GSJames

At my very first 424 contest, it was obvious that my engine was WAY down on rpm compared with the rest of the pack. Since then I have done only the obvious LEGAL things to improve engine performance, i.e., shims, plugs, back out the idle needle, de-burr the baffle, etc. in preparation for a contest next month. However, after getting out both my micrometer and the rulebook, I find that the rules state that the maximum carburetor inlet diameter is 0.325" and the maximum muffler outlet diameter is equivalent to a 9/32" round hole (.0621 sq.in.). The carb inlet and the muffler outlet of the TT 40 Pro, AS DELIVERED, are both significantly smaller than that.

Since bringing the inlet and outlet up to their maximum rulebook diameters would involve removing metal, the question is:

Is it, or is it not legal to drill/machine the carburetor inlet and the muffler outlet to the maximum diameters clearly spelled out in the rulebook? If it is legal and I am not doing it, then I am a fool for giving up engine performance. On the other hand, I do not want to make illegal modifications to this "stock" engine class, because if you win by cheating you are not a winner, only a cheater.

Opinions? OFFICIAL opinions?

daven

My Feedback: (1)

It is illegal, you would be modifying a stock motor.

In 422, it IS legal to modify the Venturi, but it has to still maintain a 9mm no go test.

fizzwater2

My Feedback: (61)

How much time do you have on the engine? It takes a while to limber up a TTpro 40 from what I can tell.

I break mine in with an 8/6 prop, or with a 9/6 with 1/2" cut off each tip, on the ground, running just slightly rich at 16.5 K rpm or so. I run 'em for a bit, (30 sec to a min) and shut down, let it cool off, do it again, gradually stretching out the run length. Do that for a while, before ever putting it on a plane. You want to break it in at near the same RPM you'll be running later, but with a lighter load so it's just a little bit rich yet. Not slobbery rich, but definitely on the fat side of peak.

I've NEVER opened up a carb or muffler, never felt the need. Like Dave said, modifying anything by metal removal isn't kosher.

Have you measured the RPM of your engine? If it's not too old, I'd go back with the light prop and run it some more before I gave up on it. I've heard of engines continuing to get better and better after the first gallon or two.

When I'm on the starting line, I always feel like my engine is down from everyone else's, too.. but they do ok. I know for a fact that one little bobble on the race course while flying will negate the effects of an awful lot of RPM difference...

GSJames

OK, thanks! I appreciate and respect both of you and your opinions. Your past postings show that you are both knowledgeable and honorable competitors. I rather figured that it would be illegal, but wanted to know if I was missing out on something obvious. As an old CL Combat flier, I know how to make an engine honk, but only want to do those things that are allowed.

As far as the engines go, both of my 424 engines are fairly new with only a limited amount of run time. Hopefully, now that the bearings have been re-set, shims changed etc., if I get a little more run time on them perhaps they will pick up a bit.

I believe that Duane Hulen told me that they typically see 16,800 on their engines with an APC 9-6 and 15% fuel. Does this sound correct? I was down at about 15,800 last time, so I've got a lot of catching up to do.

fizzwater2

My Feedback: (61)

16,800 would be a pretty good one. It might take a while to limber up to that point.

One of my old ones was a little better than that, even - completely stock, down to the bearings with the metal cages. You can guess what happened to it.... i was going to treat it to new bearings after one last race. The bearing cage failed, the pieces went up through it, and carved up a perfectly good piston/cylinder/head...

So, I have a spare muffler now... always a good thing.

Scorpion Racing

Gary, I would also suggest sealing up the muffler with high temp ATV or JB weld at the seam and the bolt head/nut area to avoid any pressure leaks. This will help it stay together (especially the JB weld! ) and improve the RPM's a little bit too. I also like using the brass muffler washers between the engine & the muffler to make sure it seals tight.

Scott Smith

Bill Vargas

My Feedback: (5)

Must also remember,,, not all 9x6 props are the same. Some will rev higher or lower than others. And never take off at peak RPM's.


BV

fizzwater2

My Feedback: (61)

True - the props do vary a bit.

At the local 424 races, we typically pass out APC 9/6 props, dyed special colors so we know that everyone is using an out of the box prop without any special selecting going on, etc. The ones that don't break get turned back in at the end of the race for the next go around.

daven

My Feedback: (1)

Gary, when I went to my first Nats, I was happy not to get lapped. Felt all of my stuff was slow compared to everyone else.

Stick with it, ask questions, and continue to fly and practice. There really are no secrets other than common sense, and I know you have that.

If you not competitive in a couple years, you may need a new hobby

Bill Vargas

My Feedback: (5)

changing hobbies ain t so bad


BV

diggs_74

Gary,

You'll get it.. Don't worry about the RPM, just concentrate on getting/staying on the course. As was mentioned, you'll find out a lot of RPM gains are lost flying big..

Keep practicing so you're ready for that AR-6 you'll be getting.. Eventually