I've got two of the VDO spark plug type head temp sensors. One on cyl 1 and one on cyl 3. I know that cyl 3 is the hottest, but i'm seeing differences of ballpark 30-40 degrees between the two. From the drivers seat I can't accurately read the gauges, so i'm going to move the gauges somewhere else and see how hot the temps get.

On the track the #1 cyl gets to about 365F. The #3 gets hot, unconfortably hot for me. I thinking around 400. I tried retarding timing 5-6 degrees total, didn't seem to make a difference.

A little history: the PO was doing a hill climb and literally melted the right side of the engine (cyl 3 side). He had the head rebuilt and slapped it back together. Before I converted to megasquirt the carbs had all this metal crap (aluminum maybe) welded to the inside of the butterfly valves. I'm thinking this head really had a melt down.

#3 always run hotter than #1, even at idle. I'm going check reading on my stock injection head temp sensor and see what Megasquirt says.

It was bugging the shit out of me at the track. I kept looking over instead of concentrating on the course.

Thanks

pesonally i do not think that the vdo works yery well at all, putting it on the spark plug just does not make sence, on my car i have both one under the plug and one placed in the original hole. the one under the spark plug with a gadge is weard to me, as i push the gas the gadge goes down if i fan the gas peddle the gadge will bounce quite a few degrees 50 to 75. there is no way the head temp can go up and down that fast. so i just do not believe it, all i use it for is to check for high temps, and try to ignore the rest of its actions. i guess the air flow has a great deal of effect on the sensor.

The stock cooling system has imbalance issues between the left and right bank of cylinders. I have monitored these temp differences at a max of 120F between the two banks of cylinders, but they generally average a delta of 60F. Number 3 will always be hotter than #1, it's the nature of the cooling system- until you swap to a DTM.

YES, head temps can raise and lower VERY quickly dependant upon load!!! You can easily see differences of 100F in less than one mile.

THE SENDER MUST BE PLACED UNDER THE PLUG!!! In 2003 I put two solid weeks of test time into finding the best possible place for the sender and under the spark plug gave use the most stable readings. The spark plug is the best place because it is not surrounded by cooling air and it is not located in the cooling fin area like the original sensor is.

I have seen a 200F difference in readings between the original sensor location and the spark plug location on the same cylinder. Anyone that uses the stock sensor location can add a minimum of 70F onto their values for "real" head temp.

The stock location was made for the stock FI system. All that system needed to know was "hot or cold" as an incremental reading was not needed. The other reason that the sensor was placed in a certain location was to monitor the cool down of an engine, basically to keep the engine from going full rich at each start up, even if you just drive 15 minutes, shut down and fire back up in 5 minutes.

The stock CHT location is for FI sensors, not CHT thermocouples. 41 Ghost, those CHT fluctuations you see are real, thats what the heads live through daily and thats why properly monitoring their status is such a good idea.

Yes, I have entire folders full of data logs to support my feelings on this topic. Understanding cooling systems and etc was my life from May of 2003 clear into late 2005, I didn't do much of anything else.

Well, is anyone going to ask it? I know this will probably start a flame war of opinions, but what is the nominal temperature range for T4 cylinder heads? Is there a factory spec for "don't get over..'' such and such. Is 400 degrees bad or not? I mean, VDO did make a 600 degree gauge for Porsche, which leads me to believe that they were expecting it to be most accurate between 300 and 400 degrees. 400 may be on the high side, but is it at the breaking point of the metal? Fire away.
Mark D.

when i say the temp fluxuates up and down to a point that is not believably i mean it in seconds 1 or 2 not a mile or a min.

you probably have a loose connection somewhere. try a different sensor or a different gauge. my temps move, but over the course of a minute. the needle does not bounce around over 1-2 secs.

Jake - thanks for your reply. I'll pay close attention to the actual reading from #3 under the plug and see what temps i'm running. I know its hot, probably too hot.

Also, aren't there 2 different VDO sensor/gauges out there?
One set goes to 600F and the other goes to 500F or 550F?

