Larger intake valves
 

The Model A engine I’m working on rebuilding has larger intake valves and stock exhaust valves. All are inserted.

What’s the point?

I’m a big believer in stock engines. That said, is their some advantage to this configuration?

Re: Larger intake valves
 

Makes the engine breather better for more power and maybe even more horsepower

Re: Larger intake valves
 

It's to keep up with the Japanese who refused to make under-stressed engines.

Re: Larger intake valves
 

Doesn't what comes in have to go out through the exhaust valve? Seems to me the only way to get more out the exhaust is to either make it bigger or increase the pressure, no?

Is anybody out there running an engine like this? My first inclination is to have the inserts redone back to stock.

Re: Larger intake valves
 

nothing scientific, just, my opinion. The air coming in is pushed in - thru the intake valve - by atmosperic pressure and the vacumn caused by piston moving downard. The air moving out - the exhaust - is being pushed by the piston and the explosion and the expanding of the air inside the cylinder. The exploison and resulting expanding air move a lot faster than the air being sucked in by vacumn. clear as mud???


as said above the engine breathes better and is more efficient which equals more hp. I have built several engines with the larger valves and the one currently on the table will have the larger valves. Iffen I wus u I would leave the larger valves alone. Iffen you have insert main or rods I wud also leve dem alone. It would be VERY expensive to conver an engine back to babbit bearings. ken

Re: Larger intake valves
 

If the valves and seats are in good shape, just do the usual regrind. Being larger won't hurt a thing (and by itself probably not help much for power). It may have been done to repair worn out seats. But without a performance cam and bigger carb and intake (such as a B), not likely to make a difference.

Re: Larger intake valves
 

http://modelaparts.net/special.htm/1.700flowsheets.html

The exhaust valves are too big.

Re: Larger intake valves
 

On a FLATHEAD 4 or V8, people that believe the exhaust should NOT be as big or bigger than the intake are living in the past.
Ideally, the exhaust system should be big enough so that NO pressure wave is reflected past the reverse flow dam to the valve seat.

Re: Larger intake valves
 

I agree, in fact, when I was sold a set of valves with larger inlets, I put them in the lathe and turned them down. Instead of 0.2" larger, they ended up at 0.1 larger. I'm quite happy with the result but that said, I'm no less happy with an engine I did with standard sized valves. Everything else is the same.

Re: Larger intake valves
 

Exhaust valves (for reasons mentioned above) are usually smaller than intake valves. Increasing valve size does not always increase flow, depending on the engine/head design it sometimes reduces flow across the whole valve lift. An example is the 305 Chevy engine, increasing intake size alone often reduces flow, there is a delicate balance between value size, lift, porting, intake and headers etc.

Re: Larger intake valves
 

Don't overlook the fact that the 305 has smaller cylinder bores, thus shrouding the intake valves.