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Back Plate Hole size for Venturi (behind screen) Babe_b10
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Cox Engine of The Month
September-2020
sosam117's

"Traded an electric charger for this "Complete" (original) Cox Thimble Drome Olympic .15 engine. Right corner is photo as received."



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Back Plate Hole size for Venturi (behind screen)

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Back Plate Hole size for Venturi (behind screen) Empty Back Plate Hole size for Venturi (behind screen)

Post  sosam117 on Thu May 14, 2020 5:56 pm

While cleaning up my Texaco engines I noticed the Venturi hole sizes were different.
The original "Texaco .049" engine with the black back plate has a venturi hole size of .086"
The original "Texaco .049 Jr." engine with the gray back plate has a venturi hole size of .063"
So, I looked at my old Thimble Drone .049 with the metal back plate and the venturi size is .063"
I modified the Texaco .049 back plate with a 3/32 o.d. aluminum tubing 1/16 long
and press it into the .086 diameter venturi hole.
The i.d. on the aluminum tubing is 1/16 dia.

Why were the larger 8cc Texaco engines have the larger .086" holes on the venturi?
Is that why the Texaco Jr. had the "gray" back plates to tell the different venturi size?
And now I know why when I flew in the SAM Texaco event, some of the other contestants had the old metal back plates.
They got longer runs than what I was getting.
I tried to get longer runs by removing some of the plastic inside the back plate but that was still not enough.
You can see the removed plastic in the photos.

I'm guessing the larger venturi hole was for the Black Widow and other hotter engines?
What are your ideas?

Original Cox Texaco .049 back plate with a .086" diameter venturi --- plastic removed inside for more fuel.
Back Plate Hole size for Venturi (behind screen) Origin10

Original Cox Texaco Jr. .049 back plate with a .063" diameter venturi.
Back Plate Hole size for Venturi (behind screen) New_ai10

Original Cox Texaco .049 back plate modified with a 3/32 o.d. x 1/16 i.d. x 1/16 long aluminum tubing.
Back Plate Hole size for Venturi (behind screen) Mod_ai10
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Post  aspeed on Thu May 14, 2020 7:09 pm

As long as you are allowed to put inserts in, you could go even smaller than .062" Just make it the right size to lift the plane up steady.
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Post  sosam117 on Fri May 15, 2020 5:35 am

aspeed wrote:As long as you are allowed to put inserts in, you could go even smaller than .062"  Just make it the right size to lift the plane up steady.

The 3/32 aluminum tubing is a nice press fit into the larger hole.
So the I.D. of the tubing automaticly is 1/16 of an inch.
Only way it would work out is try to find thick walled tubing to reduce the I.D.
Haven't found any thick walled aluminum tubing?
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Post  aspeed on Fri May 15, 2020 4:25 pm

I guess I am spoiled with a lathe. A piece of welding rod with an appropriate size hole would work. It would need to be fairly centred, or solder up brass tube and drill it out.
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Post  Basswood on Fri May 15, 2020 8:21 pm

Sosam 117
My thinking is when you pressed in the aluminum tubing you probably almost took away as much volume as you created removing the plastic in the fuel tank.
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Post  aspeed on Sat May 16, 2020 3:06 am

Basswood wrote:Sosam 117
My thinking is when you pressed in the aluminum tubing you probably almost took away as much volume as you created removing the plastic in the fuel tank.
Basswood
I think it is in a separate passage before the tank, so it would be a plus still. You can also turn away some of the centre part, as it tapers down to about 3/32" so you could get a few more drops of running time.
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Post  Basswood on Sat May 16, 2020 7:36 am

[quote="sosam117"]While cleaning up my Texaco engines I noticed the Venturi hole sizes were different.
The original "Texaco .049" engine with the black back plate has a venturi hole size of .086"
The original "Texaco .049 Jr." engine with the gray back plate has a venturi hole size of .063"
So, I looked at my old Thimble Drone .049 with the metal back plate and the venturi size is .063"
I modified the Texaco .049 back plate with a 3/32 o.d. aluminum tubing 1/16 long
and press it into the .086 diameter venturi hole.
The i.d. on the aluminum tubing is 1/16 dia.

Why were the larger 8cc Texaco engines have the larger .086" holes on the venturi?
Is that why the Texaco Jr. had the "gray" back plates to tell the different venturi size?
And now I know why when I flew in the SAM Texaco event, some of the other contestants had the old metal back plates.
They got longer runs than what I was getting.
I tried to get longer runs by removing some of the plastic inside the back plate but that was still not enough.
You can see the removed plastic in the photos.

I'm guessing the larger venturi hole was for the Black Widow and other hotter engines?
What are your ideas?

Original Cox Texaco .049 back plate with a .086" diameter venturi --- plastic removed inside for more fuel.
Back Plate Hole size for Venturi (behind screen) Origin10



It looks to me like the aluminum fill stem is down inside the fuel tanks cavity. It would have to be to fill the tank.anything you put down inside that cavity (fuel tank)is going to reduce its capacity.
So if you pull that aluminum stem out to the edge of the back plate it will increase the volume in the tank.just like cutting out the plastic.try to find a glue that's fuel safe and glue it.
Now what I'm trying to understand is. are you trying to reduce the engines fuel consumption by restricting the air flow of the fill tube/stem?
otherwise use .014 tubing rather .035 or .045 thick wall tubing and you may get a drop or two in the tank fill tubing/stem.If you used an end mill you could remove more plastic in the back plate also.but it looks like you did a pretty good job of that already.
Just my two cents.
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Post  getback on Sat May 16, 2020 12:35 pm

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Post  Basswood on Sun May 17, 2020 7:08 pm

Thanks for the link getback. lots of great info in it. keep in mind I'm looking at this strictly from an engineering point of view only.
you could also make tiny thin washers out soda pop can with different size drilled center holes to slip between and venturi stem seal and the fuel tank back plate.
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