Keith,

I don't mind at all; I'm glad you jumped in here. Lots of good info,
and I
learned some, too! But I'm still not an "engine guy." :)


David J. Gall
PS Maybe I'm a "wannabe engine guy" ;^}

-----Original Message-----
From: Keith L Welsh [mailto:welshq1@...]
Sent: Monday, June 09, 2003 1:02 AM
To: Q-LIST@...
Subject: Re: [Q-LIST] Onan, 18 or 20?


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

minor

modification to the carburetter.

QAC never used a 20hp engine. They didn't appear on the market till

1983

and
then as the B48M-GAO20 which is the predecessor to the P220 and the
Linamar
LX 790. The 22.5hp was a modified B48M-GAO18 and was the only

version QAC

used.
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
versions.

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

didn't
and had the same heads. Moral is don't shave or anything else to a

set of

heads.

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

also

require a special graphite head gasket -- don't skimp!

Sorry but I haven't been able to see a power difference between the
shaved
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

new
carburators can produce, additional power is just imagination.

If you stick with the Onan there are some definite changes

("deviations

from

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

QuickTalk/Q-Talk

for the mods.

Keep in mind you don't read much about the original 18's just those
stupid
22.5's. Terry Crouch proves the original baffeling works just fine

and I

imagine
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

with
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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