Just a thought.

--- bill

The following temps are based on the readings I have seen both on my dyno, in my test car as well as every car I own as I do 92 miles round trip daily, all aircooled. I have been able to see the effects of different temps on engines, MPG and come up with basic rules.

These readings and parameters are ONLY accurate when your sender is the same place as mine, under the spark plug.

Less than 300F- Your engine is well configured, properly tuned and you probably are not driving it hard enough!

300/340F- This is what I consider average for the Performance TIV engine that is well tuned and properly configured. With these temps you'll be getting the best MPG and efficiency from the engine, it will live a long powerful life and you can do valve adjustments at the normal 6K mile interval, since heat isn't sinking your seats and stretching your valves.

350/375- These temps are standard for a STOCK engine with a stock cam at cruise speed, if it is properly tuned. Generally if you are cruising at these speeds as soon as you hit a hill you'll see a spike to a higher level, approaching 400F. If you experience temps of 375 in 5th gear, you may be doing the engine an injustice by keeping the tranny in 5th, hitting a hill with a shift to 4th will keep load off the engine and that reduces engine heat and increases cooling fan speed- 4th gear is a friend to stock engines.

If you see temps in the 375 neighborhoodconstantly you'll be having to reduce the intervals between valve adjustments to maybe 3K miles due to the extreme heat cycling the engine and valve train parts are seeing. Engines that cruise at 375 are generally out of tune or may be misconfigured creating more heat.

375/400- No engine should "cruise" at these temps, not for any reason. Cruising at these temps is a guarantee that under load up hills that you'll be over 400F and thats not acceptable.

400/420- At these temps the rules change, 400 is the magic number that stretches valve train parts and really where cracking issues begin. 400F is OK for a few seconds if climbing to the top of a hill, but don't allow the engine to stay there long. In a perfect world as the needle starts to climb toward 400 the smart driver will grab a lower gear and remove load to cool the engine some, the extra blast from the fan also helps.

420+ Well your engine has been damaged. Go home, do a valve adjustment and see how many valves have "tightened up" from your baking experience. Those valves are the ones that have seen the most damage, so make a note of it and upon teardown (that probably isn't far away) see how those chambers and exhaust ports look as cracks will more than likely be notable with the naked eye.

The other big issue is with guys that get their engine real;ly hot and then immediately pull over and leave the engine running! THIS CRACKS HEADS!!!!!!!! If you get the heads over 400F sustained the best bet is to slow down and use the tranny to moderate the cool down period. If you do stop on the side of the road shut the engine off and allow it to go through it's heat cycle naturally, this will keep the heads from cracking in most cases. The dumb guys pull over leave the engine running and watch the needle drop down to 250F and drive again, those guys cool the engine to quickly and crack heads.

So, there ya go... Thats a short summary of what I have seen for temps and how they effect the engine. Once again THESE ARE ONLY FOR GUYS WITH THE SENDER UNDER THE SPARK PLUG!!! If your sender is there you are not lazy and want the best readings for accurate monitoring of the engine. If your sensor is not there, put it there, OR remove the gauge and throw it away, because the readings you are getting are worthless.

very usefull info , thanks again jake.

Jake,
Thanks for taking the time to better explain the type IV motors and head temps. From reading numerous posts in the past I knew much of what you were saying, but I have to admit....I would have been one of those guys who would have pulled over with the engine running to allow for cool down if my motor got really hot...not that it has...yet. I would of thought that leaving the motor running at the side of the road would have been good...keeping air flow around the heads to bring it back down. I guess that comes from my experience with water coolled motors...when you turn off a hot water coolled motor...the temps skyrocket.
Anyway...thanks,
Mark

This question might seem counter-productive, but are there any ways you can modify or improve the stock cooling system to keep those head temps better under control?

I had heard somewhere about a person modifying a type 3 fan and fitting it with our impeller for a noticeable drop in overall head temps.

