Compression Ratio explained by Scott The Viking

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scott the viking
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Post by scott the viking » Sun Feb 25, 2007 12:30 am

I re-read what you are saying...and correct me if I'm wrong...I think what your are saying is...
In a perfect world where gas would not detonate on a 15 to1 compression engine and the head design and quench area was so perfect that it would not cause hot spots...or any kind of ill behavior...in that world...a higher compression engine would not build much more heat. Right?

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Post by wildthings » Sun Feb 25, 2007 1:17 pm

I too would like to see some numbers that show heads run hotter (or cooler) with increases in C/R. I have never seen anything that proves they run hotter, yet maybe they do, I do not know, hence the reason for my posts. It has all the trimmings of well accepted urban legend to me.

At this point I have spend quite a bit of time on the net searching for any graph or test that would support increased head temperature and haven't found them. When I search "compression ratio head temperatures" I find nothing that shows head temps go up with compression.

There is a lot of information that shows that high head temperatures and piston temperatures cause detonation with increases in C/R but that is not the arguement here, as I think all would agree that if you get hot spots you get detonation.

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Post by scott the viking » Sun Feb 25, 2007 2:16 pm

Probably the reason you don't find much about it...is that it is somewhat unrealistic. You don't need to find any test...I can tell you...than in a perfect world were detonation would never exist...High compression would not raise heat enough to matter. If no detonation existed...an engine running 11 to 1 compression (or higher) would be great. Unfortunately...it does exist, thus...no real reason to test things in a world where it does not exist.
Many auto makers are experimenting with all sorts of different head designs in an attempt to run as much compression possible without the usual adverse affects. A good example of this would be today's four stroke dirt-bikes. They have figured out how to run a gob of compression (due to new head designs) and still run pump gas.
At this particular point in time...there are still limits to how much compression we can run. So if your argument is...compression is not the killer...detonation is...that is true...until you get to a certain point, then compression becomes the heat builder. So...if you run a certain compression ratio...with a certain head design, then yes...it will be just fine. The problem with that is that it does not really apply to most guys building a air-cooled VW's unless he is a cylinder head designer and could make a combustion chamber piston combo that would safely run more compression without detonation. So...I guess we could say...."if the laws of physics ceased to exist on our VW engines for a day...then when could run as high as 12 to 1 before the heat of the compressed air affected the engine head temp. Anything past that and the act of the compression alone would affect the head temp. So, perfect world, 12 to 1, no head temp increase is the new rule.

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Post by Bugfuel » Sun Feb 25, 2007 3:51 pm

<sigh>.


There is nothing strange about running high compression.
There is no reason that for example 11:1 would be the ultimate maximum.

You could have 11:1 CR and not even have enough to get the engine to run properly.


I have routinely ran 13:1 and more in race engines, and my cam grinder makes cams for race engines that have 22:1 and more compression ratio. Yes, gasoline not diesel. No detonation, no overheating.
See how this leaves you wondering "no way, how could that be possible?" It's because I do not give the missing details.

Compression ratio alone means absolutely nothing. STILL most of the discussion on it on these forums make you think it a certain value is dangerous, and take it totally out of context.

My 10.5:1 2liter+ engine runs fine and cool, all day on the highway or in the city. No external coolers. Not even perfectly tuned carbs yet (No dyno testing)

Jan

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Post by scott the viking » Sun Feb 25, 2007 4:17 pm

That's a dynamic compression ratio?

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Post by Bugfuel » Sun Feb 25, 2007 6:45 pm

scott the viking wrote:That's a dynamic compression ratio?

There is no such thing as dynamic compression ratio.
(nobody ever talks about it, although it could be calculated somehow I'm sure, after you measure actual compression pressure)

The numbers I wrote above are Compression Ratio numbers. (Static).
That is how much the air WOULD compress, if it was squeezed into a closed chamber. (No valves, no cam). That is what CR means.

Dynamic compression is a measure of maximum pressure inside the cylinder & combustion chamber before ignition. It is not a ratio, it's a solid, measurable pressure reading.

