[mkube396]

running a 396 with a Holley 3310 with a LM-1 i do need to RPM convertor to get more data but dont have the $80 for one just yeat
I am getting at WOT 13.3~12.5 < looks good to me.

but when driving at 35~55 with throttle just enough to holed speed 15.2~15.4

I don't think I am hearing any dentnating at the 15.4 but I feel I would be more comfortable at 14.5 unless I am just nuts and 15.4 is OK at all most no load.. when I press on the throttle any more to gain speed it drops down to 13.*

so i am thinking i need to mess with the air bleeds i was going to stick some fine wire in them to see if i can ritchen them up abit what do you guys think?

 

[Tom Mobley]

I wouldn't do anything like that. It's near perfect as-is. If you're worrying abut the lean cruise being leaner than 14.7 give it up. There's nothing magic about that number. My 406/Q-jet setup is cruising at 16.1. good mileage, clean plugs.

 

[427L88]

Or bump jet size +2 in the primary side. If you're right, it will take less throttle opening to keep the car at a steady rpm/mph. I use 73's in the center of my tripower, which is like a 750 cfm with two secondaries. It made the car feel more like a torquey Caddy around town.

 

[mkube396]

I wouldn't do anything like that. It's near perfect as-is. If you're worrying abut the lean cruise being leaner than 14.7 give it up. There's nothing magic about that number. My 406/Q-jet setup is cruising at 16.1. good mileage, clean plugs.

ok i can go work that...

 

[kirkwoodken]

Lean it out until it starts getting lean missing. It pretty obvious when it happens. Then richen up till it quits.

I know this comment isn't relevant today, but years ago when Sun made exhaust analyzers, we would set the lean stops on the Rochester FI's where the needle ran off the scale, which stopped at 18:1. Now, I'm not saying it WAS 18:1, but plugs AND TAILPIPES came out white. Off the end of the scale was the starting point for leaness. The old 283 FI Corvettes got 23 MPG with 097 cams, and 3.70 rear gears. Radial tires hadn't been invented yet.

It is a heat engine. Think about it: when you cruise, you are only using about 30-40 HP. And that at minimal compression since the cylinder doesn't get a full gulp of air when the throttle blades are barely open. You are making 10-15 times as much heat at WOT, and that doesn't hurt your engine. Retarded timing at cruise and too little octane can hurt your engine. Lean cruise mixtures are your wallet's friend.

 

[Camaro_fever68]

Lean it out until it starts getting lean missing. It pretty obvious when it happens. Then richen up till it quits.

I know this comment isn't relevant today, but years ago when Sun made exhaust analyzers, we would set the lean stops on the Rochester FI's where the needle ran off the scale, which stopped at 18:1. Now, I'm not saying it WAS 18:1, but plugs AND TAILPIPES came out white. Off the end of the scale was the starting point for leaness. The old 283 FI Corvettes got 23 MPG with 097 cams, and 3.70 rear gears. Radial tires hadn't been invented yet.

It is a heat engine. Think about it: when you cruise, you are only using about 30-40 HP. And that at minimal compression since the cylinder doesn't get a full gulp of air when the throttle blades are barely open. You are making 10-15 times as much heat at WOT, and that doesn't hurt your engine. Retarded timing at cruise and too little octane can hurt your engine. Lean cruise mixtures are your wallet's friend.

It's all relevant, just a little dated (I just got a 4 exhaust analyzer) The old mechanical injections were great. You can do the same thing with a carb. My 10.40 ET naturally aspirated car gets 21 mpg on the freeway keeping up with traffic with a turbo 350 and 4.11 gear. (28" tire). I can average 16-18 in town when I keep my foot out of it. Your first sentence sets my cruise AFR's. No magical numbers for any given combo.

 

[kirkwoodken]

Chocolate brown sparkplugs indicating "just right" carb jetting is one of the worst magazine myths. The first thing the average rodder was told to do was install plugs two heat ranges colder and jet up 4 sizes. That would give you those great brown plugs. That was/is totally wrong! Plugs are supposed to be white, which means they are hot enough to burn off deposites, but not so hot as to provide preignition, which is pretty hot.

Thinking about it, the old Sun exhaust analyzers were probably calibrated for carbs, not FI. Since FI gives better fuel distribution than carbs IN NONRACE APPLICATIONS, FI is generally 7-10% leaner than a stock carb setup because FI does not need to be rich enough for the leanest running cylinder. SO, 18:1 on the Sun analyzer would be closer to 16.5:1 with a FI setup. I never thought about that until this post. (Just so you don't think I'm blowing smoke.)

http://www.craigboyce.com/w/2011/03/tobacco-smoke-enema/

 

[Milan]

Keep in mind unless you are using a 4 gas analyzer, o-2 sensors only measure oxygen. an occasional misfire (not uncommon at cruise) will read as lean. if the plugs look good leave it alone

 

[livewire69]

At 35-55 mph you probably are still running on the transfer slots in the carb. My car seems to be the same way. I also have 3.42 gears.

 

[rponfick]

Is jet size even a factor at cruise? I thought it was all idle mixture and tranfer ports at cruise. So, what effect would changing jet size have at cruise?.

Just asking. Ralph.

 

[Tom Mobley]

>>> what effect would changing jet size have at cruise?.

None, unless your cruise RPM is high enough that it starts the mains metering. most cars, no.