I've been told it's best not to run an intake like an RPM Air Gap because it will cause throttle body icing in cold weather.

What I don't understand is why a throttle body will ice up on that intake, but not a carb?

I was also wondering if you could use one of those heated 454 throttle body spacers, hooking up the water hoses in the winter, and bypassing them in warmer weather.

Can anyone help me?

 

When you compress things they get hot, when you decompress things they get cold. Basic principal that air conditioning works on.

Decompressing fuel being atomized and sprayed into a cold throttle body with cold air flowing through it is the perfect environment to promote freezing of the fuel. I've never had problems with my RPM intake although it's not an air gap, the crossover is blocked off. I also run a 1/2" spacer that is made of a wood composite that does a very good job of keeping the TB very cold to the touch.

Others have had problems, I guess the problem is just having a high idle since the throttle blades won't go clear shut with the frozen fuel on them.

 

But, what I don't understand is why are there problems with this in TBI throttle bodies, but not in carbs, with the smae intake?

 

because noone on this forum drives a thirdgen in the arctic circle during winter.

 

There's always one

 

The icing is due to the fact that gasoline acts like a refrigerant in low pressure environments. In the vacuum of the manifold, the lighter components (or all of it if its warm enough) flash over to a vapor. This pulls lots of heat out of the intake charge, manifold, and throttle body. I frequently have icing in the winter and its bad enough that the casting will shrink so much that it locks the butterflies in the bore and the throttle jams shut. Even on hot days, I have alot of sweat on the TBI and upper plenum. It actually forms a puddle around the TBI if I let the car idle for awhile. This same thing also causes lots of other issues. If its bad enough, itll make tuning teh P/T fueling and accel enrich impossible. Speaking from experience, your best bet is to run a fully heated TBI manifold or similar. See my "had it with AE" thread for what happens with a standard carb manifold. In order to use an unheated manifold like the airgap, you need to switch to mass air flow and add extensive code to properly compensate for the variable manifold and intake charge temps. Otherwise, it wont work well.

 

Carbs have icing issues as well...

 

I've also noticed how AE is GREATLY effected by intake temps. I drive my car mostly in the summer so I just put up with crappy throttle response in the winter and during warm up in the summer.

Sean,

With TBI you are going from 12psi to negative pressure in one fell swoop. With carbs the fuel is at only 4 or 5 psi in the fuel line and goes to atmospheric pressure in the fuel bowls before being sucked into the intake. It can still ice up but the decompression of the gas is a much more gradual process in a carb.

 

That makes sense.

Now, the question is, couldn't you just use one of those 454 TB spacers that have water radiator water running through them, and simply bypass the water flow in the warmer months?

 

The fuel pressure doesnt have much to do with it as the gas isnt very compressable as a liquid. The temperature of the fuel and the pressure in the manifold seem to be the driving factors. The low pressure causes teh fuel to go to a vapor once it passes into the manifold.

 

It cant hurt... Itll help keep the TBI warm. Id still run a heated manifold, though. That will only be a token ammount of preheating with a carb manifold in the winter.

 

living in alaska the only time i had my tbi freeze is when it was 20 + below. had to hook the hot air pipe backup to keep it from freezeing up. when it gets above zero i take it back off.

 

Ok, I'm lost. What I'm discussing here is running an RPM Air Gap manifold for the HP benefits it offers. I've been told that, because of the air gap, the chances of throttle body icing are much greater with this intake. Since it's the air gap that is responsible for the power increases, the only way to get the benefits would be to find a way to mitigate the problems. Other than the 454 TB spacer I mentioned above, how would you heat this intake?

 

The power difference between the RPM and the RPM airgap is marginal. I think most of the power difference is because the airgap is actually a little taller than the RPM which give the runner a little straiter shot at the head.

Your worrying about 2-4 HP probably.

 

I disagree. I've personally been involved in a swap where an rpm air gap was worth 14 hp and and 22 lb/ft at the rear wheels over a performer rpm intake. A buddy of mine had his car at the body shop for over 2 months. When they went to bring it in to finally go to work on it, they found out someone had stolen the intake, carb and distributor. The body shop's insurance carrier paid for the replacements; he got the same carb (750 Speed Demon) and distributor (DUI HEI), but kicked in the difference to get the Air Gap over the Performer RPM.

Not only were the peak numbers higher, but the torque curve was flatter, as well. Drives like a completely different car. There's a reason virtually every car mag out there has been screaming about that intake since it was introduced.

 

I wouldn't recommend using the water donut from the truck apps.

It was designed to be used on a spreadbore manifold and will cause funky distribution on an RPM AirGap. Ask me how I know

I believe Holley makes a TBI to carb flange adapter that can have water circulated thru it.

You could seal off the opening under the plenum to retain some heat.

FWIW, I had icing of the throttle blades on a 50* day with an Air Gap and CAI.

 

By running a heated manifold, I mean running a different manifold with a water jacket or some other form of heating. A standard low rise carb manifold will also work ok.

The icing isnt as much of an issue as the radical changes that are needed to run an airgap through the four seasons. The gas is going to cause the manifold temp to go all over the place and it may cause the car to run poorly in certain conditions. If your running an open element and you dont have a cowl induction hood, then the hot air off the headers will help regulate the manifold temperature. If your running a CAI, then you'll have issues at low temps. From my experience, the fuel distribution is terrible when the manifold is cold and the engine runs unevenly.

The very things that make the airgap desirable for power make it undesirable on a TBI car as theres no heating and the computer has no means to compensate for the ever changing manifold temps. Itll work fine during normal use, and is a good choice for a fair weather car, but during the winter or after the engine is totally heat soaked on a hot restart, itll run funky untill its had a chance to level out.

FWIW, I do have this manifold, and Ive been using the car as my daily driver as my other car is broken down, and the engine can be coerced into running decently right off the bat with an airgap when its cold (its around 20 degrees here at the moment), but I had to pull out all the stops to make it work.

 

EXACTLY

Also in this one particular case, I can see things other than the intake change the HP/TQ of Sean's friends car. Carbs and Distributers, even with the same part number are not always equal. The float level could be a little different in one carb. The distributer could have a slightly different advance curve. etc.

 

I tried the RPM Air Gap after using an Extrudehoned Edel TBI manifold.

Gained nothing performancewise, but this was prior to doing my own tuning.

On the other hand, switching to a lowrise singleplane caused me to go way lean from 4K and up.