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LOWERING COMPRESSION

Old 02-05-2005, 05:32 AM
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olgoat
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Default LOWERING COMPRESSION

I read somewhere about adding spacers to the head to lower the compression in order to use more nitro. Can someone explain the advantage to lowering the compression to use more nitro apposed to leaving the compression higher and using less nitro? Wouldn't lowering the compression lower the power just to use more expensive nitro to restore it? There must be something I'm missing.

Lonnie
Old 02-05-2005, 08:48 AM
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khodges
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Default RE: LOWERING COMPRESSION

It's helpful to know what engine you're planning to modify, and just how much nitro you want to use. Most r/c engines will do fine from 5-30% nitro. There are some engines designed to run on low, or no nitro (Laser 4-strokes, for instance). There's more than compression affected by changing nitro content.
Old 02-05-2005, 10:31 AM
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blikseme300
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Default RE: LOWERING COMPRESSION

Running more nitro in an engine that has not been designed for it will wear it out sooner. Pre-ignition and detonation will occur. The firing timing on glow motors is an interesting subject. Read up on it and you will be amased at the things that affect good running.

The European motors are typically designed for no-nitro use. We are spoiled here in the US that nitro is easy to get.

The main reason that MDS engines have a bad reputation among most US RC pilots is that they are designed for little or no nitro. When run with the almost standard 15% nitro fuel they will overheat and also bang up the conrod bushes. This is because the ignition occurs too early.

Also, in the '80-90's, the Hanno special OS .60 pattern engines were notorious to destroy themselves if the combination of high nitro and tuned pipes were used. We would decrease the compression by adding extra shims under the head. This did not rob any power and the engines lasted.

If you need more horsepower, get a larger engine.

Safe flying!
Old 02-05-2005, 11:17 AM
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britbrat
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Default RE: LOWERING COMPRESSION

Removing shims will reduce both the compression ratio & the compression number -- you will lose power. Restoring the lost power by adding nitro, at considerably increased increased fuelling cost, is not a smart way to do things. If you want a bit more power, leave the shims alone & add nitro until you get detonation, then back off the nitro content until it stops. As already posted, excess nitro is bad news for your engine, and without additional tuning tricks (pipe, etc), the power boost with a sport engine from extra nitro is not really very noticeable.

Live within the limitations of what you have, or buy a more powerfull engine.
Old 02-05-2005, 01:14 PM
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Robert Bauer
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Default RE: LOWERING COMPRESSION

It has been my understanding that adding head shims lowers compression. Also the lower compression requires less nitro. US engines are designed for higher compression and more nitro than Europe. Please correct me if I am wrong. Bob
Old 02-05-2005, 02:01 PM
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Default RE: LOWERING COMPRESSION

I have several Fox and they are known for their disdain for more than 5% nitro. Saito engines respond well to more nitro but don't really need it. YS engines run lousy without 20% nitro.
Old 02-05-2005, 02:24 PM
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Default RE: LOWERING COMPRESSION

Using a lower compression ratio, with higher nitro fuel has a double negative effect, but also a positive effect.

As a rule of thermodynamics, the higher the compression ratio, the higher the power and the higher the efficiency.
This would mean you must always run the highest possible compression ratio your fuel will tolerate.

In full size engine terms, Diesels have an efficiency level of about 40% and gasoline engines about 32%, because Diesels use compression ratios of around 20:1 and gasoline engines about 9.5:1.

A high compression model engine, designed to run on methanol/oil mix, with no nitro at all, may develop as much power as a lower compression engine, run on methanol/oil/nitro mix. This because it is not only the BTUs the fuel has, but also the ability of the engine to utilize all those BTUs, converting them into horsepower.

High compression engines do it better.

Also, higher nitro fuel does not only cost more. Because of its significantly lower stoichiometric ratio, you must use a lot more fuel when it is a nitro mix, than when it is not. The main needle must be open 1/2-1 more turn, when using 15% nitro, than when using 0%.


But there is also a down side, a definite one.
High compression engines are much harder to adjust properly, on methanol, then lower compression engines are, on methanol/nitro.

Some of us are unusually spoiled while others can tolerate a little more finicky engine.


In Europe and other places, most people fly their planes on 0-5% nitro.

In the North American continent live the 'spoiled brats' ( ) that need the easy adjustments possible with 15% nitro.


