Valve size question

[Barnbikes]

2 questions.

Is there a basic rule for figuring out valve size on a engine?
How much room do I need between piston and valve head?

I am building a engine with a 3.265" dia. piston and a 1.900" stroke. Would 1.000" dia. valves be big enough?

Also the engine is air cooled (no fan). How much expansion should I expect from a aluminum piston so I don't hit valve head?

Right now I have .250" head to piston figured but that can be changed.

Thanks,
jon

 

[AussieJimG]

I don't know enough to comment on the valve size. I think it depends on the purpose of the engine - are you after maximum power, lots of revs, or just an engine that runs.

You need to allow enough room between the valve and the piston at top dead centre for the valve to lift 0.25xdia. This gives an opening area around the valve equal to the area of the valve seat and is the maximum lift needed.

With a 1.9" stroke and 0.25" head to piston clearance (at tdc), you have a theoretical maximum compression ration of 8.6:1 which is reasonable but the piston would just hit the valve if the lift is 0.25"

Dunno if this helps or not.

Jim

 

[Brian Rupnow]

Yes, there is a formulae--I have it here in a book. It is not so much the diameter of the valve as it is a function of how large the volume of the gap between the valve and seat is when the valve is fully open, thus the amount of valve lift. It must be a large enough gap to flow the same amount of air as the valve port. The calculation for this is easy, but you have to assume a valve diameter and cam lift to work it through. My book suggests that the volume of this opening produced by the opened valve is reached when the valve has lifted 1/4 of the port diameter. Assume a 1" diameter valve, which will give you an approximate 7/8" diameter port size. The valve would have to lift 7/32" off the seat to match 1/4 of the 7/8" port diameter. That is a lot of valve lift. To get a smaller lift, you must go to a smaller valve. Assume a 7/8" diameter valve which would give an approximate 3/4" port size. Then your valve lift would only have to be 3/16 of an inch. This is all very subjective of course, and does not take the diameter of the valve stem into consideration.

 

[t.l.a.r. eng]

A long, long, time ago, somewhere it was taught just as a general rule that 1/4th the size of the bore is a great place to start with valve head size. That would mean if your bore is 3.265 then the valve head diameter is going to be .812
The port obviously would be slightly smaller. This is just a thumb in the wind place to start!
Not exact science but if you measure the relation of valve head size to bore size on an existing engine, it sometimes is amazingly close. [on older engines]

If you are after high performance then that all goes out the window, but for small model engines it will work just fine.

Room between the piston and valve head can vary depending on how strong are the springs, how high are you going to rev it, how heavy are the valves, push rods or OHC, valves at an angle to the piston or straight, obviously more clearance is better.
Again, not sure if that is too critical in the smaller engines but increases with engine size due to weight of the parts.

 

[Barnbikes]

Going to rip apart some old Briggs I have and see what size the valves are and what the lift on the cams are. Maybe I will just reuse some parts and make the project easier.

 

[stevehuckss396]

2 questions.

Is there a basic rule for figuring out valve size on a engine?
How much room do I need between piston and valve head?

I am building a engine with a 3.265" dia. piston and a 1.900" stroke. Would 1.000" dia. valves be big enough?

Also the engine is air cooled (no fan). How much expansion should I expect from a aluminum piston so I don't hit valve head?

Right now I have .250" head to piston figured but that can be changed.

Thanks,
jon

My rule of thumb is to multiply the bore by .425 and the 2 valves will fit comfy. My V8 was .625 bore so that worked out to .265 so I rounded down to .250. Your bore is 3.265 so that works out to be 1.387 so I would round that to 1.375. 2- 1-3/8 valves will fit real nice. There should be plenty of room for a full sized sparkplug. As far as valve lift goes you would need .343 to get full benefit. Using a 1.6:1 rocker arm your cam would only need .215 lift (about the same as a 3HP briggs) so you could play with that and see what you could do.

Piston clearance is a whole other issue. Once you settle on the cam you will need to measure the clearance through the entire 720 degree cycle. With good springs and low revs .050 should be plenty. If you are less than that you could experiment with different head gasket thicknesses (changes compression) or cut eyebrows in the piston to keep things from crashing.

 

[BronxFigs]

Very interesting to read about how to select valve sizes for various bore diameters. This information is hard to find, at least I have a hard trying to find it, and even better, the math is easy to follow.

Thanks guys, for the theory, and logic. It helps.


Frank

 

[johnny1320]

My motorcycle engine has a bore and stroke of 3.00" x 2.6" valves are 1.47" and 1.26" I run .25" clearance vlave to piston and .040" piston to head clearance