Richer AFR in higher gears

[GRTfast]

I've noticed on some of my "closed course" test drives (big block, 4 barrel carb, 4 speed trans) that the afr is progressively more rich in the higher gears at WOT.

first, 13:1 (tires spinning)
second 12.5:1 (tires spinning but eventually hooking)
third, 12:1 (hooked up)
fourth 11.5:1 (hooked up)

(these AFR readings are approximations, it varies a little bit)

I get the same result if I start from first gear and accelerate through the gears, or if I roll along in each gear and punch it at 2000 rpm or so. AFR is steady through the power band, just richer in each successively higher gear. If I really lug it in 4th, say going to wide open throttle at 1200 rpm, it will even go as rich as 10:1, then move into the 11.5:1 range as it comes on the cam.

The only difference I can see in each scenario is the load on the engine is increased as I move to each successive gear, and the resultant sweep rate through the RPM is slower. I'm trying to understand exactly what is happening though. It runs great and pulls like a freight train. Thoughts?

[strokersix]

Aerodynamic pressure perhaps?

[GRTfast]

Aerodynamic pressure perhaps?

Could be, it has one of those old school cadillac style air filters with the inlets facing forward (car has no hood). Seems like if anything that would make it leaner though.

[prairiehotrodder]

I personally would try 2 sizes bigger high speed air bleeds. I've found that adjusting the HSAB to flatten out the curve works good on my race car. I could be wrong and its an easy thing to try which shouldn't hurt anything.
Brian

[GRTfast]

I personally would try 2 sizes bigger high speed air bleeds. I've found that adjusting the HSAB to flatten out the curve works good on my race car. I could be wrong and its an easy thing to try which shouldn't hurt anything.
Brian

I will give that a try. What's interesting it, it doesn't get richer as the rpm sweeps in a given gear, only from one gear to the next.

[slo-svt]

I noticed this very phenomenon 7-8 years ago when I started tuning cars. It happens with every fuel injected platform I have messed with (gm, Ford, and Chrysler) This is why I always tune them in the 1:1 (or close to) gear ratio. I haven’t put much thought into it but believe it to be load related.

[GRTfast]

I noticed this very phenomenon 7-8 years ago when I started tuning cars. It happens with every fuel injected platform I have messed with (gm, Ford, and Chrysler) This is why I always tune them in the 1:1 (or close to) gear ratio. I haven’t put much thought into it but believe it to be load related.

I believe it is load related as well, but I don't understand what physics is taking place. When the load is higher, the piston is more difficult to push down and the difference in piston speed from one stroke to the next is less. How that makes the mixture richer is eluding me.

[slo-svt]

Maybe someone that's a little better with physics and weigh in. My assumption is the slower sweep rate of the engine gives the fuel more time to get in the cylinder. In lower gears you might experience 1000rpm/sec sweep where in you 1:1 gear might be something like 3-400/second. Weather it actual liquid evaporating from the walls of the intake or just more raw fuel making it in I don't know. One way to check would be to see if this is present on a direct injected car but I haven't tuned any in multiple gears or logged at the track. Interesting phenomenon for sure though.

[F-BIRD'88]

Fix the air cleaner and fix the air entry point of the air cleaner.
AT higher speed the air cleaner is getting a progressively negative relative air pressure.
Less that ambient air pressure at the air cleaner entry.
This is the functional opposite of RAM AIR/cold air induction, like a hood scoop does.

The same relative negative air pressure under hood that allows air to flow thru your radiator at speed
also creates a relative negative less that ambient air pressure at the under hood air cleaner inlets.

thats why cars have forward facing hood scoops with a carb box.
Thats why cars have cowl induction with carb box to feed higher than ambient relative air pressure to the carb at speed.

Its like FREE BOOST and FREE horsepower.
Then you can adjust the carb jetting and HS air bleeds.

FIX the air cleaner and fix the air entry pick up location to a high pressure zone at car speed.

[GRTfast]

Fix the air cleaner and fix the air entry point of the air cleaner.
AT higher speed the air cleaner is getting a progressively negative relative air pressure.
Less that ambient air pressure at the air cleaner entry.
This is the functional opposite of RAM AIR/cold air induction, like a hood scoop does.

The same relative negative air pressure under hood that allows air to flow thru your radiator at speed
also creates a relative negative less that ambient air pressure at the under hood air cleaner inlets.

thats why cars have forward facing hood scoops with a carb box.
Thats why cars have cowl induction with carb box to feed higher than ambient relative air pressure to the carb at speed.

Its like FREE BOOST and FREE horsepower.
Then you can adjust the carb jetting and HS air bleeds.

The holes in my air cleaner housing are facing forward, i.e., it is a scoop.

[F-BIRD'88]

But it is in a LOW pressure ZONE when at high car speed.
And or the air cleaner itself is creating a LOW pressure at the inlet.

Not sure on that air cleaner but looks like a air flow nightmare.

The faster you go the worse it gets.

[GRTfast]

But it is in a LOW pressure ZONE when at high car speed.
And or the air cleaner itself is creating a LOW pressure at the inlet.

Not sure on that air cleaner but looks like a air flow nightmare.

I see what you mean, the radiator shroud is likely creating a big low pressure zone at the air cleaner housing inlet holes. That said it doesn't seem to be speed dependent as much as it is load dependent. If I use third or fourth gear at the same speed, it is richer in fourth.

[F-BIRD'88]

A simple water tube MANOMETER test set up can show you the effect on relative air pressure in the air cleaner at high car speed. You want at least AMBIENT air pressure at speed. A hair higher pressure ( relative) creates a RAM AIR
slight supercharging effect at speed which creates free horsepower at high speed.

[F-BIRD'88]

Look at how a air foil ( air plane wing shape works.
To create a low pressure area above the wing.
Your air cleaner SHAPE is acting like the top side ( low pressure) of a air plane WING.
AIR FOIL.

[David Redszus]

first, 13:1 (tires spinning)
second 12.5:1 (tires spinning but eventually hooking)
third, 12:1 (hooked up)
fourth 11.5:1 (hooked up)

Do you have data indicating mixture plotted against rpm and speed?