Inlet- and exhaust-port sizes

[geoff p]

I trying to make my own version of a horizontal petrol-engine, in the style of the Amanco etc.

With few exceptions, I'm casting everything in aluminium and I'm aiming for a bore: 25mm, stroke 30mm.

What sort of sizes would you recommend for 1/ the valves and 2/ the ports to the outside-world?

Geoff
Thailand

 

[cfellows]

I'm not an expert but I've built engines of about that size. A 9mm - 10mm valve diameter should be about right. The exhaust port should be 7mm - 8mm and the intake port shoul be 5mm - 6mm. Valve stem size of up to 4mm should be OK.

Chuck

 

[Jasonb]

I finished a vertical IHC at christmas with same bore and 38mm stroke.

Valve heads were 3/8" (10mm) and the void behind 5/16 (8mm) and passages were 5/16" (8mm) exhause, 1/4(6mm) inlet .valve stems were 1/8" (3mm)

J

 

[gbritnell]

Hi Geoff,
That's a very hard question to answer. The old hot rodding rule of thumb was to put as large of valve/s as possible into the combustion chamber/cylinder head. The intake always being larger than the exhaust because of the need to charge the cylinder better on the intake stroke.
Most hit and miss and simple designed single cylinder engines have both valves of the same diameter.
As far as size I have never seen anyone experiment with a given engine to see whether going from a large valve to a smaller valve improves or reduces the performance. That said I would say with a 25mm. bore I would go no larger than 9.5mm. with the heads of the valves. Equally spaced on a 25mm diameter would give you 2mm spacing from the edge and between valves. Now when I say the head of the valve this doesn't mean the seat area which would be a little bit smaller than the head diameter so around .75mm smaller. That's the diameter at the top of the seat. Now subtract the width of the seat, say 2.5mm smaller in diameter. Now you end up with a port size of say 6.25.
I would say this would give you a good starting point. Could they be larger or smaller, I guess. Like I said there's no easy answer.
gbritnell

 

[geoff p]

Gentlemen,
Thanks for your replies, they confirm what I was sort-of thinking, though never having built an IC engine before, I worried the passages would be too small.

I seriously doubt this engine will take any hot-rodding prizes - I shall be pleased to get it to even go!

Geoff

 

[John Rudd]

I'm not an expert but I've built engines of about that size. A 9mm - 10mm valve diameter should be about right. The exhaust port should be 7mm - 8mm and the intake port shoul be 5mm - 6mm. Valve stem size of up to 4mm should be OK.

Chuck

Chuck,
Far from it for me to be a critic as I've never designed an engine form the ground up...but..

Shouldnt the inlet port and valve be bigger than the exhaust?
I'd have thought that the inlet be bigger due to the fact that the incoming charge ( fuel/air mix ) is at atmospheric pressure rather than the exhaust gases which are hot and contain more energy...?

Again I bow to your experience and stand to be educated...and of course will help me with the engine I'm currently building ( under wraps all top secret... )

 

[Jasonb]

Most seem to have the exhaust bigger, eg the ones I have

1/6th Galloway 0.187 inlet, 0.250 ex
1/5th IHC 0.25 inlet, 0.312 ex
1/2 Domestic 0.375 inlet, 0.625 ex
Zero-six 0.312inlet, 0.312 ex

Possibley as the exhaust valve also needs to allow the piston to suck in air on the miss strokes it needs to be larger.

J