Speedtoys

" the obviously thought "

You don't know that...fuc* sake, its not there on production cars.....

It could just be a "we need to do this fast because racecar" modification. No time for a drive cycle and check the level.

mark kibort

You missed the time line.......... I'll say it again. i run cool as a cucumber on 70F days and normal temps up to 90 F days. i overheated on the Home-dyno and i over heated on a day where i had air in the hoses to start. I forgot to bleed it out and the temps were 100degrees plus. (several years ago) . it's not like jello, its rock solid. you just need to keep the conditions straight. I'm providing data. actual data. replicated while on the home-dyno. i thought it was interesting that all the things that "couldnt " happen, did happen. there are no issues, no leaks, no caps mal function, no t-stat issues., newish water pump all are in perfect working order.

like i said, i havent taking the car through multiple drive cycles, only one to full temp and about 20mins of driving and there was little change.. still needed bleeding. so i bled. im not sure if multiple cycles might do it. of course it is not stock... I'm saying im not the only one that has tried to address this modification. it was done by Porsche motorsport, its how the car was acquired from Porsche by the first owner with 18,000miles on the odometer. maybe the IMSA folks did it. dont know. all i know is that the lower feed to the radiator is down to near 20" the water bridge is 23" and the lower radiator hose is 21". the half way mark on the reservoir is 25".

GregBBRD

And the top of the reservoir is 28"....you forgot that little detail (we're not stupid, Mark.)

944 engines have bleeders, because the top of the reservoir is lower than the elbow coming out of the front of the head. There's a bleeder in that elbow or at the base of the elbow (on 951 engines.)

928 engines don't have bleeders, because the top of the reservior "towers" over the thermostat housing and all air is forced to the top of that reservior.

Sure, there's a tiny air bubble possible from above the water return hose from the thermostat housing to the reservior, but that bubble is flushed out in the first couple of seconds of running, when the thermostat is still closed.

​​There is a potential for air to get trapped in the back of the cylinder heads, if the front of the car is not "raised".....like on all V-8 engines.
The solution to "fix" this problem is about 50 years old...I used to do this on 427 Ford race engines, BBC race engines, and SBC race engines. I do this on 928 race engines, also.

I conservatively fill about 200 928's a year with coolant/water (calling it one a day is very conservative.) A grand total of zero of these engines have issues with air being trapped....we do absolutely nothing but fill them up and run them. Yes, after they warm up and cool down the first time, we top off the reservior, like should be done.

4. Why do you think your car/engine is so special/different? (#1-#3 still unanswered, BTW.)

mark kibort

Good question.... it's not that my car is ANY different, its the use case. If you dont have a heat cycle or two, not all the air will be pushed up . you have not seen any issues, because you are not talking racing out of the gate after a radiator fill........and generally, it is good enough .... on a hot day 100f and there is even a little air still in the system, it could cause an overheat, which causes more gas, which causes water to force out the reservoir. running for multiple laps, at normal temp and 0 pressure with air in the lines and the water level raised is the repeatable observation. make sense? it is repeatable. I've helped scot and others with the same issue. you boil and push out a small amount of coolant, and cool down .......you will see the same characteristics.

Now, question 1-3... i dont know the water flow. rates (i assume very high) i dont know and assume local boiling and dont know the reasons for the boiling or the location. assuming that it starts with the heads. it doesnt change the observation or theory. the air in the main water hoses doesnt go away in a few seconds by the way. i think most of it goes away after a complete cool down. i express the situation by birping the system before starting and while its warming up. (before any pressure builds..........then it is done.

no, i didnt forget any detail. i said the mid reservoir was 25". With the curves and bends its no guarantee that the air just is forced up to the container.

GregBBRD

Sorry, I thought you drove your car to events still, which would certainly bleed out any remaining bubbles of air, in route, right?

I also missed the detail that you had emptied and re-filled your cooling system, leaving trapped air, before your home dyno (whatever the hell that is) testing. Because if the car had been running and driven a heat cycle before this, there certainly would not be any trapped air.

