Is Blocking a Cast Iron Intake Heat Crossover worth any H.P.?

[rapidride2]

Is Blocking a Cast Iron Intake Heat Crossover worth any H.P.? 70% street 30% quarter mile trips.

Mild horsepower sleeper 305 (280-ish hp,) deal but I want what i can get if it means getting more by blocking off the heat crossover. If its not worth anything I won't bother.

Car has an electric choke quadra-jet and will be driven primarily in spring, summer and fall months in Southern Illinois.

[statsystems]

I block them. Just don't see the need for all that heat unless you live in Alaska. As long as you mean block it with the gasket. I've never found any benefit by filling the crossover.

[rapidride2]

ya if i do it i was planning on using the metal block off plates.

[pamotorman]

ya if i do it i was planning on using the metal block off plates.

i used .005 thick stainless steel shim stock. all you got to do is drain the rad and loosen the manifold bolts and slip in the shim stock. bend a 90 degree 1/4" lip on the top end and to can remove it if you don't like it

[peejay]

I block them. Just don't see the need for all that heat unless you live in Alaska.

Fuel economy. You need a lot less accelerator pump with a hot intake manifold.

I see fuel economy as important in a pleasure car. If I can get 25mpg instead of 20mpg, that means I can drive 25% more on the same amount of fuel. That means I get to enjoy the car 25% more. 25% more driving pleasure is a very good thing.

[maxracesoftware]

Is Blocking a Cast Iron Intake Heat Crossover worth any H.P.? 70% street 30% quarter mile trips.

Mild horsepower sleeper 305 (280-ish hp,) deal but I want what i can get if it means getting more by blocking off the heat crossover. If its not worth anything I won't bother.

Car has an electric choke quadra-jet and will be driven primarily in spring, summer and fall months in Southern Illinois.

not exactly for your 305cid ( thats if this is a SBC 350 w/#416 castings ... disregard this Info if otherwise )
but i had a 305cid Chevy #416 SBC SuperStock Heads that have the dual-port center heat riser passages
that LOST = 40 Peak HP when i forgot or did not realize those dual Port Heat Riser passages
are not actually blocked-off even with a Single-plane Race Edelbrock aluminum Victor E Manifold
the exhaust gases can still cross-over during Overlap Period --> all 4 middle Cylinder's EGT's were very much colder temps than the outers )
that Engine had 271/280 Cam At .050"
so it had plenty enough Overlap Period to act as an open EGR Valve and kill 40 HP on a 514 HP 305 SuperStock engine

i had to pour in HardBlock grout to fill in the Heat Riser's dual passages

as Cam's Overlap Period gets less and less ... the less HP you can lose to Heat Riser dual-passage acting like EGR Valve
in your application is probably not worth much HP to block it ??

[rapidride2]

It's a 305 with the 416 h.o. heads.

[RDY4WAR]

I block them. Just don't see the need for all that heat unless you live in Alaska.

Fuel economy. You need a lot less accelerator pump with a hot intake manifold.

I see fuel economy as important in a pleasure car. If I can get 25mpg instead of 20mpg, that means I can drive 25% more on the same amount of fuel. That means I get to enjoy the car 25% more. 25% more driving pleasure is a very good thing.

Is this simply a matter of a hot plenum floor keeping the fuel suspended in the air or is there more to it?

[statsystems]

I block them. Just don't see the need for all that heat unless you live in Alaska.

Fuel economy. You need a lot less accelerator pump with a hot intake manifold.

I see fuel economy as important in a pleasure car. If I can get 25mpg instead of 20mpg, that means I can drive 25% more on the same amount of fuel. That means I get to enjoy the car 25% more. 25% more driving pleasure is a very good thing.

If he is looking for HP I doubt losing .05 MPG is an issue. Relying on intake heat to atomize fuel is a crutch IMO.

[F-BIRD'88]

Without heat absorption there is no fuel vapourization. Liquid fuel will not burn.
Blocking the intake heat requires rejetting.
Keep the heat risers open on a cast iron intake street car.
( A aluminum intake can used reduced heat riser flow as it warms faster.)
As Meaux said there is good benefit by blocking one or both of the twinned (4416 heads)
center exhaust ports EGR passages to stop cross talk of the center twin ex ports.
This requires intake removal (and maybe head removal too). Then stuffing the center exhaust port EGR passages with heavy duty (thick) aluminum foil (cooking catering pans) may be easier than Hardblocking them. (on a radical racy motor).
There is big power to be had by porting those heads if you care to.
Then blocking heat risers may be worth it.
If its just a driver don't bother.

[maxracesoftware]

not exactly for your 305cid ( thats if this is a SBC 350 w/#416 castings ... disregard this Info if otherwise )

Edit : i just saw i typed SBC 350 w/#416 castings
i meant SBC 305 w/#416 castings

happy to see F-BIRD'88 has seen same sort of thing

i did not charge that Customer any money for that Dyno that Day for my stupid mistake on the Heat-Risers

they came back a few Days later and we made 514 Peak HP w/HardBlocked Heat Risers
Peak HP pretty flat = 7300 to 7500 RPM most all Tests

F-BIRD'88 = there is big power to be had by porting those heads if you care to.

they respond a lot to Porting , especially if you have no Rules or CC's to adhere to

[Geoff2]

There is a reason race intake manifolds do not use the exh xover....

[pamotorman]

There is a reason race intake manifolds do not use the exh xover....

the exh cross over port in the heads can cause overheating in race engines. a friend was vintage racing a 67 FI corvette and was having over heating problems i told him to fill the ex cross over ports in the heads and the problem went away

[rapidride2]

This is a fresh rebuild. Longblock is assembled. Looks like I may stick with the open heat riser. I was thinking about doing it if were somewhere north of 5hp.

I also have restrictor plates as well with the smaller hole.

This is in a bone stock 2-tone 78 malibu. Engine looks as stock.as it did when new. Except now a 4 barrel, 9:1 compression, home ported 416 h.o. heads and a mild Jones hydraulic.

[PackardV8]

I block them. Just don't see the need for all that heat unless you live in Alaska.

Fuel economy. You need a lot less accelerator pump with a hot intake manifold.

I see fuel economy as important in a pleasure car. If I can get 25mpg instead of 20mpg, that means I can drive 25% more on the same amount of fuel. That means I get to enjoy the car 25% more. 25% more driving pleasure is a very good thing.

Your opinion, but does anyone else think a blocked exhaust crossover would cost 25% penalty in fuel economy? FWIW, I blocked the exhaust crossover on a build with which I'm quite familiar. There was very little difference in overall fuel consumption.

One major difference not yet touched on is sensitivity to pinging on the hottest days. As with the OPs build, this is iron head, iron intake. On 100+ days, it would ping. With the exhaust crossover blocked, pinging was reduced. Adding a cold air intake, the pinging was gone.

A secondary benefit is reduced fuel percolation in hot soak conditions. Some feel today's gasahol boils at a lower temp. Keeping the intake cooler reduces hot cranking times in some applications.