Repost from Extreme Speed forum -

I've got a couple of pipe issues. I had the solutions from the former forum but now can't find them. I'm hoping that some of you can help me out with dimensions (Mr. Auger...).
On a Shrike .40 I've got an older O.S. 46 VFR? Rear exhaust with a MACS kick up header and pipe. What length should I be going for to start tuning? I'm running an AP 9x9 but could run an 8X8 which I think is the optimal prop.
On a Shrike .10 I've got an MVVS .21 with the silencer muffler bolted directly to the engine (no header length). What prop and pipe should I be using? Before I go and modify the fuse for the silencer I'd like to know if a non-silencing pipe is advantagious. I also have an MVVS .26 Combat Special that I could put oon instead but need to come up with a pipe for it.
On a Direct Connection F-20 (F-5) .40 size, I've got a Jett .50 side exaust with the Jett sport muffler. Would it be worthwhile to put it on a pipe and what prop?
Summer's on it's way and we'll start airshow racing soon, I just want to go fast. Are there pre-set headers and pipes availible?

Thanks for your help,
Chris

Hi Chris.....

On the F-20 and jett 50, a full size pipe would gain you about 800-1000 rpm over the jettstream muffler. It is not really worth it in that installation. The pipe installation creates more drag, and weighs a bit more. Prop selection also becomes more critical to keep in the big pipe RPM band. For this side exhaust installation, I would keep what you have. As for prop, anything the SJ50 will turn over 17,500 rpm is fair game. I prefer to use an APC 9x8 on my Patriot installation. Needles easily. Unloads nicely in the air.

For the OS 46 engine, is this the standard VFR rear intake rear exhaust, or the fan engine ?? What RPM are you expecting to run? Typically, the fan engine likes to turn way up in the 22K rpm range. It is very high-timed for fan use. The standard VR rear exhaust engine is time for around the 16K-17K peak rpm range. Pipe length and RPM are completely co-dependant. The pipe determins the RPM the engine will run at (can function as a governor). But, if the engine/prop is not capable of getting to that RPM the engine will die trying (over-heat, struggle).

On the MVVS, you know the pipe length. Measure that. It will determine the RPM you are shooting for, and you then select the prop accordingly to allow the engine to run there. Let me know what it measures, and I can assist.

Bob Brassell

I ran the OS 46 VF in a 40 size MK Arrow pattern plane, with a Macs pipe and header. As I recall, the header length was around 3", with a quarter inch gap between the header and pipe. This set-up turned 15,500 on the ground with a Zinger 10-7 cut down to about 9 3/4" as I recall. In the air this would unload to its max hp rating of 17000. Absolutely awesome performance. I flew it in Advanced pattern one fall just for kicks, and it was running circles around the piped 60s in the go fast pattern days Oh, this was on 10% Cool Power or Red Max.

I think your prop sizes mentioned are way too little for this engine.

Clair
AMA 15654

Thanks for the reply's
The OS .46 is the VFR, not the fan engine. What size pipe? I've seen reference to the 6.5 and 7.5 pipes. Is there a way to measure the pipe to determine which one I have? I've got two of these .46's and two pipes but am not sure of the size.
On the MVVS Do you have a formula for determining the rpm range via the pipe length? I'm wondering if a non-silenced pipe would be advantageous vs. the MVVS silencer?
Thanks again, this is really helpful.

Chris

Bob,

I don't think a full length pipe will create such a large gain over the JettStream muffler, on the low load props recommended by Jett (10x6 APC) for the .50.

Dub Jet writes in his web site, that since the JettStream is already a tuned exhaust, the boost afforded by the full length pipe will be 200-300 RPM. Maybe not enough to justify the added weight and cumbersome size, compared with the JettStream.

If you use larger props, which run at a lower RPM level, for which the JettStream is out-of-tune, you could adjust their length to get a boost of close to 2,000 RPM, over the JettStream.

Clarence Lee made the same observation, testing MVVS .61 and .77 engines, in RCM. But in this case, both the full length pipe and the tuned silencer are made to be effective in the same RPM range. So the small 200-300 RPM gain stays.

You are correct. For a 10x6, the rpm gain is minimal.

On the F-20, you would typically run a higher RPM prop, and it will unload more in the air. Properly set up, and with correct prop selection, the gain on the full pipe is closer to 800 in that case. The down side, is the operational RPM range gets very narrow.

(for reference, I am webmaster for jett engineering, and jett tech support in the engines forum here on RCU).

As I noted (be it a 200 rpm gain or 800 rpm gain) in that particular instalaltion, it is not worth the hassle for a marginal gain in performance.

The BSE 50 and SJ50 are unique animals. They enjoy all the RPM you can let them turn. The 50 in particular does not like large props. The Jettstream muffler is tuned to provide performance enhancement between 16,500 and 19,000. Pretty broad range.

We have in the past set up 'long" pipes on the rear exhaust SJ-50 to allow users to fly heavy 9x9 and 9x10 props down below 16000 rpm.... in an effort to make them quieter. It worked as expected, but the engine did not like it very much. The end user there eventually shortend the pipe, and went to a smaller prop. It was an interesting experiment.

Chris...

on that 46, use the 7.5 pipe for best performance. The 6.5 will work, but it likes the extra volume. The 8.5 works well too.
Hard to tell you the pipe length on this. Generally, for around 16K rpm, you would want to set it from the plug to fat part of the pipe, at about 12". You can work from there trimming the header until you hit peak with your selected prop.

Non-silenced pipes (or nitro pipes) give you the top performance... but they are LOUD!!! Be sure your flying buddies are not going to mind that.

In general, unless you are racing or in some other serious competition, do not strive the very final 100 rpms. Its not worth it. Run the stock or factor available mufflers. MACS sells some very nice pre-tuned pipe kits. There web site and catalog have some valuable tuning info on it. although, I can not seem to get on their web site for some reason. Hmm....

I know about Jett.

This is why I mentioned the fact.

Coming from F-1 and other racing classes does get people to disregard low RPM torque and thrust, or noise problems and to worship only high RPM.

16,000 RPM and northward with a prop, is not for everyone, or for every place, for that matter.


Where I fly, whenever a 14,500 RPM sound is heard, the whole field comes to where it is coming from.

An MVVS Q-500, spinning a 9x6 MA at 18,300, caused everyone who was flying to land and come see...