Blocking exhaust Crossover issues???
 

I see the Procomp, or whatever else it is branded as, intake for a reasonable amount but they do not have an exhaust crossover and are blocking that area off. Are there any issues blocking that crossover? Reversion issues? I know it is best to fill the head to block that area and I would likely be switching to E-heads at some point, but for short term, I am thinking about purchasing the RPM copy procomp intake. Thoughts?

They are fine so don't hesitate to get one.

I had my crossovers blocked off at one point then had them filled....

Helps keep heat away from intake

That was my thought as well, just did not want to create any issues from now till the time I switch heads.

Blocking the holes will not cause any problems. I used the Mr. Gasket intake gaskets with the thin metal block-off plates, on lots of engines. And I used homemade block-off plates, with a couple of factory alum intakes.

Never had any problems, street or race. But I have read that there is a slight performance advantage, when filling the holes. I've also heard that it changes the sound of the exhaust. Can't verify.

So whats the best method to fill the exhaust cross over,. and does it apply to most pontiac cast iron heads... :popcorn:

Molten aluminum pored from the combustion chamber side.

There is no easy way to fill the crossovers in my opinion. To fill properly with molten metal, the heads must be completely disassembled, crossover ports cleaned, preheated and poured with your choice of aluminum, zinc or mix of metals like they use at Mondello. With this method, you need to have a crucible to melt the metal and a ladle to pour it into the crossover. It needs to be done quickly and in 1 pour to prevent it from solidifying in layers that could break loose and work their way into the exhaust valve. Unlikely that it would, but not impossible.

An alternative to aluminum is a high temp resin called Resbond 940HT. This is a powder that is mixed with an activator in a ratio of 100 parts powder to 30 parts activator that can be pored into the port and allowed to solidify. This will provide a nice fill that will withstand 2800 degrees continually. Again, the heads will need to be disassembled and the crossover ports cleaned and scuffed for adhesion before poring in the mixture. This is an adhesive, so cleanliness is important. It is necessary to block off the port at the intake manifold side and pour the mixture through the exhaust valves until the cavity is full. Depending on the temperature of your work area, it will take 24-48 hours for the mix to completely set, at which time you can use sand paper to shape the ports if required. The next step is a final cure in the oven at around 400 degrees for a period of time.

I have never actually poured a crossover full with molten aluminum, but I do know the risks involved. Without proper preheat, it is possible to crack the cast iron due to the rapid heating of the head. I have also seen where the pour solidified in layers, causing small pieces to become loose in the ports. Many people have done and continue to do this with success, but without the right equipment, this can be risky and dangerous. I have used the Resbond to fill the crossover cavities, and found it to be a very easy alternative, but expensive at around $120.

Here is a very good step by step thread from the Olds forum: http://classicoldsmobile.com/forums/...ond-940ht.html

Blocking the exhaust crossover or filling in the exhaust crossover passage in the heads is NOT a good idea for engines that are driven in cool/cold weather.

None of the factory divorced or hot air style chokes will work with that deal either.

If your car is casually driven in warm/hot weather only, then not having a working crossover is OK, but it will still increase the time required to heat the intake up enough so fuel doesn't "glob" up on the runners and come out of suspension.

A working choke is a nice feature on most street driven vehicles, but certainly not a deal breaker if you don't mid "patting" the throttle and waiting a few minutes for things to heat up so the engine will idle without some additional help from the operator.

For "max-effort" set-ups blocked or not having crossovers at all, or a choke of any kind aren't really a big deal. Very few folks call on those vehicles to take you down for groceries in the cold Winter months up North when a working choke and open crossovers become necessary.

My own engine hasn't had open crossovers for quite a few years, since going to the KRE heads, but I did put an electric choke on the carb after not having one for a few years. It's one of the best upgrades I've done in the past 10 years, fires up instantly in any weather, fast idle for a minute or so till it warms up some, and didn't slow it down at all at the track......Cliff

Thanks for the reply's fella's,..

Where can I get some Resbond,.. and could this stuff also be used for some fill in port work on heads and intakes...

My 64 GTO is just a warm climate car here in Michigan,.. I'll probably block the crossover with a small piece of metal,..

Good info...

Since this question was asked in the "street" forum, will chime in.

If I had a nickel for every telephone call we have had in the last 25 years from individuals that have blocked the crossover on street engines, and had cold driveability issues; I could buy Florida and retire!!!

25 years ago, I listened to the same stuff about blocking the crossover and did on my own Pontiac. Ended up fitting a manual choke carburetor to be able to drive the car in town for the first 30 minutes after start-up. And yes, I have a 4-speed; would not like to even think about doing it to an automatic. After warm-up, it runs great!

Jon.

I purchased it direct from Cotronics. Their tech line is helpful and they are familiar with this application.

http://www.cotronics.com/vo/cotr/ca_fastset.htm

Pay attention to the final curing part of that Olds page. Bake out moisture for final shrinkage & hardening. Heads can be heated in a gas grill, or the kitchen oven if your significant other is game.

Jon, I'm with you on this as we are in the "Street" section, and most folks don't realize that heat is actually your friend when it comes to these things, and that the intake is going to heat up anyhow, you are just prolonging the process, and using more fuel the entire time.

I get the SAME calls here, from folks who tried the "trick of the week" and blocked the cross-overs, then wonder why the choke no longer works, and why the engine runs poorly until it reaches full operating temperature and heat soaks a bit......Cliff

Since the exhaust gas that circulates under the manifold is a lot hotter than coolant, wont the manifold get hotter at full operating temperature then the rest of the motor?

