Chassis Dyno experiences

F

Florida Fiesta

New member
I'm looking for some feedback on the chassis dyno process.

I took my ITC to the dyno, they hooked it up, warmed the engine a bit, got it up to 3rd gear and floored it unitl it hit 6,500 rpm, then shut it down. Then they tell me I'm off the chart on the lean side so we increased the secondary fuel jets on the weber. We repeated this a couple of times until horsepower and torque stopped increasing.

Does this make sense? Is this going to relate to running WOT at Daytona for a mile long stretch?
 
Did you increase the main jet or DECREASE the air correctors?

The Air corrector has much more influence on mixture strength at the top. If it is smaller, the less air in the mixture.

The main jets affects all RPM (above idle), but at top end, its the air correctors.

What Weber? A 34 DMTR by chance?

You should be around 145 on the mains and 195/160 on the airs with F30 emulsion tubes for that carb.
 
That's the normal process. Did they use a wideband O2 sensor for readings? Did they give you a print out? If they got it to the 12s at 6500 you're good for any track.
Chuck
 
Sounds like you were on an inertia type dyno - DynoJet and Mustang are two of the more common brands. On this dyno you use the wheels to accellerate a roller of known mass, and using math derrive the torque required to achieve the measured rate of accelleration at each rpm. Then using math derrive hp from torque and rpm data.

This is better than nothing, better than tuning by ear, but as you noted really only tunes for WOT conditions at the resulting accelleration rate.

The other type of dyno is a load dyno. They apply resistance to the car via different methods and measure the actual torque the engine 'pushes back' with. Dyno Dynamics and Dyna Pak are two popular brands of this type of chassis dyno. Most engine dynos are this type as well.

They have the ability to hold the engine at a specific rpm while you operate the engine at different load/throttle openings, so that you can tune all the 'buckets' of load/vs. rpm. Imagine a graph with manifold pressure on the y axis and rpm on the x axis. Lower manifold pressure = lower load (at WOT you should approach atmospheric pressure, any thing less and you have vacuum behind the butterflys). You can then adjust the fueling with whatever method your car makes available based on the power it makes in each of those buckets, and can do the same with timing. Generally you will end up with a more driveable car with more area under the curve when tuning on a load dyno, but you get pretty close with an inertia dyno.
 
I last tuned on a Dyno-dynamics, and went throught the same proceedures. Since my tunner was very experienced, I haven't had any drivability probles and didn't do any constant speed runs either.
 
Well here's some more info..
The dyno was a DynoJet.. I don't know the model.
the carb is a weber 32/36 dgv with f6 emulsions on both sides.
First run we saw AFR around 14 after 3,500 rpm. the guy said to open the fuel jet. We did it by 10-15 increments a couple of times until we used the largest fuel jet I own. then did a run with a lower air jet, but the afr/bhp/torque all stayed about the same.
I'll have to open it up a check, but I think we ended up with 205 fuel and 165 air in the secondary.
then we moved the timing a couple of times - retared 2 degrees from where we started seemed to be the sweet spot.
Again, seems like great info if I'm drag racing my Fiesta (watch out all you SMART cars!), but I'm sceptical of what will happen halfway down the back straight with what I think are huge fuel jets - seems like it would bog down.
 
It won't bog...just run out of steam!

I figure the 205 is a bit large.

Unfortunately, when you run out of jets to test, it becomes a bit of a wasted effort.

Beg or buy more of a jet collection around what you have presently.
 
AFR will vary based on where you are taking the sample from. ratio can change just be moving your sampling location by a few feet.
 
mossaidis said:
AFR will vary based on where you are taking the sample from. ratio can change just be moving your sampling location by a few feet.
Click to expand...

It certainly shouldn't. If it does you have issues. Assuming all cylinders are running at the same AFR (they never do) you should be able to measure the same AFR at the exhaust port as you do at the collector or at the tail pipe. If you aren't then you most likely have an exhaust leak at some point in the stream.

Ignoring how real world airflow at speed on the track might affect the engine there should be minimal difference between the tune you see on the dyno and what you see on the track. Tuning is what dynos are used for... If they didn't provide repeatable results no one would use them. One of the most basic rules of tuning is to give the engine what it wants which means sometimes you have to step outside of the box to optimize a tune. Now that may mean that something is wrong with the engine package and you're compensating for it. But the dyno and test equipment, when properly calibrated and used, does not lie.
 
Weber jetting

Just be aware that the "Power Valve" of the Weber carbs is what you see at the bottom of the float bowl. That is a valve that allows passage of fuel downward when the vacuum signal is lost (acceleration, full throttle), then horizontally toward the emulsion tubes. It is in this passage (horizontal) where the actual fixed high speed jet is put. It is only accessable by removing the lead soft plug in the front of the carb...that is when you will see it. It is about half way down the passage. the jet is what meters the amount of fuel.

Increasing the size of the valve will not increase the amount of fuel entering the power jet.

Thought I'd pass that on.

Good racing.

Bill:024:
 
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