[Boost Junkie]

It's been widely claimed that the stock intake flows more air to #6 cylinder at high boost on big turbos. I've never seen any real evidence to support this notion, so I did some testing the other day with the Heraeus EGT monitoring system to see how EGTs changed from cylinder to cylinder as the boost pressure was raised. I did 9 tests at increasing boost pressures, starting with a steady state 3K RPM cruise and then making 8 pulls between 18 PSI and 28 PSI, recording the peak EGT from each cylinder after each pull. I did the testing on my car with the SP74, RPS header, SP fuel with 720cc injectors, VPC, SFC, FMIC...

At a steady state cruise, all the cylinders appeared to be running at more or less the same EGT. Under boost, cylinders 1 and 3 appeared to be the hottest and cylinders 2 and 6 the coolest, and the differential seemed to increase a bit as the boost pressure was increased. Obviously this does not support the common belief that #6 runs leaner than the rest at high boost.

I'd like to have the time and opportunity to test all of the aftermarket/modified intakes on my car and see how they compare...

Steve
FJO dealer

 

[White Supra]

Actually I believe that #3 is going to run leaner. That's just based on which piston I smoked most often

 

[Boost Junkie]

That fits with what I saw. If anything, #3 may tend to run leanest of all -- either #3 or #1, but as boost gets on up around 28 PSI (as far as I went with my testing) #3 seems to be getting even leaner than #1.

I have a wideband O2 port in #3 header runner. I'm planning to move my wideband O2 to that port and see what the results are. I'm not sure if the wideband will be accurate at the higher pressures encountered in the header, but I'll give it a try. I've HEARD that high pressures tend to throw off the accuracy of some wideband sensors, but then that may just be something that has been repeated often enough that it has become accepted as fact even though it is not fact -- just like the claim that #6 runs leaner.

Steve

Actually I believe that #3 is going to run leaner. That's just based on which piston I smoked most often

 

[BLK-SP64]

Steve I love your Scientific approach to testing and choosing parts. Great job. #3 went on mine too when I blew my engine.

 

[Boost Junkie]

Hmmm.... I'm planning to have my head and intake worked. I wonder if it would be a good idea to have #3 and #1 intake runners left a little less "worked" than the others.... I'd want to try to do some more testing before making that decision, but it sure seems like #3 and #1 run leaner. I guess a good flow bench that operates under pressure instead of vacuum is what's really needed, although there might be other factors such as how much back pressure is in the exhaust side -- is it the same on all cylinders?....

More questions than answers.

 

[BLK-SP64]

More questions than answers.

Or more specifically, ......each new answer leads to a whole new set of questions....

 

[AnArKey]

Boost Junkie....I can tell you that the whole Turbo Buick (GN, TR) community who use FAST aftermarket ignition systems with a wideband agree that you MUST run the sensor post turbo. The extream high temperatures in the runners can damage the sensor, and the massive pressure changes greatly effect the accuracy and consistency of readings.

 

[Boost Junkie]

AnArKey,

Thanks for the info. I think I'd be willing to take the chance on the temperature side -- some of the O2 sensor suppliers recommend 500-800 C as an ideal operating range for non-wideband sensors, but I don't know if that temperature is different for widebands or not. If not, I'd think that if I hit 875 - 900 or so a time or two just running the tests it'd have a good chance of surviving.

But that still leaves the pressure issue... Do you know what type of sensor the GN guys were using, or how much exhaust back pressure they were seeing? All I want to do is make a couple short pulls with it in the runner. I'm not TOO worried about ruining the sensor because worst case, I could get another one if I could scape up the money. But if the AFR information I get won't be reliable, then I don't want to waste my time doing the test...

Steve