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Old 03-14-2008, 06:01 AM
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Intake Crossover, To be blocked or not to be blocked

Intake Crossover, To be blocked or not to be blocked, that is the question. Whether I should put in a set of gaskets like the blocked set (like Fel pro 1204) that was on the intake when I removed it or put the unblocked type (like Fel pro 1256) on instead. While I will be driving this only on warm sunny days as it has no roof, I thought that the unblocked set would still be better, as it is a stock SBC 305 High Performance crate motor and thus would allow the carburetor to be more efficient. The only modifications to the motor are an Edelbrock Dual plane manifold, a Mallory Electronic Magnetic Distributor, a Holley 600 CFM and Zoomie exhaust headers with Harley baffles. Also it has been over 35 years since I have changed an intake so other ideas are welcome. Also should they be .060 or .120 thick for stock applications? Any comments?

Last edited by 2000jack; 03-14-2008 at 06:25 AM.
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Old 03-14-2008, 07:38 AM
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Intake Crossover, To be blocked or not to be blocked

does the intake have the holes through it , most after market alumn ones don't .
and as thickness you asked , is this for the block off plates ?
and if it's been a long time like you stated , punch dimples in block surface and on intake where front and back meet in the middle and just use a 1/4 inch trail of good red heat type silicone after cleaning off bought surfaces with alcohol .
squeez out 1/4 bead across front and back and lower intake straight to all , put in your bolts and loose tight them and let sit for 24 hours .
then torque to specs , but i just hand feel mine tight .
don't forget to clean the 4 inner bolts that usely leak in center .
i clean bolt threads and spray brake cleaner down holes to get all oil gone and use silicone on them .
this will stop oil leaks in that common problem area .
carb cleaners have trans oil in them now , but brake cleaner hasn't changed .
it won't hurt your motor and it evaporates clean .
after i do the intake tight is when i remove the center bolts and then do this procedure .
good luck .
oh yea , i use a shop vac when cleaning off heads where gaskets where .
minimizes crap getting in your block down below .
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Old 03-14-2008, 07:57 AM
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does the intake have the holes through it , most after market alumn ones don't . Yes

and as thickness you asked , is this for the block off plates ? This part I am not sure what you are asking

I was going to buy new bolts as the old ones are not chrome
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Old 03-14-2008, 12:07 PM
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Quote:
Originally Posted by 2000jack
Intake Crossover, To be blocked or not to be blocked, that is the question. Whether I should put in a set of gaskets like the blocked set (like Fel pro 1204) that was on the intake when I removed it or put the unblocked type (like Fel pro 1256) on instead. While I will be driving this only on warm sunny days as it has no roof, I thought that the unblocked set would still be better, as it is a stock SBC 305 High Performance crate motor and thus would allow the carburetor to be more efficient. The only modifications to the motor are an Edelbrock Dual plane manifold, a Mallory Electronic Magnetic Distributor, a Holley 600 CFM and Zoomie exhaust headers with Harley baffles. Also it has been over 35 years since I have changed an intake so other ideas are welcome. Also should they be .060 or .120 thick for stock applications? Any comments?
For what you want to do blocked will work OK though the general reason to block them is to cool the incoming charge for more power, hardly sounds like one of your goals.

Blocking will cost some mileage as equal mixture distribution becomes more iffy which is made up for by throwing enough fuel into the motor to insure the leanest cylinder, usually #7 on a Chevy, is rich enough not to missfire or run too hot. The heated crossover while theoretically costing a little power does a lot to balance out the engine which makes power in reality. A race car is a different situation regarding this for a lot of reasons having to do with high speed intake dynamics which a street cruiser doesn't or seldom sees.

Bogie
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Old 03-14-2008, 02:18 PM
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Quote:
Originally Posted by oldbogie
For what you want to do blocked will work OK though the general reason to block them is to cool the incoming charge for more power, hardly sounds like one of your goals.

Blocking will cost some mileage as equal mixture distribution becomes more iffy which is made up for by throwing enough fuel into the motor to insure the leanest cylinder, usually #7 on a Chevy, is rich enough not to missfire or run too hot. The heated crossover while theoretically costing a little power does a lot to balance out the engine which makes power in reality. A race car is a different situation regarding this for a lot of reasons having to do with high speed intake dynamics which a street cruiser doesn't or seldom sees.

Bogie
The car only weighs 2000 pounds or 2250 when I am in it. So the 200 or so hp this gives out is fine. I think I would rather have the motor run as it came from the factory than to get a few more horses. "Thanks" for you and burnt olds
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Old 03-15-2008, 06:39 AM
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I wouldn't block them, for years I have, thinking I was getting free horsepower but my engines have always been cold blooded and tempermental until there good and warm even on nice warm days. But on the other hand I don't run an electric choke either.
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Old 03-15-2008, 07:41 AM
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Quote:
Originally Posted by 6426yy
I wouldn't block them, for years I have, thinking I was getting free horsepower but my engines have always been cold blooded and tempermental until there good and warm even on nice warm days. But on the other hand I don't run an electric choke either.
That was where I was going- unblocked. I have an electric choke on this too. I think I will have enough bugs to work out when the car gets on the road, so I want to get the motor to run as smoothly as I can. If I want more horsepower I will probably go the 350/383 route later on. I would throw a photo on, but I can never get the http://hotrodders.com/forum/images/e...nsertimage.gif thing to work
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Old 03-15-2008, 12:27 PM
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The factory heat riser passages were designed to aid quick carb/plenum warm up on a cast iron intake manifold. I find with the Edelbrock performer manifold in particular that it gets a bit too warm if both exhaust passages are fully open.
I've found thru trial and error that what works best on this manifold is one side blocked off and one side restricted to allow a moderate amount of exhaust heat to reach the manifold plenum. About a 1/2" hole on one side and the other side completely blocked off is just about right for non northern winter driving.
This manifold is cast with pretty thin aluminum and heats up plently quick, requiring less than full exhaust riser flow under the carb plenum.
try it.
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Old 03-15-2008, 05:47 PM
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Quote:
Originally Posted by F-BIRD'88
The factory heat riser passages were designed to aid quick carb/plenum warm up on a cast iron intake manifold. I find with the Edelbrock performer manifold in particular that it gets a bit too warm if both exhaust passages are fully open.
I've found thru trial and error that what works best on this manifold is one side blocked off and one side restricted to allow a moderate amount of exhaust heat to reach the manifold plenum. About a 1/2" hole on one side and the other side completely blocked off is just about right for non northern winter driving.
This manifold is cast with pretty thin aluminum and heats up plently quick, requiring less than full exhaust riser flow under the carb plenum.
try it.
That's great info, I just bought some Mr Gaskets at the local speed shop and they are complete blocked off. I asked him about that and he said that I just cut them to suit. I was thinking of using a hole saw on them, so I will cut just one. I will be driving in the summer or warmish weather as it does not have a roof.
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