CHT sending unit question.

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geddes66
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CHT sending unit question.

Post by geddes66 » Sun Jan 17, 2016 1:21 am

OK, The engine in in a Bradley GT kitcar BUUuuUUT I built it with type 3 cooling system. So it got posted here. I have a VDO (Very Darned Overpriced) CHT guage and need to get a 10mm thermocouple to make it work. When I go on Ebay I see type K and type J's which one will give the correct voltage and or current to make it read right?

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Piledriver
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Re: CHT sending unit question.

Post by Piledriver » Sun Jan 17, 2016 1:36 am

I'm pretty sure all VW CHTs are K type, as are ~all EGT probes, as K has a wider range.
Some aircraft use J type for CHT.

You have 10mm spark plugs?

You can put on your own ends on TC wire and save yourself a lot of $$$, many hardware stores sell the crimp on rings in many sizes, twist the ends of the TC wire together and crimp properly.
(just take the plastic collars off and use "non insulated" crimp die with the bump on one side)
That's how the commercial units are made anyway.

For the engine side you want the stranded TC wire in fiberglass insulation,, contain the ends with some heat shrink. The solid wire is OK for fixed runs but will break ~quickly with engine vibration/movement.

You can splice sections of TC wire the same way, just twist "like" wires together (same color) and crimp in one end of an insulated butt splice.

You may need to ream out the rings for 14mm plugs, they are commonly available off the shelf up to 1/2" ID.
Last edited by Piledriver on Sun Jan 17, 2016 1:46 am, edited 2 times in total.
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Marc
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Re: CHT sending unit question.

Post by Marc » Sun Jan 17, 2016 1:40 am

I'm guessing you want 10mm to put under a head washer or at the stock D-Jet sensor location? Either's probably going to give a lower reading than at the sparkplug, but it's just a relative (and not particularly accurate) measurement anyway...and sensors under the plug are a real P.I.T.A at maintenance time.

Sez here that a K-type thermocouple is compatible:
http://www.ratwell.com/technical/VDOGauges.html

(scroll down a page or so to get to the CHT discussion) ...I've never used anything but VDO senders with VDO gauges so I can't promise you it'll work, but the contributors at that page seem to know what they're talking about.

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Re: CHT sending unit question.

Post by Piledriver » Sun Jan 17, 2016 1:50 am

CHT under a head washer will read a long way off the proper location under the plug, and respond far more slowly.
...Better than nothing, but not by much.

All the "safe" and reference CHT temps you have probably ever read about are from the under plug location.
The "factory" CHT sensor was only for EFI warmup purposes, which just needs to be reasonably consistent, is not remotely the same as safe max CHT values.
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Re: CHT sending unit question.

Post by Marc » Sun Jan 17, 2016 1:55 am

I use an eBay 14mm sparkplug sender on my trike, along with a cheap eBay digital readout panel. Works well, but BOY is it hard to get in place. They're larger-O.D. than the VDO senders, and even after trimming need to have the terminal tab bent & positioned just so, holding in place with a thin screwdriver while installing the plug. I fussed with it for almost an hour before I gave up on getting it installed at #3 and settled for #4.

That Richard Atwell page I cited shows a comparison between a VDO CHT gauge reading and a "laboratory-quality" one - the VDO is horrendously inaccurate for reasons explained there - there's almost no point in taking anyone's recommended max CHT as gospel. But once you know what's normal under given conditions for your engine, the gauge still has some value since it'll let you see if something odd is going on - even if the sender's mounted in a less-than-ideal location.
Last edited by Marc on Sun Jan 17, 2016 2:04 am, edited 1 time in total.

geddes66
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Re: CHT sending unit question.

Post by geddes66 » Sun Jan 17, 2016 1:56 am

The engine was built by "Old Speed" in the Los Angeles area in 2006. It is a 2007cc running Del Lorto carbs. I am not sure if the plugs are 10mm but they are VERY small. the smallest I have ever seen. I was really surprised. My old SU would not come close to fitting, so I cut it down and JB Welded it as close as I could (on the head) to the plug. The gauge now moves only to 100 at its hottest, clearly not right. So it is time to get it fixed. Ebay has them for 5.99. It will go on plug 3.

Thanks for the help.

That Information is EXACTLY what I needed.

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Re: CHT sending unit question.

Post by Marc » Sun Jan 17, 2016 2:10 am

There is such a thing as a 12mm plug, but most likely they're 10mm if they're the smallest you've seen. You shouldn't have the trouble I did shoehorning the 14mm eBay sender in.

I once ran a VDO CHT gauge on one of my circletrack cars, and putting the sender on a plug was out of the question since they had to be removed & installed frequently for readings (we raced even if it rained in Seattle and had to do a lot of jet changes to stay at peak performance). I just put an m6x1.0 stud into one of the o'cylinder tin screw holes on the top of the head and sandwiched a 14mm VDO sender between two ¼" fender washers, held down with a shakeproof nut. It read a little lower than when installed at the plug, but it was better than nothing and still served to alert me if the fanbelt came off (no charging system on that ride).

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Re: CHT sending unit question.

Post by Piledriver » Sun Jan 17, 2016 2:29 am

Mr Atwell just explained why I have not run "extra" gauges in the past, i just instrument the oil pressure/temp and CHT on an engine install with accurate sensors/gauges (my old Suntune DVM and TCs (is temp compensated and I have verified cal at work), 0-5v industrial pressure sensor or good quality mechanical gauge) TEMPORARILY, and if all is working as expected I pull them and don't worry about it anymore.

OTOH tossing or a loose fan belt on a stock cooling T4 or T3 will just result in a dead battery, as the fans are bolted to the crank snouts.

While I disagree with some parts of Gene Bergs Little Blue Book, but his writings on "automotive" grade gauges are pretty much sane:
The (very) condensed version:
Pretty dash filler, not to be trusted, distracting, so whats the point?

Dakota Digital and equivalent gauges are a clear exception, as they are of industrial quality and cost real money.
VDO does not go near there, but somehow manages to still charge a lot for "pretty".

It may be possible to get a "good reading" very like the under plug location on the bottom of the head opposite the plug with a little work.
You could use a small, short screw and crimp in that case, it just needs to be in the meat of the roof of the chamber just opposite the plug, results should be sane vs. plug location. Is also largely shielded from direct cooling airflow in that locale.

I have only really considered/looked at doing T4 heads that way, haven't looked at T1 heads in ages, but worth giving it an eyeball.

I have also heard of folks waxing the plug (to prevent sticking during cure only) and using a smear of JBweld to secure the TC loop to the head. (Jake Raby IIRC)
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