1955 Pontiac (advice for setting efficient but driveable mixture?)

[KrautBurner]

Originally Posted by dtmerritt

OK, here is where I need your help! I know manufacturers set the mixture at 14.7 for the catalytic converter, which I don't have. I know leaner ratios are better for mileage, but without running too lean, what ratio should T strive for?

I'm fairly confident there was no cat on a car built in 1955

but a wideband O2 sensor could be helpful, and or a Pyrometer
you may also want to look into a vacuum gauge

[Cobb]

Just remember running lean and cranking up the timing will raise your NOX levels if you are subject to any emissions testing.

[elhigh]

Those older rides are usually grandfathered out of emissions testing. I can imagine some nanny state looking over your shoulder but I don't realistically expect it, not with a '55.

The O2 sensor won't help his car but it could go a long way to helping with the tuning, hooked up to an AFR gauge on the dash or even just a handheld.

I wonder what it would take to retrofit an on-the-fly mix control like in light planes? They have to be adjustable while running to compensate for air density changes as the plane gains altitude. I know we have some light plane pilots on this forum, anyone want to weigh in on that idea?

My truck has a carb. Leaning it out for highway cruising would be pretty sweet.

[slownugly]

Personally I would adjust carb, points, ignition timing to stock for the trip. I wouldn't risk doing damage.

However when you get it home then I would have some fun. I'm on the same page I'm building a 68 rambler American and I can't wait to start playing around with the carb for fuel economy.

Couple notes-
Don't waste money on a 30 dollar afr gauge. They only come with the gauge and are made to wire into the signal wire of a narrow band o2 sensor. Get a good wideband afr gauge that comes with the o2 sensor.

Invest in a vaccum gauge first!

On my car not only am I planning on messing with the carb but the entire fuel system. I ditched the mechanical pump and I'm running an electric with a relay system grounded by an oil pressure safety switch (much like gms of the late 80s early 90s) if I shut the engine off for engine off coasting the pump will shut off. Im also running a fuel pressure regulator. I'm going to set the float to factory spec but I'm going to play with pressure at the carb. Im not counting on that making a difference at all but who knows. It's a carter yf 1 barrel.

Edit: also your only going to see gains from playing with jets rather than idle mixture screws. the idle mixture circuit isnt even used at cruise speed which is where you want your lean condition. the main jet circuit is what you want to modify. only problem with that is your timing curve is set at a certain spot and guess what, if you jet it wayyy lean your going to deal with detonation and chance melting a piston or 8. msd makes an adjustable timing knob where you can retard your timing up to 15 degrees on the fly. it requires driver feedback and constant monitoring but i think it would be fun as heck to have lean burn in a carbed car!! i should pm pgfpro about it he could give some insight on timing a lean burn car.




Also I'm ditching points. I'm 25 and I know how to set these cars up but I really want to take advantage of a stronger spark if I can get it, so I opted for the pertronix conversion. Oh yea and reliability and not having to change them after 20k will be nice too

Take that mechanical fan off and toss an electric fan on via a switch or thermostatically controlled switch. I'm running an adjustable thermo switch.

[Hersbird]

I'm glad you asked this as well. Often I see tuning related to modern cars but these pre-emission cars get to benefit from tuning best for pure economy. Plymouth made a feather Duster in 1976 that with the slant 6 and overdrive manual got great mileage. I wonder how much better a car like that could do with some modern fuel injection and aero mods.

[Frank Lee]

Planes use exhaust temp as the guide for leaning.

http://flighttraining.aopa.org/stude...ls/engine.html

[dtmerritt]

I would like a wide band in a few months when my wallet recovers, but for now I purchased a narrow band and the mechanic there will install it for me. I'd rather, but both me and my wife are crippled and have to use scooters to get around. Because of an auto accident I have almost no cartilage in my right hip, add in a stroke on the left side, well I can walk into the bathroom by hanging on (from my electric wheelchair). The stroke made my left hand pretty useless however, so I do what I can, and pay someone to do what I used to.

[dtmerritt]

In AZ no emission test is required, IF. You have to belong to an organization and have classic car insurance, it must only be used for car shows and kept in a locked garage.

[dtmerritt]

I am thinking EFI.

[KrautBurner]

Originally Posted by iveyjh

Maybe I missed it somewhere in the reading of this post nobody ever answered your question about cruising AFR. I find that a ratio of about 16.5 to 18.5 to 1 yields good economy. I don't think you can reach that ratio by just adjusting the idle jets though.
Good luck

that seems aweful lean to me
but all my AFR tuning was done at WOT

thats why I mentioned a EGT (Pyrometer) could be a good safety gauge