Blurry94
Matt C.
You were seeing 11-11.5 AFR's with 60psi of fuel pressure, which is a good number for boost. What is it now at 100psi
V2 upgrades on speed density?
- Thread starter Travis017
- Start date
Last two pulls I made on my way home sunday we’re 10 on the afr with about 6psi and 100+ fuel pressure. I didn’t hear any detonation but I realize that’s lean
Actually 10 AFR is VERY rich, I'd personally run it around 11.5-11.7
I guess I’m thinking of it backwards then. How do I lean it out with my current setup?
Well, you don’t. It is rich and should be. That’s the thing with doing it old school. If all us carb guys had AFR gauges we’d been crappin bricks.
If the problem is boost look for leaks and belt slippage.
I guess if I get the proper boost numbers it will correct itself. I’ll see if I can get some more tension on the belt and check all my clamps.
ASAP
Fore!
You can order discs to change out in the FMU to correct AFR. With your combo, I would shoot for 11.5-11.8 as Ray said above. A motor running too rich isn’t good either. My truck didn’t like anything below 11.2. I targeted 11.8 at 10psi and 11.5 at 14. Ran perfectly. I would change the pulley set up to a smaller SC pulley to get your boost up to around 10psi. Get in touch with shaunditty on here. He has a similar set up to what your looking for.
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oilwell1415
Well-known member
I've seen a few dyno tests run on carb'd engines with a wideband in every cylinder. They can range from 9:1 to 16:1 on the same engine. Turns out all that time we were told we could only run 8.5:1 compression on pump gas probably had nothing to do with the compression, it had to do with the mixture.
What I mean is don’t worry too much about it until you have everything else buttoned up. Better to be rich while your working things out, then dial in WOT AFR stuff.
Blurry94
Matt C.
As stated above, 10.0 is too rich. It may be even richer if the meter doesn't read less than that, which is really pointless to wash the cylinder walls down with that much fuel. I personally like to keep mine between 11.3-11.5 AFR's, given that all other components are working properly.
Ok so I had some time to mess with the truck this morning. I drilled a new hole and moved the pulley on the adjustable tensioner to get a better wrap on the supercharger pulley. Now it will get up to 8-10psi but I’m still running 10:1 afr. I started playing with the Hobbs switch and adjusting it to come on later. I have it around 5psi activation but until it comes on I start creeping lean. It will start around 11.5:1 then start creeping to 14:1. About that time the switch activates the pump and I end up back at 10:1. Im wondering if my afr readings are a little skewed as I put the sensor in the pipe next to the original sensor at the Y pipe joint. I ordered two more weld in bunds and will try moving it up close to the flange where the manifold and pipe mount. Ordered two bungs so I’ll probably put one on both sides so I have options down the road
That’s cool, sounds like you are making progress. I’ve run an FMU back in 2003 and again last year on both my trucks. I just lived with it. I didn’t get into the discs or opening it up - mostly because I had plans to swap ECU and injectors but also it ran good and I didn’t worry about it.
The AFR is probably close man, mine was same, in the 10 range on long 1/4 mile pulls.
I’m sure there’s no way to dial it in 100% but as long as it’s close and not gonna blow the motor I’m good with it.
This. Washing down the cylinder walls isn't good either.
Ordered a 10:1 disc. I’ll see what that does. What’s an acceptable range for afr? At what point should I be getting out of it? 14.7:1 is stoic as we all know but under load im sure that’s too lean. Should I not let it go above 13:1
You are targeting 11.5 AFR rock steady above 2-3lbs boost. Any leaner and you risk unexpected disassembly of your engine.
Blurry94
Matt C.
If all you're doing is replacing the FMU disc from 12:1 to 10:1, then the lean condition you're getting, before the inline pump turns on, will only get worse: 14+ is already on the edge.
You can set the Hobbs switch to come on at a lower boost range, like 2 psi (or replace it with one in that range), to stave off that lean condition it gets before it comes on at 5psi. Then swap your disc out to a 10:1 and monitor your A/F ratios.
Or, upgrade the in tank pumps to 255lph units. The higher volume pumps, along with an FMU, may allow you to possibly ditch the Hobbs switch, pump relay and inline pump. The plus side to ditching the inline pump is that there are less components to fail and you get rid of that pesky rich spike when the inline pump comes on.
You can set the Hobbs switch to come on at a lower boost range, like 2 psi (or replace it with one in that range), to stave off that lean condition it gets before it comes on at 5psi. Then swap your disc out to a 10:1 and monitor your A/F ratios.
Or, upgrade the in tank pumps to 255lph units. The higher volume pumps, along with an FMU, may allow you to possibly ditch the Hobbs switch, pump relay and inline pump. The plus side to ditching the inline pump is that there are less components to fail and you get rid of that pesky rich spike when the inline pump comes on.