[Cliff R]

I certainly don't know all the "physics" involved here, but do not what happens on the user end with these things.

For about 90 percent of the folks who get involved with blocking crossovers it's a complete waste of time/funds, and will only prolong intake warm-up times and hurt engine efficiency.

Keep in mind the basics of the internal combustion engine N/A. Heat, fuel, and spark.

Heat is created by compressing a very lean mixture then adding a spark at the correct moment to achieve combustion. To do this we compress the volume of air/fuel above the piston past the intake valve closing point. The piston compresses the mixture, heats it up, and raises the LEL so we can ignite it.

In a wet flow system we rely on all the items involved with the intake system to effectively mix the fuel added to the air before it starts getting compressed. Things done to cool that mixture certainly don't help the process, and cold engine part take some of that fuel out of suspension and increase fuel demand to achieve the LEL.

Now, we're torn between wanting to induce a cool dense air charge which helps pack more air into the cylinders vs needing to raise the temperature of that charge high enough so that it needs the minimum amount of fuel added to it for maximum efficiency.

So the theory of having a cold intake to help things out pushes folks to block off the crossovers and take other steps to reduce the temperature of the incoming air charge.

The problem here is that you just can't get around the basic laws of physics with these things. Taking the heat away from the intake is a superficial benefit, and it disappears after heat soak and hot under hood temps go ahead and heat up all the parts of the intake tract anyhow.

Now the only point to argue or consider is how much lower the actual intake temperature is without the added heat from the exhaust, and if that lower temperature is beneficial at all to a street driven vehicle?

What I do here is not to worry about it, and we do NOT block off crossovers, nor do we eliminate the vacuum advance, nor do we run rich mixtures from the carburetor to compensate. I have found with my own vehicle, even when I was racing it couple of times a week, that it was best to run the engine FULLY heat soaked and up to FULL operating temperature, vs trying to run it cooler, or cool down any of the engine parts involved between rounds, etc.

Yep, I'm the guy you come up against in final rounds that can return immediately to staging when they start "hot-lapping" the field, and leave the engine idling in the staging lanes until we make the next run. 99.9 percent of the time my next opponent is frantically running around at his pit site trying to cool down the engine, running the cooling fans in staging, and having all his buddies push the car in the lanes until he's called to race it. I've even had folks beside me ask for more time, pack ice on the intake, hit it lightly with water from a bug sprayer, etc, all in vain, as that deal just has you all over the map when it comes to trying to reproduce the same ET/MPH from the last run...IMHO.

The cooling things down deal for the most part works fine early on, but in later rounds it becomes difficult, if not near impossible to get the engine back down to the same temperature that you achieved in earlier rounds, so you are going to be hard pressed to run your number.

So I'm giving up some secrets here, but at the same time showing how heat can be your friend with these things, on the street or at the track.......Cliff

[carbking]

Bottom line is whether one blocks the crossover or leaves it open is entirely up to the individual.

Personal opinion is that FOR STREET ONLY DRIVEN VEHICLES, the only benefit is the paint won't burn off the crossover! Trailered race vehicles are a different story.

Cliff said it very well (over a couple of different posts) in that based on ones' tolerence of cold driveability issues and how rich one is willing to tune the carb, and when the vehicle is driven; one can get away with blocking the crossover.

The solution on my own GTO on which I blocked the crossover was a manual choke carburetor. Even in 75 degree weather, the engine likes some choke for up to 30 minutes driving around town. (Or, I can push the choke off, and restart the engine at each stop light!). Too much HP/CI on a street engine and tuning is responsible. I chose NOT to just throw too much fuel at the engine! On the highway, I get fuel economy most of you never even dream of with no detonation on pump gasoline (93 octane).

And I would NEVER again block the crossover on my own street vehicle.

But the choice is left to each individual.

Jon.

[Half-Inch Stud]

Well stated. Agreed.

[Blued and Painted]

Being the crossover starts right behind the exhaust valve, then anything off idle will be throwing Fireballs back and forth from 4 cylinders directly under the carb.

Could it be restricted?

Could a 3/8th tube be installed from one exhaust port on each head when filling the head passage to supply restricted heat?

Are any here pouring the head passage then plumbing coolant through the intake?

[Bruce Meyer]

Originally Posted by carbking

Bottom line is whether one blocks the crossover or leaves it open is entirely up to the individual.

Personal opinion is that FOR STREET ONLY DRIVEN VEHICLES, the only benefit is the paint won't burn off the crossover! Trailered race vehicles are a different story.

Cliff said it very well (over a couple of different posts) in that based on ones' tolerence of cold driveability issues and how rich one is willing to tune the carb, and when the vehicle is driven; one can get away with blocking the crossover.

The solution on my own GTO on which I blocked the crossover was a manual choke carburetor. Even in 75 degree weather, the engine likes some choke for up to 30 minutes driving around town. (Or, I can push the choke off, and restart the engine at each stop light!). Too much HP/CI on a street engine and tuning is responsible. I chose NOT to just throw too much fuel at the engine! On the highway, I get fuel economy most of you never even dream of with no detonation on pump gasoline (93 octane).

And I would NEVER again block the crossover on my own street vehicle.

But the choice is left to each individual.

Jon.

If you lived in Phoenix you might think differently. My one GTO is all stock with a qjet and open crossovers. When I shut the car off in the summer the fuel boils in the carb and stinks up the garage. Also it takes a while to start it the next time because the fuel had boiled away. My other GTO has the crossovers blocked and a Holley 4150. No stink and starts easy every time. It also has no choke. Takes slightly longer to warm up.

[Overkillphil]

On one of my Pontiacs that actually gets driven occasionally in colder weather, I opened up one of the block off plates (pass side) and left the drivers side intact. Seems to have been a good compromise and one of those experiments that worked.

[captain42082]

how about using the block offs that came in the intake gasket set? anyone use them?

[Overkillphil]

That's what I was referring to in my post. Have used them for decades in various Pontiacs.

[TedRamAirII]

i run a Q-Jet with open cross over. Lately, if I park for 5-15 minutes after driving a while, my car wont start. It turns over fine but wont run. Seems like the gas is boiled out? I have not had the chance to verify if the carb is out of gas. I should get the air filter lid off and see if the squirters are doing anything. E-10 I assume? I see these 1/4 Spacer plates that insulate the carb, are they any good? anyone try it? http://www.coolcarb.com/order-products.html