating. You also have to consider what happens when you shut down the engine: The coolant stops flowing and the heat soaked engine dumps heat into the stationary coolant causing localised boiling. These are all good reasons to keep the coolant temperature lower than standard production values on a modified engine that is producing more reject heat. In the end, the cooling system exists to dissipate heat to the outside world. The radiator will be able to reject more heat if it is run at a higher temperature. The system will find its own equilibrium. If you are attempting to thermostatically control the coolant temperature below the required radiator temperature for dissipating the engine's reject heat, then the temperature of the system will rise. If the radiator size is inadequate or the airflow is inadequate, then the temperature will rise. So if you fit a 72degree opening thermostat you should not necessarily be surprised to find your coolant temperature fluctuating up to 76 degrees, 82 degrees or whatever. It is possible that under your usual running conditions (high ambient temps, track use, slipstreaming etc.), the temperature may fluctuate significantly above the fully open temperature of the thermostat. This is a problem worth fixing, but you have to assess whether the problem is one of... ... compromised airflow ... inadequate radiator size ... inadequate coolant flow ... unnecessarily specifying too low a temperature thermostat. The reason it is worth fixing this is because steady temperatures give the engine management computer a much easier time in correctly fuelling the engine and are generally easier on the engine materials. Sometimes the correct answer is to accept that a higher running temperature is not necessarily a bad thing, and fit the higher temperature thermostat. If you are attempting to control the temperature at standstill with an electric fan, it will just put more load on your alternator and battery if you try to maintain temps too low. Don't bother. Set the fan switching temperatures higher so that the fan really does cycle in and out and when you get on the move again, allow the temperatures to settle before giving the engine maximum death. So going back to the quote: The higher the pressure, the higher the temp the coolant should be during normal running conditions - you don't want the fan cutting in too soon. In the end, I think this quote is just a bit of loosely phrased logic although its heart is in the right place. You have a design temperature and pressure. If you redesign the system to run at lower temperatures (and pressure by consequence), then you have distanced yourself from lots of the potentially harmful effects of overheating, although you may give yourself other problems in rejecting the heat from the engine. If you redesign by increasing the pressure (reducing the air gap and beefing up all the pressure seals and the pressure cap), you have distanced yourself from potentially harmful effects of overheating. Link to comment Share on other sites More sharing options...
Myles Posted April 27, 2005 Author Share Posted April 27, 2005 The bit I'm now most 'worried about' (in the 'a little knowledge...' stakes) is the warning many people give about creating an unnecessarily-large temperature-differential across the head and/or the effects of cycling the thermostat and any associated thermal shock... Some/all of this is probably confused in my mind - but if I'm only getting a 6-8 degree swing in temps, I'm probably OK, aren't I? Project Scope-Creep is live... Alcester Racing 7's Equipe - 🙆🏻™ Alcester-Racing-Sevens.com Link to comment Share on other sites More sharing options...
Peter Carmichael Posted April 27, 2005 Share Posted April 27, 2005 You minimise the temperature change across the head by having adequate coolant flow. The production coolant flow is adequate even for modified engines (i.e. no need to look at different coolant pumps or larger hoses). The thermal shocks that people keep going on about are... unmeasurable in the normal configuration. You need to be measuring temperatures entering the block after the water pump and there is no easy way to get a temp sender in there. You could in theory measure flow rates as well to see this happening, but that requires a specialised rig and a dyno setup. Unless you are talking to the bloke who has done the investigative dyno work through a series of structured experiments, I suggest that most of what you will hear is rumour and superstition. A relatively slow temperature swing measured in the bypass flow is not an effect worth getting concerned about. As long as the bypass flow is adequately fast (and it is), the effect of an opening thermostat will be minimised (in theory). I've got many hours of datalogging that show entirely consistent coolant system behaviour. The people with the most problems are those that bleed their systems by running the engine with the pressure cap off and then go for a drive. That is asking for trouble. Link to comment Share on other sites More sharing options...
elise_s1 Posted April 27, 2005 Share Posted April 27, 2005 Peter, thanks for the explaination. Personally, I think the right approach to a good cooling setup should be on 2 steps: - first you decide the correct running temperature, considering the criteria of critical maximum temperature you describe along with some other relevant temperature criteria (like the expansion coefficient of the liners and block or the thermodynamic efficiency) - then you "build" the system to work at the given temperature, not just replacing the thermostat but also choosing the right radiator, air ducts and fan, etc. taking into account the engine needs in terms of heat rejection. Only this way you could have a relatively constant temperature both in different operating situations and in different engine areas. Cheers Carlo Link to comment Share on other sites More sharing options...
Colin Mill Posted May 2, 2005 Share Posted May 2, 2005 I think the header tank arrangements on the K are all wrong for getting the system under pressure because the header tank is cold and has a large airspace. One might think that the expansion of the coolant into the header tank might compress the air in the header to provide the pressure but with a system of 4.5 litres capacity only about 50cc of water will be displaced as the system warms from 20 to 100C so this is simply too little to pressurise the system significantly. The system could be pressurised by the increase in vapour pressure in the header tank as it warms up. However, with no circulation through it and located where it gets a good cooling blast I can't see the header getting above 60C which will only give about 2.8psi with water (and about 2psi with a typical antifreeze mix with its elevated boiling point) If course you can always pressurise the system by arranging a small leak from the cylinders to the cooling system I think we should adopt a system with two pressure caps. The first controlling flow from the engine into the expansion bottle and which has no airspace under it. This would allow almost immediate pressurisation as the water expands. Obviously the expansion bottle would need a pressure relief cap on it for safety in an overheat situation. Edited by - Colin Mill on 2 May 2005 11:22:25 Link to comment Share on other sites More sharing options...
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