Engine for my Viper?
 

I have two set up for Q25 racing, and a spare.... Engine choices I have are .... TT.40 Pro ... Webra Speed .40 ... old Super Tigre .45, the round/6 bolt head. What do you recommend? Prop size and quess on speeds?

TT. 40 Pro. APC 9x6 110mph.

Kurt

Will the TT be faster than the Webra? This one won't be used for any racing, looking to go as fast as it can, with what I have. I thought maybe the ST .45 propped right would be the fastest.

I would go with the Webra before the TT. I used to get close to 17K out of mine running a rev up wood 9x7. I'm betting it would do at least 17,300 with an APC pylon 9x7 with a Jett muffler.

if you can work on the engines then you should. With some mods you can get 17,000 with a APC 9x8. open up the carb, exhuast port, might have to lower the sleeve, and might have to mess with the crank timing. now if you dont want to do that then a great option is to sell them and buy a newer super tiger 45 from tower and order a "black" jett muffler. this combo stock will spin a 9x8 APC prop 17,000. this will pull that plane around 130-140.

I had a new ST .45 on it a few years ago with a red Jett muffler and a little work, and it went low 130's I got rid of that engine when it started losing some compression, and made all of them Q25 legal. I have an UltraThrust muffler for the Webra, I think I'll stick that on it for now. I don't really want to buy anything right now.

try the black Jett muffler. the red one doesnt work near as well as the black one. sure the black one is for the Q-40's but for some reason on a bone stock St 45 it screams about 1,000 more rpm than the red one in the 17,000 rpm range. really unloads in the air.

also dont try those Q-500 Pylon props on sport engines unless you can spin them 18,000 or more on the ground. if not your not as fast as you can be....

Awesome, thanks!

Speaking of which, if I can slightly hijack for one question - I know APC's "190,000/dia" rule, that suggests the Q500 props are good for up to about 21.5k. In practice how fast do people consider them safe in use?

I dont think i would want to spin them over 23 on the ground when you are next to them. I think that rule is a safe standing. I have ran some 11" prop over 18,000 with no issues but doesnt mean it is safe.. What are you looking to run them on that can spin them over 20,000

If that is the case no problem. I'm talking 21-22k tops on assorted engines - kind of a general safety question.

I've never failed an APC prop from shear RPM.
I had an OS 1.20 four stroke shed the blades of a 15 x 6 MAS in a vertical climb, but that prop might have been defective..