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Old 09-24-2009, 12:36 PM   #16
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But doesn't pinch allow you to have compression??????????????
No. Take a brand new motor with tons of pinch and drill a hole in the piston or grind a big flat spot down the side. It will still have lots of pinch, but no compression.
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Old 09-24-2009, 01:15 PM   #17
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Like many have said you know when compression is low when the engine starts to flame out at idle or when it gets hot. This of course pertains to properly tuned engines.

The 3 oclock test is a good one. It could be very useful if your engine has flamed repeatedly and your trying to figure whether its a tuning or compression problem.

Its more important to me that an engine run properly than whether it feels like it has compression or not. Ive been running OS engines for the last 3 years and compared to the Novas and RBs I ran prior they run on much less compression. Therefore I have learned to pay attention to how the engine runs rather than how it feels.

Also there needs to be an understanding that pinch and compression are not the same. Pinch is when an engine locks or is very hard to turn over caused by an overly tight unbroken in piston and sleeve. Compression is the engine compressing air between the piston and head button. An engine with no pinch can still have compression in fact this is how it should be and how you can tell your engine is broken in.
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Old 11-27-2009, 05:13 PM   #18
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Pinch :When the sleeve is manufactured it has a slight taper from the bottom(largest) of the sleeve to the top(smallest. The reason for the taper is that as the engine heats-up, the top will get hotter than the bottom of the sleeve and will expand more than the bottom, so when the the engine is hot the sleeve has the same diameter from top to bottom. The same principle is used in airplane engines(real airplanes, piston powered).
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Old 11-27-2009, 06:08 PM   #19
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Old 11-28-2009, 09:12 AM   #20
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I would say when it looses top end and the torque go away. This tells me time for a new sleeve and piston.


A good accurate resizing process should result the "propor seal" from porting window to TDC, rebreakin easy, and run hard + strong for quit a few more gallons. RayAracing

Last edited by RayA; 11-28-2009 at 09:32 AM. Reason: also
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Old 11-28-2009, 09:53 AM   #21
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A good accurate resizing process should result the "propor seal" from porting window to TDC, rebreakin easy, and run hard + strong for quit a few more gallons. RayAracing

If I have to go through the breakin process all over again and it's only going to run for 2-3 gallons then why not buy a new piston and sleeve and get 8-10 gallons out of it? $$ is not a very good answer. You show me somethings that's inexpensive about this hobby, it doesn't exist. Even the spec slash racing is expensive. The batteries that are competitive are over $100 each and you better have 2. Then you need the correct charger another $100 a transponder $100, a receiver $100, a new engine every 4-5 runs and I am sure I forgot something. I have at least 600 into my spec slash and I am sure it's not done yet. This hobby costs $$ no way around it. So what do you gain by a repinch?? (off topic sorry)

The 3 o'clock test and performance are the beat indicators IMO
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Old 11-28-2009, 10:00 AM   #22
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Break in on a pinched motor is idling a tank or two and then run the snot out of it.
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Old 11-28-2009, 10:25 AM   #23
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Break in on a pinched motor is idling a tank or two and then run the snot out of it.
Ohh that makes sense. Thanks.
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Old 11-28-2009, 12:15 PM   #24
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Easy way to tell, is pull the head, turn the motor over and if the piston pushes the sleeve out you still have pinch. If the piston dosnt move the sleeve and slides past tdc with no resistance then your sleeve and piston are shot. I have a losi 454 with almost 6 gallons through it and still has plenty of pinch and compression but the motor itself hasnt run worth crap since day 1.
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Old 11-28-2009, 12:20 PM   #25
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Just remember there is more than just a piston and sleeve in a nitro engine

The tell tale signs of low compression are erratic tuning .

richening up the low speed needle to compensate for lack of compression causing you to have to raise the idle .

Low compression in the p/s set will cause loss of torque and random flameouts .

If you want to just resize your p/s set then ray is the man , if you need more than p/s resize you will either send your engine to a reputable engine person or try to work on it yourself . (Keep in mind that there are ways the engine could be rendered useless if you don't know what you are doing)


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Old 11-28-2009, 12:31 PM   #26
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Just remember there is more than just a piston and sleeve in a nitro engine

The tell tale signs of low compression are erratic tuning .

richening up the low speed needle to compensate for lack of compression causing you to have to raise the idle .

Low compression in the p/s set will cause loss of torque and random flameouts .

If you want to just resize your p/s set then ray is the man , if you need more than p/s resize you will either send your engine to a reputable engine person or try to work on it yourself . (Keep in mind that there are ways the engine could be rendered useless if you don't know what you are doing)


Monty
Hi Monty...quick ? We need to rich the idle , is it bcz we need to put extra fuel to seal the p/s?

Sometimes i feel this low compression engine cant idle? May i plz know why?

I have ran some low compression mills; but they tend to run fast on top speed...
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Old 11-28-2009, 02:43 PM   #27
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Hi Monty...quick ? We need to rich the idle , is it bcz we need to put extra fuel to seal the p/s?

Sometimes i feel this low compression engine cant idle? May i plz know why?

I have ran some low compression mills; but they tend to run fast on top speed...
Yes , extra fuel/oil to fill the volume of the combustion chamber to create some compression .

If an engine has low compression it is probably in worse shape than you think it is as far as the rod slop , bearing wear and other parts of the engine that wear out.

The engine design that I have noticed to run decent with little to no compression is the v spec design , big bore with short stroke .
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Old 11-29-2009, 07:32 AM   #28
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I used to think pinch and compression were synonymous...they're not. I replaced 2 motors because I thought they had lost compression when in fact they had only lost pinch. I'm no longer concerned with either and just worry about performance. My V-spec just passed the 7 gallon mark and I'm still pleased with how she perfoms. She still holds a tune, doesn't flame out, and idles smooth.
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Old 12-02-2009, 10:48 AM   #29
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I used to think pinch and compression were synonymous...they're not. I replaced 2 motors because I thought they had lost compression when in fact they had only lost pinch. I'm no longer concerned with either and just worry about performance. My V-spec just passed the 7 gallon mark and I'm still pleased with how she perfoms. She still holds a tune, doesn't flame out, and idles smooth.
Thats an engine has served you well in my book. You took care of it.

The engine makes best power, will rev into its RPM's, and will continue to combust a fresh fuel-air mixture best with minimal piston to cylinder clearance. Cylinder liner and piston surface finish conditions I find is equally important....."Fit + Finish". Half the work I receive daily "A Pinch Is Not Enough". We all know ON A NIB ENGINE, The "two" areas of physical resistance deminishes right above porting windows and at TDC and we then are in the "sweet spot"....usually with atleast 1/2 gallon , or more breakin process. I can share my opinions on alloy work hardening / stress relieving / piston to liner lapping relative a NIB engine breakin but thats out of context this tread about Engine Compression, so I won't. My point here is, I find the optimum running nitro engine cranking 40,000 RPM's 1) Gets the compression needed to make horse power well after 30% piston travel past porting windows (as is felt by hand) BUT still needs to seal well the engines total stroke for gas-air mixing 2) Will have great torque/gain rpm's quickly (engine inertia) with minimal physical resistance at point of combustion, TDC . In summary, Unless you are a racer with quite a bit experience and hundreds of engines under your belt its hard to feel that "sweet spot" testing a cold engine / piston+sleeve set in your hand.engine designs and their running charactoristis varry quite a bit, and half tenth one thousands in fit makes difference in some but not all. Watch the performance on the track / lap times / tunability.

RayAracing

Last edited by RayA; 12-02-2009 at 10:50 AM. Reason: name
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