Valve size guidelines?

[Bob Farr]

Is there a general rule or ratio between displacement and valve size in model IC engines? I have the cylinder and crankcase casting from an old air compressor that I'd like to turn into an IC engine. Throttled carb, dual OHC's, & pressure lube are what I'd like, but maybe I'm aiming too high. I'm definitely running out of space to fit everything. I don't know how you guys manage to build engines so small!

The pump body has a 2" bore and the original crank had a 1.5" stroke, so 4.7124-CuIn's is what I calculate for displacement. I can put a hemispherical combustion chamber in the head to give more room for larger valves, but the biggest is a pair of 1-inchers. That's probably much more than necessary, and it's hard to squeeze those in there among head studs, intake/exhaust ports, bolt holes, etc. A pair of 3/4-inchers make life easier, but 1/2-inchers seem proportionally too small. I'm currently drawing a 45-degree angle between the cam banks but if I narrow that it will make more room for the ports. I'd like to keep it at 45-degrees if I can, just because it looks like a little Offenhauser head.

Are there any rough design guidelines to help me make everything fit without having to find out later the hard way that the valves I ended up with won't work?

Thanks,

Bob

The Victim:

 

[Peter.]

Hi Bob.

4.7cu-in is about 77cc. If you look at a typical OHV engine such as a Honda ATC or Cub90 engine, you'll find both valves are smaller than an inch, at a guess I would say the Cub90 valves are about 3/4" inlet and 5/8" exhaust in diameter (the 'big valve' heads taken from the more powerful pit bike engines are 23/21mm valves). It's a very frugal running engine and an easy starter too. Going by that I would say your engine will run very well on 3/4" or smaller diameter valves.

 

[mu38&Bg#]

Intake .43-.46 x bore exhaust .35-.37 x bore. Anything that fits will result in a running engine. if you want to make a lot of power you need big valves. If it's never going to pull a load, leave yourself some room to work with so cutter clearances etc, aren't tight.

Greg

 

[Maryak]

Greg,

What a wonderfully simple and very valuable piece of information. :bow: :bow:

Thanks You

Best Regards
Bob

 

[mu38&Bg#]

One other concern I should have noted is that if valves, with a lot of lift, are highly canted like in a Hemi, you need to pay attention to valve interference during the overlap period. Highly canted valves can be larger than I posted above. Reference: "Internal Combustion Engine Fundamentals" John B. Heywood pg 222.

Thanks Bob, I have a lot of books on engine design. I probably read and design more than I build.

 

[Bob Farr]

Gentlemen,

Thank you for sharing the helpful information. Small motorbike valves will be a good source of parts in this size range, and the guidelines give be a target to shoot for. Thanks also for the reminder about valve lift/overlap/timing issues in hemi-head motors.

The engine is for display. It will not need to pull a load or make a usable amount of power. However, I won't consider it a complete success unless it makes a suitably impressive amount of quality noise!

Thanks,

Bob

 

[Lakc]

Some of us horsepower freaks dont consider the valve too big until you run out of room notching the top of the bore ;D

Just to make noise, however, Greg gave great guidelines, stick to the smaller end.

 

[stevehuckss396]

Hi Bob!!

In my bugatti engine I have .375 diameter valves in a cone shaped combustion chamber. With the spark plug in the center. If the design is blown up to a 2" bore you could get a .875 valve in there very easily. If you want I could try to cad a drawing of the head and chamber so you could blow it up some 2.? times. Hell, If you come over here for a few hours with the block we could design the entire head, cam box, and valve covers if we just blow up the bugatti design.

 

[Bob Farr]

I don't know why I didn't think of your Bugatti but it would be a great OHC design to get some ideas from. The head for this little compressor motor should be easier to fit things into: with just one cylinder, I should be able to relocate the spark plug to the back of the head instead of between the valves.

Now that I know your preferred beverage I may take you up on that offer for a visit. I still haven't worked out whether to gear or belt drive the cams, so don't start drawing yet. We'll talk at the next club meeting. I've been meaning to ask you a bit more about Alibre CAD. I'm still in the 2D pencil and graph paper mode and that may need to change soon