stuffing the Fox.35 bypass

This has been discussed many times. I wanted to try an attempt at filling the bypass with the basswood slug. I know its been done with JB weld as well. I'm looking for an easy alternative if all doesn't work well. Currently,I'm using the engine on a original Tutor. This particular Fox has been working out real well as opposed to my last Tutor with a Fox. This time around this one is lighter and I'm using a plastic uniflow clunk tank. I get a terrific break using this tank returning to a 4 cycle the instant it levels out. As soon as the plane goes inverted, it has a pretty good burp. I'm using a Sig r/c plug which typically cures this. Its not causing a bad hesitation, you just have to be ready for it. I cleaned up the inside of this case smoothing and polishing the bottom of the bypass.I'm not sure if this is contributing to a more than usual burp. I just wanted to know how high this slug would go and how much of the bypass do you actually fill. I would imagine as long as fuel could reach its port destination everything should be fine. Ken Cook

Plug runs from the bottom to the top of the bypass port. Basswood plug is 3/8 inch wide by the thickness of the port.

5/16 wide not 3/8

>5/16 wide not 3/8


Ah, Frank. That's within the manufacturing tolerances.

Ted

Ken,
You mentioned you cleaned up the bypass. Actually the little casting lip at the bottom of the bypass is a good place for the bottom of the plug to “rest”. Somewhere in the archives here in SS someone has some nice photographs of the installation of the plug. I always put a touch of JB Weld on the back of the plug to tack it to the case wall, but if its carved properly you want it to be a snug fit between case and sleeve. The lighter, tighter turning plane is probably why you are seeing the "burp" now.

Ted,
I’m still intrigued by how much bypass area that we PA guys need. I can see how speed people would think in terms of “bigger is better”, but the experience with the Fox 35 might say that there may be a better size. Obviously, if we completely close off the bypass the engine power will be diminished to zero. So I’m pretty sure there exists a lower limit to bypass area that comprises power output. But, the 5/16" plug doesn't hurt the power at all.

I still think that in the case of the Fox, that the reduced area accelerates the gases up the bypass and that somehow the quality of the run is related to how fast we are “slinging around” the cylinder head in space and its relation to the gasses trying to get into the chamber. The delta in the power curve of the before and after of this mod is nil, but the quality of the run certainly isn’t. PA has some high acceleration rates while trying to run a fairly rich setting. No other discipline tries to do this.

So I ask myself, “self, why do the $400 specially built PA motors have three big ports all the way around the sleeve, with great big volumes in the bypasses?” “Maybe we’d be better off with one smallish port that would get make the velocity of the incoming gas really really fast.” Maybe all the guys pouring the aluminum are racers. Maybe after I win the lottery I’ll try to make some of these.

this.
>
>So I ask myself, “self, why do the $400 specially built PA
>motors have three big ports all the way around the sleeve,
>with great big volumes in the bypasses?”

These motors are run at a greater RPM than a typical Fox .35. This helps keep the mixture velocity up at larger volumes.

Larry, NorCal Circle Jerks

>Ted,
>I’m still intrigued by how much bypass area that we PA guys
>need. I can see how speed people would think in terms of
>“bigger is better”, but the experience with the Fox 35 might
>say that there may be a better size. Obviously, if we
>completely close off the bypass the engine power will be
>diminished to zero. So I’m pretty sure there exists a lower
>limit to bypass area that comprises power output. But, the
>5/16" plug doesn't hurt the power at all.
>
>I still think that in the case of the Fox, that the reduced
>area accelerates the gases up the bypass and that somehow the
>quality of the run is related to how fast we are “slinging
>around” the cylinder head in space and its relation to the
>gasses trying to get into the chamber. The delta in the power
>curve of the before and after of this mod is nil, but the
>quality of the run certainly isn’t. PA has some high
>acceleration rates while trying to run a fairly rich setting.
>No other discipline tries to do this.
>
>So I ask myself, “self, why do the $400 specially built PA
>motors have three big ports all the way around the sleeve,
>with great big volumes in the bypasses?” “Maybe we’d be
>better off with one smallish port that would get make the
>velocity of the incoming gas really really fast.” Maybe all
>the guys pouring the aluminum are racers. Maybe after I win
>the lottery I’ll try to make some of these.
>

I am kind of agreeing with you here. One answer, given already, is rpm. The faster running engines get higher speed charges due to rpm (the piston closes down faster squirting the fuel up faster). Still, even the pipe ships are not running appreciably faster than the low(er) rpm engines. We run a Fox 35 at what? 7-8 thousand? We launch our pipe ships at maybe 10 thousand or a bit less.

