Problem with AFR

[DaveW]

Hi guys, the collective knowledge on this forum is amazing. I hope you can help me with an issue.

While dyno’ing my race motor, I notice a large drop in AFR at about 4000 RPM. It hovers around 10:1 until 5000 RPM, at which point it increases back to around 12.5:1 by 6000 RPM (and relatively flat after that). As you can imagine, the torque drops off significantly during this time.

I have done dozens of pulls while changing main jets, air jets, venturies, and emulsion tubes. None of this made any significant difference. Additionally, I have verified that the intake and exhaust lengths match Larry Meaux’s Pipemax calculations.

The only possibility left that I can think of, are the cam specs.

Intake duration (0.50”): 266 deg
Exhaust duration (0.50”): 248 deg
Lobe centers: 104 deg

I’m guessing that a possible fix could be to retard/advance the cams. Any suggestions would be welcome. BTW, this is a road race application, which needs a good power band from about 4500-7500.

Thanks,
Dave

[CamKing]

What engine?
What bore and stroke ?
What size carb ?

[mag2555]

Sounds like the motor is scavenging really hard between 4K and 5K.

[DaveW]

Mike,

The engine is a 2.7 liter Porsche 911.

Bore is 3.54" (90mm), stroke is 2.77" (70.4mm).

Carbs are 46mm Weber IDA (40mm venturies).

Dave

[CamKing]

2 valve or 4 valve ?

[DaveW]

Mike, 2 valve.

[CamKing]

IMO, the cam is way too big, and you have way too much overlap.
I would have recommended something in the area of 240@.050" on both the intake and exhaust, with a 106-108 LSA

[Kevin Johnson]

I am guessing you have already explored any torsional issues with the crankshaft? The engines have been around a while so I am sure this has been discussed in depth. The IDAs would be affected if the engine was moving because of this.


http://forums.pelicanparts.com/porsche- ... amper.html

Also, as you increase the power in boxer engines the rocking couple becomes more of an issue and then again the IDAs would be affected.

With the Subaru boxer fours the increased power output from tuners induced cracking of the oem oil pickup tube/head assembly. Again, think how it would influence nice big triple IDAs sitting on top of vertical manifolds. Do they make isolation gasket assemblies?

[DaveW]

Mike,

I'm not ready to change out the cams at this time. Is there anything I can do with regard to changing the cam timing that might help?

[F-BIRD'88]

To reduce the overlap and intake duration try opening up the intake valve lash (for a short test run) as much as is practical. If you see a improvement try the exhaust lash too.
If you see a improvement trend contact CamKing. He will get you set up with a correct cam.

[CamKing]

Mike,

I'm not ready to change out the cams at this time. Is there anything I can do with regard to changing the cam timing that might help?

Since you can't change the LSA, there's not much you can do.
Try loosening up the lash .004", like F-bird recommended. Just for a pull or 2, to see if it helps.
Maybe advancing the cam 4 degrees would help a little.

[DaveW]

Mike,

I'm not ready to change out the cams at this time. Is there anything I can do with regard to changing the cam timing that might help?

Since you can't change the LSA, there's not much you can do.
Try loosening up the lash .004", like F-bird recommended. Just for a pull or 2, to see if it helps.
Maybe advancing the cam 4 degrees would help a little.

Mike,

It's my understanding that advancing the cam lowers the peak torque RPM.

[CamKing]

Mike,

It's my understanding that advancing the cam lowers the peak torque RPM.

When the cam is close in duration, to what's needed, advancing the cam normally makes the power come in sooner, and fall off sooner.
Your cam is so much bigger then what's needed, I don't know what will happen. My WAG is that the earlier exhaust closing will lessen the reversion, and the earlier intake closing will build a little more cyl pressure.

[MadBill]

Can you measure airflow on your dyno? A drop in that RPM range would indicate reversion, which richens the mixture since fuel is entrained each of the three times such air passes through the venturi. Should be visible with filters/airbox removed.

We had a similar issue with a doen-on-power twin cam 1600 cc Toyota-powered Sports Racer: When we pulled the airbox, huge standoff of fuel droplets was observed. Cam adjustments had modest effect. Later I noticed the merge collector/ Supertrapp (yeah, I know, but we left out all the baffles) had a very large throat for the engine size and power. Cutting it down from 2 -1/4" to 1-3/4" knocked almost 2 sec off the lap time but we never got it back on the dyno to quantify the difference. Turned out the former owner had bolted on that system before selling the car but never ran it in that configuration.

[David Vizard]

Hi guys, the collective knowledge on this forum is amazing. I hope you can help me with an issue.

While dyno’ing my race motor, I notice a large drop in AFR at about 4000 RPM. It hovers around 10:1 until 5000 RPM, at which point it increases back to around 12.5:1 by 6000 RPM (and relatively flat after that). As you can imagine, the torque drops off significantly during this time.

I have done dozens of pulls while changing main jets, air jets, venturies, and emulsion tubes. None of this made any significant difference. Additionally, I have verified that the intake and exhaust lengths match Larry Meaux’s Pipemax calculations.

The only possibility left that I can think of, are the cam specs.

Intake duration (0.50”): 266 deg
Exhaust duration (0.50”): 248 deg
Lobe centers: 104 deg

I’m guessing that a possible fix could be to retard/advance the cams. Any suggestions would be welcome. BTW, this is a road race application, which needs a good power band from about 4500-7500.

Thanks,
Dave

Dave - Post a picture of the engines carb setup as you run it but with the air filter off.

DV