Dynamic peak pressure in a car engine is much lower than the dynamic compression pressure inside a closed chamber, because the valves are open during the compression stroke, allowing air to escape.

The more aggressive cam you have, the more valve timing and overlap it has. In other words, the valves stay open longer, during the compression stroke. If the valves are open at all during compression stroke, the engine produces zero compression pressure. It uses compression RATIO from the moment the piston starts to travel up from BTDC, but actual cylinder pressure doesn't start bulding until both valves are completely shut.

This means that the piston travels "useless" amount of distance until it starts making pressure. The longer the valves stay open, the less travel distance the piston has to build desired actual pressure. This is why we must raise the compression RATIO, if we increase the time the valves are open, to allow the piston to create enough pressure in less amount of travel, so it can compensate for the actual pressure loss that escaped through an open valve.


Jan

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Post by scott the viking » Sun Feb 25, 2007 8:17 pm

The first thing to understand is that "compression ratio" (CR) as it is usually talked about is best termed "static compression ratio". This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). For example, if a hypothetical cylinder had a displacement of 450cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. If we were to mill the head so that the volume above the piston crown was decreased to 40cc, the CR would now be 490/40, or 12.25:1. Conversely, if we hogged the chamber out to 60cc, the CR would now be 510/60, or 8.5:1.



Everyone knows that high performance engines typically have higher compression ratios. Simply put, higher compression makes more hp. Higher CR also improves fuel efficiency and throttle response. So why not bump up the CR even further? Once CR exceeds a certain point, detonation will occur. Detonation kills power and it kills engine. The amount of compression a given engine can handle is determined by many factors. These include combustion chamber design, head material, use of combustion chamber coatings, etc. Once these mechanical aspects of the engine have been fixed, the main variable is fuel octane. Higher octane = more resistance to detonation and the ability to tolerate more compression.



The above brings up the question that is often on the mind of performance enthusiasts and engine builders: how high should my CR be? Even if you know all about your engine and have decided what fuel you are going to use, the question cannot be answered as phrased. Why? Because without reference to the camshaft specs, talking about (static) CR is next to meaningless!



How is this so? Well, think about the Otto cycle and how a four stroke engine works. The power stroke has been completed and the piston is heading up in the bore. The intake valve is closed and the exhaust valve is open. As the piston rises it is helping to push the spent combustion gasses out the exhaust port. The piston reaches TDC and starts back down. The exhaust valve closes and the intake valve opens. Fresh fuel and air are drawn into the cylinder. The piston reaches bottom dead enter (BDC) and starts back up. This is the critical point as far as understanding DCR. At BDC. the intake valve is still open. Consequently, even though the piston is rising up the bore, there is no compression actually occurring because of the open intake valve. Compression does not begin until the intake valve closes (IVC). Once IVC is reached, the air fuel mixture starts to compress. The ratio of the cylinder volume at IVC over the volume above the piston at TDC represents the dynamic compression ratio. The DCR is what the air fuel mixture actually "sees" and is what "counts", not the static CR. Because DCR is dependent upon IVC, cam specs have as much effect on DCR as does the mechanical specifications of the motor.



DCR is much lower than static CR. Most performance street and street/track motors have DCR in the range of 8-8.5:1. With typical cams, this translates into static CR in the 10.0-12.0:1 range. Higher than this, there may be detonation problems with pump gas. Engines with "small" cams will need a lower static CR to avoid detonation. Engines with "big" cams have a later IVC point and can tolerate a higher static CR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Of course, race motors also have much larger camshafts which is another reason they can get away with such high static CR, often in the 13-15:1 range.



Note: there is some confusion about use of the term "Dynamic Compression Ratio". Some people use it to refer to the characteristics of an engine combo running at high speed. In that case, the engines volumetric efficiency will have a major effect on cylinder pressure. In this case, a larger cam will increase cylinder pressure when within its' rev range. Thus, more power and more cylinder pressure will be created. We prefer to think of this concept as "cylinder pressure" to avoid confusion.

