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Old 10-01-2009, 07:48 PM   #1 (permalink)
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Carb: cold air vs hot air

Been mulling on this for a couple of days now.

The basis of fuel metering in a carb is dependant upon the volume of air passing through irrespective of mass.

So, if our main jet is fixed, the air/fuel ratio will vary dependant upon the density of the incoming air.

Now, why do some carb cars seem to run better in the cooler weather?
Now with my vehicle, it feels like it gets better fuel economy during the warmer months (when highway driving), which would hint at perhaps fitting a HAI. however, given the dirty aerodynamics of my vehicle, i think that this phenonmenon is more due to the warmer air being less dense, therefore less aerodynamic drag (easier to cut through) than during colder air temps.

Since the carb is not adjusting the amount of fuel to take advantage of the the extra oxygen associated with denser air, my thought is that the main jet (from the factory) is probably set to the rich side (to cater for worst case scenario) and cooler air is in effect 'leaning' out the air/fuel ratio promoting a more complete burn and giving the feeling of more 'power'.

i understand that warm air assists in atomising the fuel, however when at cruise (where i spend most of my time), but wouldnt the light load/high vaccuum conditions contribute more to fuel atomisation through the carby than warm air will? after all, most factory warm air systems revert to fresh air once the engine has reached operating temperature.

So, in order to get my a/F ratio closer to stoichiometric, the first step should be to direct air that is as dense as possible into the standard air intake? Either read the colour of the plugs or use an A/F gauge (o2 sensor stuffed up the exhaust)?

With the leaner mixture, and fitment of a higher energy ignition system, a more advanced timing profile (more vaccuum advance?) will also be possible?(lean mix takes longer to burn, but needs more spark to ignite).

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Old 10-01-2009, 10:19 PM   #2 (permalink)
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EFI
MegaSquirt - Electronic Fuel Injection Computer by Bowling & Grippo * * 2009
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Old 10-01-2009, 11:43 PM   #3 (permalink)
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Gee, thanks for the advert. If i wanted to fit EFI I would have done it.

Does anyone have any comment about the proposed question?
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Old 10-02-2009, 12:32 AM   #4 (permalink)
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You seem to have a grasp of it but you will always get poorer FE in the cold irrespective of whether you are tuned rich or lean (within reason). Fluids are more viscous and the wall of air you are pushing is just that much more dense.

I think the warm air could help with atomisation and allow you to open up the throttle more for reduced pumping losses but a couple of caveats. Denpending on the carb, more throttle may pull on the enrichment circuit in the carb, also get too warm and vapour lock becomes a problem. Also carbs are more prone to pinging than EFI. Lean detonation is a sure way to toast your engine.
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Old 10-02-2009, 12:46 AM   #5 (permalink)
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Cold Fuel and Warm air

Another reason I think mileage improves in warmer weather is the fact that you are pumping relatively cold gas that then warms up in the tank and expands...
In other words you pumped in say 10 gallons of fuel that was at a temp of 50 degrees F. It is then heated up by the warmer air and presto 10 gallons slowly expands to say 11 gallons...

Also, Mopars (chrysler, dodge, plymouth) from pre FI days were known to pump a lot of heat into the manifold; there is a passage that allows hot exhaust gases to pass thru the manifold under the carb... and no I do not mean EGR. This is in addition to the warm air ducted into the carb during cold engine running. The heat in the manifold even caused a lot of hot start issues especially on the 60's and 70's era Mopars.
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Old 10-02-2009, 01:12 AM   #6 (permalink)
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Quote:
Originally Posted by aussie_modder View Post
Been mulling on this for a couple of days now.

The basis of fuel metering in a carb is dependant upon the volume of air passing through irrespective of mass.

So, if our main jet is fixed, the air/fuel ratio will vary dependant upon the density of the incoming air.

Now, why do some carb cars seem to run better in the cooler weather?
Now with my vehicle, it feels like it gets better fuel economy during the warmer months (when highway driving), which would hint at perhaps fitting a HAI. however, given the dirty aerodynamics of my vehicle, i think that this phenonmenon is more due to the warmer air being less dense, therefore less aerodynamic drag (easier to cut through) than during colder air temps.

Since the carb is not adjusting the amount of fuel to take advantage of the the extra oxygen associated with denser air, my thought is that the main jet (from the factory) is probably set to the rich side (to cater for worst case scenario) and cooler air is in effect 'leaning' out the air/fuel ratio promoting a more complete burn and giving the feeling of more 'power'.

i understand that warm air assists in atomising the fuel, however when at cruise (where i spend most of my time), but wouldnt the light load/high vaccuum conditions contribute more to fuel atomisation through the carby than warm air will? after all, most factory warm air systems revert to fresh air once the engine has reached operating temperature.

