Ok guys I need help. I am trying to put a Magnum 91 4s in my P51. Problem I am having is how do I find the center of the cowl. I have posted some pictures and if you need more just tell me. I guess I thought the opening in the firewall was the center for the engine, not so. I have the right distance from the firewall to the backplate spinner.

Is that an ARF?
From my experience the firewall will have an a thrust offset angle built in so if you center the engine mount on the hole the angle of the engine will actually center the drive hub of the engine in the cowl.

Yes it is a ARF- I think the hole that you see in the firewall is much lower than the opening in the cowl. Maybe 1/2" or so.

The skimpy instructions and picture shows the engine mounted sideways. Is there any problem mounting it inverted? The only things that would be sticking out of the cowl would be a very small part of the head and rocker arm cover. If I do mount inverted do I then remove the part of the cowling below the prop output hole?

Yes, remove that part of the cowling.

And do yourself a favor... Mount the engine sideways. It will create a lot less headaches for you in the long run. Besides, you'll never see it at 500 ft

Four stroke engines, inverted, tend to kick back when you start them. Side mount is better.

Ok side mount it is. Should I put the engine on the table and then slide the cowl over it to see where I have to cut out, then mount it on the firewall.

I have an H9 p51 with an invert mount. Runs and starts fine with Saito 100. It will be a neater installation if you invert, since the fuse on a '51 is so narrow.

I wonder if they did'nt install the firewall backwards, as there is usually scribe lines for the engine mount.

I would suspend the plane vertically on a ladder so the firewall is flat, then you can position the engine to get it lined up with the cowl. You will probably have to lay the engine inverted in order to slide the cowl over it for positioning. The centerline will be offset a little to allow for the right engine thrust.

Cut the cowl after the engine is mounted.

I have a pumped O.S. 91 4 stroke running inverted, runs just fine. put a remote glow igniter on it for easy access.

To cut cowl holes.
1. mount engine
2. tape pieces of printer paper to fuselage to extend over engine
3. cut paper to where needed for engine, exaust, needle valve ect...
4. remove engine (leave paper in place)
5. install cowl and mark where to cut using the paper cutouts as a guide.(paper should slide over outside of cowl, ie.. slide cowl under paper.)
6. remove cowl, make cuts, imstall engine, install cowl and make trim adjustments as needed.

GunfighterII
I don't see any lines on the firewall. I have taken a tube (from a roll of monokote and hot glued it to the firewall and then put the cowl on to make sure I have the center lined up. Now I think I can mount the engine and will try what misslieman suggested. Hope I don't screw it up.

Some ARFs have a template you cut out of the instruction manual to mark for engine mounting, usually toward the back of the manual.

looks like one of the nitro models arf's.
all my 4st engines are inverted they run well,havent had a kick back any time running them upside down,having that big head sticking out the side will look poor on a warbird.
a tip for getting the engine to line up with the center of the cowl tape a small stick of balsa to the top of the fuse, on the center line, hold engine so the hub lines up with stick mark the locations of the screw holes and make vertical lines.
fit cowl as close to the finished position as you can without screwing it to the fuse, and tape the stick to side of fuse at the center of the cowl opening,remove cowl, and line up engine hub with the stick, make horizontal lines where the lines intersect drill holes for the screws simple and easy then you can do the final tweeking,and adjusting when you fit and cut the cowl for the air inlets.
finished product, notice the head barely sticking out of the bottom of the cowl,OS 91fs

goirish,
That does look like a Nitro models P-51. I'm building one for a buddy, the Lou IV, and it has the scribe lines on the firewall.

I thought if you looked inside at the back of the firewall, you would see the lines. of course that won't help much anyway.[:@]

The paper trick is the way to go with the cowl cutout, makes it foolproof.

I would mount the engine inverted, as long as your engine is well broken in, much cleaner look.

Just make sure your fuel tank is not too high.

Hey, Hey, Hey!! Missilman--want to thank you for the tip on using paper to make a pattern for my cowling. 'Worked GREAT!!! Looks like it was made by the professionals from accros the sea. Always get great help on this forum.

BigTim--Did you cut out the airscoop below the prop spinner--hard to tell from the picture but it looks like maybe you did. Thanks for the nice picture. Hope my flys as well as your's looks like it is.

Really?

All of mine are inverted and I've had no problems with this.

OS's, Magnum's, Saitos, etc.

Sounds like you may have had a fuel siphoning issue, flooding the head, caused by using the wrong fuel tank height.

yes I did open the air scoop you definatly need the extra air flow for cooling,the Hangar-9 planes fly very good not your typical warbird.
I also have a P-40, there solid in the air I haven't seen any of the nitro models clones up close but they seem to look about the same,I have read good and bad about them,mainly the not so great instructs and suspect glue joints something to take a good look at.
another thing is to make sure there is tri-stock in the corners at the firewall area,add some if there isn't, and to fuel proof the fuel tank area with thinned epoxy for strength as well as in case you get a leak in the tank,the fuel can weaken the wood fast and a 91fs isn't a weak engine for sure.