I just re-read your post with the video!!! I need to pay better attention to what you say. Sorry about that!!!

Definitely need to get 100% on the TPS during full throttle with gas pedal before doing anything more with this volumetric efficiency stuff. I got side-tracked doing the calculations. This could have affected the snap tests too, especially if using the gas pedal. Are the blades vertical when TPS reads 85%? Can you open it further by hand, when someone is pressing the gas pedal to the floor?

 

Thanks a bunch for this Les and George, there's some really good info there. So I live in Kelowna, yesterday it was very hot, around 38 to 40 degrees C. I played with the numbers a bit to calculate air density and always ended up around 1.1xxx g/L, even playing with a temp a change of 10 degrees C does not yield much difference. I have a small snip below of the VE I calculated, using your air density Les as well as two that I calculated. So the VE doesn't look the best definitely. Now, I watched scanerdanners video on diagnosing a restricted exhaust, on the car he was working on he pulled the upstream o2 sensor to see if it would run better, and it didn't really, but when he put a backpressure gauge on it surely showed the cat was clogged. I ended up just getting a back pressure tester, I will be able to test for certain and not play any guessing games. With this said though it is supposed to get here on Monday at the latest, well my curiosity is going to get the better of me in that time and I think I will want to try the test again back to back with taking out the upstream o2's, hopefully on the weekend. What you said Les about fire with the upstream o2's removed has me thinking though, I will make something work. OK guys, I'm just going to come out and say this too, I know it's a bit 'from my opinion' so I've held off from saying it as I try to just say facts on here. There's a good chance it runs worse with the exhaust gasket back in, it seems to give me a long crank much more often, like every day now at least once. When I turned it on after putting the gasket in I had this exact thought, "is it running rougher right now?" and it very well might be. But first, as you said Les I will take a look at my TPS more closely first when I get home from work today, I'll let you guys know what I come up with about that.

 

Okay guys, a quick update. I checked throttle blade at WOT and it still read 84.3 degrees on the tablet with me holding the blade open as much as it would go at the throttle body. However, I noticed this is the GM PID, so I checked the SAE PID and it, sure enough, reads 100% at WOT. Maybe you can do this Les to confirm my readings are in check. There's also a video below of the two PIDs, I also did a quick knock on the cat today to see if anyone was home, and it definitely sounded like there's some loose stuff in there (some rattling when I was hitting the cat). I will see soon enough guys, soon enough.

 

Looks like the units for the GM PID is degrees and the units for the SAE PID is % of full throttle opening - never noticed that before. So a little less than 90 degrees rotation would be full throttle on the GM PID (vertical angle at full throttle 90, minus tilt of blade angle at idle 05). Apparently this is why you are seeing 85.

P.S. in your calculations, I would bet that you used ambient (outside) air temp if you came up with air density of 1.15 on a hot day. You should be using the air temp as measured by the vehicle MAT sensor, which would most likely be hotter - and this drives down the air density value/target MAF value. I think this would probably make your %VE a little above 70%.

Here is an interesting fact (at least I think it's fact anyways). If you can raise your VE by 15% (from 70% to 85%), you should raise your torque/horsepower approximately 15%. On this 4.3, that is about 30 horsepower. It's all about the amount of oxygen present in the cylinder when it is ready to fire. Put more O2 in with higher VE - then burn more fuel and make more power. Did you know that race engines can exceed 100% VE by playing with pressure/flow in intake/exhaust systems and valve timing/valve overlap? (See post 157)

It seems to be commonly thought that %VE changes with air density (usually altitude is referenced). But I think this is BS. Since VE% is a PERCENT you have to use the same air density in calculation of 100% flow, and then compare to the flow you are experiencing to arrive at %VE. I was always taught actual/theoretical * 100 = percent. Hey George, please take a look at this and see if you agree with me or with Summit? https://help.summitracing.com/app/an...iency-%28ve%29 I say %VE stays the same - sea level or mountains. Its the air density that changes. Why no one understands this I cannot figure (maybe I'm wrong?)

