Finicky pipe

[tracerbob]

Hey guys,

I'm trying a new pipe put and I can't seen to find the sweet spot. The boat is a Microburst hull approximately 42" long, .67 silver head Mac running a X357/3 prop. My pipe is a Brown parabolic initally set at 11.5 inches from the center of the cylinder to the seam on the pipe. At this setting the rpms were down in the straights and it would speed up in the corners. So I shortened it ot 11.0 inches and the it was faster in the straights and slower in the corners. I finally pulled it out to 11.25 with not much improvment over the last setting. I have read that some people are opening up the hole inside the pipe at the base of the stinger to make the pipe a little more managable. Anybody ever heard of this? If you have, what size did you open the hole up too? Andy, are you out there?

 

[AndyBrown]

Yes Robert,

Bore out the stinger to full diameter, which is .470" or something close to that.

Andy

 

[tracerbob]

Yes Robert,Bore out the stinger to full diameter, which is .470" or something close to that.

Andy

Ok, let me double check here. Were talking about the bare metal pipe with the smooth shape, not the black pipe. Just wanna make sure were on the same sheet of music before I attack this thing with a drill bit. Will you be in Atlanta for the Spring Nats?

 

[John Knight]

Bob,

I have a number of these pipes, from .21 to .90. Have drilled all of them out. Use a drill bit just small enough to fit inside the stinger and drill out the internal baffle. Do NOT change the size of the stinger by drilling it out! Be extra careful to flush out the metal filings. Don't want them sucked into the engine. Drilling them relieves the backpressure and does not really affect the DB (At least I have not noticed any changes). I have MAC's, NR,s Picco's and CMB's and all of them run better with the baffle drilled.

I will be in Atlanta so look me up and I can show you what I have.

John

 

[[email protected]]

Interesting!

 

[John Knight]

One other note I missed earlier. The Crapshooter guys from the midwest told me at Atlanta's spring race that the drilled AB parabolic pipes can be shortened. As a baseline, their 45, Picco and MAC powered engines were flying as if on rails. Drill the baffle out, and a modified engine can run a 8.5" parabolic pipe. Stu Barr (Crapshooter owner) is reported to run a 8.250" pipe. I shortened my pipe over an inch and it does work! An increase of 7+ mph.

I then starting drilling and testing all my others. An improvement as noted above was seen.

 

[Don Ferrette]

Like John said, drill the baffle out using a bit just a touch smaller than the stinger i.d. DO NOT make the stinger i.d. any larger.

 

[John Knight]

Bob,

One final note for you. I have a rigger with a 67 MAC bluehead (mfg in 2002) with the AB parabolic aluminum finish (not the black one). I know you run a mono but this will blow your mind. My pipe length is set a 10 1/8" (just measured it). But a rigger can run a shorter pipe. And I know that I can run it shorter and get more speed.

Maybe some of the mono guys running a similar setup can help. I am strictly a rigger racer but I suspect that your pipe length is about right. Is the prop too large for the engine to turn on really well? Might try a slightly smaller prop to test with and see what it does.

John

 

[tracerbob]

Interesting answers. I ran the boat this weekend with the drilled out pipe set at 11.00 center of cylinder to the weld. There was a marked difference and a new set of problems. At this length, it would peak in the straights, but started to sag in the turns. I reset the pipe to 11.25 and tried it again. Now it only sags beginning at halfway through the turn and doesn't peak untill it's about 8 feet or so exiting the turn. I would of tried it at 11.50, I ran out of adjustment on my pipe bracket, so I'll try that next weekend. I measured the inner diameter of the stinger and found a 29/64 drill bit that fit. It's slightly smaller than the inside of the pipe and worked fine. Metal chips were a big pain in the butt though. What drill sizes did you use on the 20 and 40 pipes?

 

[Don Ferrette]

Interesting answers. I ran the boat this weekend with the drilled out pipe set at 11.00 center of cylinder to the weld. There was a marked difference and a new set of problems. At this length, it would peak in the straights, but started to sag in the turns. I reset the pipe to 11.25 and tried it again. Now it only sags beginning at halfway through the turn and doesn't peak untill it's about 8 feet or so exiting the turn. I would of tried it at 11.50, I ran out of adjustment on my pipe bracket, so I'll try that next weekend. I measured the inner diameter of the stinger and found a 29/64 drill bit that fit. It's slightly smaller than the inside of the pipe and worked fine. Metal chips were a big pain in the butt though. What drill sizes did you use on the 20 and 40 pipes?

Go down just a touch on the prop size or pitch, the MAC's like to rev a little. Sounds like your straightaway speed is there but just a touch too much bite in the corners. If you keep stretching the pipe you kill off top speed. Another thing to try is to take the prop you ran & relieve it slightly right where each blade meets the hub so it will unload a little more in the turns.

 

[TimD]

Interesting answers. I ran the boat this weekend with the drilled out pipe set at 11.00 center of cylinder to the weld. There was a marked difference and a new set of problems. At this length, it would peak in the straights, but started to sag in the turns. I reset the pipe to 11.25 and tried it again. Now it only sags beginning at halfway through the turn and doesn't peak untill it's about 8 feet or so exiting the turn.

To me that's not another set of problems - that is a good sign.

As Don suggested - prop down slightly and use the rev's. I'd even go as far as setting the pipe back to your starting length as it was still getting on pipe that short - and prop it accordingly. Smaller prop doesn't mean slower You might find you have to run it slightly richer than before.

Please let us know your progress!

Tim.

 

[tracerbob]

That sounds like a logical next step. I'm running a box stock X457 right now. Any suggestions? Very good input folks, I feel brain cells growing?

 

[tracerbob]

Forgot to mention...It's a X457/3. How bought switching to the 2 bladed version?

 

[Don Ferrette]

If you're happy with the handling I would just relieve it at the hub. The 2 blade gives slightly less lift & may change the way the boat acts. B)

 

[tracerbob]

Don, Do you have a picture of this mod?

 

[Don Ferrette]

How's this----

 

[bsmtboy2]

I am still confused about this, drill out the internal stinger inside the pipe how far in ? You also mentioned not to make the outer opening any larger . I noticed my CMB pipes also have a internal stinger will this mod work on those as well ? :blink:

 

[TimD]

I am still confused about this, drill out the internal stinger inside the pipe how far in ?

Drill right thru. It's not a tube like the outer stinger.

 

[tracerbob]

Hey guys, there is an actual baffle of sorts at the base of the stinger inside the pipe. Like there was a washer welded to it. it's really the end of the rear cone of the pipe. They just don't open it up to the inner diameter of the stinger. What that leaves you with is a small hole about and 1/8 of an inch to let all the gases out. I don't recall any other pipes having this feature.

 

[tracerbob]

Don, did you remove any material off the trailing edge of the blade other than the notch at the base?