How much will 40 HP matter???

[Nomex Maximus]

Hello,

I am trying to decide which engine to select for my 7A.

I am pretty well decided on a superior XP engine. I can't decide on the horsepower. I expect that I will mostly do cross country flying with little aerobatics. I had been thinking that a simple 180 HP engine with a fixed pitch prop was the way to go or even the 320, but then I have sort of gotten the itch for the novelty of the XP-400 engine (with CS of course). My question: How much difference will the extra 40 HP make in the flying characteristics of the plane? Easier or harder to fly? What kind of fuel consumption will it draw? What kind of performance difference might I see?

Or, to quantify it differently, the XP-360 will set me back $21000 whereas the XP-400 will cost $32000, so if I didn't buy the big engine I could buy a high end Garmin unit along with the normal sized engine... decisions, decisions... could you all give me your thoughts?

-- John,
Wayland Twp, MI
empennage nearing completion

[Kyle Boatright]

What will 40 hp give you? Probably 15 mph and 500+ fpm in climb over a 180 hp installation.

It will also burn roughly 20% more fuel if you push the throttle to the firewall.

One concern I have developed with low volume products (i.e. the XP-400 at this point) is that if the manufacturer goes out of business or drops the product, the spare/replacement part situation could get ugly.

[N941WR]

Better yet, do a search.

Doug changed the O-320 to an O-360 in Flash some time back and had some good insight into how the same plane handled differently with a bigger engine.

[Christopher Murphy]

build it light and use the "320" C/S prop and it will perform on less fuel.

[Ron Lee]

I would be concerned about the XP400 (mainly weight) but a strong O-360 and CS prop would be a great performer.

One more time: O-360 (or IO360) and CS prop

[tonyjohnson]

I am unfamiliar with the XP400 and do not know its weight. If the weight is substantially more than the 360 it may have a dramatic effect on the weight and balance.

Personally, I don't know why you would need more HP unless your mission requires it. The 360 seems to be plenty enough and much cheaper. You can buy a lot of avionics for the price delta between the 360 and 400.

[groucho]

From a performance perspective, your climb rate is proportional to excess power divided by weight, so an extra 40hp will make a fairly big increase in climb rate. Your top speed won't go up all that much though. Drag increases proportional to your airspeed squared...Team Rocket claims the top speed of their 250+hp F1 to be 250mph. My RV-6 will go about 218mph in level flight with a 180hp & constant speed prop. So, with a straight, light airplane yours should go between 218 & 250 if you have more than 180hp.

If you do the math (with lots of assumptions)...if my plane with 180hp does 218, it'll do 250 with 237hp, so that works out about right.

Now, I can't imagine saying I couldn't use more power...BUT, the 2-seat RVs fly extremely well on a 360 (and 320 for that matter). As far as acro is concerned, with my C/S 360 I can pretty much pull up into any acro maneuver from cruise flight. It doesn't have enough power to do a double Immelman though.

I do know that if my Rocket friends pull their 300hp monsters back to cruise with me, their fuel burn is pretty close to what I get. Not that I could pull the power back if I had it.

Hope this helps!

p.s. Don't do 218mph in your RV-6. That's above the Vne.

[Andy_RR]

Originally Posted by groucho

If you do the math (with lots of assumptions)...if my plane with 180hp does 218, it'll do 250 with 237hp, so that works out about right.

Drag increases with the square of the speed, but drag power (drag force x speed) increases with the cube of the speed, so you'll be needing more than 270hp to get to 250mph

An extra 40hp would, in theory, take you to 233mph

A

[Mike S]

40 HP used to get my old VW to 65-70 MPH, sometimes a bit more.

Too bad it wont add that much to an airplane.

Mike

[Mike S]

40 HP used to get my old VW to 65-70 MPH, sometimes a bit more.

Too bad it wont add that much to an airplane.

Mike