Leester :    I have 2 magnum 36XLS  and one has  a .025 and the other .026 both
run great I tryed a .027 and .028 the resultwas not as good. one engine has a Randy Smith needle valve and the other has a super tigre eather one will work great.

Leester...I'm confused. The "Pro" designation sounds more like a Thunder Tiger, as does the small venturi bore. Magnum would be XL or XLS, unless it was a real old one, that was made in Taiwan, and then it'd really be a Thunder Tiger with the Magnum name. The main difference is whether or not the spraybar blocks part of the venturi bore or not...

Then too, the early Magnums made in Taiwan (that later became TT's) have the offset "ST-type" venturi clamping bar, as do current TT's...mostly. That would make sense, if you have the small bore venturi, with the NV offset and not passing through the venturi bore center.

It sounds like Larry Wong is quoting the choke area...I'd have to check on my Leonard Neumann venturi chart to be sure. I could offer you a copy of it. I modified Leonard's chart a little, to make it more clear and understandable. Email me if you want me to email it to you. (sbasser at yahoo dot com)  If you can post or email me a photo of the engine, it would also clear up the ID.

Meanwhile...an idea struck...copy and paste the two charts for Magnum and TT .36's. Where it says "Modified", that's for blocked boost port, per Leonard's SSW rework. Unfortunately, the columns get botched up by the software.  

Hope this helps you figure it out...but I think you're ok on venturi bore to start with. IF you decide you need to open it up, go slow. Going up 1/64" will make a lot of difference, so you should investigate number drills, primarily. Letter drills will be useful for engines with the spraybar through the venturi bore center.  Drilling the Delrin will be interesting, as the drill will suck into the plastic in a fraction of a second, very uncontrollable.

Anyway, the chart....I reorganized the entries in vertical blocks for the first (smallest) size venturi for each engine, but only labled the first set of 4 for each engine. You're on your own after that! The first number is always the venturi bore, the second always the spraybar diameter, the third always the choke area, and 4th is always choke area per cu.in.

The idea here is to be able to predict how big you can go, and what will happen.  Leonard (and GMA) use large venturi bore to regain power lost by blocking the boost port (SSW) or detuning (timing and compression reduction in GMA's case). Fuel consumption is therefore quite high.

There are many approaches to making any given engine work for stunt. All work, apparently, but everybody has their preferences, and it's probably a regional thing. What works is copied/recommended. Locally, we've been seeing a lot of R/C conversions with blocked boost ports. It's easy to do, and settles the engine down real well. Very much like the difference between an FP and an LA.   Steve

Magnum 36   
.270 (restrictor-type venturi bore, spraybar through)
.155 (spraybar)  
.0154 (venturi area)         
.04279 (choke area per cubic inch)


.274      
.155   
.01649455   
.0458

.278   
.155   
.0176   
.0489

.281
.155   
.01846
.05128

.287
.155
.02020
.056

.296
.155
.022933      
.0637

Thunder Tiger 36   
.144 (True Venturi, no spraybar through)   
.155 (spraybar dia., who cares?)
.016286 (choke area)   
.046 (choke area per cubic inch)

.147
.155
.01697      
.04714

.154
.155
.0186265
.05174   

.160
.155
.0201
.05585

.166
.155
.02164
.06011   

(modified)
.169
.155
.02243
.0623

.172
.155
.0232352
.06454

.173
.155
.0235
.065295

.177
.155
.0246
.068333

.199
.155
.0311
.086

.228
.155
.040828
.134

Hi every body

I've been searching for the proper dimensions of a venturi for TT 36. To modify my R/C engine into control line.
The mentioned spec by Steve are very helpful
But the question is:
As I know the profile of a venturi is just like an hour glass with a restricted area at the middle and wide areas at the ends, I guess the mentioned dimensions are for the middle restricted area, then what whould be the diameter of the two ends and also the length of each cone?
The length of the cone and the diameter of the restricted and wide ends also affect the taper ratio of the cone which affects the air flow and pressure in the Venturi.

Some say I can use a simple cylindrical hole instead, does it work?

Any opinion is appreciated,

Regards,
Ehsan

Hi every body

I've been searching for the proper dimensions of a venturi for TT 36. To modify my R/C engine into control line.
The mentioned spec by Steve are very helpful
But the question is:
As I know the profile of a venturi is just like an hour glass with a restricted area at the middle and wide areas at the ends, I guess the mentioned dimensions are for the middle restricted area, then what whould be the diameter of the two ends and also the length of each cone?
The length of the cone and the diameter of the restricted and wide ends also affect the taper ratio of the cone which affects the air flow and pressure in the Venturi.

Some say I can use a simple cylindrical hole instead, does it work?

Any opinion is appreciated,

Regards,
Ehsan

The important thing is that there is a restriction in the middle, with more open ends.  Lots of folks don't understand how venturis work and give bad advise -- I can't say that I'm a super-expert, but I'll let you know what I know:

If you have an engine that uses a "pipe with a spraybar in the middle" style of venturi, then you can just make a constant-sized hole and bung a spraybar through the middle of it.  This will work.  I feel that things work better if you taper the inlet and the outlet so that the section with the spraybar is constant-diameter for about 1/4", tapering out to the outside world on one side, and tapering out to the engine on the other.

If you have a "true" venturi, then you need a tapered section, and you need to make sure that the fuel gets to the edge of it.  Also, some feel that it works best to have a sprinkler of some sort that extends at least some distance into the venturi -- see if you can learn how to use the search bar and look for posts on the subject.

As for size, the important thing about venturi sizing is that you get the total area of the throat correct.  Folks will often just talk about the diameter of the hole, totally ignoring the size of the spraybar -- this can lead to confusion, and to poorly-sized venturis.  I can't remember the exact number, but for "full sized" stunt planes you want a total venturi area of around 0.018 square inches (0.281 diameter with a 0.156" spray bar).  If you achieve that with a different-sized spraybar, or with a "true" venturi then you need to keep the overall area about the same (the Magnum 36 venturi cited has an area of about 0.023 square inches; less if there's a sprinkler intruding into it).

Inneresting.  And yet, your old Magnum 36 runs great (it did a nice pattern yesterday, in fact - but it had to be attached to an airplane to do so).

John Thompson and Bruce Hunt both tried mightily to get that TT .36 to stop the "runaways", but it always turned into 4.5 second laps on the ex-McClave "Black Bird". Which, of course, was originally powered by an Aero Tiger .36, and didn't do silly stuff like that.

The Magnum XLS .36's are sweet engines, no doubt. That engine you have surely has 20+ gallons of fuel through it, and I'm sure it'll still do 1 flip starts if you get the procedure right. Can't complain about that!  Steve

I have a Magnum Pro 36, the venturi on it is Delrin. The opening is 11/64th's this seems awful small. I have not run this engine. Is this actually to small? If so what would be the suggested size? Can I drill this out on a drill press? If so what precautions should I take when drilling it (if neccesary)Now if you say a size of 283 or what ever and I go to True Value is that the size bit I ask for?  All help appreciated I need to learn this venturi stuff.

HI

I set them up with a venturi that has a number 17 or 18 drill bit size  hole, this is around the equivalent of a  .275 restrictor. They run very well with a true venturi  that size. There are hundreds out there that I did before  they were replaced  with  Thunder Tiger Pro 36 engines

Randy