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CH Temperatures on VW


Old Koreelah
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I have found a hopefully reliable article on the net which recommends the best temperature range for VW CHT's in order to ensure long exhaust valve life. This authority recommended 412-428°F as acceptable on climb and 338-345°F on cruise.

 

My problem is that when testing my cooling modifications with ground running, all four cylinders rapidly climb to 420°F. I can't find a recommended temp for this mode, but it does seem high.

 

Any advice?

 

Regards,

 

Old Koreelah

 

 

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Hi,

 

Checking my old records of the VP-1, at a temperature of 41 degrees and 47% relative humidity nil wind with a/c standing in the sun two hours then after 15 minutes of ground run the CHT,s were 370-380 with cylinder 4 about 5 degrees hotter. On fast taxi the temperatures dropped to around 360 degrees and stayed there for the whole takeoff climb(3300rpm) on leveling off and reducing power they were 310 to 320 degrees. Motor had 961 hours on it and heads off twice, second time had valve grind, new plugs and leads just fitted.

 

Bob.

 

 

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Hi,

 

Forgot to ask what brand of oil are you using?

 

I noticed when i changed to castrol GTX2 the running temperatures were lower. I do oil and filter changes every 200 hours. I did notice there was less sludge and deposits after changing oil brands. As well some rise in temprature can be attributed to the heads, incorrect valve clearances or wronge/weak valve springs(using after market springs) valve seat damage or internal seal leakages.

 

Bob.

 

 

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Thanks for the reply, Icebob. I am using the oil recommended by my VW guru (can't remember brand, its locked away now) but I doubt it would make the sort of difference I'm seeing.

 

My old VW donk was just sticking out in the wind and lacked gauges; assessment of temp. was guesswork.

 

With the knowledge that the exhaust valves area has vertical fins and needs the air shoved down past it I spent weeks building a pressure cowl with carefully-designed ducts channel the air. It sure doesn't seem to be working.

 

What sort of cowling arrangement did your VP have when you gained the results quoted above?

 

 

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Hi,

 

I went down that path too with little success.

 

I went to an "overall" cowl with guides inside to redirect the air, I found the exits were too small and air was choking at the exits, not helpful.

 

mine had the heads slightly redesigned where the fins are with an airflow ram tube about 80mm opening and directing the air onto those fins you talk about and that did not work either.

 

I was advised to check the oil cooler tempratures as that may be the sourse of some loss of heat exchange and to a degree it was, i got another built and it dropped my overall enging tempratures about 9 degrees but not enough for me to be happy with.

 

In the end i purchased from Great Lakes in the USA a set of 4 heads for my 2100cc motor the fins are longer and slightly thiner than standard heads and are made from cast aluminum and have the screw in point for the temperature gauges inbuilt above and between the exhaust valves, it was not until i spoke to someone from Brisbane that i learnt the dune buggy re manufactured heads are the same thing and that would have saved me just over $300, bugger!

 

The smaller motors still should not give the temperatures you are getting unless there is another issue hidden there even the old cast iron heads were cooler than that.

 

The imported heads and a bigger oil cooler (40% bigger with thermostat)fixed my heat problem so my cowl is just the conventional form conforming with about a 30mm air gap all round and the exit size slightly bigger than the airflow entry size.

 

Bob.

 

 

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Hello Icebob,

 

It's reassuring to hear I'm not alone in wasting lots of effort with cooling. I sure wish I'd seen the Great Plains and Aerovee systems before. Looks simple as. Just a big skirt running horizontally around engine to ensure air has to go down past fins to get out. Perhaps I'll tear off my complex ducts and build something like that, or just buy a set of laser-cut fences from Aerovee.

 

Regards,

 

Old Koreelah

 

 

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Morning Icebob, sounds like your success should be my guide.

 

When I conservatively applied Tony Bingellis' air inlet formula (0.35 sq inches X hp) I got a figure of 124 sq cm for 55hp (which I'm sure my 1600 will never approach) My inlets are heaps bigger than that, and the exits total 280 sq cm. I was more worried that at cruise the engine would run too cool!

 

Instead it seems that overheating whilst ground running is a certainty.

 

Did your cooling ducts follow the formula?

 

Regards

 

Old Koreelah

 

 

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Hi,

 

Yes that is the formula I used too however my old notes say .76 sq inches per rated horsepower to the next highest HP (not actual HP). 1600cc VW motor is 52 HP, and 2100cc is 68HP by my old notes for calculating.

 

Your calculation is for the smallest opening for cooling not the largest.

 

I did trials at altitude to balance out ground and flight temperatures by slightly modifying the cowl and airflow guides.

 

I found in summer i could only idle on the ground for about 20 minutes before an overheat condition started, in winter all day if i needed. flight was different i did get over cool a few times and once i fitted the oil thermostat that fixed the issue. The thermostat is basically a bypass valve that sends the not cooled oil back to the sump to mix with the cooled oil.

 

Bob.

 

 

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