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Building a street 455. The Mondello crossover plates look MUCH easier to install than the Edelbrock plugs. Which begs the question...do they work? Why on earth would anyone install the plugs when those plates are so easy to install. Is blocking the crossover worth the trouble on a built street engine running electric choke? Am I wasting my time worrying about it?
So I guess, since I have already done my head work, the best option is to leave it open? I can't really imagine the gain would be worth that much work for a street engine. Am I correct in that assumption?
Is blocking the crossover worth the trouble on a built street engine running electric choke?
Edelbrock recommends that the plugs be put in place so that the intense exhaust heat does not warp the aluminum manifold. If you live in a cold-weather state, they say you can drill a small hole in each plug to get some manifold heat.
Even if you don't have an aluminum intake, understand that heat under the carb lowers the power output of the engine (hot air expands, so less mass of air enters the cylinders to make power).
When driving for longer time, wouldnt heat soak get the whole engine package to pretty uniform temperature anyways?
If you go and search some reeally old topics, there are things which make you wonder, like guys driving plenty fast way back then, who state that they didnt see the gain from filling the crossover versus just plugging them.
Personally, for street, me nor the experienced head-guy didnt see its worth the effort. Of course im "hampering" my motor already with dual-quad so..
When most of us have upgraded to dual-exhaust, and usually some sort of headers, how much do it really even crossflow between heads when you have some rpms at engine, when theres no choke-point on either side of exhaust possibly promoting some crossflow?
Last edited by Inline; January 4th, 2019 at 11:48 AM.
Edelbrock recommends that the plugs be put in place so that the intense exhaust heat does not warp the aluminum manifold.
Does that mean that E-brock intakes are so inferior to factory aluminum? Aluminum W-30, W-31, 260, 307, and Seville 350 EFI intakes did not require plugs and did not warp. The reality is that unless you are running the car at the track regularly and are adjusting jetting to take advantage of the reduced inlet temp, you will never see a difference (though I'm sure the butt dyno will register an improvement after doing all that work). If you really want to plug the crossover, cast aluminum into the exhaust ports in the heads and properly port the exhaust ports. THAT will provide a real improvement.
if you really want to plug the crossover, cast aluminum into the exhaust ports in the heads and properly port the exhaust ports. That will provide a real improvement.
x2. And it’ll sound different as well.
Last edited by cutlassefi; January 4th, 2019 at 06:39 PM.
Interesting. Is it correct to say that blocking the crossover also inhibits the discoloration especially on freshly painted cast iron intakes?
What are you guys thoughts on filling the crossovers on the unique heads such as D, F and H? Would that decrease the value of the heads from a purist standpoint?
Blocking will stop the related paint issues, will also slow choke response on divorced choke but most of these cars are not used in cold weather. Would not go thru the trouble of blocking heat passage in head, remember the KISS principle.
Don't modify rare heads by filling the crossover. Their value could be ruined.
It's too easy to block the exhaust crossover with a plate. You can even use an aluminum beer/soda can cut to fit the intake manifold gasket at the crossover. I did this on my 71 98 and it was worth at least 10-20 hp on the butt dyno.
I blocked my crossovers when I went to a 4 barrel set up on my Cutlass. It was a hassle driving during cool weather. When a leak was sprung after 4 or so years I put the crossovers back into service. Quicker warm up during late fall and early spring. My plates were also beginning to burn through when I took the manifold off. Now I don't need to worry about it.
I have used 2 part steel putty to block off the crossover port. Plan on 2-3 tubes to completely plug the holes. Even though the package says it’s good to 500 degrees, after many years the putty is still there.
How handy are you? When I put the intake on my 455 I wanted to block off the crossover with thin stainless. So I went to dollar tree and bought 2 stainless steel bowls for a $1 each and cut the bottom out of them to make my block off plates. Seems to work great used a little exhaust sealer to hold them in place while installing the intake. Haven't used the car enough to know how it will last. I do know 2 bucks beat spending $20.
Thanks for the responses. I've decided that the gain from plugging the crossover effectively isn't worth the effort. I'm not risking rattles and leaks for next to no gain.
Use the block off plates unless it is a daily driver then you might want to re-think blocking the cross over. Fill with aluminum only if the heads are un-machined and you are looking for every last bit of performance out of iron heads.
We ran plates and they helped keep the intake cool always a good idea in my opinion.
Have used pieces of stainless steel fitted into intake gasket to block crossover.
