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Aug 25, 2019, 09:55 PM
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New 424 Engine Option

The 424 Engine Committee of the RC Pylon Contest Board has been working to deal with the lack of an available engine for the 424 Quickie Sport class since the loss of availability of the Evolution 46NX. The Committee is made up of five members of the AMA Contest Board.

Looking for options on the market for the class made it clear there arent many available. A key goal in choosing a new option for 424 was to find an engine that had equal performance to the Thunder Tiger Pro 40s and Evolution 46NXs that are currently approved and being used.

The OS 46AXII was suggested by some. We looked at it carefully. Obviously its a high quality engine but its widely believed that it would out perform the currently approved engines and require everyone to buy new engines to be competitive. Also, they cost $160 each and parts are not cheap either, for example a replacement carb is $90 and a piston and liner set is $90. OS is a strong company that is likely to be around as long as any glow airplane engine manufacturers still selling engines. This is because they have a very strong presence in the RC car engine business. We really did not like that this engine would basically obsolete all of the current engines.

Looking for other options, we talked to Dub Jett. The discussion ended up with Dub agreeing to see if he could come up with a version of the engine he manufacturers for the 426 Quickie class that would be basically the same performance as the TT40s and Evo46NXs currently in use. Another goal was to achieve this performance with minimal differences compared to the 426 version.

Dub went to work and earlier this year, sent the Committee a muffler and venturi to test on existing 426 engines. The muffler has a shorter can and no internal pipe. The venturi has a 0.250 bore. Both the can of the muffler and the venturi are anodized red for identification purposes.

Extensive bench testing was done, comparing to a baseline TT40. The baseline TT40 used is the one used when the Evo 46NXs were tested and approved in 2015 and had not run since. That baseline TT40 repeated the performance seen in 2015 with the same test props. Testing with a current APC 9-6 and four other test props of different loads were used to understand the performance and how the engines unload. Three different Jett 426 engines were run on the bench with the 424 muffler and venturi. The performance turned out to be very close on the bench to the TT40 performance, both with the 9-6 and unloaded.

Next, a quickie equipped with telemetry, recording airspeed, RPM, altitude and control stick positions was used for flight testing and comparison between the TT40 and Jett 424 engine configuration. Once again, the performance worked out to be very close to the baseline TT40 in airspeed.

More testing was done in North Carolina at Old Julian Airport at their Spring race in May. A Jett 424 engine package was flown on the course with a current 424 plane. They looked very similar. Then, at the May CAPS race in Muncie, with the permission of the other racers in the 424 class, a Jett 424 engine was entered in the matrix by a Contest Board member and flown for comparison without being given any points. The second day, another pilot flew the setup. The end result was that the engine appeared competitive but did not look like it had an obvious performance advantage.

So, with the performance confirmed to be right about where the class needed it to be, we looked at the benefits vs the drawbacks of approving the Jett 424 Q-500 engine for 424 class.

- Dub has been willing to work with us to dial his 424 package right to the TT and Evo performance level. This will allow folks that still have The TT and Evo engines to continue running them. The Jett 424 engine being proposed will simply be another option that is available.
- The engine is available, including very good availability of replacement parts and service as Dub has demonstrated over the years we've been flying 426.
- The engine is quite reliable at 426 performance level and should be even more so at 424 level.
- The intake is much simpler, more reliable and cheaper to replace than the carbs on either the Evo or the TT and, you can't get parts for those engines any more.
- Racers with 426 engines currently, can easily buy and switch the venturi and muffler and enter 424 contests with engines they already have.
- Newer racers can start by buying a high quality engine in 424 configuration and later buy a 426 muffler and venturi to move up to that class when they're ready.
- The Jett 424 engine is supplied only with a remote needle valve assembly, improving safety
- The Jett 424 option is lighter weight than the TT or Evo, giving more weight margin for the airplane.
- A key is, this engine will come with Dub's great support for parts and service.

- Cost. This is the same basic engine that Dub sells for Event 426, with the exception of the muffler and venturi. The price for the engine for 424 will be $275. The current cost for the 426 version is $285. Separately, the 424 muffler will be $55 and the red 0.250 venturi is $15 for those that already have a 426 engine.

