'69 350 - blocking intake heat passage

[Tom M.]

I'm planning to block the intake manifold heat passage on my '69 intake manifold under the Rochester carb. Does anyone have a manifold without carb that can measure the hole size? I've read the plugs needed are either 3/8" or 7/16" - which is the right size?

[Bill W.]

Re: '69 350 - blocking intake heat passage

Tom,
Not sure what part of the country you live in but unless you are using some sort of fuel injection, blocking intake heat did not work well for me. I blocked the heat on my '65 300 horse motor thinking I would get a little more horsepower. The motor would hardly run till it warmed up, which took several minutes. Moderate acceleration during this warm-up period was poor. Even after warmup it would die when taking off in low gear.
I live at 5000 ft. elevation (North of Albuquerque, NM) and blocked the manifold in early December, ambient temps during the week I did this were in the 20* to 50* range. If you live in a warmer climate it MIGHT work for you. I went back to heated intake a week after first blocking the heat. Good luck. Bill

[Joe L.]

Re: '69 350 - blocking intake heat passage

I'm planning to block the intake manifold heat passage on my '69 intake manifold under the Rochester carb. Does anyone have a manifold without carb that can measure the hole size? I've read the plugs needed are either 3/8" or 7/16" - which is the right size?

Tom------

If you're talking about installing threaded plugs, you need 3/8" NPT.

[Joe L.]

Re: '69 350 - blocking intake heat passage

Tom,
Not sure what part of the country you live in but unless you are using some sort of fuel injection, blocking intake heat did not work well for me. I blocked the heat on my '65 300 horse motor thinking I would get a little more horsepower. The motor would hardly run till it warmed up, which took several minutes. Moderate acceleration during this warm-up period was poor. Even after warmup it would die when taking off in low gear.
I live at 5000 ft. elevation (North of Albuquerque, NM) and blocked the manifold in early December, ambient temps during the week I did this were in the 20* to 50* range. If you live in a warmer climate it MIGHT work for you. I went back to heated intake a week after first blocking the heat. Good luck. Bill

Bill-----


I think he's talking about blocking the "heat slot" found on certain Chevrolet engines including 1968-69 Rochester Q-Jet manifolds. Also, used on some earlier small blocks.

I believe what you are referring to is blocking the heat riser cross-over passage in the intake manifold. That's a whole different story.

[Tom M.]

Re: '69 350 - blocking intake heat passage

Joe - correct - plan is to block the heat slot immediately below the ' '68-'69 carb. Do the holes need to be threaded for screw-in plugs (are thy already threaded?) or is there an alternative to use drive-in "freeze plugs"?

Bill - the plan is to leave the exhaust heat crossover fully operational so the manifold will still be warmed from cold start. I'm in the Northeast and eliminated the exhaust pipe crossover valve on my 66 Jeep with 225V6 (Buick) and have the same issues you mention.

Thanks both

[Joe L.]

Re: '69 350 - blocking intake heat passage

Joe - correct - plan is to block the heat slot immediately below the ' '68-'69 carb. Do the holes need to be threaded for screw-in plugs (are thy already threaded?) or is there an alternative to use drive-in "freeze plugs"?

Bill - the plan is to leave the exhaust heat crossover fully operational so the manifold will still be warmed from cold start. I'm in the Northeast and eliminated the exhaust pipe crossover valve on my 66 Jeep with 225V6 (Buick) and have the same issues you mention.

Thanks both

Tom------

The holes are not threaded. They have to be threaded. However, my recollection is that I just use a 3/8" NPT pipe tap without even having to drill before tapping. Cup shape drive plugs can also be used. I think the size is 1/2".

[Bob W.]

Re: '69 350 - blocking intake heat passage

Joe, In post #5 you said that you can use cup shape drive plugs are they the same as soft plugs?


Thanks Bob

[Joe L.]

Re: '69 350 - blocking intake heat passage

Joe, In post #5 you said that you can use cup shape drive plugs are they the same as soft plugs?


Thanks Bob

Bob------



Yes, they are also called soft plugs. Easily available at auto parts stores. They are the same type of plug used for the front oil gallery plugs on small blocks.

[Bob W.]

Re: '69 350 - blocking intake heat passage

Joe, Again thank you for the quick reply. Bob