There actually are two "full time bleeders" on a 928, one from the top of the radiator and one from the thermostat housing. There's nothing to open or close...they "bleed" all the time. Both of these go to the top of the reservior. Once you see a solid steam of water squirting out if these hoses into the reservior, the system is bled.
​​​​​

hacker-pschorr

Day one of Navy Seal training: "Bubbles Go Up"

Long story how I know that.

mark kibort

Greg.. no more driving to the track. so, i guess i havent had a chance to see how the system reacts to just driving on the road for more than 30mins. i know heat cycling the system doest get it close, but i still feel air in the lines after it cools down, and am able to bleed a little of the remaining air out. makes me think its not a perfect system, but adaquate.

as far as the Home-dyno (thats just the car on jack stands, running the car WOT, while controlling RPM with rear brakes) it introduces a high heat load on the engine , similar to a dynojet and with no fans, you cant do more than a couple of runs or risk overheat. during the experimentation on what RPM , gear, and ramp up to test WOT effects on the timing, the engine got into the overheat range, and ejected some water. As the engine cooled via the fans and still runningunning, the gas /air space in the hoses remained , there was no pressure, and the overflow container was 1pint higher. (from mid line, to completely filled to the to top)

I think the confusion is created by the conditions for overheat and the results. At the track, i had a mild over heat, that raised water level in the container the day before....forgot to bleed it (i noticed it but needed to wait for cool down, and forgot the next AM).. i started the race with air in the lines on a 100 degree day, and it over heated.....so, i backed off for a few laps and then there was a checker. in the pits, i noticed the lines soft, air in the lines, overflow container full,and engine running at just below mid line. (below fan engagement temps) . this is the part we are debating........... i say its possible that the air in the lines isnt immediately filled by the water bottle being higher and that trapped air doesnt escape . yes, fluid can still fill (circulate) from the top of the radiator feed line while trapped air is in the upper radiator hose seems to feed also from the bleed line above the T-stat. however, i have not seen the system exchange air in the lines for fluid in the reservoir very effectively this is the obsevation.. however, i find it very easy to push the air into the bottom of the reservior by "burping " the system (squeezing the hoses) which bubbles up to the reservoir and then when released, water replaces the air..... this can only happen with the cap removed.

so, based on your responses and experience, its quite possible that i have not allowed for a heat cycle or enough running time for the system to self bleed, or if there is air in the system to start and you begin a race, there is some impedance of flow and it can contribute to an overheat condition before it has a time to self bleed. you can also create air in the system by overheating, and as it cools the pressure in the lines will be ambient, the air will not move out of the main hoses, and the overflow container will be resting at the overfill level. over several laps and running in the pits this didnt change.. after a cool down... a bleeding . burping of the system, it all returns to normal. engine runs, pressurizes system in a few mins and runs normally.......

mark kibort

Yes, and sometimes in sunken ships (and cars) there are air pockets where air doesn't or cant go up. Sometimes hard to move at an angle up too if there are obstructions. We are talking about air pockets in the system.

andy-gts

sooooo Hacker you are a seal..........

hacker-pschorr

You are forgetting the fact there are no areas for such air pockets in the 928 unless something isn't working at intended.

This is what you are famous for, taking a relatively simple situation and over complicating it to the max. I recall yeas ago you going on and on and on for days proclaiming there was no adjustment to a double disc clutch. Then suddenly your brain switch chapters and you became the self-proclaimed expert on adjusting these clutches.

How to we change your channel to the one that has a better understanding of the 928 cooling system?
The fact remains, if you think the 928 is difficult to "burp" it's only because you lack experience working on anything else. Go try to burp an 80's 302 Ford and get back to me.

If there are any air "pocket's" it could be in the heads on a racing engine with the rear caps blocked off (even that is a thin theory) - and even if true, would not be effected or "fixed" by adding the bleeders you are suggesting in this thread.

