Hi all,

I have a set of 081 heads that I'm planning to use on a 350. As I understand, I can put 1.94 and 1.6 valves in them no problem. My question is, will 2.02 and 1.6 valves fit in them. If they do, would new valve seat insert have to be used?

 

What is the desire to put 305 heads on a perfectly good 350?

The limiting factor is the chamber size of the 081 head is designed for the smaller cylinder bore of the 305 (3.750)

You cannot run larger valves than the combustion chamber was designed for.

I believe 1.94 is the practical limit for those 305 heads.


If want larger valves I would use heads made for the 4.00 bore of the 350.

 

Thanks for the input. I actually agree with you. I was just wondering because I happen to have a set of 081 heads totally rebuilt and fresh from the machine shop that I planned to match to an old 350 short block I picked up for a song. However, after I posted this and before I got your response, I realized two things:
1. The flow difference between 1.94 and 2.02 valves only shows up after 5,000 rpm. This is a TPI motor so...
2. The compression ratio with these heads would come out over 10:1 may be even 11:1 so I can't really use the 081's after all.

 

Thanks for the input. I actually agree with you. I was just wondering because I happen to have a set of 081 heads totally rebuilt and fresh from the machine shop that I planned to match to an old 350 short block I picked up for a song. However, after I posted this and before I got your response, I realized two things:
1. The flow difference between 1.94 and 2.02 valves only shows up after 5,000 rpm. This is a TPI motor so...
2. The compression ratio with these heads would come out over 10:1 may be even 11:1 so I can't really use the 081's after all.

 

Take this under consideration.The intake port/runner is more efficient than the exhaust port.Cam timing/compression dependent,there is exhaust gases force into the intake polluting the intake mixture.Enlarging the intake valve without touching the exhaust port could make that much worse.

The overlap causes exhaust gases to be sucked in before the intake valve closes.

 

This is an 081, with 1.94"/1.6" valves in it.

Note how the chamber that's still stock, the intake is UP AGAINST the vertical "wall of the chamber. In this case, the larger valve will actually flow LESS THAN the original 1.84", because it's shrouded so bad by the chamber.

Now imagine what that situation would look like if the valve was EVEN BIGGER.

By the time you fix that, as it is in the one that's been laid back in that area, the chambers are usually up to the 61 - 62cc kind of range.

By the time you took out enough metal to fix it for 2.02" intakes, you'd be running a SERIOUS risk of scrapping the casting, for hitting water.

Just jamming larger valves into a set of stock heads is a great way to pry money loose from beginners who think that "larger is always better"; and there's ALWAYS at least one unscrupulous "machinist" lined up to make it happen. In this case as in so many others, just because a thing "can be" done, does not automatically make doing it "A Good Idea".

 

Sofa... excellent picuture !

this should be a "sticky" in the FAQ section and or this section

titled practical limits for 305 heads or something like that.

This photo speaks volumes...

I have been explaining why 305 heads on a 350 block are a bad idea for years...it gets old.

 

Thanks for all the feedback. That picture says it all.