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Author Topic: Questions about higher nitro use in Magnum 25A  (Read 3346 times)

Offline Skip Chernoff

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Questions about higher nitro use in Magnum 25A
« on: May 08, 2017, 04:30:42 AM »
I thought I'd ask the speed merchants these questions. I'm running a Magnum 25A in a small R/C pylon racer. On an APC 8x6 she's turning 17K on the ground and giving me good reliable runs. I'm using my home brew 15% Nitro fuel 20% oil 50/50 castor /synthetic. I'd like to go faster so I'd figured I'd bump up the Nitro. Here are my questions:

(1)Where do you get head shims for this engine?

(2) How much Nitro can I put thru it before needing head shims?

(3) I'd like to try 30% Nitro. Can this engine stay together  with 30%?

(4) I've got a bunch of ST "Racing" plugs (cold) would they work ok?

                                                                            Thanks,PhillySkip

Offline Paul Smith

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Re: Questions about higher nitro use in Magnum 25A
« Reply #1 on: May 08, 2017, 09:07:58 AM »
You might be able to use OS 25 head shims from Tower Hobby.

More nitro does not automatically mean you need to raise the head.  RC engines have excess head clearance due to the back pressure from the muffler.  If you did away with the muffler you are probably under-compressed.

If you want to win shorten the prop and up the nitro.  It's only a model plane engine.  Why baby a losing engine?
Paul Smith

Online bill bischoff

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Re: Questions about higher nitro use in Magnum 25A
« Reply #2 on: May 08, 2017, 01:31:11 PM »
This engine is probably the same as the ASP S25A. The ASP has a 17.4 mm bore. The closest thing I found from OS is the 25XZ car engine. it has a 17.5 mm bore. The gasket set includes a .003 and a .004 head shim. You may possibly have to trim the OD. Tower P/N LXYZT6

Offline Ken Burdick

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Re: Questions about higher nitro use in Magnum 25A
« Reply #3 on: May 08, 2017, 01:54:22 PM »
the best clue you have so far is the soft crank. With enough go, it will shear the pin or break at the port. You can if you have the tools, radius the crank ports so the corners are round. It will help prevent cracks from beginning.
The Magnum should run well on 30% as per your mix, leave the head where it is.
 If you do go to 30% , you might try going to a larger prop since you will now have a bit mo-go. That is a tough call though, if you don't see much improvement in speed, go up to a 7" pitch and begin fethering the tips for your corner speed. The Magnum will wind up and show good rpm quickly followed by a broken crank so maybe keeping the rpm down and the pitch up.....but like Paul said, it's a disposable engine.
Have fun

K

Offline Skip Chernoff

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Re: Questions about higher nitro use in Magnum 25A
« Reply #4 on: May 08, 2017, 08:53:46 PM »
Thanks all for jumping in to the discussion.

Offline kenneth cook

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Re: Questions about higher nitro use in Magnum 25A
« Reply #5 on: May 10, 2017, 01:58:27 PM »
              Skip I have run 35% in both the square head version and older round head version with a 8x4 prop. They're the engine I use for speed limit combat. The crank pin is pressed into the crank. I never had a failure but if one was to take place, I think this is where it's going to happen.

Offline Skip Chernoff

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Re: Questions about higher nitro use in Magnum 25A
« Reply #6 on: May 11, 2017, 06:43:38 PM »
Thanks Ken,I'll try the 30% that we used in Danny's 1/2A Speed Plane. BTW Shawn was correct about the Skylark.I added an ounce of tail weight and it turned like a different plane. Skip

Offline Skip Chernoff

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Re: Questions about higher nitro use in Magnum 25A
« Reply #7 on: October 23, 2017, 07:04:45 AM »
Fellows I tried the 30 % in my Magnum 25A in my Quickie 500 Dodger and she didn't run well at all. She didn't transition well from extended idle to full speed,and was sensitive to which glow plug was used etc etc. I went back to my normal 15% and she worked beautifully.
In this class of R/C racing it is critical that the engine can idle for quite some time and transition without hiccup to full speed. The 15% was just the ticket......PhillySkip

Offline john e. holliday

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Re: Questions about higher nitro use in Magnum 25A
« Reply #8 on: October 23, 2017, 10:21:36 AM »
I remember in the CL racing events there was some high nitro used in some of the engines by the competitors.   The first year they required only 10% nitro a team that won Scale Race, The Lamberts,  looked at their time and stated that was the fastest race they ever had run.  And the speeds just kept going higher and higher. H^^
John E. "DOC" Holliday
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AMA 23530  Have fun as I have and I am still breaking a record.