 

Revised post immediately above

 

Thanks Les, so you are correct in that I did use ambient temp for my air density calculation. I saw on the summit website for highly modified engines the VE should be over 100%! Wouldnt this only be possible with forced induction though? Because you would need to be forcing more air into the engine than it would usually hold? I think your logic makes more sense on the VE Les, the air density has atmospheric pressure in the calculation then you use that number to calculate the expected 100% VE MAF rate, the atmospheric pressure being in the air density calculation would automatically account for the altitude difference specific to where you are, right? Then you use that calculated MAF rate against the actual MAF rate that's AT THAT ALTITUDE. My point is both of the numbers (calc MAF and real MAF) are based on the same atmospheric pressure for your specific area and altitude.

 

I think that Summit and others miss the point I am trying to make about volumetric efficiency not changing due to altitude. This is because they calculate in simple CFM via a carb hat on a dyno and compare to theoretical volume displacement by the engine at same RPM. But this does not seem to take Mass Air Flow into account.

Indeed a race engine can certainly approach true 100%VE at specific RPM if cam timing and intake ports and exhaust is optimal and working to fill the cylinder most efficiency. I also think it is possible for more than 100% of theoretical air to pass through the cylinder head during the time when both valves are open (valve overlap) - but I think that no more than 100% can be trapped when the intake valve is closed. But if you can flow (waste if you will) an extra 15% through the head before that valve closes, you sweep the old exhaust out of that chamber in the head above the piston more completely and make a bunch more power. I think that as you approach 100% and higher you start wasting some fuel/air mix to flush the cylinders. It does work for power, but not because 115% is crammed into the cylinder. At least thats my belief until someone explains it better to me. There is so much BS on the Internet. My rule is it has to make sense, even if an "expert" says so. Oops I just proved myself wrong in post 154 above by thinking about it more! I have crossed out the misleading text!!

 

Thanks Les, it is definitely interesting to me learning more about volumetric efficiency. So guys... Exhaust backpressure test has been done and, no dice. It's 0psi idling and maybe 1.25psi on snap throttle to 2500rpm, both of these are in spec and signal my cat is not restricted, my cat may not be working efficiently anymore, but I know it's not restricted now. So guys, I've been over exhaust pressure, checking o2 sensors to see if they respond to a lean condition I create (and they did), I've been over compression rigorously, looked over all sensors the computer takes in to calculate how much fuel to give (MAF, MAP, ECT, O2 sensors, TPS), and been over fuel pressure (all in spec). The only thing I'm not 100% on is the MAP sensor, I may need to look into a way to definitively test it. I'm not too sure a MAP reading incorrect would give me this condition anyways. Guys I am really starting to get stuck here, let's go over this again. Rich fuel trims on both banks (the same amount), intermittent starting issue, and a very rough idle. I keep going back over my fuel pressure test because I just don't believe it, something has to be going on here and I don't know how I'm not finding it! Maybe I should do the leak-down test for longer? A possible pinhole leak in the regulator that gets bigger when the vacuum is sucking on it? Let me know if this is possible guys, I'm kind of ruling out a problem with a single injector because it's affecting both banks equally, but possibly a problem with the regulator still? Let me tell you guys this as well, I think I have narrowed down a pattern at last for when it gives me a long crank time, it seems after I drive home from work (warmed up fully 30 min drive) then let it sit for maybe an hour-ish, come back and it will give a long crank time. Let me try it again later today and see if it does it again, I will get a video if it does do it. Now, why would this happen? It will cold start the first start of the day with no prime and have no problem at all? This is where I'm getting stuck guys, what's changing to make it not start after it sits for an hour-ish, but it will start right up for me after 8+ hours? That would be the million-dollar question I guess.

 

I had to break out the big gun scanner to help another member on his evap/fuel trim problem. Remembering this discussion I watched fuel trims at different operating conditions. The engine computer moves around to different fuel cells as operating conditions change (mostly rpm and load I believe) and each time you move to a different cell number (idle was fuel cell 17) then LTFT immediately changes to the stored value for that cell. So if your watching fuel trims while your driving around trims will constantly change abruptly with each fuel cell assignment change.

George

 

MAF is reading much lower than expected. Yours 130. Mine 155. If exhaust is not restricted, baro pressure reading is correct, and MAT is correct - then MAF is likely bad. I suppose outside chance of bad PCM not interpreting things correctly.