While MVVS engines were originally designed for 0-5% nitro, adding ONE head shim allows this engine too to be run very well on 15% nitro, with no ill effects (besides higher fuel costs). MVVS acknowledged this just lately.
Old 02-05-2005, 03:30 PM
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Default RE: LOWERING COMPRESSION

y'all are are forgeting about the squish clearance and how important it can be in a high performance engine. adding shims can really screw this up and result in loss of power despite an increase in nitro %.



dave
Old 02-05-2005, 03:30 PM
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Default RE: LOWERING COMPRESSION

Lowering compression effectively retards ignition timing. Adding nitro effectively advances it back again. The extra nitro will make for a more reliable idle.
Old 02-05-2005, 03:34 PM
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Default RE: LOWERING COMPRESSION

ORIGINAL: britbrat

Removing shims will reduce both the compression ratio & the compression number -- you will lose power.
Hmmm Removing the shims will increase compression ratio and compression number. You must recalculate the compression ratio again.

In Europa are fuel with nitro not cheap therefore we has the engines with higher compression ratio and running cheaper fuel without nitro.

Jens Eirik
Old 02-05-2005, 04:09 PM
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Default RE: LOWERING COMPRESSION

ORIGINAL: ZAGNUT

y'all are are forgeting about the squish clearance and how important it can be in a high performance engine. adding shims can really screw this up and result in loss of power despite an increase in nitro %.

dave
Dave,


The words of MVVS head engineer and engine designer...
One extra shim will mess up nothing; more will.

I was also opposed to this in the past.
Old 02-05-2005, 07:32 PM
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olgoat
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Default RE: LOWERING COMPRESSION

Thank You for all the replys, I now have a better understanding of the compression and nitro question.
One last question, how do I determine if I have a Pre-ignition (too much nitro) and detonation problem?
I had a Magnum .61 and I ran 10% nitro, the engine ran great for about 1 gal. then the engine blew (rod or crankshaft broke), Magnum did replace the motor with no questions, could I have been the cause? I use 10% in all my engines (GMS .76, Evolution .48, ASP 1.08) except my Supertiger 2300 which I use 15%nitro and 15% oil and #F plug to make it a little more reliable.

Lonnie
Old 02-05-2005, 09:27 PM
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downunder
 
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Default RE: LOWERING COMPRESSION

Lowering the compression does reduce power (with the same fuel) because of the lower efficiency, as Dar said, but also because the ignition timing is then retarded. You can regain a little of this lost power by using a hotter plug. Nitro however adds more power than you lose with the reduced efficiency which is why it's so commonly used. The cost of course is the initial expense plus the far higher fuel consumption compared to straight methanol.

The reason for lowering compressions to use nitro is that nitro won't tolerate high compressions like methanol does before it detonates. Straight methanol will handle up to about 17:1 where straight nitro can only handle around 6.5:1 so with a blend you need to use a compression somewhere in between these two.

There's really only one way to find out how much nitro an engine can tolerate and that's by using a good tacho and gradually increasing the nitro content. When there's no discernible rev rise use the fuel from the previous run. This way you'll never get to the point of detonation which apparently sounds like frying eggs (although I've never heard it myself). Don't forget though that you should also play with cooler plugs to really make the most of it. It's a real juggling act

Squish clearance has no real affect on sport type engines, even good ones like a Rossi, because any change to compression/fuel far outweighs any change in squish clearance (unless you're raising compression and put the head too close to the piston ).

I have to disagree with Dar on one thing though, high compression engines on zero nitro are dead easy to adjust. So are low compression on zero nitro.....
Old 02-06-2005, 01:15 AM
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Default RE: LOWERING COMPRESSION

Brian,


Believe it or not, with all my experience, I never ran an engine on FAI 80/20 fuel.
Over 95% of the fuel I ever used was 5% nitro.

So I was speaking from experience of others, model magazine engine gurus among them.

Adjusting nearly any engine to use 5% is very easy.

I can take your word for the ease of adjusting, but I guess it is the placement of that 'easy/difficult break-point', for each individual modeler.


Specifically for MVVS, using 5% nitro makes the ideal idle needle adjustment quite narrow.
Maybe 15% will make the range a bit wider, after adding that head shim...
Old 02-06-2005, 11:21 PM
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Default RE: LOWERING COMPRESSION

Restoring the lost power by adding nitro, at considerably increased increased fuelling cost, is not a smart way to do things.
So millions of people who live in a country where nitro is much cheaper are stupid? Adding nitro to make up for lowering the compression ratio does a lot more than just restore the power, it increases it beyond that which is lost, in some cases much more. Especially if the engine is designed for more nitro with proper squish and porting for the increase speed. Efficiency is lost, but because it carries its own oxygen it still burns clean.

