Don't forget to request a wideband 02 sensor hookup to your exhaust while it is on the dyno to measure the air fuel ratio. That way you'll know how rich or lean your engine is running and see if any improvements can be made to your car's power AND engine longevity. Without this info a dynoplot is just a pretty graph.

 

I thought the mixture is automatically handled by the DME and can't be adjusted. If it's out, presumably this would just tell you that there was a fault on one of the sensors?

To be honest - I'm mostly interested in the pretty graph. I have no immediate plans for power upgrades, I just want to check the car is making the kind of power it should.

 

No, at wide open throttle the dme basically ignores any input from the oxygen sensor.

The only way to know if your car is making the kind of power it should is to measure the AFR in addition to engine torque. It will tell you just how out of whack your engine is and where in your engines RPM range the problems exist and/or improvements could be made.

 

OK - I did know that at WOT the air / oxygen sensors are ignored but wasn't thinking straight.

Let's say that the AFR is way out. How is it controlled by the DME given that it ignores air / O2 sensors - and what would I be able to do about it?

 

yes good point Dave. In order to measure engine power, WOT is used in most likely 4th gear. What the hell does the O2 sensor have to do with it?

 

At WOT, I think the DME looks at engine RPM & squirts fuel accordingly based on figures in a look up table. I don't think it uses air flow measurements at WOT, which is why looking at the chip after exhaust, inlet, head & cam changes is so important.

 

Dear John,
You are correct at wide open throttle the O2 sensor input and the airflow sensor input are ignored. The airflow sensor input is approx 4.6VDC and as long as the foot is to the floor this will not change so the DME says okay I am at WOT the airflow sensor is wide open but I know it is going to sort of vibrate a bit due to air turbulence so I will ignore it from now on. If you ever get to see an engine on the test bench you can actually observe (with some disassembly) the vane chattering in the airflow.
Ciao,
Adrian
911C4

 

I went to my session at G-Force today (the running problems I had over the weekend which I mentioned in another thread didn't appear this morning). The result : 250.6bhp - so not as good as Phil, but not bad.

Now a much longer explanation :

I stopped at GT-One on the way to get their opinion on the problems I saw on Saturday. While I was there, I mentioned that I was heading to a rolling road and they said they had a spare DME. Since mine is chipped I borrowed their spare so I could try back to back tests - but unfortunately, that didn't work after all.

Initial runs with my DME showed a saw tooth effect (and only about 215bhp). I was warned about the possibility of this before we started - basically because of the dynamic AWD, if the wheels start to slip, the power shifts to the other wheels - which inevitable makes them slip etc etc. We tried about 5 or 6 runs all of which had this problem. Apparently, some C4s don't suffer from this so much - the guy doing the run said it seems to vary with wheel type as much as anything. I guess Phils was OK. They said that if they have cars long term - e.g. for tuning - then they often disconnect the prop shaft to make it 2wd. Nissan Skylines have a similar system and similar problems (bet you didn't know that!).

I then pulled the fuse to stop the power shifting, and got two good smooth runs which is where the figure above comes from.

We then tried swapping DMEs but got very odd results. Two almost identical runs - power was building much lower than with my DME (looked like it would only reach about 210) - but then at about 6K rpm the power suddenly spiked. Apparently, this usually means the wheels have been slipping and have suddenly found grip - which may explain the low power reading up to that point. We eventually gave up.

I used GT-Ones spare DME on the way home to see if I could feel a difference - and surprisingly I thought I could (although I suspect it's all in my head). It felt to me like their DME was crisper at lower revs (e.g. 2.5K rpm) but mine had the edge above 4Krpm.

Shame we couldn't get good results with the stock DME as a back to back test would have been interesting. At least my car seems to be making about the right power (won't race Phil though.....).

One thing - similar to Phils comments at the start of this thread, I saw the graphs for the Mass Airflow sensor. I'm now thinking about that.... but not this year.

One totally unrelated thought that popped into my head while doing this testing. If the accelerometers should show warning lights when they stick, wouldn't this mean they should light up on a rolling road? The wheel speed sensors etc. must say I'm moving but the accelerometer won't be moving at all. Anyway - it didn't.

Finally - best part of all. Because of the hassle getting decent readings and because I couldn't get the readings I wanted with the stock DME - they didn't charge me! I spent about 1.5 hours there, with probably 45 minutes on the rolling road and it was completely free. Bargain.

I recommend the place - apart from the fact that they didn't charge me - they were very friendly and did seem to know their stuff.

 

Hi Dave

Great result and great price!

Sorry, I should have mentioned that my car had the power-shifting problem, too. I must confess I was rather worried when I saw 216bhp come up!

Did Chris the aussie guy do the test? He was telling me about these rolling-road shootouts in Oz where guys build these monster trucks simply to get mega horsepower reading but they can't be driven! Mad!

Anyway, glad you had a good day. Who's next on the rollers, eh?

Cheers

Phil

 

It was indeed Chris.

I was actually quite happy when I first saw figures of 218 come up, because I assumed they were power at the wheels. Only after he came and told me it was the calculated flywheel power did I get worried!