I have had my car recently completed with the TTi Twin Turbo. With the Boost Controller set to high (16lbs) boost (580/690+ rwhp/rwtq), at the top of the RPM there is a backfire...I am running 104 octane with the hi boost setting. What could be causing the backfire. I am using the MLS gaskets w/ARP Heads studs. A leak down and Compression test was done to confirm any motor damage or head gasket blow outs...compression was good 167-176 on all cylinders and the leakdown was 4-6%..with the exception of #3 cyclinder having 11% leakdown..but ironically #3 also had the highest compression. So I don't think it is a head gasket or piston problem. I have TR6 plugs in the car gapped to .035..they all looked good....What could it be???
Thanks

 

what was the a/f?

how bout the data from your datalogger?

you haven't given much info to go on

 

Air/Fuel is 12.9-13.0 on 104 octane....zero knock retard.
The interesting thing is that it only does it on the hi boost setting 16lbs not the low setting of 12lbs

Could I be blowing out the spark??

Should I be looking for something more specific???

 

Does an overly rich state cause backfires in the exhaust on exit to the heated exhaust?

 

Could be just spinning up to the rev limiter before the rpm gage does. It didn't back fire once on the dyno...just the street..but you have to think about shifting way before your there. Tires light off instantly.

 

It could be the rev limiter. I thought I had a backfire the first few times it cut loose on me in the cool weather, then I realized it is just how it sounds when the rev limiter hits really hard. The tach seems to lag quite a bit when it revs up that fast.

 

the dyno doesn't load up the motor like when the car is on the street.
try setting the rev limiter 200 rpm higher but shift where you do now. if the back fire goes away, it most likely meand you where hitting the limiter.
if not , try taking the spark gap down to 28thou.

 

You're kidding, right? Unless this is a RACE car, you should not run this lean for forced induction. Additionally, if this was the measured A/F on a dynojet you may find that your actual A/F under load on the streets is different. Just because you're not showing any KR doesn't mean the engine's 100% happy. Do you know if the max KR was set to "0" in your programming?

Greg Banish
President, Detroit Speedworks, Inc.
Livernois Motorsports Calibrator

 

Whoa!! Yea, you're running lean man! If it's 13 on the dyno it's most likely 13.5 on the street! I run my car at 11.5 on the dyno to be safe, I'd lean that bad boy out!!

- Dug

 

I believe Dug meant to fatten it up!

SC-

 

Hmmm
I agree with all of your statements.
I will make sure that the AFR is not > 12.5.(but this would not cause a back fire would it)
The AF is only 12.5 using the 104 octane at 16lbs of boost (using the boost controller) w/ the boost controller off I get 12lbs of boost and the AF is 10.5 using 91 octane. I have had other FI cars and agree that they usually will run more rich on a dynojet than on the street.

Doug,
Have you seen some of LPE's dynographs for their TT's...the last one I saw was 13.1...it seemed crazy to me too. I am running all forged components..but I know that those can break too.

I was thinking of regapping the plugs too .028 instead of .035 just incase the spark is getting blown out at 16lbs of boost.

I will have the rev limiter set a little higher too.

I appreciate your input and look fwd. to hearing more.

Thanks
Kevin

 

DOH!! You are correct, I meant fatten it up!

 

Kevin, I have not seen the dyno graphs . . . but I definitely think 13:1 for an FI car is too lean. It's also a definite possibility that you're blowing the spark out . . . I've got mine gapped at .034, but I'm only seeing 11psi. I would try to drop the gap too.

 

Too lean.

I2.9 on the Dynjojet might be like 14:1 on the street you have no margin for error now.

It backfires while you are giving it gas, decell'ing or between shifts?

 

may be a stupid thing to suggest, but if you have not cats, make sure your COT is turned off. it can cause weird problems in the 5000 rpm range.. But i think you're higher than that. let me know what you find out, i'm having similar problems, but i'm only running 9lbs

 

The backfire is occurying between shifts at redline..and I have a feeling it is because since it is mainly in First and Second gear that the motor is revving faster than the Tach can indicate and it is hitting the rev limiter which I know will create a popping noise.
Regardless I agree with what has been talked about here and am going to take a look at the Air/Fuel and the Spark Plug Gap. There are not cats currently on the car, on off road test pipe is in place.

Need further clarification.
If the A/F is 12.5-12.9 on the dynojet and there is ZERO KR and the timing is 14 degrees...do I really need to fatten it up a little. I mean that is what we look for right is how much timing and A/F we can add without getting KR? or am I wrong?

Thanks

 

I had a very similar issue when I was dialing in my Powerdyne setup. It would feel like a backfire, and it would actually cause the car to "nose down" by the time I was in third gear at the track. It wouldn't do it in first, and only would show sometimes in second, but by third gear it would feel like you hit a rev limiter. On the dyno, it pulled fine too, only showing less than one degree of knock every once in a while.

I ended up just pulling out a couple degrees of timing and it was fine. I went from 13 to 11 at peak torque, blended the timing in and out, and the problem went away. Just something else for to try for you (once you get that a/f dialed in first of course).

-Geoff

 

Shoot for an 11.5 A/F on a dynojet. Gotta wet that **** down with FI to make sure it's safe, period!

Who tuned your car for a 13.0 A/F? Those are numbers one would like to see for an all motor application, not forced induction.

 

Once again the back firing occurs at the top of the rpm range between shifts...this is all with tires spinnning on the street....600+rwtq between 3000-4500 rpms is hard to control on street tires..that is the only time I hear the popping is trying to get through 1st and 2nd.

You guys think it is the rev limiter and the tach is not catching up before I shift or the spark being blown out by boost and a rich condition is created causing a back fire.

I know what detonation sounds like and this is not it. Once again would to lean of an A/F ratio create a backfire...I always thought to rich of an A/F would cause backfire. Keep the info coming guys..its good stuff.

 

I'm guessing it is the rev limiter.

As far as a/f goes, slap an onboard WB on it and be done with it. You have spent this much time and money ($350 more for www.innovativemotorsports.com is worth it). You will need an RPM pickup also so you can log.

Although I have seen leaner a/f on the street (in a high gear) than on the dyno, the low gears are typically fatter. If you are spinning (you are probably much fatter than you think).