Venom clone

It looks like a Black Widow with a regular new SPI cylinder/piston set and a KB or DD crank. Same venturi as the BW. So if I change the crank and put a new style SPI set with a TD head on my BW it's a venom? Am I missing something special?
From what I guess from that link is that you have to use parts hand chosen for a specific fit and feel. What else does it take to make a Venom? I want one. I have all that stuff except for the crank, but I don't like the loose fit of my new SPI set. It's been run once on my Medallion. I have another that feels the same on my semi-Killer Bee( minus the crank and head).


EDIT: I looked again, I see it has the SPI set with the slit bar removed. I thought Bernie said they found that caused too much SPI and lowered the performance. That's why the one they call the TD has a regular long skirted piston.

If you look at a #4 or #5 piston/cylinder assembly installed on an engine you will find very little SPI, probably only about .0010" or so (just an estimate). I believe that the SPI comes from the exhaust opening being cut slightly lower than on most cylinders, not from the pistons being shortened. I have never actually measured this so I could be completely mistaken.

Bob

fit90 wrote:If you look at a #4 or #5 piston/cylinder assembly installed on an engine you will find very little SPI, probably only about .0010" or so (just an estimate). I believe that the SPI comes from the exhaust opening being cut slightly lower than on most cylinders, not from the pistons being shortened. I have never actually measured this so I could be completely mistaken.

Bob

I was talking about people using the shortened piston from this: http://coxengines.ca/product.php?productid=122&cat=13&page=1 with this cylinder: http://coxengines.ca/product.php?productid=119&cat=13&page=1

Yep, the short piston on the left installed in the cylinder on the right.

That's the combo Bernie says doesn't work well. That short piston gives way more SPI than any old style setup like #1 or a #4 TD. The cylinder on the right is just the one on the left with the slit bar removed. The SPI is a huge gap. Look through the slits and you'll see the skirt completely opens the lower gap. The air resistance of the slit restricts the air flow so as to prevent too much induction.

I questioned him about it starting in in post #16, read his replies. He's talking about trying it in his Venom.
http://www.rcgroups.com/forums/showthread.php?t=1533873&page=2

fit90 wrote:If you look at a #4 or #5 piston/cylinder assembly installed on an engine you will find very little SPI, probably only about .0010" or so (just an estimate). I believe that the SPI comes from the exhaust opening being cut slightly lower than on most cylinders, not from the pistons being shortened. I have never actually measured this so I could be completely mistaken.

Bob

You are not mistaken, that's exactly right. I've had Q&A a couple of times over the past year about this with Bernie at RCG.

It seems you can run an open port cylinder with a non SPI piston, or you can run an SPI piston with a slit port. Combining open ports with the short piston is a dog.

I'm headed to the shop to play with my new Sullivan 1 ounce clunk tank. And to dig up my old postage stamp backplates, seems I have a taker on my offer to unload them. See y'all later.

RknRusty wrote:Yep, the short piston on the left installed in the cylinder on the right.

That's the combo Bernie says doesn't work well. That short piston gives way more SPI than any old style setup like #1 or a #4 TD. The cylinder on the right is just the one on the left with the slit bar removed. The SPI is a huge gap. Look through the slits and you'll see the skirt completely opens the lower gap. The air resistance of the slit restricts the air flow so as to prevent too much induction.

I questioned him about it starting in in post #16, read his replies. He's talking about trying it in his Venom.
http://www.rcgroups.com/forums/showthread.php?t=1533873&page=2

Yeah, in my plans that I had available on my site, I said to only use a TD .049 #4 cylinder and piston assembly. The lightened piston (remove material without shortening skirt) would be optional but would probably increase the RPM. I remember when Matt tried the Venom clone with the TD/SureStart cylinder and SPI piston and he said it didn't run worth the crap. Bernie also told me awhile back that the combination worked but not very well at all. I managed to get my hands on a few 1775 #4 assemblies from Estes when they still had most of the engine parts available on their site but once they went away, the TD cylinder became very very hard to get. I only have one cylinder assembly left and it is on my clone. The other 3 I had were all sold to old buggers out at the field.

I'd like to have at least one spare new #4 set, but it probably won't happen. Bernie swears his work as well, but I'm skeptical. I'm sure I'll try one one of these days.

I just did that- combined a SPI piston+ the TD/SS cylinder... But if it doesn't go well, I'll just install the piston coming with the TD cyl

I coudn't wait, so I just ran it...



It performs well, but I was expecting some higher pitched noise... I guess it was around 18-19k, but can't say it with precision...
It certainly must come from the lightened piston as you guys said me, but the fuel was 10% nitro (10% castor+10% synthetic, home mixed), and it just had 2 tanks throught it, so It's maybe not completely broken in. I also installed 3 heads gasket, as it wouldn't start without it...


Oh well, I'm already happy to run it, and if I find a #4 cyl+piston, I'll just install it right in!


