Would installation of headers cause part throttle BLMs to increase slightly? Well that is what I have seen at least. I just installed Hooker 2055s coated by Jet Hot. Now my part throttle BLMs are 130-132. I had it pretty good before the headers went in. This is with a $6E tdf ARAP modified bin. The only other change I made was install of a new Bosch O2 sensor. I now get .93-.94v out at WOT.

But my idle BLM is now at 132. It was 124-128 before. I allready made a slight change to the FI constant, and it brought some of the BLMs closer to 128, but idle and low MAF values still have 130-132 numbers.

Could it be the new sensor, or do I have different airflow thru the engine?

 

Changing the exhaust lowers the back pressure, which allows less restriction to the air flow thru the engine, and reduces the self EGR'ing effect the exhaust has on the engine. Kind of a ying, and yang thing.
It apprears that in your case, the resulting decrease in backpressure and increase of air flow thur the engine up'd the engines need for fuel to match the increase.
Lessening of the self EGR'ing taken by it's self would also see an improvement with less timing.

Bosch has had a long history of problems with their O2 sensors. I'd suggest sticking with A/C's.

 

Ok Grump, I think I got it. The Blms aren't off by too much, just a little high at lower MAF values. Just got to tweak the tables a bit. Been taking some timing out as well.

Funny about the Bosch sensors, I thougt they were decent. Some folks on these boards like them. Bosch is what came stock on our cars, yes?

 

Bruce,

What problems should I encounter when using Bosch? I have been using them for some time. I recently converted to a heated O2 for emissions purposes using an O2 sensor from a Bonneville SC motor.
The other question is could you explain further the "reduction" in self EGR when using freer flowing exhaust? I would think that if exhaust was freer flowing you would increase the possibility of EGR effect on intake charge. I just put on Magnaflow cat back on my 84 Vette. Its got 2.5" Y-pipe and inlet mufflers. This was an increase from the 2.25" Dynomax Y-pipe and inlet Flowmasters. SOTP, it feels stronger and appears to rev more easily. I know SOTP doesn't amount to much but I hope to find out more at the track next week.

 

The Bosch have had a known internal ground problem since day one, and often go intermittent. The other things are they more EGT, and more back pressure sensitive then AC's. So other then being prone to failure, and erratic they're fine.

Self-EGRing of the engine.
The less the back pressure the less exhaust gets pushed back into the engine as the exhaust valvle is closeing and the intake is opeiniong at overlap.
The greater the pressure difference across the EGR valve the more it will flow. So reducing the backpressure reduces the amount it flows.

Fatter, shorter exhausts allow the engine to change rpm quicker, just the same as changing runner lenght, and cross sectional area at the intake end of things. And muffler design is often over looked and critical, since some while only 19" long have the gases change direction and the path can be much longer.

 

Bruce,

Thanks for answering that. So mufflers like the Flowmasters would actually lengthen the exhaust path and consquently increase backpressure if I understood correctly. With lower backpressure, is there a possibility of "overscavenging" the chamber?

 

A) yes.
Straight thurs are the best answer, but sometimes it takes running two to dampen the noise, sometimes turn-downs can help. I'll save ya looking under the GN, it's got two straight thurs, and the turn down ends right past the rear axle.

B) It can happen.
It takes tuning for a finite rpm band, and then things going out of range when out of that rpm band. Typically not a problem with a street car. Note, street not just streetable.
We're excluding motor homes and 20' long exhuast systems.

 

"We're excluding motor homes and 20' long exhuast systems."

 

Grumpy,

Could the backpressure thing you mentioned be making my car run so bad? I just installed the new Catco dual cat setup on my car along with the Hedmann headers. BUT, there is no exhaust after the cats(no cat-back). Could this be causing my car to idle "rough" since the lack of backpressure is keeping my EGR from doing it's job???? I also changed to a new Bosch 02 sensor.....but I ruled that out because it ran the same with the original 13 yr old 02 sensor...lol Just thought Id ask to see if maybe that would make my car idle rough. I just havent made enough $ to get my catback yet..lol Plus, it sounds pretty good without it....