I know we have some top tier Carb guys on this thread, but would it make a difference if you are going with a TB EFI setup?

crossover
 

I have two factory intakes. One is bone stock, just cleaned up ports up to match gaskets.
And the second I removed the crossover completely and made blockoff plates.
Well lets just say I preferred the stock one with crossover intact. My Firebird
has good manners cold and HOT with it. Heck the exhaust note changed and
even my kids noticed the difference. The goal was to keep carb as cool as possible
and having to wait 1/2 hour on a 70 degree morning to get smooth idle was a deal breaker.
But the modified intake does make great garage art.

Gerry

Happily driven with blocked crossovers. Resbond-filled is easier overall. Aluminum pour into the intake gasket side worked well for me. I use 0000 steel wool stuffed into the bowls to stop the alumin flow to result in a nice as-cast bowl shape. Easy easy.

Electric choke ought to solve all the Cold warm-up concerns. Yet I use the thermostatic spring with carefull adjust for choke action. Or just been leaving the chokeplate off & deal with a stubborn cold start.

Blocked crossover Benefit is reduced running temps on the highway. Probably reduced ping too. Blocked crossovers are indicated for the hobbiest that affords time and effort to tune the combo, or endure a rough spell during warm-up.

OTHERWISE keep with the other's advice to keep the crossovers unblocked for that high quality cold-start and run-ability.

Heat is your friend with a "wet-flow" intake system. Cool/cold engine parts in the intake tract cause fuel to come out of suspension and require richer mixtures (a working choke) until the parts heat up.

The only reason folks are able to get away with not having a working choke on these engines is how they are used. Most NEVER set out in below freezing or sub-zero weather. It is difficult, if not near IMPOSSIBLE to start one of these engines and have it continue to run till warmed up in really cold weather without a working choke.

You have a fighting chance at that deal if you are using an aftermarket gas-sucking pig carburetor that's putting down way to much fuel right to start with, but with a well tuned carburetor you will find your engine stalling out, and requiring a half dozen or so shots from the accl pump to restart it, then it will stall again and again, running a few more seconds each time until you get enough heat in it so it stays running. (Sound familiar).

The factory really did know what they were doing running exhaust gas thru the bottom of the intake to heat it up quickly. This greatly reduces engine warm-up time, and allows for leaner mixtures to be used for "normal" operation.

The last couple of time this topic has shown up we see opinions from one end of the spectrum to the other. My opinion is that you need to leave the cross-overs open if you have a divorced or hot air style choke system. You need to leave them open if you plan on using the vehicle to fetch your groceries year round AND live where it falls and stays well below freezing for weeks/months at a time.

If you are just pissing around on nice weekends with one of these cars, and almost NEVER drive it in cool/cold weather, it simply becomes a matter of how much you can deal with when it relates to cold start-ups. I personally ran my own engine for a few years without a working choke, and it was fine. I was younger and could deal with having to sit there and do 2-3 pumping the hell out of it restarts, and "patting" the gas to keep it running until it would idle on it's own.

The BEST thing I ever did for my engine was to install an electric choke on the carburetor. Now I have INSTANT starts in any weather, and fast idle for warm-up. I even tested that deal last Winter at -26 degrees outside and it roared to life and warmed up just fine. That same day my Nissan Altima BARELY started, and my 2000 Tahoe refused to start!!!

One thing I will not do here is argue about this topic. All sorts of good theory, and opinions, etc, but it really boils down to personal preference and where you live and how you use your vehicle. IF you are in the dessert in the SW, and it's always hot, you may not need a choke or an open cross-over, and could actually run into scenarios where the added heat cooks the fuel in the carb on hot shut-downs, not sure about that but common sense tells me this could be a potential issue.

I say all that because the factory did not only run exhaust thru the bottom of the intake, they also had an additional heat channel under the carburetor in 1967 (Pontiac's with q-jets) that put heat DIRECTLY on the steel shim under the carburetor. That deal is certainly over-kill, and at the time was probably a good idea to run even leaner mixtures and heat up the fuel some, but in actual use we get enough complaints about those intakes that we recommend to customers to block off the supply holes to the channel under the carb if/when you are having issues related to cooking the fuel out of your carburetor......Cliff

"I know we have some top tier Carb guys on this thread, but would it make a difference if you are going with a TB EFI setup?"

Those are just electronic carburetors, same rules apply. I'm a NOT a fan of any of them to date. I really do NOT like the fact that they dry up the intake in certain conditions. Never all that fond of the factory TBI set-ups either, and have a few great stories to tell about how we've out ran them every place with q-jets and tuned HEI distributors, but that's another topic.

It seems like the aftermarket TBI's are are getting better, but I've cause the demise of so many of them, I'm surprised a few of the companies making them haven't put our a "contract" on me!......Cliff

CLIFF,
Electric choke indeed.
hot manifolf floor for vapor release sounds good. But consider;

I want to propose that even the ice-cold start event though warm-up should show the fuel never touching the manifold floor at fast-idle: pethaps cold-start fuel goes from throttleplate cracks to runner having the demand.
Highway cruise ought to show the same dryness.
Half-throttle through WOT is a sure situation for fuel slamming tne manifold floor.

And surely the valve bowl & long-turn Cyl wall wett with fuel at all conditions, with severity depending on porting efforts aimed at reaching "wet flow" goals.

Any good?

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

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.

Well stated. Agreed.

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?

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.

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.

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

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

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