TheSamba.com :: View topic - Is this the Dakota CHT gauge people always recommend?

Tue Oct 27, 2009 4:37 pm

73kombi wrote: theizzardking wrote: yeah i had them and was driving by them, lol i didn;t need a speedo,tach,or even a windshield sometimes as i was looking at that dam thing so much!

i'm now part of the feel it know it drive it crowd

that's my point......

you can drive around looking at this...and worrying.

or seeing things like this...out my front window...

I only watch the CHT if I "think" its getting hot, like climbing a mountain grade like in your pic. Lots of those grades in Washington/Oregon/BC...they don't have those on the other coast....just little hills. 415F is the hottest I have ever been...230F oil temps then too.

and wondering if I am gonna pull that next grade...(HWY 20 NW WA)

any guesses on what my CHT was?

I bet hot!

Wed Oct 28, 2009 9:08 am

hi-way 20 seems to be the beast in washington for the bus, i take i-90 and baker hiway all the time with no problems but going up 20 i would see 400+ when i had my gauges installed on my old engine, nothing as nerve racking as trucking up a really steep grade @ 35 mph with a little green light flashing "warning warning dr.robinson warning!"

you guys gotta ask your self if cht temps and oil temps and yadda yadda where so crucial why did VW leave them out of their buses all the way till water cooled? if it's properly tuned,fully sealed, and everything else is with-in spec you should be good if you don't push it and just let the bus do the driving.

we really really need a samba WA WA winter camping trip or something so many of us on here!

Wed Oct 28, 2009 9:32 am

Since the thread seems to have petered out, let me step in with a mild thread-jack: I know ratwell proved the superiority of the Dakota Digital gauge, but are there any other gauges and senders that approach the same accuracy without the steep price and definitely non-period appearance?

I'm not wedded to VDO by any means, but I can't imagine that for the dozen or so companies out there who make and market gauges, there isn't similar accuracy available in an analog style gauge.

M

Wed Oct 28, 2009 9:46 am

The people that make air cooled CHT gauges are few.

For $150 or so, the Dakota is a pretty good deal in the big picture.

Wed Oct 28, 2009 9:55 am

I agree that VDO/analog style is MUCH more desirable, but as far as my research goes, nothing comes close to the Dakota for accuracy, and in the end it's the data that counts.

Wed Oct 28, 2009 10:37 am

I have a MGL avionics EMS engine monitor. Definitely NOT period nor cheap but it is very accurate and monitors all 4 CHT's.

If I was going for a "period" analog look, and cheap, then I would go for a Micro-1000 gauge. They are temperature compensated and actually cheaper than the VDO gauge. Take a look at the dual CHT, you could put one on each cylinder head.

http://www.aircraftspruce.com/menus/in/chtgauges_micro1000.html

Wed Oct 28, 2009 1:24 pm

The reason I asked is because I've seen temp sensors for AC motorcycles that a pretty analog gauges and was wondering why the DD was preferred.

Well, after a little research, I know why: the range is too narrow on every other product, topping out at 340F. Not nearly a wide enough range for a bus.

QED.

Wed Oct 28, 2009 2:47 pm

From what understand (and correct me if I'm wrong) the DD CHT gauge compares two sensors to get it's reading: (1) the cylinder head temperature sensor (ring around spark plug) with the (2) sensor in the gauge itself. My question is why compare the cylinder head temp with the cabin air? Shouldn't it be compared with the outside air - the air that is cooling the engine. For those who have heat that drive in the cold weather it's comparing the warm cabin air to the cylinder head temp which I think would make the CHT on the DD gauge read low.

How does the VDO gauge get it's reading?

Wed Oct 28, 2009 4:18 pm

56ovalbug wrote: From what understand (and correct me if I'm wrong) the DD CHT gauge compares two sensors to get it's reading: (1) the cylinder head temperature sensor (ring around spark plug) with the (2) sensor in the gauge itself. My question is why compare the cylinder head temp with the cabin air? Shouldn't it be compared with the outside air - the air that is cooling the engine. For those who have heat that drive in the cold weather it's comparing the warm cabin air to the cylinder head temp which I think would make the CHT on the DD gauge read low.

How does the VDO gauge get it's reading?

vdo is analog single signal from the sparkplug hole.

what your saying above is void anyways in that situation the gauge would read hotter than actual and everything would be ok,

but thats besides that point, the only way to achieve what your getting at here is to take an ambient air temp inside the engine compartment (which again would be next to impossible to find the optimal location) then do the math with your cyc head temps, i don't think you'd find a gauge that does that , possibly something in the Avionics field might , but then again unless your equipping your bus with doppler and infrared i don't see the point.....

cabin air temp should be close enough to outside air temp. it at least feels that way....

average between the cht and the cabin temp brings that
margin or error down even further than just trying to sense the cht alone.
so something is better than nothing...even if the data is slightly off course.

bottom line, is there a better gauge out there for our buses?
no not really unless you start getting really expensive........
it's as simple as that, is it perfect no, is it better than anything offered for our bus? yes. (unless you want to spend $$$$ updating your 30 year old vehicle with state of the art aviation electronics)

Wed Oct 28, 2009 5:16 pm

56ovalbug wrote: From what understand (and correct me if I'm wrong) the DD CHT gauge compares two sensors to get it's reading: (1) the cylinder head temperature sensor (ring around spark plug) with the (2) sensor in the gauge itself. My question is why compare the cylinder head temp with the cabin air? Shouldn't it be compared with the outside air - the air that is cooling the engine. For those who have heat that drive in the cold weather it's comparing the warm cabin air to the cylinder head temp which I think would make the CHT on the DD gauge read low.

