810 Briggs Rabbit Hole - New Head Branch


Help Support HomeBuiltAirplanes.com:


Well-Known Member
Jul 15, 2014
North Carolina
Stock valves would be a negative for me. I'd rather use better ones. As it's a new head, there is scope for better ports and smaller valves. I'm rather leery of aftermarket stuff. Turbo car valves seem a good idea to me. My car has Inconel exhaust valves as stock and is designed for 930C EGT. The NA version puts out 120hp through 16 valves. Tuned, 160. Same head as mine, lower grade valves. So that's a potential 30-40 hp of NA flow capacity on a 2 valve twin. Exhausts are 23mm dia and 118mm long, intakes 30mm dia and 110mm long. There are many other engines that could supply valves, guides and seats. I've even seen nimonic valves in a stock engine. Inconel is far better than the usual hotrod 214NS valves.

A hemi wedge chamber would be nice, but that would be a real
PITA to machine...

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Nov 14, 2009
Rocky Mountains
Stock valves would be a negative for me. I'd rather use better ones.
If this were a true clean sheet project then I'd agree.
To me, this is getting into serious mission creep. Just a new head is far enough down that path from "inexpensive industrial engine" to question the value of the project. My goal(s) is/are better packaging, increased cooling capacity, and a simple bolt together package.
The stock bits seem to have proven themselves in real world service at OEM power levels. With increased cooling capacity they should survive at slightly increased power levels. If they don't then swapping in better becomes an option.

The stock valve (my measurement) are 89mm in length. If you know of a better valve with a similar stem length in the 6mm or 7mm stem diameter range I'd take a look.

Chamber shape? Strictly TLAR based on a few decades of reading and experience and strongly influenced by the 2 valve water-cooled VW heads. Vertical valve stems and a good squish/flow ratio are my primary parameters.


Mar 30, 2015
Clovis, NM
This is a table comparing some of the industrial engines that have been used or proposed for aircraft use over the last 10 or so years.

If you compare this data to "proper" aircraft direct-drive engines, the achievable power is around 40hp/lt and maybe 43hp/lt with everything optimised. The power/displacement is normalised to 3,600rpm and based on max power (not all manufacturers have max cont power advertised).
View attachment 100003

Aiming for 43hp/lt (35hp out of the Briggs 49) is already at the high-end. A set of new heads optimised for flow at the lower rpm might improve the output but I'm not sure by how much and if it is worth the effort. It would be very nice to try.
I dontt know how to use your chart. What HP do you think that Scott's 1200 CC half VW would make?
James Fuller


Well-Known Member
Log Member
Aug 25, 2019
Peeramon (AUS)
Hi James, it is just a proportional extrapolation:
Hummel 1/2 VW, 917cm3, 32hp @ 3,600rpm
1/2 VW, 1,200cm3: 32/917*1200 = 42hp
This is assuming that all variables stay the same (intake port ratio, carburetor size ratio etc).
This would give you 42/1.2 = 35hp/lt for the 1,200cm3 engine (achievable)