SCCA RX7 RACER

Hi All

Took the race car to the track and had a good time. The mega squirt worked well and did some remapping for my new intake (had to add fuel which adds up to more HP). During the pace laps each time I lifted off the thottle a about 5000 RPM I would get a large back fire. Also when I lift the throttle entering a turn. It seams all related to when back off the throttle plate close up. It seams that there is fuel being dump in on decell how would I correct that? I have only S5 intake streetport, 550 prim Inject, 750 sec inject, custom header with 3" exaust, not using Mazda TPS I think it is a GM type one (calibrated). MS1 V3 with pull up resitors. missing 2 tooth on bottom gear set up.

Thanks in advance
SCCA RX7 Racer

Eagle7

It's always a crap shoot to try to help without an msq. Also a log of the issue if possible.

muythaibxr

The deceleration fuel amount would probably help, as would making sure the areas you go into for the VE table are correct when you lift.

Ken

toplessFC3Sman

iTrader: (1)

yea, low MAP areas may be too rich, spark may not be advanced enough at low MAP, and the fuel cut on decel (negative TPS_dot) might not be working. However, with the plunger-type of TPS, I tend to get a lot of jitter at WOT, so I never turn on the decel fuel cut to avoid pulling fuel under boost. I never had much success with tuning accel enrichments with MAP_dot. EAE / wall wetting fuel accel correction (MS2) does seem to help considerably, since it doesn't rely on the TPS for fueling.

SCCA RX7 RACER

Here is my program settings from my last race.

toplessFC3Sman

iTrader: (1)

All of the VE values in the lower MAP bins are way too high (rich). When you get to low load (20-30 kPa), you're basically just telling the injectors to dump fuel in, and when it can't burn b/c its too rich, it gets spit out the exhaust and burns when it hits the fresh air; thus the backfire. For my VE's, at 15 kPa I go from ~25 at 700 rpm to ~50 at 8000 rpm, the next row up (25 kPa) is about 15% higher, and the next row (35 kPa) is about 6 - 8% higher than 25 kPa. My settings have the AFR slightly rich at very low loads so that drivability is smooth (especially when recovering from overrun fuel shutoff, which you don't have enabled), and your map is throwing double the amount of fuel into the engine at the same loads, making it extremely rich.

If you datalog the problem & when it happens, or even just watch on the 3D VE table under "Tuning", whenever the engine drops down into that region you'll see tons of extra fuel and a backfire

SCCA RX7 RACER

I did not consider the lower area of my map. I been focusing on tuneing the mid range and top end at the track I will play with that area to remove the back fire issue.

arghx

iTrader: (3)

You do realize that Mazda used an anti-afterburn system to prevent this? On the 2nd gen extra air is injected into the intake manifold via the ACV. On the FD, the idle speed control valve allows extra air to bypass the throttle plates.

Backfires can be reduced but on a pre-Renesis engine they are going to happen at least some times.

Eagle7

Real race cars spit flames

My 2nd gen ITS car doesn't have a major backfire issue. A few pops here and there, but does spit flames. Like was said, lower the values in the bottom row of the VE table.

SCCA RX7 RACER

I have the engine stripped of all the factory extras no oil pressure regulators (external system that is adjustable). Oil injection system for lubrication of apex seals has been removed (Mix gas with two stroke oil at 150:1). I only have a TPS calibrate closed at 36 to full at 128. Factory throttle body (No flow meter), S5 intake modified ported (No power valves) with stock primary injectors (550cc) and aftermarket secondary injectors (750cc). As you can see I have just the basics to race around the road circuits.

I mention this because I need some fuel on deceleration to lubricate the apex seals because the lubrication is mix in with the fuel with the two stroke oil. In my case I feel the idle circuit would be enough. At the same time I do not want the engine to go to lean prior to applying throttle for acceleration and have it stubble on exiting the corners. On up shifts drivers from behind say I do have a flame coming out but no major back fires (the odd small pop).

I do not have the over run fuel shut off enabled in fear of running the apex seals dry. But in stating this I do not understand exactly how it works so I have not set it up. Can some one explain this to me and may be I can incorporate it in to my set up. Or is this the right move on my part because I mix the oil in with the fuel. Please give me you feed back in this area.

Thanks
SCCA RX7 RACER

Eagle7

I'm no expert, but I also have overrun turned off, for exactly the reason you described. Since overrun cut is off, it injects based on the VE table.

Jobro

iTrader: (2)

You are getting backfires because the mixture is incorrect. Notice I did not say rich or lean.

See its possible to get a lean backfire too, it mis-fires, then the partially burned cycle can still ignite later down the exhaust pipe.

I've not had problems with backfires rich because I'm usually trying to conserve petrol, well other than at full load. At wide open I'm using 13.2:1 mixture, mixture heads rich, to 11.5:1 by 13 or so PSI.

