Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

polecat w/40jett motor

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Old 09-09-2007, 11:00 AM
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SACKRC
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Default polecat w/40jett motor

Got a pole cat pylon plane and put a 40 jet carb mtr. in it. Gettin ready to fly next week.
Question is I bench ran the motor with the sugested prop with 30% nitro. prop 8.8/8.5. and a mc 9 plug . also with a full pipe (mac pipe) from jett eng. The motor is taching 18,300 18,500. After shorting pipe. throtle response is still good. MY question is I dont want to cut the pipe to short is this a good rpm.
Motor is stock, talking to dub jett he states I can go up on the exhaust .030 ( rpm should increase) with out to much trouble. but if I do it will response still be good , dont need any stalls due to the motor trying to get on pipe. Also will the pipe stay at the same lenght when I rasise the exhaust, hope fully..


Thanks Rich B.
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Old 09-09-2007, 08:04 PM
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Default RE: polecat w/40jett motor

leave the pipe alone..... Dub does a good job setting up the full size piped engines.

if you want to change the effecive length.... change the prop instead. There are a lot of props in the D1 series to work with.

Which engine is this ? A 428 Q-500 .40 ? or another model ?

The pipe length defines the rpm. Anywhere around 18.5-19K is a great point to work with. You have a baseline rpm and prop. If it runs well where it is now... consider not changing it any more. If you wish..... try a few props.

Load the engine up with more prop until you see the engien drop in rpm.... or where it starts becoming difficult to needle. If it gets cranky or struggles to get on the pipe, do not continue or in any way force the engine... it will get hot rapidly, and that is rarely good. Try up to the 8.75x9W and 8.8x9. (Should probably run there ok qirg rgw 9w) At that point where you see performance drop, the pipe is effectively too short.. and you know the rpm/prop that is now that limit.

Unless you are looking to go from 170 to 180 mph for a real good reason ....... i would leave the timing alone..... keep the pipe where it is.... and then try a variety of props.

Otherwise... yes, you can go with the timing change..... and yes, it will affect the engines ability to idle and transition with the higher timing. It will still run great, especially with the light prop..but consider it will become just a bit less user friendly and more critical on the pipe/prop setup... and a missed needle setting will cost you a plug.

Bob
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Old 09-10-2007, 05:20 PM
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Default RE: polecat w/40jett motor

Bob, bought it last year from dubb jet.side exhaust 40 with red carb and black mini pipe. The pipe and header was sent to me an inch too long, after cutting a quarter a shot rpm when up and settled at 18.5 With your experence am I in a good range on rpm. Last year I had it in a matney zoomer and did 160 mph on 30%. The pole cat is liter and has less wing area, less air foil . Hopfully more speed to follow. l
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Old 09-10-2007, 08:05 PM
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Default RE: polecat w/40jett motor

Which polecat are you talking about? I assume one from www.supertrc.com or www.bigbruceracing.com ?
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Old 09-10-2007, 08:07 PM
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Default RE: polecat w/40jett motor

BTW, I ran that same prop on a stock Nelson Q500 (SS) motor this weekend and it peaked at 19,800 with 15% Powermaster fuel.

Just for comparison. I have never used a full tuned pipe setup, just the standard quarterwave Ultrathrusts and JettStreams.
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Old 09-11-2007, 06:06 AM
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Default RE: polecat w/40jett motor

Thats strong rpm, on that nelson. A full pipe has better range and with less stagger as much when getting on pipe. and more rpm (on the right engine). I have to check my exhast timing. I think its 160 or 165. Jet state 170 max. Youve got 1000 rpm more and less nitro that strong performance in comparison. I do known the nelson is a superior motor in quality, had no idea on how much. thanks for info got a better bench mark to go for. Got to check timing on crank , exhaust and intake. If you got numbers on the nelson, love to know them!!!!! thank. rich b.
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Old 09-11-2007, 08:44 AM
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Default RE: polecat w/40jett motor

Thats just it, I'm only good at full throttle

The Jetts are of equal quality to the Nelsons, I several of each. My motor is a standard Nelson Q500 motor used in 428 racing, I have no idea in regards to your timing questions. Just stick with what Dub and Bob advise and you will be good to go. Tough to improve on what they know.
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Old 09-11-2007, 10:59 AM
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Default RE: polecat w/40jett motor

I gotya on the engine...

That is one of the Jett 428 racing engines....... black muffler...... but with the throttle carb installed.
Dub sells a bunch of those. Great for sport Q-500 flying.
They only turn 300-400 rpm off of the full racing version.

Since such is the case.....
you want to use a 8.8x8.5, 8.8x8.75 or 8.75x9W prop.

If you have 15% fuel..... I would suggest using that. The 30% is ok, but I have absolutely no experience running the 428 engine on anything besides the 15% fuel.... so it would be hard to compair numbers.

