I don't think it's that critical. IIRC, the MAP sensor reads in .01kPa increments and AFR doesn't change dramatically from vertex-to-vertex. So long as it's hitting the AFR target say +/- .1, it should be ok.

Frank

Hi Guys,
I haven't had much chance to be around but wanted to make a comment on VE with respect to tuning. The VE you see in the fuel table in Megasquirt has little to no respect to the volumetric efficiency of an engine.
In Megasquirt the VE is a scale of 1 to 255. It is simply a number in a look up table the computer references to decide what to do next. It is not uncommon to see normally aspirated engines tuned with numbers over 200 in some of the Bins. We all know very well the engine isn't drawing in that much air on it's own. The VE number is scalable with a great respect to "Fuel Required" that is a rough calculation of injector size to an estimate of how much fuel the engine will want.

I have been through some Code issues with Megasquirt in the last month getting the Twin Trigger code working for motorcycles but I now have a nice running BMW K100 turbo conversion ready to go on the dyno.

Thanks, Charlie. I knew it was used as a modifier for the REQ_FUEL measure - and knew it wasn't an exact measurement of the engine's true VE, but I thought it was closer to reality than what you're saying. Thanks for the insight.

On that thought, I would imagine the VE number also has a great deal to do with fuel pressure - particularly if REQD_FUEL is calculated incorrectly - or if it's calculated initially, then later changed - which would cause the VE tables to change on follow-up use of megatune. I suppose you could massage the REQD_FUEL number to obtain a VE table closer to reality.

Here's what the manual says should be theoretically achievable:



How true in practice that might be is not something I can say with any authority, but it would seem to me that if I was seeing 200 numbers, I'd probably look for an incorrect REQD_FUEL value calculation. I'll take a closer look at the table values when the tune is better.

Now, let's see some pictures of that turbo bike. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Frank

That all makes sense to me. I like that last paragraph.

Charlie, I used to dream about an R100 with a small turbo and a microsquirt, but then I'm kinda old school...

This is some cool stuff...


Frank, hope you didn't get affected by all that crazy rain in Columbia yesterday. <img src="/forums/images/graemlins/scared.gif" alt="" />

Brendan, we have a driveway issue at the yard that the water didn't help.... other than that, business as usual.

Frank

So am I, new thread over in off topic with a few pics for starters.

Just another bit about the MS use of VE tables, this is a direct copy paste from the MSX forum, I felt it was well written.

The VE can be anywhere from 0 to 255. The way the fueling works is that Required Fuel is your base pulse width. Then it's multiplied by the entry in the VE table. For example, if you have a Required Fuel of 10 and a 50% VE, that scales the pulse width down to 5. After that, it's multiplied by air density correction and other factors to get the final pulse width.

So while both Required Fuel and the VE table do theoretically mean things about how the engine behaves, in the end they are just numbers that either work or do not.

Agreed, Charlie.

All of the numbers in the ECU lookup tables are just numbers. I always get a kick out of folks who say that a particular engine should have "X" ignition timing at "X" RPM - or some other such nonsense. The best one is the valve timing statement - the one that says to set cam timing to a certain mark on the adjustable sprockets without consideration for what else has been done to the engine. <img src="/forums/images/graemlins/lol.gif" alt="" />

Speaking of adjustable cam sprockets, I gave some instructions to my machinist for a set and mine are now finished. I should pick them up tomorrow if I have time. I stopped by this afternoon to resolve a question while he was putting the final touches on them and they look terrific.

I killed a lot of time there today, looking at some hot rods - '37 Chevy delivery panel truck, '36 Ford pickup with a 302, 2 Camaros being rebuilt, 1 '32 Ford Roadster with a big ol' blower, a '64 Buick Riviera (brought back memories of one I once owned), a '55 Buick Roadmaster and a '64 Corvette convertible - in for disc brakes, side pipes and a drive-by-wire LS1. The 'Vette's probably worth $70,000 and it sure sounds nice!

Frank

I love shops like that. I used to lust after the 63 Riviera. I'll be in the Muscle Shoals area this weekend for a wedding, and I'll pay a courtesy call on one of my favorite cars, belonging to my wife's uncle, my former opthalmologist, a red 63 'Vette roadster, 327 300hp 4spd, completely stock, all original, one owner from new, and one of the world's great guys, who let me drive it when I turned 16, and who calls his wife of better than 50 years, Martha Ann, "Poopsie". If he just wasn't a damn Tennessee fan. At least the car is not orange, like the Cal24 sailboat, the V tail Bonanza airplane, and the Chris Craft cruiser and the Doberman who gets a dye job every fall...

I agree on the "common knowledge" numbers. I once had two Osca engines, 849cc dohc 4's, both blueprinted to exactly the same specs with the same cams, both degreed, same combustion chamber cc's to within 5/100ths, same piston weights, same rod weights, same everything, and the Webers had to be jetted two jet numbers different on the main jets, and 3 numbers different on the emulsion tubes. If you swapped the carbs, you had to swap the jets back to the original motor. You could swap the cams/intakes/headers and it made no difference. I never was able to explain that one, unless there was some other differences in the carbs themselves.