EDIT: I just watched this other video from an actual venom, and it's finally not so bad!

So which cylinder/piston combo is on yours?
It sounds a little faster than most old BWs, though I do have one that inexplicably runs a lot faster than the rest. Was that a 5" prop?

Another thing that I wonder about; the short light pistons achieve higher RPM, but I wonder if they are lower on torque due to the reduced inertia in the rotating assembly.

RknRusty wrote:
Another thing that I wonder about; the short light pistons achieve higher RPM, but I wonder if they are lower on torque due to the reduced inertia in the rotating assembly.

Just as a guess, I would think not, since the KB crank is more substantial than the standard crank and the piston represents reciprocating weight vs rotational weight of the crank.

But, it's just a guess.

I don't how much of an impact prop diameters have on torque, but we do know that prop weight is important (kickbacks and load reversal). Since the KB/Venom clones are usually propped with smaller diameter (and lighter) props, that variable may have a greater impact on torque than the piston weight. My understanding of torque vs. HP vs. RPM is limited, so I'm playing more of a devil's advocate here.

andrew

It ran with a 5"

So basically I just built myself an expensive Black Widow ... Well, just by the looks of the engine, it's good enough for me

If the engine has less torque, what would be the consequences? I can't really figure it out, as I guess it's not like a car where the load will vary a lot... Here it's a (nearly) constant load, isn't it?

ideeman1994 wrote:It ran with a 5"

So basically I just built myself an expensive Black Widow ... Well, just by the looks of the engine, it's good enough for me

If the engine has less torque, what would be the consequences? I can't really figure it out, as I guess it's not like a car where the load will vary a lot... Here it's a (nearly) constant load, isn't it?

Great to see that it runs and actually sounds good and not sluggish. You really need a tach to see what it is really dong but the pitch of the whine sounds close to Reggie's Venom.

You would need the torque to overcome the air resistance and pull the plane around.

ideeman1994 wrote:It ran with a 5"

So basically I just built myself an expensive Black Widow ... Well, just by the looks of the engine, it's good enough for me

If the engine has less torque, what would be the consequences? I can't really figure it out, as I guess it's not like a car where the load will vary a lot... Here it's a (nearly) constant load, isn't it?

The load when on a test stand is constant. Depending on what aircraft you chose to fly it with will vary the load. I'ts a nice engine no matter how you slice it. You did a great job! Have fun with it.

Thanks! I'll sure will

I built my copy of Venom like this but have a true #4 P/L. Davis killer crank and enlarged venturi size in backplate. I also did same on a Black Widow but have not run either one at this time. It does look purdy.
Dan

Bernie may I ask (if you dont mind sharing here)
How may Piston cylinders does Paul buy to make one engine?
Because the chances are if you just "slap" an engine together the chances are you wont have a perfect cylinder piston fit.
I have noticed if you buy 3 sets yo will get one or two with absolute perfect fits.

Cox International wrote:I don't recall Paul buying any large #'s. It's more like 2-3 at a time.

There are persistent rumours that the Surestart engines do not have good compression in general and I have to disagree. If I had a buck for every buyer that complains his engine would not start (because the compression was too high) it would make for a nice dinner at a restaurant.

We especially run into overcompression issues with this product:

http://coxengines.ca/product.php?productid=362&cat=27&page=1

where some modellers have to use up to 4 glow head gaskets to get the engine to start. I mean, it's a nice problem to have.

By the same token, I advocate using that product over the OEM Cox high-compression head as it does lead to even higher RPM. I have had no problems getting up to 22,000 RPM out of a reedie with SPI piston and Killer Bee crank.

Bernie
www.coxinternational.ca

Good to know!

GENTLEMEN,

To end the speculation.... Somehow over many years I accumulated at most~ 30 pistons in my piston drawer. Years ago, I was fortunate to be able to buy single TeeDee pistons from the late Joe Klause when I visited with him. Mine are graded (only by hand feel) into 4 groups: small, medium, large & orig Venom. The original (lightened) Cox Venom pistons I won't use as the failure rate is just too high. I only compete once or twice a year, so I can't be bothered using unreliable pistons. I think my selection is now down to perhaps 15 after producing a batch of ~20 Mouse Race engines which I've since sold.
SOoo...since it's a certain "hand feel" that I'm looking for, I try everything in the size group. So in a say medium size group of ten, I'll try them all. Of the 10, perhaps 3 or 4 will fit well. Then I'll do one last final solvent cleaning & try the last 4 again. I'll settle on perhaps 1 & a spare. Then I'll do a bench run. If I like the TACH result I'll leave it. If no good, I'll swap with the other (spare piston) & try again. Certainly it's a somewhat tedious endeavor, but it does give me powerful ready to race engines right from the test stand with little or no need for a break-in.
....and now you know the REST of the story!

Cheers, Paul

Welcome to the forums Paul.