Pipe tuning is simple.... follow the instructions on the Jett website. The full pipe on the Q-500 engine can allow you to really push the engine a bit..... but you will want to keep it a bit user friendly.

Put the 8.8x8.75 on there and keep cutting 1/4" at a time until it is running over 19K rpm...... then cut 1/8" at a time. Should get to around 19,800 rpm or so, but watch for that next 1/8" cut where it will stop rising in rpm or where it drops off. Stop there. The needle valve may be hard to set at that point. That is ok. Only try to find peak rpm BRIEFLY.... find it.... and back the needle out 1/2 a turn quickly. If the engine gets hot or lean at any point in the process... let it cool down completely before proceeding or you will get useless rpm numbers. For flying the first few times, switch down to the 8.8x8.5 ..... but leave the pipe where you set it as above. Find the ground peak, back off on the needle 800-1000 rpm.... and launch it.

Of note...... and a question...

Why did you choose to use a pipe instead of the black racing muffler?
The engine runs great with the racing muffler....... you should see it turn close to 20K rpm with the 8.8x8.5 installed with the engine exactly as Dub provided it. And in flight it will re-stage on the muffler upward closer to 21,000.

The pipe just adds compexity and weight to this particular setup. It is much harder to install. And any performance gain above what the muffler is capable of will be a few hundred rpms... not a 1000 rpm. It also will not have the multi-stage capablity that we have found with the mini-pipes/racing muffler.

Also there is no value at all considering timing or compression changes from the stock engine setup. You can measure the timing aspects of the engine..... it is good to learn about stuff like this but Id strongly suggest not messing with anything.... this particular engine is running on the edge as it is now. Nearly all of the magic and reliability of the 428 engines (Both nelson and Jett) comes from the muffler, materials, quality control and the simple design.
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Old 09-11-2007, 11:03 AM
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Default RE: polecat w/40jett motor


ORIGINAL: SACKRC

I do known the nelson is a superior motor in quality, had no idea on how much. thanks for info got a better bench mark to go for.
That is an inaccurate statement...
let me leave it at that.
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Old 09-11-2007, 11:05 AM
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Default RE: polecat w/40jett motor



Both are fine engines, just different mousetraps, I've beaten, and been beaten by both
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Old 09-11-2007, 11:07 AM
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Default RE: polecat w/40jett motor

Yep... me too ...
Ive also kicked butt with an Edmunds [&:] once upon a time.
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Old 09-11-2007, 11:29 AM
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Default RE: polecat w/40jett motor

And soon we have MB Profi's to contend with
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Old 09-11-2007, 11:38 AM
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Default RE: polecat w/40jett motor

thats why I have a few nice running TT40pro engines so I can relax for a change.
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Old 09-11-2007, 03:48 PM
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Default RE: polecat w/40jett motor

Dave, are you sure about that?

There's been some talk about an MB Q500 engine. But I talked to Metkemeijer a couple of weeks ago, and he wasn't too sure about the whole Q500 thing as I understand it. He had been thinking about it though.
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Old 09-11-2007, 04:16 PM
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Default RE: polecat w/40jett motor

Yeah, I'm pretty sure.
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Old 09-11-2007, 08:48 PM
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Default RE: polecat w/40jett motor

Just did a bench run on 15% vs 30% the 15 ran 17500 and the 30 ran 18500. the 30 has 20 % klotz technoplate that has some caster in it.. know the fuel is good, mix it my self. (experence from running nitro boats). Daven with this prop im using and your using and getting another 1000rpm more, is this correct or is your prop cut down. OH and i ran the mini and bob is right the performance is the same in rpm as the full pipe.. got go the the mini pipe, simple install..
Checked my book i had taken the timing on my jett 50 and its 125 intake and 165 exh.
my jett 90 is 125 in and 175 exh. didnt ever do the 40.. but it should be atleast 165 /170 exh. thanks again for feed back Rich B.
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Old 09-11-2007, 11:55 PM
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Default RE: polecat w/40jett motor

Rich, I trust the experts, namely Dub and Dave.

Yes, a standard Q500 Nelson motor will turn a 8.8x 8.5 over 19.5, some closer to 20k.

Don't get all hung up about ground rpm though, it really means NOTHING in regards to top speed.
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Old 09-14-2007, 12:11 PM
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Default RE: polecat w/40jett motor

Rudeboy,

I just got the Profi thing clarified to me. It is a profi motor, not a MB motor. Rob was not involved with its design. Profi is the company that manufactures Rob's motors.
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Old 09-14-2007, 12:26 PM
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Default RE: polecat w/40jett motor

Yeah Dave. That's kinda what I heard. Rob had been thinking about the Q500 thing... but it basically boiled down to the fact that he didn't want to upset his relationship with Profi.
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Old 09-16-2007, 11:48 PM
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Default RE: polecat w/40jett motor

Rudeboy, actually it is because Rob Metkemeijer doesn't want to upset his relationship with Henry Nelson. They've been good friends for several years.
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