Backpressure importance
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 1,043
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2023 C3 of the Year Finalist - Modified
Backpressure importance
A phenomena I’ve noticed and I would appreciate if someone could explain.
I have cutouts in my exhaust, I can toggle between open header and mufflers.
when the engine is at not ideal timing, about 28 degrees total, there is a noticeable difference in idle quality with cutouts shut versus open.
when open, it’s about 500-550rpm, acting like it’s about to die, shut the cutouts and rpms come up to 600ish and the engine is stable.
In my mind, open headers improves the scavenging ability of the motor. Less backpressure means more exhaust gasses exit and it should run better.
why does providing back pressure seem to help the motor idle?
of note, it is a longer duration cam(292/300) and it is installed 4 degrees delayed.
I have cutouts in my exhaust, I can toggle between open header and mufflers.
when the engine is at not ideal timing, about 28 degrees total, there is a noticeable difference in idle quality with cutouts shut versus open.
when open, it’s about 500-550rpm, acting like it’s about to die, shut the cutouts and rpms come up to 600ish and the engine is stable.
In my mind, open headers improves the scavenging ability of the motor. Less backpressure means more exhaust gasses exit and it should run better.
why does providing back pressure seem to help the motor idle?
of note, it is a longer duration cam(292/300) and it is installed 4 degrees delayed.
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 1,043
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2023 C3 of the Year Finalist - Modified
Thanks Lars
Racer
Joined: Jan 2020
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From: Tacoma, Wa
I was nervous about the fuel mixture differences with my cutouts, so I installed them right before the muffler so the exhaust travels a decent length either way. The muffler is the only difference. There is a change, but its very minor and no one would notice but me.
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
Lars
Disabled by request 8 JUN 2025
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From: St. Augustine, FL
On "run what you brung" nights at the local dragstrip, every street driven car with headers has to open them up and make a pass down the drag strip. Most drivers are disappointed when their car starts "shooting ducks" all the way down the strip from running lean.
Drifting
Joined: May 2020
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From: Baton Rouge, Louisiana, metro area
So Lars,
With ECM/Injector/Knock Sensor motors, does this open headers problem get corrected solely via ECM control measures? Is there enough system variation to control injector pulse and timing?
With ECM/Injector/Knock Sensor motors, does this open headers problem get corrected solely via ECM control measures? Is there enough system variation to control injector pulse and timing?
Thread Starter
Burning Brakes
Joined: Dec 2017
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2023 C3 of the Year Finalist - Modified
On the main metering circuit the lean-out is not too bad, although still there, when you open up the exhaust: The main metering circuit uses the Bernoulli effect to pull fuel out of the bowl and out the discharge nozzles, so the more air (mass) goes through the venturi, the more fuel is pulled with it. However, the idle and transition circuits do not use the Bernoulli principle to pull fuel out of the carb - they rely entirely on manifold vacuum to pull fuel out of the carb below the throttle plates - the increased airflow has no effect on the fuel being discharged - it remains unchanged if manifold vacuum is relatively unchanged, so the carb leans out. Since the engine gets most of its fuel from the transition circuit during light throttle cruise, the lean-out effect can be significant when improving exhaust performance, such as when swapping over to headers or installing dual exhaust on a car that had a single exhaust. When upgrading the exhaust you typically want to increase jetting in the carb and richen up the idle mixture.
Lars
Lars
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Le Mans Master
Joined: Mar 2004
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From: Little Rock AR
Engine Masters did a test on cutouts and placement. They pretty much needed a header collector extension in any open header application for best power. They mounted the cutout in various distances and got good results with setups close to the end of the various length extensions but really bad results at the back near the axle.
I have never used a cutout but the episode was worth a watch for sure.
I have never used a cutout but the episode was worth a watch for sure.
Thread Starter
Burning Brakes
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2023 C3 of the Year Finalist - Modified
the nitrous easily makes up for the lost power of pulling 6 degrees.
when I’m normal street driving I run 36 degrees total
Is my vette stock?? HAHA
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If you are running such a big cam why are you running such a low idle....I know it sounds cool but you can run it above 900 and adjust your idle mixture to pull in more fuel and richen up the mixture itself and have a higher vacuum at idle. It will improve your cruise A/FR and give it better street manors and not run lean when you open them up...atleast not as lean
Thread Starter
Burning Brakes
Joined: Dec 2017
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2023 C3 of the Year Finalist - Modified
If you are running such a big cam why are you running such a low idle....I know it sounds cool but you can run it above 900 and adjust your idle mixture to pull in more fuel and richen up the mixture itself and have a higher vacuum at idle. It will improve your cruise A/FR and give it better street manors and not run lean when you open them up...atleast not as lean
I think that would be rough on the trans shifting from park to drive at 1300 rpm.
At WOT my A/F is 13.5
Is my vette stock?? HAHA
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is it 13.5 with the nitrous shot....I would think thats kind of lean for racing..... but I realize lean makes power
Thread Starter
Burning Brakes
Joined: Dec 2017
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2023 C3 of the Year Finalist - Modified