It is possible to get it reving cleanly, and lift off at 7000rpm smash it into next gear and keep going with no flames smoke noise whatever.

I'd put money on if, if you autotune using a wb02 and some of the statisical software to 13.2:1 AFR for the entire operating region 2500rpm and up you would get no exhaust flame.

13.2:1 is an easy mixture to ignite and burn completely at partial load.

toplessFC3Sman

iTrader: (1)

Well, his problem was that he had about 60% VE down at 20 kPa MAP, so that it went very rich as soon as he let off. I'll get the occasional pop when letting off quickly from full boost if the exhaust system is nice & hot, but I know I'm a bit richer than I should be at full boost (10.5 - 11:1 at 14 psi), since I still haven't made it to a dyno to dial things perfectly yet (altho I feel I'm reasonably close from data-logging).

The stock ignition system in the 2nd gen will reliably ignite part-load mixtures out to 18:1 (if you've got spark timing ball-park correct), although this can make accel enrichment tricky because the engine is much more prone to surging & hunting if there is a change in fueling. In cruising from 30 to 80 kPa MAP I try to maintain 16:1, since thermodynamically thats around the peak for efficiency (with no EGR, slightly earlier spark timing to account for slower flame speed), and it still keeps good drivability. Below 30 I richen up to about 14:1 so that when the engine returns from over-run fuel shutoff, it will almost certainly catch & run correctly on the first cycle instead of bucking, plus combustion is less stable at very low MAP. Above 80 I richen up to 13.5ish at atmospheric (100 kPa), and then continue to richen up to about 10.5 to 11 at 200 kPa. This is all from 1500 RPM upwards; below 1500 is richer for a stable idle, warm-up, and return to idle, as well as initial tip-in response.

Also, you're right on the overrun fuel shutoff... if the MAP & TPS get below the values, and you're above the specified RPM, no fuel will be injected, so you'd lose lubrication to the apex seals if you're just premixing

SCCA RX7 RACER

Thanks for the replies so far. What about the Decel Fuel Amount % that I have set at 90% in the Acceleration Wizard? Is that setting of 90% a factor since I have the Turn Decel Enrich off when set to never in the Accel decal Trigger dot table so it will not cut fuel on deceleration?
SCCA RX7 RACER

Jobro

iTrader: (2)

On the topic of cruising at 16:1 AFR or cruising at 18:1 AFR. My engine has never willingly done this. Even when I had the S5 TII intake on my 12A and injectors very close to intake ports it would not cruise lean. Mid 14 AFR's was the most consistent. Pushing for leaner mixtures caused misfires and used more fuel because you would have to trigger throttle pump to cruise smooth using more fuel...

SCCA disable overrun fuel cut off, it is not going to cause backfires if your mixtures are correct.

Also make decell fuel amount 100% Seeing as its a non turbo race car.

Any of the 'niche' economy improving features should be left for absolute last.

Jobro

iTrader: (2)

Those tables look tasty. Whats your req fuel and primary injector size to go with that fuel map. I bring in more fuel faster in the mid boost ranges. Also you have a big swing from 0psi to 20psi. My mixtures change from 13.2 to 11.5. Thats like a change from 115% VE @0psi to 135%VE @ 13psi.

SCCA RX7 RACER

I will make use of all the good information that you guys have provided. I will go with reducing my fuel map in the lower bins to what has been suggested. I will increase my timing to reflect other spark tables in the low bins as well. As mentioned by several people if the mix is right and the spark is there then the fuel will burn in the rotor rather than in the exhaust system removing the back firing problem.

I have a race in Portland next weekend so I will have time to adjust it then. In the area of acceleration (4500 to 9000 rpm) the AFR seems to be good with 40% to 100% throttle opening. I still have a few small areas to clean up to improve throttle response. At this point I do not have to worry about the motor going POP and I can still run in the top 3 position of my class.

If I can get this bug of back firing out I will post my changes so people can see how I cleaned it out to help the next guy.

Thanks again

toplessFC3Sman

iTrader: (1)

I don't know what prevented you from cruising at any leaner than 14:1 (ignition system maybe?), but having played with it on a couple of 1200 mile round trips, 16:1 seemed to produce the best results fuel-economy wise. Best of 27 mpg highway, but that was driving like a granny... realistically 22 -23 highway, although it falls quickly as you use the loud pedal

The req_ruel I'm using is 5.0 ms (2.50 after squirts/cyc & # of injector corrections applied), with 870 primaries and 1000 secondaries.

I wasn't trying to say that you should have over-run fuel shutoff on; its a fuel economy thing and a preference, but for your car, I'd keep it turned off (keep fueling on overrun)