The stock system is not bad, WHEN IT IS SET UP PROPERLY. These days few people understand the proper way it all goes together.

The other big issue is keeping the cooling fan clean, dirty fans can elevate temps by 30-50F easily.

Modifying the cooling system to work better than factory is NOT an easy task. Generally any attempt to do this without extensive means of monitoring changes will result in negative results and I have certainly had my share of those.

The key is not generating heat, thats the easiest way to make the stock system more effective!

I have been working on some mods for the stock cooling system, primarily for VW Bus owners as there are more of those on the road than 914s and they have 4,500 pounds to push around (they also open their wallets for things that work) but the mods are the same for the 914 engine.

Honestly an engine should not run hot, even in extreme climates if it is configured correctly and in tune. The stock engine is the hottest running of all-

does it help for cooling to change your 914 engine lid so that it has a screen on the entire lid instead of just half.

It helps to remove the rain tray by 10-20F on the CHT gauge. This is in 90F air temps and also it helps your oil temps by 10-20F to pop open the engine lid while driving.

At 70F everything should be plenty cool....oil -200-220F - CHT's 325-375 MAX

Then there is the question of accuracy of the VDO gauges themselves.

C

No factory spec. When asked, Porsche said "Go ask VW, it's their engine." VW then said, "We never considered it to be a problem."

Go fig.

--DD

Exactly... both visual resolution and accuracy. One tick = 50 degrees. Kind of hard to tell if i'm safe or melting away..

I've seen a few people post about the Dakota Series II Digital Head Temp System (link)

Good bad? It's ambient temp compensated which is a huge advantage over the VDO. It also has a high temp alarm.

Maximum temperature reading - 750°F (400°C)
Gauge Resolution - 3-4°F (2-3°C)
Gauge accuracy - ±10°F

Anyone use this thing? Comments?



This is the cheapest price I could find..

i have one....works great!

The VDO is not the best instrument for total accuracy, but it does have repeatability....

The VDO gauges of yesterday were much better than todays, but none of them are temperature compensated..

I have 9 different head temp gauges to finish studies on over the summer, I plan on doing this while my Engineer intern is here from the UK as he is more analytical than even I am...

The fellas at Georgia Tech and I have chatted about this and they have agreed to offer up some of thier gadgets for the test work when the time is right.
These SPA design instruments have proven to be the best money can buy in the past, thats why I use them to drive the RDT Labs data logger, not cheap, about 300.00 each.

Yarin, you can always back up your VDO gauges with your contact tire pyrometer.
I stuck the probe down the sparkplug hole in the tin getting the point as close to the pug base as I could. Comes out pretty darn close to what the gauge reads.
I also got the Westach dual gauge. I run one thermocouple on each side.
1-3 Usually 20-30 degree difference side to side.

Joe - I don't have a pyrometer.. yet

Do you leave the pyrometer there when on the track or autoX? I'm a numbers guy myself, I like seeing reading as an indicator of whats ok and not OK.

I think i'll order the Dakota gauge.

I got my gauge. Slight problem....

The spark plug doesn't fit in the sensor. The inner diameter of the sensor is about 1mm too small.

What did you guys do? Drill it? I don't want to mess with the calibration of the sensor.

Thanks

That's good info, I have not tried to fit mine yet. A dremmel tool with a sanding drum should work. I doubt it will mess up the calibration.

BTW Yarin. I should have pics. of the inj. setup tomorrow. i finished the second rail today. all that's left to do is make the support bracket

Yarin - As for being able to see both gauges at the same time, Morphenspectra (sp?) sells a left windshield pillar gauge holder - holds 3 gauges and is ideal for you auto cross types b/c rather than having to look down at the gauge, you just look slightly left...

just a thought.

-pt

I decided to ditch one VDO and go with a digital head temp gauge. I can't do a gauge pillar pod because of my roll cage.

I emailed tech support, got this response:

"If the sender doesn't fit around your spark plug, you can either drill
it out so it will fit or you could cut the end and widen the loop."