The Murphy Builders Site

Posted: **Fri Feb 17, 2012 11:28 pm**

Hey Ken,

Run that one by me again. What's the formula? I've never heard that
stated before. Very interesting though!!

Thanks.

Brian Lawson,
Bothwell, Ontario.

ps.... anybody ever use the grass strip closest to me, the field at
Highgate, CNA2? WUZZIT like? Good for a Rebel?

XXXXXXXXXXXXXXXXXXXXXXXXXX
On Thu, 14 Aug 2003 16:13:28 -0400, you wrote:

I would guess that when we see the rotax V6 numbers, at least some folks

will decide that it blows the doors off a 300 hp Lycoming on an SR.
Somewhat like a 912 tends to perform better than most equivalent rated
Lycosaurs on a Rebel. The larger slow turning prop will give more
takeoff thrust per hp. The turbo likely means no reduction in takeoff
power due to altitude to at least a few thousand feet.

From Bob's comments, if the normally aspirated engine is only burning
11gph at .42 sfc then it is only making 11 gph * 6 lb/gal / .42 hp-hr/lb

= 157 hp.

Off the top of my head I would guess that at 18 gph the radial engine is

making about 215 hp. leaned out. Is that in the ballpark?? If it is
getting the same cruise speed, that will give an indication of the
difference. More efficient prop, a lot less cooling drag, significantly
less fuel weight for an equivalent mission, etc.

Ken

Not sure its fair to compare that
larry173 wrote:
Exactly we need a 350 to 400 hp At this point LS1, Ls^ is our only

good

option. Rumers are of a V8 400hp Bombi but in about 5 years.
----- Original Message -----
From: <Nielsenbe@aol.com>
To: <murphy-rebel@dcsol.com>
Sent: Wednesday, August 13, 2003 11:36 PM
Subject: Re: New at Murphy

why 300HP? It seems they would go for higher HP. There seems to be

alot of

engines around 300 but alot fewer at 350-400. Is it because they are
offering a

certified replacement for the 540-550 class engine? Seems to be

lacking on

the

moose though.

Brad

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Posted: **Fri Feb 17, 2012 11:28 pm**

I'll second the BS meter going off with some of the auto conversions out
there! I did a little excel spread sheet to plug in numbers for different
engines, and the numbers on Crossflow web page almost broke the BS meter.
According to their web site their 320 HP 3.3L Subaru at 75% has a SFC of
.3085 gph.

Not only the SFC appears stretched... but using commonly accepted formulas
to calculate HP and torque I compared their web sites claims. They claim
300 HP and 680 lbs of torque at 5600 engine RPM with a 2.17:1.00 PSRU.
Again, using the commonly accepted formula to calculate torque from HP and
HP from torque, 300 HP with this PSRU should give about 610 lbs of torque...
not 680. Another way to look at it is to get 680 lbs of torque with this
PSRU you would need about 334 HP, not 300.

So however you look at it, you would have to say any of the information
offered is "optimistic" at best. I have no doubt these engines will out
perform their air cooled counter part... but take the manufacturers claims
with a grain of salt.

Mike

----- Original Message -----
From: <klehman@albedo.net>
To: <murphy-rebel@dcsol.com>
Sent: Tuesday, August 19, 2003 7:03 AM
Subject: Re: GPH = HP?? was Re: New at Murphy

Brian

Actually I'll bet you have heard of this. The formula is just the
definition of SFC or specific fuel consumption. Typically the Lycomings
run a SFC of about 0.5 lb per hour which is to say they will burn 0.5
pounds of fuel per hour for each horsepower that they produce for an
hour. It will be a little higher during full rich takeoff and a little
lower during leaned cruise. But the number doesn't vary much for a
piston spark plug engine. I can explain why if you like. Higher
compression engines do a little better, liquid cooled engines also
generally do a little better, but SFC never varies by more than about 10%.

Anytime you hear a claim that works out to better than about 0.44 SFC,
even for a modern liquid cooled engine, you should start getting
skeptical (OK maybe that should be 0.42 if the V6 rotax pans out which
I'm sure it will). Some auto engine converters absolutely peg my BS
meter when you apply this test. A lot of claims for engine performance
are based on greatly exaggerated assumptions of power output and this
little calculation helps one keep perspective. Fuel flow (lbs per hour)
tells the story!

Diesels of course have a lower SFC. They are more efficient ie. they do
more work per lb of fuel consumed. (more hp for an hour per lb of fuel
burned in an hour).

In the example, the units of SFC are hp.-hr / lb. which makes all the
units of measure cancel out leaving hp. as the final unit.

Ken

Brian Lawson wrote:
Hey Ken,

Run that one by me again. What's the formula? I've never heard that
stated before. Very interesting though!!

snip
From Bob's comments, if the normally aspirated engine is only burning
11gph at .42 SFC then it is only making 11 gph * 6 lb/gal / .42 hp-hr/lb
= 157 hp.

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Posted: **Fri Feb 17, 2012 11:28 pm**

Brian

Actually I'll bet you have heard of this. The formula is just the
definition of SFC or specific fuel consumption. Typically the Lycomings
run a SFC of about 0.5 lb per hour which is to say they will burn 0.5
pounds of fuel per hour for each horsepower that they produce for an
hour. It will be a little higher during full rich takeoff and a little
lower during leaned cruise. But the number doesn't vary much for a
piston spark plug engine. I can explain why if you like. Higher
compression engines do a little better, liquid cooled engines also
generally do a little better, but SFC never varies by more than about 10%.

Anytime you hear a claim that works out to better than about 0.44 SFC,
even for a modern liquid cooled engine, you should start getting
skeptical (OK maybe that should be 0.42 if the V6 rotax pans out which
I'm sure it will). Some auto engine converters absolutely peg my BS
meter when you apply this test. A lot of claims for engine performance
are based on greatly exaggerated assumptions of power output and this
little calculation helps one keep perspective. Fuel flow (lbs per hour)
tells the story!

Diesels of course have a lower SFC. They are more efficient ie. they do
more work per lb of fuel consumed. (more hp for an hour per lb of fuel
burned in an hour).

In the example, the units of SFC are hp.-hr / lb. which makes all the
units of measure cancel out leaving hp. as the final unit.

Ken

Brian Lawson wrote:

Hey Ken,

Run that one by me again. What's the formula? I've never heard that
stated before. Very interesting though!!

snip
From Bob's comments, if the normally aspirated engine is only burning
11gph at .42 SFC then it is only making 11 gph * 6 lb/gal / .42 hp-hr/lb
= 157 hp.

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