No, I am a Jimmy Buffet fan (where the story came from).

mark kibort

actually there is... proof is in having air in the coolant lines before start up. why doest the fluid fill the gaps naturally? sit there for a year and the air pockets will not move. fluid flow via the water pump will help, but its not perfect. water can move quite easily under the trapped air. the expanding and contracting of the fluid, does move the air around , but as i have found, not completely. i have helped many 928s with this issue. a few at the CT automotive shop too! nothing wrong with the system and over time it eventually bleeds out. but if you are doing a track day after a radiator fill, be careful and expect a circulation issue.. OR, you can burp the system and have it be rock solid. its just experience.

as far as the double disc clutch, yes, i spent a LOT of time studying the dual disc adjustments as mine was problematic. GB was talking about high hp cars breaking off "ears" of the "H" and other nonsense based on clutch release speed etc, , and when i finally looked at the system completely and the facts and looked where the forces were applied, i did gain knowledge of the system, no different than others here have done on other fronts.... in the end. (and GB private messaged me after all of that and agreed) ive looked at all the factors, and ended up PINNING the INT PLATE in place, rather than buying a new one and it has lasted on a 420rwt engine for over 10 years now with no adjustments (and still working perfectly) I dont think there is anything wrong with a deep dive into technology to understand it better and make adjustments accordingly. remember all the people saying, including DEVEK to just push the adjusters all the way back and they will self adjust? i found that to be a HUGE problem and proved that with my car, and scots car. too far or not enough and the int plate touches the flywheel or pressure plade and drags on the driveline.

i never suggested adding bleeders. those were added by the porsche team or porsche because they obviously saw a need. they didnt just do one, but two. ive never used them.. i dont believe in "bleeders" but do believe in burping as it SO easy to push the air out to the reservoir when cold for a no air pocket coolant flow. again, you can see this yourself after you are filling the radiator from a job, or have an overheat occurrence. the air remains but i suspect after a few heat cycles , bleeds itself. at the track, you cant wait for that process and any air can contribute to poor flow and make it run warmer and possibly cause overheat. this is a LOT of experience here talking. look at my temp gauge on my videos on the hottest days. it runs unusually cool when things are solid and set up well.

You can feel if the hoses have air in them (pockets) even when hot. when without air, they are fluid solid. its a very easy thing to feel for.

mark kibort

Let me just say, i didnt intend this thread to be an argument , just an observation and theory of causes , to what ive seen a few times. its quite certainly possible that the system can bleed itself over time and heat cycles, but ive found, on race day if there is air in the system , it doesnt go away "naturally" without burping the system and if you are racing and the engine gets near the overheat range, pressures can rise and push out water from the overflow, creating air in the system that doesnt seem to fix itself, even after the engine has cooled down to below fan turn on temps. (this i have seen multiple times). the system contents contract, the system is then NOT under pressure, and thee is impedance to the flow due to air in the lines that doesnt bleed itself out. upon seeing this in the pits after a race.. a quick burp by opening the overflow cap, and squeezing out the air, and a replacement of the cap, allows the system to be ready for the next race. for street driving, the cooling demands dont seem to be as great, and possibly the length of time of operation, or number of heat cycles is enough to fix the issue without attention to it.

Mrmerlin

what kind of cooling fluid are you using while doing all of this testing?

mark kibort

All of it, on track for the last 20 years and this last test on the garage has been done with pure distilled water and water wetter, with a splash (pint) of antifreeze.

GregBBRD

Not much mystery, here.

The engine overheated. The pressure went up and a small amount of fluid was pushed out the cap. If the cap was good and held pressure, it would have quit venting with 1 bar of pressure remaining.

Immediately removing the cap would cause the fluid to boil (due to the reduction in pressure) would result in and "Old Faithful" event. If you, Mark, were able to immediately remove the cap....it's really simple.....the cap did not hold 1 bar of pressure (once it started venting) and it needs to be replaced. (This is a common failure mode.)

If the engine was allowed to cool down, after it was shut off, the pressure would drop fairly quickly and the cap could then be removed without ejection of fluid. This would take 5-10 minutes, depending on what volume of air was going past the engine (once the system temperature drops below the boiling point at atmospheric, the pressure drops very quickly, allowing the cap to be removed without drama.)