What is the proper cyl head temp to aim for when setting a TT .40 for a race. Mine was running about 240 or so the other day and we were wondering if that was about normal. We are running 9x6 props and 15 % fuel.

The proper head temperature occurs when the engine is warmed up and is adjusted at about 200 to 300 rpm below peak. A TT40 generally takes several seconds to come up to temperature and you can tell when it gets there because it quits gaining rpm when adjusted as above. The head is now at the correct temperature what ever that may be. Out of curiosity I have looked at it with one of those little but very inaccurate IR temperature gages and yes the readings were about 230 to 240 degrees F. Even if the little gages were accurate I don't know how you would use the information. I mean if the engine was properly adjusted to just a little below peak I certainly wouldn't start squeezing on the needle get get the head temp to 240. Maybe there's some guys out there that use head temperature to help determine optimum head shim thickness but I doubt it.

Denis

So you just run it up to peak and back it off 200 rpm? Does it lean out some as the tank empties out? We have talked about that at the field and some say yes and some say no. I have seen some people run them so lean that they lock up during the race and I don't want that to happen to me. We are all pretty new to this racing thing and none of us know much about the proper way to tune the engine for a race.

If your using a standard tank, I would back it off 400-500, as it will lean out as you get down to the bottom or experience any foaming. If your using a bladder type tank getting down to 300 off peak would be about right. Bladder tanks virtually eliminate air and foaming from the equation, allowing you to maintain a similar needle setting throughout the run.

Head temp readings are not the way to go. Its easier to tell if you are rich or lean by listening to the plane in the air, watching for a smoke trail, checking your plug after you land, and checking the exhaust on the bottom of your airplane.

You don't want your motor to lock up during a race, but in racing people tend to try and get that last few rpms out of it and push them too far. How far you want to push yours, is completely up to you. I try to sneak up a good needle setting starting rich in my practice flight, and slowly trying to dial in the perfect setting with each succeeding flight. If your caught in a heat and you think your starting to go lean, you can back the throttle off a little and it should help richen back up and not smoke your motor.

Thanks for the info. What size tank do people normally run? I have the stock viper tank and I usually have more than half tank left after the practice and races.

With a TT40 you can get along just fine with 4oz.

Denis

I believe the tank is a 6 oz. I may try a 4 just to save some weight. Thanks.

For the TT40, I'd go w/ a 4 or 4.5 oz tettra bladder tank. You'll have to hog out F1 a tiny bit to get it to fit, but it's really easy to do. If you move up to a Nelson or Jett (428) you're going to have to go to a 6oz tank. For those I use a Jett 6oz slim. You'll have to move your throttle servo w/ that setup, but again it's pretty easy.

Make sure you read the Viper modification threads that are around. Even w/ a TT40 you'll pull the firewall out of them.

Tim

Yes I already knocked the firewall loose. I pinned it with toothpicks and it seems to be holding better. Who sells the tettra tanks?

Chris - First off, take my advise and get a bubbless tank. Either a tetra or a Jett. It can and will save you an engine at some time. 4 oz is plenty for a race.

Start the engine and get to high rpm for 10 - 15 seconds. Then using a tach. peak the engine to max RPM. Now richen it up 300 - 400 rpm. Pinch the fuel line briefly and the rpm will peak again. When this "Pinch" brings the engine up to with-in 100 rpm of your peak reading, take off and fly.

You will hear the engine lean out as you pull through the turns and then richen up in the straight. If the engine stutters in the turn, richen it up just a little. If it does not pick up quite a few rpm in the turns, lean it out just a little. After a couple of flights, you will get the needle where you want it.

Good luck!

Tetra tanks -
http://www.darrolcady.com/Accessories/accessories.htm

jett tanks -
http://jettengineering.com/

Both are good but the Jett slim line 4 oz. tank requires less modification to get them installed near the CG.

Thanks I will try that 4 oz. tank and see what happens!

something else you might consider is to add a splash(1oz) of castor oil to your gal of fuel for when you are practicing,,,


BV

If this is happening, then your tank is too low. If you can't raise the tank, then the engine needs to be rotated to lower the carb.

You should also add some fillets on the inside of the tank compartment between the sides of the fuse and the firewall. Mix up some 30 minute epoxy with some flox or milled carbon, then make a nice fillet in the corners. Make sure you don't clog the blind nuts though! I put a coat of vaseline on my screws and leave them in the blind nuts while I make the fillets.

Tim