Effects of exhaust pressure on Methanol tuning?

[engineermike]

I have a strange problem with my LT1 Camaro. A brief run-down of parts is: 363 cid LT1, AFR210 heads, 224/236-114 cam, PT-76GTS turbo, 450 ci water/air intercooler, home-built piping.

The way it WAS: 21 psi boost, through full exhaust including 3.5" piping over the axle and a 3.5" Hooker Aerochamber muffler. This made 680 rwhp through a TH350 and loose converter. The Alkycontrol Methanol was set to come on at 10 psi. The "Ramp" knob was on max and the "Initial" knob was set on 3 o'clock. I believe I have 2 - M15 nozzles. It worked fine and ran at about 11.5/1 A/F ratio at full throttle.

The way it IS: 20 psi boost, through an open downpipe. I had to reduce the wastegate setting ALOT to get the boost back down to 20 psi, indicating my cat-back exhaust was restricting flow quite a bit. This indicates to me that the exhaust pressure has dropped significantly, from 48 psi to perhaps 30-35 psi. Since I made these changes, the car started stuttering badly when the Methanol comes on. I've had to turn the "Initial" knob all the way down to zero to cure the stuttering, though it still does it sometimes. Now, it's running 12.5/1 at full boost even with the "Ramp" knob still on max.

Why did the exhaust pressure have such a drastic effect on Methanol tolerance? What do I do about? Has anyone else experienced this?

Mike

 

[Razor]

How are you tuning this? In other words what computer is controlling the engine?

I run my car with a cutout. Open or close, there is no difference in the alky.

If your running a MAF style setup, you would have been peggin the MAF and it shouldnt make a difference.

I would of setup the system to come on at 5 PSI not 10 PSI. Also would of made sure its on a 3 bar so tracking can take place past 15 PSI.

The Initial should not be less than 12:00(half way), and the Gain in the mid-3/4 way up. Then tune from there.

Plugs/gaps play a role as well. Make sure theyre fresh, and the gaps tightened down a little.

Air fuel with alcohol should be in the low-mid 11's, and work from there. Mid 12's is dangerously lean.

Lastly, turn the boost down and work on getting it running 110%, once running correctly, crank it up.

I bet what happened when you opened the exhuast the turbo was allowed to spool faster thus making more boost at lower rpms.. that moved your VE down into tables in which where tuned differently with the restrictive exhuast. I experienced the same thing when I tightened my converter.

 

[engineermike]

How are you tuning this? In other words what computer is controlling the engine?

If your running a MAF style setup, you would have been peggin the MAF and it shouldnt make a difference.

I'm using the stock ECM. The MAF pegs at around 8 psi boost.

Also would of made sure its on a 3 bar so tracking can take place past 15 PSI.

It is a 3 bar MAP.

 

[engineermike]

I run my car with a cutout. Open or close, there is no difference in the alky.

Does your boost change when you open the cutout? How much does the backpressure change?

 

[Razor]

Does your boost change when you open the cutout? How much does the backpressure change?

Yes, it changes boost. It goes up.

Pressure.. dont know. Never tested it.

Since your MAF pegs at 8 PSI, your controlling fuel based on a specific INJ DC/per RPM. So your computer doesnt know your at 10 PSI or 30 PSI boost. And once you setup your fueling for 20 PSI, 21 will make it lean, 19 rich. This is why the push for HP tuners/EFI live 2/3 bar speed density conversions.

Turn your boost down on the motor to a safe level and see what else changed. Work it back up. The methanol shouldnt affect it other than richening the mixture up.

On a twin nozzle setup like mine, the controls are EXTREMELY sensitive to small incremental changes. A little goes a very long way, and moving the controls should be done incrementally in very small movements. This is why I dont like setting up cars on twin nozzles.

Put it like this, the LS1 Parish truck that made 1000+ RWHP has the same system you do. Takes time to learn to properly tune meth.. get the controls back to factory default.. and start from there again. And do not crank any of the knobs wide open.. as this will give you issues. Activate(turnon) at 5 PSI, set the Initial to slightly less than half way, set the blue gain knob to 1/2-3/4 way up.. and tune from there.