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Rotax Carb Balancing

T

todehnal

Well Known Member
Has anyone heard of using a non contact temperature probe to balance carbs? They measure the temperature of the exhaust stacks, and I understand that it is a much more accurate than any other method. I know that motorcycle techs use them and they say on the 4 carb engines, it is the only thing that really works.

Tom
 
More Carb Balancing

The guy I chatted with, started by saying that the real goal here is to keep one cylinder from fighting another. By the way, he is not an airplane guy. He tunes drag bikes for a living. He said that the simple and most accurate measure of power being produced by each cylinder was their respective EGTs. That single factor eliminates all other variances and is the absolute goal. Apparently these probes or extremely accurate, are around the same price as the old carb gauges, and are much easier to use. I just thought that it was interesting.

I may do some experimenting when I get my engine running. I'll probably start with the recommended method and then verify it with the temp probe and try to determine just how much easier and more accurate it is with the EGT method. It will be awhile, but I'll report back, hopefully sometime this summer.

Tom
 
If that's the case, why couldn't you just use the EGT readings on your Dynon once its warmed up? May have to try that!
 
Understand the logic - but

If the vacuum is equal on the carbs., the temps will be very close also I'm guessing. On adjustable carbs., that makes more sense to me. Since these are pretty much fixed, I don't think you will see much difference.

John Bender
 
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EGT's

Pete, I think you're right! With the Dynon displaying the EGTs for us, I'll bet we wouldn't need to hook up the vacuum gauges to do the balance. Plus, we have full time monitoring during flight. That will be part of my test plan.


Tom
 
Tom

The EGT's are not both on the front or rear cyls., therefore you will not be able to use that method. Better stick with the vacuum gauges.

John Bender
 
Add Probes

Hey John, I haven't received my engine yet; hopefully within the next couple of weeks. But, what about adding probes to the other cylinders? It might be valuable info anyway. By the way, I'm going EAB

Tom
 
Probes

We have decided that we are going to install EGT's on all 4 cylinders. Cheap insurance. It appears that the Dynon will support it. In taking to Service Center they didn't think that there was a lot to be gained by CHT's on all 4 because it is primarily measuring the system temperature with liquid cooling tending to equalize the readings on all of the heads. To do CHT we would have to go to a different style sensor. The sensors native to the 912 are attached in the cylinder heads in a way that can't be duplicated on the other cylinders. We would have to use the spark plug ring sensors.

Our electronics guru said that the dynon would display any combination of cht's and egt's.

I think that other LSA's with this engine do have 4 cylinder EGT's.
 
Be sure to check the Dynon D-180 that came with from Van's before purchasing the probes. Van's customized the D-180 and you cannot get to some of the setup screens that are mentioned in the D-180 installation manual. Ensure you can get to the screens that have to be modified to support the CHT and EGT configuration you are thinking about.
 
Hey Tom

I guess it wouldn't hurt anything, but I don't see a real value. If you lose your coolant, you'll know it. Since there is no thermostat, the temp do not vary that much between cyls. With the air induction system used here, the cyls. should be fairly uniform. That is what I have seen so far. I think there is enough info on the screen now if you are using the Dynon. You can tend to spend too much of your time looking at the dash, and not outside. The Dynon will warn you if something is out of range. I like that feature the best. If you add more things, report how they work out. I am satisfied with what Van's has chosen to display.

John Bender
 
JBPILOT said:
The EGT's are not both on the front or rear cyls., therefore you will not be able to use that method. Better stick with the vacuum gauges.

John Bender
Click to expand...

The vacuum method is the easiest and most accurate. A friend of mine who's also a 912 owner has the fancy digital balancer and it gives a very accurate result.

Doing it by EGT is a good method but on the 912 will probably be a lot more complicated and probably will cost more money and hair-pulling. Also keep in mind the intake manifolds have longer runners to cyls 1-3 and they'll always run a little richer/cooler than 2-4, but by how much is probably not equal on each side. It isn't on mine, and might not be the same on others too.

LS
 
Good Temp Probe

A good temp probe from Snap On is less than $100, and very simple to use. That eliminated the complexity and installation, but still may provide valuable information during the carb balancing process.

Tom
 
todehnal said:
A good temp probe from Snap On is less than $100, and very simple to use. That eliminated the complexity and installation, but still may provide valuable information during the carb balancing process.

Tom
Click to expand...

I still don't see it. By that time you're halfway to the cost of a good vacuum gauge and you're still not measuring the carb balance the way you're supposed to on the 912. You still have the issue of placing the probe in exactly the same spot on each cylinder with that huge prop swinging around to hinder your efforts, possible normal imbalances between the two sides, etc.

The suggested way from Rotax seems a lot simpler, easier, safe and not that much more expensive than (one) outboard EGT probe....

LS
 
Doing It The Old Way

Well, maybe you're right. Vacuum may still be the best way to go. It's hard to knock a factory suggested routine. It's just that I have watched these drag bike techs use the temp probe and fine tune a bike in a few minutes, getting cylinders within a couple degrees of each other, where a hundredth of a second is the difference between and win and coming in second.

Tom
 
todehnal said:
Well, maybe you're right. Vacuum may still be the best way to go. It's hard to knock a factory suggested routine. It's just that I have watched these drag bike techs use the temp probe and fine tune a bike in a few minutes, getting cylinders within a couple degrees of each other, where a hundredth of a second is the difference between and win and coming in second.

Tom
Click to expand...

With the 912, there's no way you would ever be able to get it nearly that exact anyway. It's going to be a little bit off somewhere in the throttle range no matter how careful you are or how good your throttle cables are. It'll be well within the accuracy obtainable with a vacuum gauge. So there's not much benefit to a more accurate system.
And you've already got the ports available at the engine to do it this way (the crossover tube), the procedure is already well worked out etc.....

LS
 
Reason for balance tube

If you have the vacuum set individually equal, the balance tube corrects it even further. It will be good if you set the vacuum equal.

John Bender
 
John Bender and I balanced the carbs on my RV-12 using the TecMate CarbMate. It is a differential pressure instrument, thus, it shows the vacuum difference between the two carbs. It worked very well. On the plus side, there is no need for dampening needle valves in each line as the instrument averages the pressure readings and then indicates the pressure difference on a set of LEDs.

My only complaint about the instrument; the hoses supplied with the unit needed adapters (not supplied) to attach the hoses to the engine carb balance lines.

Cost was about the same a quality vacuum gauge set.
 
Rotax

I understand that Rotax is close to releasing their fuel injected version of the 912. I am hoping it's available when my time to buy comes along.

Flyer58
 
What I do

Idle - 2200 RPM - 3000 RPM - 4000 RPM. If you get these RPM's set, it will not be off far at any RPM. Again, the balance tube will help correct minor differences. If you get the idle and 3000 set, the others will be close.

John Bender
 
Using exhaust temps to "balance" carbs doesn't guarantee equal airflow. learned this when tuning 4 carb v-12 jags. Exhaust temps for balancing assumes you have VERY accurate temp probes. This also has to be done with the engine under load, or in the "power band" of the engine. If you are "off the pipe" in 2-stroke terms you will have inconsistent temps from varying combustion efficiency. Don't forget the primary reason for carb balancing on the 912 application has to do with saving the gearbox from backlash and less vibration under idle condidtions more than good running. You can have the carbs off by a fair amount and not me able to tell much of a difference under 60% or more power. It also assumes the fuel system settings and conditions are PERFECTLY equal from one carb to the other. CV carbs make that difficult because the throttle plate does not determine airflow thru the carb. The slide does.
 
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