Does blocking the exhaust cross over in a 455 cause a noticeable drop in gas mileage? Thanks.

 

If you are asking if adding or deleting an H-pipe in a dual exhaust system affects fuel use, then, it's not noticeable.

 

I don't understand the question. Is this the crossover for a single exhaust or an H pipe?

 

No. I blocked my exhaust cross over (in the intake manifold) to avoid boiling the fuel in the car - Which it did. I read that the atomization from the heat helps improve gas mileage and I was hoping to quantify it. Thanks.

 

The extra heat vaporizes more fuel, which improves distribution, and therefore decreases fuel consumption. You may see a few percent decrease in consumption, but not significant.

A bigger effect will be a power increase because you will have a denser (cooler) mixture entering the cylinders.

If you live in a cold-weather state, your car will not like the absence of heat and it will not run as well in cold conditions. In that case, drill a small hole in your block-off to allow a bit of heat in. I used 3/8" holes in the aluminum that was poured into the head's exhaust crossover passages.

 

The reality is that you won't see a measurable change in either mileage or power. Blocking the crossover is for applications where you are trying to squeeze the last drop of performance out of a combo (ie, NHRA Stock class racing). You need to change the jetting in the carb to take advantage of the denser air intake, or else it's meaningless. At best there's a tenth reduction in the quarter.

 

I was on another site that Cliff Ruggles post on. He was very much against blocking the crossover. He contends that it actually hurts performance, his words not mine.

 

Thank you.

 

It could cause a significant drop in MPG if there isn't enough heat to the choke thermostat to open the choke blade.

 

He also says that on Qjets, odd = manual trans, even = automatic... I know a lot of H/O and W30 owners who would take exception with that.

In any case, you can't make a sweeping statement like that. Cooler intake charge clearly improves performance, ASSUMING that the rest of the engine is tuned to take advantage of it. If you simply block the crossover and do nothing else (which I suspect is most often the case) then yeah, the carb probably runs lean. If you take the time to properly calibrate the carb for the blocked crossover, you will improve performance. Whether or not that improvement is measurable under the conditions that the car is used is a different question. On a car that only goes to cruise night and back, it's a complete waste of time.

 

I can accept that interpretation.

The exhaust heat changes the temperature of the air on its way between the carburetor and cylinders. The density of the air going through the carburetor does not change. Therefore the metered air-fuel ratio should not change.

Changing the jetting may be required to paper over a carb's possibly inconsistent fuel metering at higher air flow. That would be a problem with a specific carb, not a fundamental, universal problem.

 

That's a fair point, but I'd argue that the carb itself is heated by the crossover, and thus the air through it has lower density (as does the fuel in the float bowl).
Power depends on mass of air and mass of fuel. Both are reduced when the crossover is hot. Again, is that change measurable? Likely not in normal driving.

 

The 2 Oldsmobiles presently in my family run fine in the cold with no crossover. Red one has been daily driven since ‘09, 24/7/365 w/ 68 OAI ducts disconnected in winter, the blue car has been driven occasionally in cold weather, down to 0*F and not a stumble. Start it, put in gear and drive off.

Edit - FWIW both cars have electric chokes.
​​​​​​….

 

There are three advantages to plugging those holes:
1. Eliminating hard starts when hot.
2. Minor performance gain.
3. The intake paint will no longer burn off.

Blocking off the crossover reduces heat soak and the associated hard starting, due to fuel boil off.(assuming everything is in good service and properly adjusted).
I had that problem. It farted, backfired, puked, snorted, and grunted and just would just not restart nicely on hot days. Embarrassing especially when smoke billowed out from under the hood.
Once I did the block-off mod (welded iron plugs into the intake), the heat soak problem was gone. Now, when warm it starts like FI with a twist of the key standing outside the car.

I could feel a seat of the pants improvement, more prevalent on hot humid 95°+ days.

I had to retune the carb(minor) and dumb-out the divorced choke. It's very cold-blooded as the divorced choke has quite a delay as you can imagine. An electric conversion is a simple solution.
Additionally, I removed the stove flap and welded the holes in a set of 69 exhaust manifolds.

Did this mod 20 years ago:

 

I would think that an engine that has a higher charge temperature would have lower volumetric efficiency. But at the same time the improve atomization should offset this to some degree. Those with modern TBI systems (Sniper/FITech...), with high pressure injectors... Does the improved atomization -vs a carb- net any noticeable effects?

I never had a pinging problem in this engine, but I would believe that a cooler air/fuel mixture should -to some degree- diminish detonation. Has anyone had any experience here?

Since I plugged my cross-over, I do have a bit of a lag in the stove coil. It used to work flawlessly, and now the choke appears to open later than it should. Not a big deal, but it's noticeable.

 

Fuel injection atomizes the fuel much more efficiently than a carburetor. FI has high pressure that pushes the fuel out as opposed to a carb with a venturi that pulls the fuel out and it then has to atomize in the air stream. So I doubt blocking the exhaust crossover would result in any significant difference.

 

Actually that’s not entirely true, sorry.
The reality is that if you look down the throat of these TBI EFI units, they really don’t atomize the fuel as well as a carb can. Think about it, the flow from a carb is pretty much continual, EFI is pulsing and not delivered in a linear fashion. HOWEVER the EFI WILL compensate for temp changes, both in engine and intake air, and is able to instantaneously change fuel metering accordingly.
Hope this helps.

 

Ah, I was thinking of more modern port fuel injection and not TBI. I forget that the “new” technology we’re discussing is actually archaic.