I agree with Dar on the tuning. Nitro is a monopropellent, that is if it can be ignited nitro will burn without added oxygen. Because of this it is very tolerant of over rich needle settings, which make it easier to adjust. It is so tolerable that with high nitro fuel it is possible to set it rich enough that the engine runs cooler, yet still have more power than lower nitro fuel. Its not that it idles better but that it is easier to find a good setting because the tolerable needle setings are wider. Still if the idle mixture needle has is fine enough setting it to idel on low or no nitro fuel is not that hard to do, especially if it has high compression. The only problem I have with high compression engines is that they are more difficult to idle slow and shake more. This because if the increased resistance during the compression stroke.
Old 02-07-2005, 09:06 AM
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Default RE: LOWERING COMPRESSION

ORIGINAL: Sport_Pilot
The only problem I have with high compression engines is that they are more difficult to idle slow and shake more. This because if the increased resistance during the compression stroke.
My experience has been different. When I raised the comp on an Enya 60X from the standard 9.75 to 13.5:1 to make the most of zero nitro fuel I found I had to lean out the idle mix drastically and it then idled much slower and very steadily.
Old 02-07-2005, 11:06 AM
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Default RE: LOWERING COMPRESSION

ORIGINAL: downunder

ORIGINAL: Sport_Pilot
The only problem I have with high compression engines is that they are more difficult to idle slow and shake more. This because if the increased resistance during the compression stroke.
My experience has been different. When I raised the comp on an Enya 60X from the standard 9.75 to 13.5:1 to make the most of zero nitro fuel I found I had to lean out the idle mix drastically and it then idled much slower and very steadily.
downunder...

More compression ratio = more hotter air/fuel mixture. And glowplug will keep warm in higher compression in idle.

In old instruction paper for SABB hot bulb engine from 1930.. how to adjust casket height for cylinder head. There are wrote: Higher casket thickness make less knocking in high revolution and poor idling cause colder hot bulb.
Thinner casket height make better idling, keeping hot bulb warm in idling and more knocking in high revolution.

Glowplug engine and hot bulb engine are familiar.

Jens Eirik
Old 02-07-2005, 11:29 AM
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Default RE: LOWERING COMPRESSION

ORIGINAL: downunder
My experience has been different. When I raised the comp on an Enya 60X from the standard 9.75 to 13.5:1 to make the most of zero nitro fuel I found I had to lean out the idle mix drastically and it then idled much slower and very steadily.
Ditto, I have been impressed with the low smooth idle of many engines designed for low nitro. The Irvine .53, for example, idles superbly with 5% nitro. It just kept idle slower and slower, and just when I thought it was going to quit, it idled slower still. Yet as soon as I goosed the throttle, it spooled up instantly. I tried 10% nitro for one tank and saw no difference in the way it ran. So why bother?

Like Dar, I run 5% exculsively, unless the particular engine wants more nitro. Even though I live in the Land of Cheap Nitro, I see no need to use more just because it's relatively cheap. More nitro still is more expensive. And the much lower stochiometric ratio of nitromethane compared to methanol means you will burn more fuel per flight, which may not be a big deal with a dinky .46, but a Moki 2.10 will see a substantial difference. I can see Dave Patrick's reasoning for modifying the Moki for lower CR, but I rarely hear the original Moki owner here in the USA complain about the way theirs ran. I think those new DPM 2.10 owners are in for a rude awakening after running it for a season on high nitro. The fuel bill sticker shock might just sent them scrambling to raise the CR back up.

Unless the engine in question runs poorly, backfires, pings with high compression ratio and all other efforts fail to correct it, keep it the high CR.
Old 02-07-2005, 04:03 PM
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Default RE: LOWERING COMPRESSION

Hi!
Robert B...You were partly right!
Lower compression requires more nitro and higher compression requiers less nitro.
Engines made for the US market has often slightly lower compresion than engines made for/in the European market which has higher compression.
Regards!
Jan K
Sweden
Old 02-08-2005, 09:40 AM
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Default RE: LOWERING COMPRESSION