Offline Paul Smith

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Re: Questions about higher nitro use in Magnum 25A
« Reply #9 on: October 23, 2017, 10:55:12 AM »
We have better engines now and they provide a lot of power even on 10%.

But back in the day, going 10, 20, 25, 30, 40, and even 50% got tangible results.  In our area 40% was the standard fuel for Fast, Slow, FAI and 1/2A.  We got it direct out the side door of Logghe Brothers Stamping Plant.  Mr. Logghe was bored with the little sheet metal parts that earned his living.  He liked high performance engines.

If he blew up an OPS 65 to win a boat race, he still won the race.  The guy who saved his engine lost the race.
Paul Smith

Offline Fredvon4

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Re: Questions about higher nitro use in Magnum 25A
« Reply #10 on: October 23, 2017, 12:07:39 PM »
One big clue here some may have missed with the R/C Pylon racing with a R/C carb

Using that engine on a combat plane with a wide open fixed venturie, bladder, and remote NVA can take a lot of vitamin N and the right prop to keep the crank for exploding

In a R/C carb and suction tank there is a much different dynamic...suction tank is assumed

I  gotta believe Randy Smith and Henry Nelson can give useful hints for modifying the R/C carb to better results and still keep the soft crank happy

"A good scare teaches more than good advice"

Fred von Gortler IV

Offline Paul Smith

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Re: Questions about higher nitro use in Magnum 25A
« Reply #11 on: October 23, 2017, 02:54:43 PM »
I wouldn't be surprised if an RC racer going straight in a descent was able to unload even more RPM than a CL model.

In my days of sharing a field with RC pylon, they sounded pretty high-REVed.
Paul Smith

Offline Dan McEntee

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Re: Questions about higher nitro use in Magnum 25A
« Reply #12 on: October 26, 2017, 07:17:19 PM »
Fellows I tried the 30 % in my Magnum 25A in my Quickie 500 Dodger and she didn't run well at all. She didn't transition well from extended idle to full speed,and was sensitive to which glow plug was used etc etc. I went back to my normal 15% and she worked beautifully.
In this class of R/C racing it is critical that the engine can idle for quite some time and transition without hiccup to full speed. The 15% was just the ticket......PhillySkip


     The main ingredient to getting good throttle response is proper low speed needle adjustment. The low speed needle is influenced by the high speed, but the low speed needle affect the run from idle up to about 3/4 throttle. With the higher nitro, you may have to make a radical departure for low speed adjustment. The ones to talk to about this is the R/C power boat guys. They are on and off the throttle a lot. R/C cars are like this also. They are not on the top end for extended periods like airplanes and boats may be, though. You may have to try different crabs to get what you want.
    Type at you later,
    Dan McEntee
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Offline Skip Chernoff

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Re: Questions about higher nitro use in Magnum 25A
« Reply #13 on: November 06, 2017, 04:00:09 AM »
I spent an afternoon adjusting the low speed needle (with at least 40 years of R/C carb experience) and this particular engine in stock trim just wasn't happy with the 30 % . Thanks,Skip

Offline Les Akre

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Re: Questions about higher nitro use in Magnum 25A
« Reply #14 on: November 20, 2017, 09:56:02 PM »
Hi Skip

Most of today's sport engines (R/C or otherwise) can definitely benefit from a compression increase. I find most of the sport engines we use are woefully under-compressed, and the plug height is way off as well, usually too high to make power serious enough to hurt the motor. This is the result of how the manufacturers view glow motors these days. They don't want to accept any more warranty claims than absolutely necessary, so they assemble a motor that short of major abuse, is difficult to damage.

If you have the capability, and the rules allow, you could modify your head for more useable power. In your case, this would be far more beneficial than adding nitro, which as you found, in many cases does little to improve performance in some engines.

Something to consider...

Les


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