Pipe length for Macs 15cc pipe on CMB90

[Ian Inverarity]

Hey guys,

On a stock CMB90 what would be the suggested pipe length using a Macs 15cc pipe(specifying also where it is measured to! B) ) ?

Thanks,

Ian.

 

[Anders_Martinelle]

12 inch and go from that is a good start,messured from the end of the cone to glowplug.

Anders

 

[Ian Inverarity]

Anders,

Is that to the end of the diverging cone or start of the converging cone? :huh:

Thanks!!

Ian.

 

[westoz]

Nitrocrazed measure to the widest point on pipe to centre of plug just got new cmb90 macs pipe coming 12inches sounds like good starting point.cheers

 

[Ian Inverarity]

The Macs 15cc pipe has a parrellel section in the middle, which is why I was asking about where to measure it, at the start or end of the parrellel section. B)

Ian.

 

[westoz]

So its not a standard nitro pipe I would probably measure from start of the converging cone.

 

[Wild Bill]

Hi Ian. In my experience, the pipe should have a slight join line in that flat section. I measure from the plug to that line. My twins run at 12" with the same pipe.

See ya.

Bill Annabel.

 

[Bob Morton]

Twelve inches :lol: :lol:

Bob Morton B) B)

 

[Mitch Graber]

What type of hull?

What prop is being run?

What percentage nitro?

What is the exhaust timing?

VERY hard to answer your question

without knowing any of the above.Mitch

 

[Scott Schneider]

If you have one of MACS older 15cc pipes that has a flat section ? then measure from the rearmost edge at start of divergant cone.

stock engine and @ 40% fuel ? 12" from case certerline / plug to rear cone. @ 12" is about right. If higher nitro or higher EX timing then a LONGER pipe is required. B) Scott

 

[Ian Inverarity]

Thanks for the tips guys! B)

Ian.

 

[John Knight]

Ian,

I have 2 highly modified CMB 90's with a MAC 15CC nitro pipe with a add on muffler. I measure from the center of the flat spot in the pipe center which is where my pressure fitting is located. Mine is 11". Running a Roadrunner rigger. Engine was modified for 10.5" but I generally run it longer as the needle gets too sensitive. Mono's will need to be longer.

John

 

[Mitch Graber]

John,curiosity has got the best of me.....how was this

CMB 90 engine "modified" to run a 10-1/2" pipe?

Also,what prop and pitch are you currently running?

Thanks.Mitch

 

[Don Ferrette]

John,curiosity has got the best of me.....how was thisCMB 90 engine "modified" to run a 10-1/2" pipe?

Also,what prop and pitch are you currently running?

Thanks.Mitch

I was kinda wondering that myself.............. :unsure:

 

[John Knight]

Since I did not do the mods myself I can only pass on what I was told. Have a gold head Evo 90 with incredible changes made to it. Both intake and exhaust timings have been raised. Sleeve has been significantly reported and piston skirted and opened up. Stan Simpson from Fla did the mods. He told me that he did the mods and matched the MAC 15cc (90) nitro pipe for a 10.5" length. Guys, I am not smart enough to figure out the physics of it all, but I can tell you it works. But I have to be right on with the needle. If it is, it will run with twin riggers. After a few frustrating days/races, I started backing the pipe out to widen the needle band. Cost a few mph (2-3) but sure reduced the angst. The boat & engine was IMPBA District 13 champion in 1999 & 2000, with 2nd's and 3rd's since then. Just for grins, Jerry Crowther and I opened it up and put new bearings in it and he said that he has never seen an engine so modified. And he is a multiple world champ!

Almost 2 years ago, I got ahold of an Andy Brown purple head CMB 90 that he owned and sold at the Huntsville Internat's. The allure of getting an Andy engine was too much to pass up. It is also highly modified but mostly through the exhaust side, sleeve reporting and piston skirting. Been working with that engine in the recent past just to see what it will do. This engine seems to like about 11" of pipe length. Also changed hulls last year so it is still a work in progress.

Have used a number of props but generally use a Octura 1667, pitched and cupped. Have several different pitches and cups. Nothing stock. Also use the Octura 4 blade S series. Have also used the Octura H-45 & H-50. Don can give you better advice on props as he is more knowledgeable than I about them. He runs F hydro and gives me advice from time to time.

 

[Mitch Graber]

John,thanks for the info,I wasn't looking for prop advice,I

was just wondering how much cup you were running on

that boat and motor combo to run the pipe that short.I am

sure if I ran 1/2 the amount of cup I am running now I could

jam the pipe that short also.......but why would you want to,

when like you said,the needle margin gets very small,and the

boat starts acting very pipey,NOT a good combo for heat

racing.Been there,done that.....very frustrating.And who cares

if you have the fastest boat during mill time,if you can't put

6 laps together and finish heats,I am all for dependablity and

wanting to finish EVERY heat.Mitch

 

[John Knight]

Mitch,

Have several 1667 props that I use. Cup starts about 5.0" and goes up to about 6.2". Don't remember which one I have on it now but will find out tomorrow in Atlanta.

I'm with you about finishing. Fastest boat for 3 laps will not cut it. That's why I do what I do. Seen too many run over the edge and not finish. They get a kick from getting all of the attention but not finishing is the same to me as throwing out the anchor!

John