scarceller

I've been doing some tuning on my all new MAF setup and am finding some very interesting results for 2nd gear WOT street runs.

I record all the following data during a run:
AFR
RPM
CHT
MPH
MAF signal 0-5vdc converted to g/min

The MAF signal is what I want to focus on. The MAF device I use can flow 22,000g/minute and this engine can't breath anywhere near this limit. At WOT 6800RPM it breaths about 13,000g/min. Also the MAF has been flow-benched so I have accurate voltage to flow curve.

What I see is that after 5800RPMs (peak torque area) air flow quickly flattens off. Meaning that up to 5800RPMs you see air flow go up in linear proportion to RPM, makes sense as RPM increases so does air flow. After 5800RPM the rate of air flow increase vs RPM drops off significantly. And after 6500RPMs the air flow actually drops! Now I fully understand that VE drops off at hi-rpm but I would not suspect it to drop this much.

Side note: While tuning stock AFM system I had assumed that the AFM was fully opened at 5600RPMs because the AFM signal no longer increased after this point. But given my new MAF system testing this seems not to be the case, what's really happening is AIR Flow really is simply dropping off!

This is a Euro 3.2L with SSIs into 2in2out M&K pipe so I know I have some exhaust restriction.

I started thinking about another possibility: The engine fan pulls tons of air through the engine bay and most of this air needs to make it through the deck lid grill. Could it be (to some extent) that the engine bay establishes negative pressure at higher RPM and road speeds? If this where the case then the intake system would suffer from lack of a good air source.

Has anyone studied this area in any detail? I plan to put a MAP (baro) sensor in the engine bay to log and record some readings. Figured I'd ask before I go down this road.

scarceller

Here's a graph of the air flow, see the green line. The white line is RPMs.

whalebird

maybe a smaller air flow meter? A "restriction" in the in/out flow of the engine may speed up the velocity in this range. Just a quick thought.

scarceller

The MAF isn't that much oversized. I get 4.18 volt range during WOT pulls. I wanted some head room in the MAF. I don't think the MAF is the issue.

What do you mean by "A 'restriction' in the in/out flow of the engine may speed up the velocity in this range." can you elaborate?

MR2Aaron

Was your plot taken with the car stationary on a dynamometer, or in motion?

scarceller

Street run car in motion the blue line is MPH

Amber Gramps

There are threads on the possibility that "the fan is starving the intake" but I don't think anyone has ever proven it.

My guess would be valve float or turbulence inside the intake/exhaust path.




Top up or top down???

With tail in your avatar???

whalebird

I'll try to elaborate...like squeezing the end of a water hose makes the water shoot out the end. In electrical terms, it's like Ohm's law - voltage(velocity) increases with resistance (restriction).
Naturally there is a fine balance in a system such as an internal combustion engine.
Your graph shows a sharp drop...a little peculiar, but probably not entirely cooling fan related.
Where's your ignition timing at this point?
Can you measure MAP, whats happening with vacuum at this point?
Cam timing?

Amber Gramps

I don't have any idea how the math works out, but any chance you have exceeded your maximum flow rate in the intake plenum???

http://www.turbokraft.com/catalog/pr...roducts_id=138

scarceller

Top up wit the tail in avatar, the car is a factory wide body.

scarceller

Yes, many things can be causing this and I agree most likely a result of several factors. The car has WebCams 20/21 grind (7* overlap). Timing is all in at 30* by 3000RPMs then at 6000RPMs timing bumps up to 32*.

I'm trying to look at one variable at a time. I'm looking at air pressure in engine bay at this time. I may simply remove the deck lid and do a run as this is very easy to do. I do not suspect pressure in engine bay is causing all of the issue. I know some of the issue is back pressure in the exhaust, I can tell this is the case from the AFRs I'm seeing above 5400RPMs. My new MAF system calculates injector pulse widths (amount of fuel) very accurately from the given (trusted) intake air measurement. The WOT AFR is commanded to be 13.0 and I calculate fuel needed to achieve this 13.0 from the given intake air but you can clearly see that after 5400RPMs I go 1 point richer into the 12.0AFR instead of the commanded 13.0AFR. Even though I know for sure that I'm adding the correct amount of fuel to the given intake air to achieve 13.0. So how can the AFR be 12.0? One very good explanation for this is that not all exhaust gasses have been expelled from the cyl because of exhaust back pressure. If the cyl has EG left in it then that EG has no O2 then the intake valve opens and the properly calibrated intake mixture is then diluted with the EG and what results is not enough O2 for the given diluted mixture and this is why the AFR goes rich! This is a known theory in internal combustion engines. You can try it very easily, simply allow EGR to feed into the intake of a running engine and the AFR goes Rich.

whalebird

Prolly approaching the limits. As well as cam grind, valve angle, compression etc.
Are you dumping fuel at this point? Injectors/duration? I ask because the abrupt flow decrease could be the egg, not the chicken. Timing or fuel could be hitting a wall and stalling the flow.

scarceller

Here's a picture of this MAF setup.

scarceller

Let's run the flow numbers for 7000RPMs with a VE=1.0

3.2L / 2 * 7000 = 11,200L/minute

Now convert from l/min to cfm = l/min * 0.0353
11,200l/min * 0.0353 = 395cfm

Flow to each cyl:
66cfm per cyl (395/6=66)

66cfm is no where near what this site shows for the stock intake manifold.
Also my intake is already extrude honed intake.

Unless you are running a turbo the stock intake seems plenty big for the 3.2L

jakeflyer

Having very little knowledge about air flow and drag based upon antique airplane tinkering, my guess is that pulling the rear bonnet off will not give you any valid information. Any way to measure air pressure inside the engine compartment, like an electronic barometer for air density?