Yes, this cost is notably more than the TT40 and Evo 46NX, when they were available. The only other currently available engine that was worth considering was the OS and its not very much cheaper to purchase than the Jett option and the replacement parts are quite expensive. Causing the obsolescence of the currently approved engines would have been a big problem with the OS. Considering the known durability and reliability of Dubs 426 engine since we started that class, and the fact that Dub worked with us to dial the performance to be basically the same as the current engines, we felt it was clear that the Jett 424 Q-500 engine option was the right choice for Event 424.

The 424 Engine Committee has voted unanimously to approve the Jett 424 engine. Next, there are some adjustments to the 424 engine rules needed to allow the engine to be used legally in the class, including allowing a carburetor or a venturi and making changes in regards to maximum price. That urgent proprosal is being run through the approval process currently and can be seen on the AMA web site at:

None of the RC Pylon Contest Board members have expressed an opinion against this proposal. There are procedures that have to be followed but the proposal will be processed as quickly as we can and is intended to take effect immediately once approved.

With all of the above said, in the mean time, if a Contest Director for an upcoming race would like to allow the Jett 424 engines to be used before the approval of the proposal, the CD should add an exception to their sanction specifically allowing the engine and make that exception known in all announcements about the event.

Attached is a photo of the Jett 424 engine configuration with a 426 muffler alongside for comparison. Also included is a pic of the venturi opening difference between the 426 version on the left and the 424 version.

We will update this thread to notify the community when the proposal process is complete.

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Aug 25, 2019, 10:47 PM
Registered User
I was the lucky raffle winner at the NATS of a 424 muffler and venturi setup for the Jett. Looking forward to trying it out, for sure.

Thanks for the informative post, Jim, and thanks to Dub for all his efforts, too!
Aug 25, 2019, 11:48 PM
Dave Yost's Avatar

Should have been done years ago.

Aug 27, 2019, 06:43 PM
Registered User

Have you all tested or considered the JBA .46A sold by I have no experience with this engine, but reviews in the late 2000s on forums are positive. At $69.95 with a USA distributor, it might be worth checking out...
Aug 27, 2019, 08:25 PM
Registered User
Thread OP

If they would like an engine to be considered for 424 they would need to submit samples for testing. We will not approve an engine that obsoletes the current engines by exceeding their performance or one that clearly is not competitive.

Last edited by PylonJim; Aug 27, 2019 at 09:45 PM.
Aug 27, 2019, 08:37 PM
Registered User
Hey Jim, how are you? I have a question for you, I had 2 Jett Super Sports which I converted to 426, through the purchase of the Head and the Muffler conversion from Dub. So in theory, since these engines became legal for 426, I could just change the venturi, and the muffler in these engines, as approved, to be legal for 424, correct? Does it belong to the rule that ' once an engine is legal, it is always legal"?
Take care,
Aug 27, 2019, 09:44 PM
Registered User
Thread OP
You would have to talk to Dub to confirm if your configuration would meet the 424 configuration. I don't know.

We do not have a once legal always legal rule.

Aug 28, 2019, 07:58 AM
Registered User
Ok Jim I understand but again, this is an item that needs to be determined by the Contest Board, is it not? So the situation is this: I do have a Jett Super Sport from the Sempra Era, for which Dub sold the Head and Muffler replacement to make it legal and competitive with the new 426 he built. In other words, that Jett Super Sport became legal for 426. It would make sense that if the Super Sport engine now competing in 426 with the modifications that Dub sold, that if I now I buy the new venturi and muffler for 424, that I should be able to race 424 with it. It makes sense to me, shouldn't the Contest Board reach out to Dub, ask this question, and if correct, that it should be included in the engine rules? If we are trying to use 424 as a class to promote pylon, there could be a number of people that have the old Super Sport, that may try, or, come back to racing, given the low cost of buying the new venturi and muffler for the old engine.
Just my two cents Jim, take care,
Sep 04, 2019, 08:54 AM
What is the specified fuel to run % nitro brand ?
Latest blog entry: Biggest Purchase in My RC Life
Sep 04, 2019, 10:30 AM
Kinetic Sculptor
Nitro content is 15%, brand is optional but Ritch's Brew is the standard among racing types. The AMA Nats and most other major contests supply Ritch's Brew.

Oct 28, 2019, 04:27 AM
Registered User
Where does one find a list of permissible engines?
Oct 28, 2019, 10:11 AM
Registered User
There are three:

Thunder Tiger Pro .40

Evolution .46, with the later one-piece muffler, not the two piece.

And now the Jett with the 424 specific venturi and muffler.

Not a long list.

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