I believe this is written in the factory guide recommending the use of “ shim” stock which is thin metal plate . My machinist welded the crossover shut on the manifold with two aluminum plates then machined the mating surfaces , he said for my application it was probably overkill and I wouldn’t notice much gain, but it’s in his recipe for Oldsmobile engines and since I added a bunch of might as well upgrades he just went ahead with the full recipe lol
Originally Posted by joe_padavano
if you really want to plug the crossover, cast aluminum into the exhaust ports in the heads and properly port the exhaust ports. That will provide a real improvement. I
X3 on this. Dick Miller told me that filling, porting and dividing the exhaust port was good for about 50 horses with my E heads. Otherwise, those center ports are constantly fighting one another.
If you really want to plug the crossover, cast aluminum into the exhaust ports in the heads and properly port the exhaust ports. THAT will provide a real improvement.[/QUOTE]
I've considered doing this but am curious to know if there are risks of breaking the head as aluminum expands at a rate 2.5 times that of cast iron. Does anyone have experience with this to confirm its effectiveness?
...are (there) risks of breaking the head as aluminum expands at a rate 2.5 times that of cast iron. Does anyone have experience with this to confirm its effectiveness?
I had aluminum poured into my heads in the 1980s. I prepared the head with casting sand so porting would be easier. After porting, I drilled a 1/2" hole through to give some heat to the intake (I drove this car during Minnesota winters).
This was on my daily driver for 10 years without problems.
Understand the expansion question this way...
As you say, aluminum expands in the range of 2 to 2.5X as much as cast iron. But it conducts heat 3 to 4X better. So, although the aluminum gets heated by exhaust gasses, the ENTIRE aluminum plug conducts this heat to the cooled cast iron that surrounds it. This limits the aluminum's temperature rise and therefore mitigates any potential expansion problem.
If you really want to plug the crossover, cast aluminum into the exhaust ports in the heads and properly port the exhaust ports. THAT will provide a real improvement.
I've considered doing this but am curious to know if there are risks of breaking the head as aluminum expands at a rate 2.5 times that of cast iron. Does anyone have experience with this to confirm its effectiveness?[/QUOTE]
Aluminum has a higher CTE, but it also has a much LOWER stiffness. The aluminum may expand, but that compliance in the aluminum will mitigate any splitting loads in the cast iron. Basically you have two springs in series, a soft aluminum one and a stiff iron one. The soft spring will compress more.
I welded the cast plugs into the intake with my MIG/C25 gas.
I cut the cast plug down on the smaller end so it recessed into the port. Heated the intake and the plug and welded away.
The plug was recessed enough to weld a nice bead and did not interfere with the sealing surface.
Been on the car for 20 years, no burnt paint. No boiling carb.
The divorced choke was tweaked to work. It's cold-blooded.
Definite improvement especially hot starts which is what drove me to do this and a few other mods.
Don't forget to remove the heat stove flap & spring in the driver's side ex mani then weld the shaft through holes.
I have used 2 part steel putty to block off the crossover port. Plan on 2-3 tubes to completely plug the holes. Even though the package says it’s good to 500 degrees, after many years the putty is still there.
Been there and done that. It works really well. No rattle
"Fill with aluminum only if the heads are un-machined and you are looking for every last bit of performance out of iron heads."
Has anyone in here tried this? Interested to know if you heated the head prior to introducing the aluminum. If so, how hot? Obviously milling the head would be necessary after completion. I'm considering trying this simply because I want to reshape the ports on 4A heads.
(Did you heat) the head prior to introducing the aluminum? If so, how hot? ...I'm considering trying this simply because I want to reshape the ports on 4A heads.
The foundry I went to heated the heads in a kiln to 800°F before pouring the aluminum.
Refer to post 22 for information on the reshaping.
I tried the shims; they burn through! When the heads were off for touching up the valve job, I cleaned them and bead blasted the center exhaust ports then filled with water mix powdered furnace cement.
Location: Colorado Springs Colorado/Thousand Oaks Ca
Posts: 1,719
Originally Posted by TAPilot
"Fill with aluminum only if the heads are un-machined and you are looking for every last bit of performance out of iron heads."
Has anyone in here tried this? Interested to know if you heated the head prior to introducing the aluminum. If so, how hot? Obviously milling the head would be necessary after completion. I'm considering trying this simply because I want to reshape the ports on 4A heads.
Use a small furnace to melt the aluminum, works great, I do not heat the head prior to filling. I use a block off plate with a .016 hole on the intake face and fill it from the bowl side. It does a prefect job.
Last edited by VORTECPRO; September 27th, 2022 at 05:11 AM.
January 4th, 2019, 08:35 AM
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