Re: Onan, 18 or 20?

Keith L Welsh <welshq1@...>

A little clarification. Hope you don't mind David.

The only way that I know of to tell the difference is to look at the
paperwork... :)
The block has some tell tale signs that it's a 20hp model. Also the true
OEM 20's have a different carburetor and is very noticable. It sets up
higher and
is about half again as large. Bigger venturi, 5 air vents to the float
bowl, larger
fuel tube.

Seriously, the "20 hp" was listed as "22.5 hp" by QAC and was the
result of shaving the cylinder heads for higher compression, and a
modification to the carburetter.
QAC never used a 20hp engine. They didn't appear on the market till 1983
then as the B48M-GAO20 which is the predecessor to the P220 and the
LX 790. The 22.5hp was a modified B48M-GAO18 and was the only version QAC
It's predecessor is the P218. Each engine is based on the 47.7 CID block.

Incidently that 47.7 CID block is the only one available in two hp

The shaved heads are junk -- don't use them. Use the 20
hp heads manufactured by Onan specifically for the B48M engine.
(These heads came out after QAC pretty much stopped selling Q1 kits.)
Those heads originated with the OEM 20hp engine. Big difference is the
placement of the spark plug and the domed combustion chamber. Note
that only the 22.5hp versions had problems the original 18hp engines
and had the same heads. Moral is don't shave or anything else to a set of

Whether you start with the 18 hp or the "22.5 hp" engine, the new Onan
20 hp heads
will give you a more powerful, more reliable engine. These new heads
require a special graphite head gasket -- don't skimp!
Sorry but I haven't been able to see a power difference between the
heads and the 20hp heads. And it takes much more than just the heads
to produce additional power. Without additional air and fuel, which the
carburators can produce, additional power is just imagination.

If you stick with the Onan there are some definite changes ("deviations
the plans") that need to be made to help reduce vibration and to
control temperatures. The installation given in the plans is very
likely to
result in an overheated engine; read the back issues of
for the mods.
Keep in mind you don't read much about the original 18's just those
22.5's. Terry Crouch proves the original baffeling works just fine and I
most Onans still sport the same as well. My problems came about to the
thinner valve stems found in the newer engines. They can't take the heat
without warping that the older ones could. The vibration was controled
the optional Kevlar engine mount in place of the aluminum one. It came
standard with the 22.5 option.

Hope this helps
Keith Welsh

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