I saw a post in another thread suggesting to avoid air gap manifolds for street use because they don't stay cool with extended run time (and so presumably don't offer any performance advantage under real world conditions) and they also give worse fuel economy.
Is the worse fuel economy caused by a longer warm up time leading to increased choke usage or is the economy still worse even once the engine is warmed up, and if so why?
This is for carb applications.
Carbs need heat to vapourize the fuel.
The colder it is, the more fuel you need so that you get enough vapourization to run. Kinda like leaving the choke on just a little bit.
I really really want an air-gap intake for my daily-driver '77 C10, but I also drive Canadian winters with it and really need the heat in the intake for the vapourization.
Once the engine and under-hood is up to temp, it shouldn't be a problem.
Keep in mind, that extra fuel can also be accelerating your cylinder wear by washing away the oil. Old carb engines wore the cylinders out out way faster than the EFI engines do.
Once had a guy drop by my shop to drive his vehicle and help him make the cold-running better and more driveable. Engine had an air-gap intake (no heat) and Vortec heads (no heat source). I told him that with this set up, it will NEVER run awesome when it's cold until it has finally warmed all the way up. My C10 has a basic Performer, heat passage intact, AND a thermal riser off the headers to get it warm quickly. Always drives awesome when cold as well as in winter.
In reply to SkinnyG (Forum Supporter) :
I get all that. My question is - assuming the intake temp is pretty much the same as a non air gap manifold once warmed up (which seems reasonable) wouldn't you get pretty much the same fuel economy at that point as well?
If the flow rate and delivery method (dual plane or single plane) are the same, yes.
If the flow rate and delivery method (dual plane or single plane) are the same, yes.
In the other thread I spoke with a little more absolute terms than is the reality. I stated that they will all get to the same temperature eventually, but that is a bit overly simplified. An air gap intake will stay a wee bit cooler depending on the application which could cause you to have to chase the A/F mixture a slight amount to the rich side. This is also dependent on the engine architecture. Something like a Pontiac uses a floating intake with a valley pan, so in theory, all of them came with air gap intakes from the factory. Something like an SBC where the bottom of the intake is directly exposed to hot oil in the valley will have different effects on runner temperature than a Pontiac.
My point was to emphasize the folly of chasing runner temperature for a street engine. The point of a street engine is to get it streetable ASAP... get it off-choke, get it up to temperature for health, get the heater working, etc. The healthiest thing for an engine is to get up to temperature as quickly as possible so that the oil is the proper viscosity, contaminants can evaporate out of the oil, bearing tolerances get to their proper expansion, and fuel consumption can return to dancing around stoich ASAP. Prior to that, you have fuel washing on the cylinder walls, adding contaminants to the oil, and richer operation. Great for a 12-second blast down a drag strip, not so much for a 15 minute morning commute.
Thousands of tests have been done on air-gap vs non-gap intakes, and the results boil down to a few extra ponies when the intake is cool (provided the carb is tuned richer) but nearly zero benefit when they are hot. Any torque differences between the two could be attributed to the fact that the plenum height is taller giving a bit more "ram" gravity effect to the harmonics inside the runners. Suffice it to say, a high-rise dual plane like an RPM without an air gap performs almost identically to an air gap intake with the same plenum height much like a Performer air-gap.
Long story short, Air gap intakes are a marketing gimmick, and many of their claimed benefits are assuming cold runners, and most of the benefits disappear as the runners get hot. Since they take longer to get hot, it means you spend more time needing a richer mixture. If you do run an air gap intake, I suggest using a hot air/coil choke as opposed to electric. An electric choke heats up at a somewhat fixed rate without regard to intake runner temps. Using a thermostatic choke will reference the actual temperature of the intake and more accurately follow what the engine needs.
The ONLY benefit to cooler engine temperatures is power production if your A/F supports it, but in an air gap intake, those benefits don't always exist. Choose an air gap for its taller height and ramming effects, but not for cooler runners. That would be entirely antithetical to the goals of street driving. We're all so focused on that last 3hp that we'll never feel that we often sacrifice drivability for phantom bragging rights of less than a handful of horses.
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