How does the VDO gauge get it's reading?

It's a complicated answer. You can read all about how thermocouple's work at the Omega Electronics website.

But the short answer, is that it is in fact the temperature of the "other end" of the thermocouple wire (usually the gauge itself) that needs to be measured in order to correct the reading from the thermocouple. This is called "cold junction compensation" for rather esoteric reasons (again read up at Omega).

Gauges which are not temperature compensated (such as the VDO) are only accurate at a specific temperature (usually room temperature). Cold Junction Compensation fixes this. The Micro-1000 gauge I posted above, the Dakota Digital, and the MGL Avionics are all temperature compensated.

Wed Oct 28, 2009 6:18 pm

While searching, I did find a westach CHT that ranged from 100-700F and was cold junction compensated. Model 2C1 (and variants). See westach.com.

This is a more traditional analog gauge that would seem to provide all of the same benefits as a digital gauge without screaming 'anacronism' at the top of its lungs. Even better, the gauge and sensor come to about $85, all told.

Obviously this is as much a matter of taste as a matter of accuracy. For a very slow moving delta (like your CHT hopefully is,) the digitally numeric gauge would be preferred for easy reading. But if you are watching your CHT as a matter of total engine management (one value among many), I'd think you'd do just fine with an analog gauge if it were CJ compensated.

M

Wed Oct 28, 2009 6:44 pm

Hard to read 400º and 425º from the drivers seat.

I like to set the DD to flash when they get to an interesting temp so I can look out that big baywindow instead of watching the gauge.

It would suck to hit a moose or soemthing.

Wed Oct 28, 2009 7:06 pm

Okay, so the DD CHT gauge is better than the VDO because of it's temperature compensated feature. But, is there any benefit from it being digital? Is it faster showing the 'correct' temperature reading than an analog gauge or is the rise & fall of the cylinder head temperature not quick enough to make a difference?

Wed Oct 28, 2009 7:10 pm

Agreed. Which is why you'd order their 2C1-LH, which has a threshold settable warning lamp.

I'm not saying this is a preferred method. Only that the DD is not the only affordable instrument out there that is still accurate. I spent enough time of westech's site to ogle some of their goodies, such as a Fuel Pressure gauge with settable low warning. I got to thinking about putting that inline on the fuel rail between the regulator and #1. Drop in Fuel pressure...either your fuel rail has sprung a leak or your pump is starved upstream. Handy.

Anyway, you could dump buckets of money in to instrumentation that would accurately tell you that you were killing your engine because you'd neglected basic maintenance (like to aforementioned fuel hoses.) If you're an information junkie like me, you like to know what your mill is doing back there. It has taken me a while (after driving water pumpers for many years) to get used to driving by ear and by feel in my 1302, without any gauges but that big dial in the middle that tells me how fast I'm not going. :wink:

M

Wed Oct 28, 2009 8:10 pm

webwalker wrote: While searching, I did find a westach CHT that ranged from 100-700F and was cold junction compensated. Model 2C1 (and variants). See westach.com.

Hmmm, according to the Westach instructions that gauge is not temperature compensated.

http://www.westach.com/instructions/2C1.JPG

Wed Oct 28, 2009 8:25 pm

Perhaps I'm thick; (its almost midnight here) but the assembly info you cite in your link starts with Quote: 'The indicator [gauge] measures the temperature difference between the hot end (probe) and the pin terminals on the other end of the thermocouple (COLD JUNCTION).

What am I missing here? Either a cold junction thermocouple is implicitly 'temperature compensating' or it serves no purpose. Did I miss a step? :?

M

Wed Oct 28, 2009 9:12 pm

webwalker wrote: Perhaps I'm thick; (its almost midnight here) but the assembly info you cite in your link starts with Quote: 'The indicator [gauge] measures the temperature difference between the hot end (probe) and the pin terminals on the other end of the thermocouple (COLD JUNCTION).

What am I missing here? Either a cold junction thermocouple is implicitly 'temperature compensating' or it serves no purpose. Did I miss a step? :?

M

All thermocouples have a cold junction. Units like the Dakota Digital are cold junction compensated. This means they compensate for the outside temperature on the "cold junction" side of the thermocouple wire (the end NOT attached to the spark plug). The DD gauge will give you the most accurate temperature at the spark plug. The VDO style gauges simply tell you the difference between the two ends of the thermocouple, calibrated with the assumption the cold junction end (ambient temperature) will be about 75 degrees. The problem is, when it is 32 degrees out, the calibration is incorrect and the gauge will read 75-32=43 degrees HOT. When it is 90 degrees out, the gauge will read 90-75=15 degrees COLD.

Wed Oct 28, 2009 9:20 pm

and that explains it!

thanks for the clarification! :)

M