I've decided to limit my fleet of engines to 5% nitro. More compression requires tighter engines which drives up the price. If you think about it ,turn an engine at 16,000 rpm instead of 12,000 your wear out your engine by 25% more so lets say at 12,000 your engine lasts 4 years and at 16,000 3 years. buy the bigger engine and turn at less RPM for power. Pocket the savings in lower cost of fuel toward the next purchase of replacement engine and at an extra year later.
Old 02-09-2005, 12:37 AM
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Default RE: LOWERING COMPRESSION

If you think about it ,turn an engine at 16,000 rpm instead of 12,000 your wear out your engine by 25% more so lets say at 12,000 your engine lasts 4 years and at 16,000 3 years. buy the bigger engine and turn at less RPM for power. Pocket the savings in lower cost of fuel toward the next purchase of replacement engine and at an extra year later.
That may be but these model engines are relatively cheap. And I have rarely worn out even the higher reving ones. I could save money by not being in this hobby and generally more power is more fun. Still I have a couple of low nitro engines. One of them has a good low idle, but it does shake a bit, but it is a lowly ASP. But generally increased compression or cylinder pinch will make it shake more, but if a well built smooth runner on a strong plastic mount I doubt it would be that noticable.
Old 02-09-2005, 02:00 AM
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Default RE: LOWERING COMPRESSION

Sport_Pilot, Speedster,


In model, tapered-bore ABC/ABN/AAC engines, life cannot be quantified as a linear function of RPM.

In a ringed engine this is not so easy either, since though the ring does ride on the cylinder wall, it is not only the number of times it reciprocates, but also the outside load applied to it.

Only a small part of the force that pushes the ring against the cylinder wall is the ring's springyness.
Much more of it is compression and combustion pressures, that bypasses the piston crown, enter between the ring and the top of its groove and build up behind the ring, being unable to continue down, between the ring and the bottom of its groove, since the piston is pushing up against it, sealing it for all practical purposes.

So the higher the compression and combustion pressures, the harder the ring is worn against the cylinder.

So higher BMEP/torque may wear the ring as fast, or faster than higher RPM.


If anyone here does not understand what I just wrote, it is not because it is incorrect.
Just look closely at full-size, drag-race pistons, from some years back and the holes drilled in them, through the crown, to pressurize the ring grooves...


Back to tapered-bore engines; the pinch does not exist, when the engine is running at its normal operating temperature, so no vibrations will be caused by this 'cold only' pinch, neither will any additional wear take place.

The piston is actually riding on a film of oil and this is the reason RPM does not directly determine its rate of wear.

High compression does increase the vibration level somewhat (greater change in rotation speed) and may require more flywheel, or more RPM, to overcome, in a single cylinder engine.
The differences are not very large, however.
Old 02-09-2005, 08:41 AM
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Default RE: LOWERING COMPRESSION

ORIGINAL: DarZeelon
If anyone here does not understand what I just wrote...
I don't

Old 02-09-2005, 09:57 AM
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Default RE: LOWERING COMPRESSION

Sure you do, Brian.

But most people think the ring seals in the cylinder due to the fact that it springs out, which is a very partial truth.

Since many members here do not have as much knowledge as you do, I mentioned the drag-race pistons, to illustrate the idea.
Some newer examples have holes drilled just above the ring and some don't relying on the pressure that blows-by the crown, as in normal engines.
Old 02-09-2005, 10:33 PM
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Default RE: LOWERING COMPRESSION

Please let me address the squish clearance as mentioned by Zagnut and Downunder. The squish clearance has a huge influence that is not readily apparent. It's affects are often confused with changes in compression. The whole point of having a squish band is to promote turbulence in the combustion chamber. How do you quantify turbulence? By calculating how fast the air/fuel mixture is "squished" out from between the head and piston. This is done easily with computer software from TSR software.
http://www.tsrsoftware.com/
Changing the squish clearance on a model airplane or car engine that is set up "tight" to begin with (less than .014 inch) by just a couple of thousandths will make a huge difference in average squish velocity as compared to compression ratio. Using results I got from TSR's squish velocity calculating software I machined diffferent heads for some of my engines and noted very positive results. I was able to keep compression very high, had great idle and throttle response and had better peak power. The motorcycle guys have known about squish properties for years. If your squish is too tight for your engine's operating range the symptoms that crop up are rough running at high rpm and even pre-ignition or detonation. If the squish is too "lazy" you will have an engine that feels a little flatter but will tolerate a slightly leaner mixture setting and how you set the needle will be less critical. We are talking differences the thickness of a sheet of notebook paper! These kind of changes have EXTREMELY small affects on the actual compression ratio. Do the math and you'll see.

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