Some years ago when we were still experimenting with blocking the by-pass on the 35-40 FP engines, Lew Woolard was experimenting with his Fox 40. He redid a Fox 40 for me that I had, and I suggested blocking the boost port. He was reluctant to try this on the Fox 40 since it has such narrow transfer ports. But he was willing to give it a try. He blocked a boost port in a case and a transfer port in a back panel and did some "mixing and matching". Blocking one trasfer port alone helped this set-up. Blocking the boost port alone was best, but the engine ran fine--and better than stock--with both the boost port and one transfer port blocked. That left one vary narrow transfer port remaining for it to run on. And with that engine we were running a 12-5 original BY&O at a higher rpm than with the FP.

Just an interesting experiment, but kind of backing up what you are thinking.

>
>So I ask myself, “self, why do the $400 specially built PA
>motors have three big ports all the way around the sleeve,
>with great big volumes in the bypasses?” “Maybe we’d be
>better off with one smallish port that would get make the
>velocity of the incoming gas really really fast.” Maybe all
>the guys pouring the aluminum are racers. Maybe after I win
>the lottery I’ll try to make some of these.


>
The PA motors are based on cases where the original design goal was HP. (IIRC the 40/51 on the pylon 40, 40 Lite on the .36 Combat motor)

Pat MacKenzie

>Ken,
>You mentioned you cleaned up the bypass. Actually the little
>casting lip at the bottom of the bypass is a good place for
>the bottom of the plug to “rest”. Somewhere in the archives
>here in SS someone has some nice photographs of the
>installation of the plug. I always put a touch of JB Weld on
>the back of the plug to tack it to the case wall, but if its
>carved properly you want it to be a snug fit between case and
>sleeve. The lighter, tighter turning plane is probably why
>you are seeing the "burp" now.
>
>Ted,
>I’m still intrigued by how much bypass area that we PA guys
>need. I can see how speed people would think in terms of
>“bigger is better”, but the experience with the Fox 35 might
>say that there may be a better size. Obviously, if we
>completely close off the bypass the engine power will be
>diminished to zero. So I’m pretty sure there exists a lower
>limit to bypass area that comprises power output. But, the
>5/16" plug doesn't hurt the power at all.
>
>I still think that in the case of the Fox, that the reduced
>area accelerates the gases up the bypass and that somehow the
>quality of the run is related to how fast we are “slinging
>around” the cylinder head in space and its relation to the
>gasses trying to get into the chamber. The delta in the power
>curve of the before and after of this mod is nil, but the
>quality of the run certainly isn’t. PA has some high
>acceleration rates while trying to run a fairly rich setting.
>No other discipline tries to do this.
>
>So I ask myself, “self, why do the $400 specially built PA
>motors have three big ports all the way around the sleeve,
>with great big volumes in the bypasses?” “Maybe we’d be
>better off with one smallish port that would get make the
>velocity of the incoming gas really really fast.” Maybe all
>the guys pouring the aluminum are racers. Maybe after I win
>the lottery I’ll try to make some of these.
>


In reference to piped engines, I believe the difference is the pipe itself. The scavaging of a piped engine is far better then that of a muffled one. In a Fox, the crankcase pumping is not just pushing the fuel charge up into the cylinder, it is also forcing the remaining exhaust gas out after the port and atmospheric pressues equalize. In the piped engine, the impulses actually cause the fresh charge to be somewhat sucked out of the crankcase into the cylinder. Decreasing the bypass area increases the transfer velocity which aids it scavanging. I don't believe the transfer port velocity itself effects the run, I believe it is in the effect it has on the scavanging that makes the difference.

I had begun a study of a number of recognised stunt engines and took silicone bypass castings with the object being to perform flow tests on them for comparison. Unfortunately, I got sidetracked, but hope to get back to that one day soon.

Randy Ryan <><
AMA 8500
SAM 36
BO all my own M's

Randy,
The flow tests would be very interesting....if you get back to them please tell all, whatever the results. I agree that the main beneficial effect of increased velocity would be additional scavenging of the exhaust gasses. Increased velocity might also result in slightly better inertial scavenging of the crankcase, which could result in higher cylinder pressure.

Randy C

>>5/16 wide not 3/8
>
>
>Ah, Frank. That's within the manufacturing tolerances.
>
>Ted

LOL! Ted, thanks for the chuckle.

>5/16 wide not 3/8


I always get that wrong.

Hi Ken, these photos were previously documented on the forum and as a Fox owner, I just happened to keep them.
Hope they help, (and hope they post) cheers - Tony

[photo not recovered: 29257.jpg]


[photo not recovered: 29258.jpg]

Attachment #1, (jpg file)
Attachment #2, (jpg file)

Hi Tony

That's exactly what the filler in my Fox looks like except mine's aluminium.

Yup, these are few of the photos for fixing the "Fox Burp". It sure cured mine. My engine would sound like it would quit running and restart as it went over inverted. Sure makes the pucker string get tight. LOL.

Paul Taylor

~I wish I knew everything you have forgotten about C/L.

Check out my Blog@
http://rootbeard.rcuniverse.com
A link to a slide show of a fun day: http://www.youtube.com/watch?v=V-_sR0ZydSQ