So, in order to get my a/F ratio closer to stoichiometric, the first step should be to direct air that is as dense as possible into the standard air intake? Either read the colour of the plugs or use an A/F gauge (o2 sensor stuffed up the exhaust)?

With the leaner mixture, and fitment of a higher energy ignition system, a more advanced timing profile (more vaccuum advance?) will also be possible?(lean mix takes longer to burn, but needs more spark to ignite).
Weld a bung on each side of the engine's exhaust header, and insert one-wire case-grounded O2 sensors (You can get them from Hondas before 1991)

This will give you a basis for a quick and dirty O2SAT test, which will tell you if you're inside a given range of A/F ratio.
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Old 10-02-2009, 05:20 AM   #7 (permalink)
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Quote:
Originally Posted by aussie_modder View Post

With the leaner mixture, and fitment of a higher energy ignition system, a more advanced timing profile (more vacuum advance?) will also be possible?(lean mix takes longer to burn, but needs more spark to ignite).
I think lean mix needs more spark, but I am not sure if it takes longer to burn. One thing for sure is it's more prone to pre ignition so more vacuum advance will probably hurt you.

The best thing you can do is get a wai and drive it like that for a couple of weeks, than put a cai and drive it like that, then you'll know for sure which one will get you better gas mileage. I think the trick here is going to be keeping air that's going to the carb the same temp and then adjust a/f mixture just for that.
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Old 10-02-2009, 11:14 AM   #8 (permalink)
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You may also consider rejetting your carb. They sell one generic setting everywhere, if you're getting specific you can get as specialized as you want with carb selection and tuning. An O2 sensor added to your exhaust path will be very helpful as well. I find that to be very helpful when dialing in the carb and monitoring your advance.

A properly tuned engine will be more powerful, have better FE, and lower emissions. Plus, I would certainly recommend having some A/F monitoring before trying any major lean/rich tweaking. A simple vaccuum gauge helps out here as well.

Plus, without spending a ton of money you could look into putting an electric fan on it. The clutch fan slows down at idle, and speeds up when the grille is bringing in air. No fan can compete with highway speed air rushing in, so it's reverse of what you want. The electric keeps the idle air higher, which tends to keep you cooler at slower speeds. It at least will allow you to stabilize your cold/hot engine compartment swings. It's not uncommon at all to run hot at idle and cool off when you're moving. Stabilizing that would seem to be of benefit to your problem. And if you really want to get tweaky you can turn it off, or find one with (or setup) a temp switch to turn it off when it's not needed, which reduces the draw on the alternator. And you'll also be reducing engine drag a tiny bit.
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Old 10-02-2009, 11:42 AM   #9 (permalink)
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Alot of carbed engines came with warm air intakes stock, with a thermostat that keeps the air at 100 degrees F.
The better cold starting you might see is often because of the enrichment valve opening up or the choke being full close.
The warm air intake is often removed by people who don't understand what it does and notice that the car still runs ok that day.
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Old 10-02-2009, 01:05 PM   #10 (permalink)
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I love carbs. Even in your post it is overthinking the owner. It is more dynamical than injection never will be...cold outside? so is the fuel, and air, it is always one world. there are extreme setups. the most strange is the crabs with cooland channels running through them. ridiculous. I hit -28F below and still do not need such madness. Need heat rise? suck air from within engine bay..but always have an outside source, direct draft ..it is for electrical physics reasons.
I like warm air mix, I can't call it 50/50, it just gets some cold with the warm.
A real thing to conquer is the real time draft. It needs very strong singular fires, upgrade coil perhaps, typical tune up towards hot rod parts in the igntitions. and I found a very smart trick with an exhaust resonator, allowing catalyst hot and gentle flow for idle circuits at the same time (its in another post here, a hollowed, welded glasspack with the steel innards still inside.)
if you own other than a japanese "feedback" carb it is junk, if attmpting feedback. that would be the last generations of eco-carbs as man knows them.All the holleys and carters and bogro jets american versions are truly engineered disasters. Old webers from the 1950s design era did better with a modern choke. Don't let bad carbs ruin true thought process simply natural genius. you get good fuel at WOT? it is good for all year, just ttinker with heat reise and temps and fires. A downdraft carb is a cold cold engine.

The carb is a genius for me with a temperate range very very below zero into traffic jams in southern new england summertime slop of 100 degrees and more...and I would still go anywhere at 40+++ mpg anytime...on an original 23 year old build. I would hope hitachis on subarus aren't the only masters.

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