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Post by Bugfuel » Sun Feb 25, 2007 8:31 pm

well said :)

Jan

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Tom in PA
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Post by Tom in PA » Sun Feb 25, 2007 9:15 pm

I believe part of the reason higher compression produces higher temps is because the fuel is being more completely burned. It is all forced closer to the spark plug and the higher pressures lead to a faster burn. It is more "efficient" in that it gets more energy from the same charge of fuel.

No testing, just a thought.

Another thought. With the charge burning more rapidly, it is shedding more of its heat into the chamber and not out the exhaust. If the charge burns more slowly, some it it may still be burning when the exhaust valve opens.

I doubt there is a single answer, likely a combination of events.

Tom in PA

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Post by david58 » Sun Feb 25, 2007 9:22 pm

Bugfuel wrote:well said :)

Jan
Well that is true to a point but there are things that make heat in the combustion chamber :wink: that you kinda left out.VBS but the piont you made is your point not one that everyone goes by so why did you complicate things in a sentance?Tuemuffler beaings have been around for tears.
Hot, humid air is less dense than cooler, drier air. This can allow a golf ball to fly through the air with greater ease, as there won't be as much resistance on the ball.

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Post by Bugfuel » Sun Feb 25, 2007 9:27 pm

David58bug wrote:
Bugfuel wrote:well said :)

Jan
Well that is true to a point but there are things that make heat in the combustion chamber :wink: that you kinda left out.VBS but the piont you made is your point not one that everyone goes by so why did you complicate things in a sentance?Tuemuffler beaings have been around for tears.
Uhh, what? Care to rephrase that?

Jan

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Post by david58 » Sun Feb 25, 2007 10:01 pm

Bugfuel wrote:
David58bug wrote:
Bugfuel wrote:well said :)

Jan
Well that is true to a point but there are things that make heat in the combustion chamber :wink: that you kinda left out.VBS but the piont you made is your point not one that everyone goes by so why did you complicate things in a sentance?Tuemuffler beaings have been around for tears.
Uhh, what? Care to rephrase that?

Jan
No I want to hear you out on this one.
Hot, humid air is less dense than cooler, drier air. This can allow a golf ball to fly through the air with greater ease, as there won't be as much resistance on the ball.

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Post by Bugfuel » Sun Feb 25, 2007 10:09 pm

David58bug wrote:
Bugfuel wrote:
David58bug wrote:
Bugfuel wrote:well said :)

Jan
Well that is true to a point but there are things that make heat in the combustion chamber :wink: that you kinda left out.VBS but the piont you made is your point not one that everyone goes by so why did you complicate things in a sentance?Tuemuffler beaings have been around for tears.
Uhh, what? Care to rephrase that?

Jan
No I want to hear you out on this one.

Dude I don't know what it is you want. I don't understand your question.

First you said I left stuff out, then you say I complicated things in one sentence. Then your "Tuemuffler beaings have been around for tears" makes no sense to me, I fail to see the context.

Maybe you could quote my text and show me what you want me to elaborate on, or ask me directly. I cannot answer a question if I cannot understand it.

Jan

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Post by david58 » Sun Feb 25, 2007 10:35 pm

Dynamic Compression Ratio
(Will my engine run on pump gas?)
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Dynamic Compression Ratio (DCR) is an important concept in high performance engines. Determining what the compression ratio is after the intake valve closes provides valuable information about how the engine will perform with a particular cam and octane.

Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio. Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.

The difference between the two can be substantial. For example, with a cam that closes the intake valve at 70º ABDC, the piston has risen 0.9053" from BDC in a stock rod 350 at the intake closing point. This decreases the sweep volume of the cylinder considerably, reducing the stroke length by almost an inch. Thereby reducing the compression ratio. This is the only difference between calculating the SCR and the DCR. All other values used in calculating the CR are the same. Note that the DCR is always lower than the SCR.

Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine.

Two important points to remember:

The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine

Determining seat timing: Since the early days of the internal combustion gasoline engine, engineers have known that the Otto four stroke engine is compression limited and that the quality of the fuel used determines the CR at which the engine could operate. However, it is not the Static CR but the actual running CR of the engine that is important. Compression of the air/fuel mixture cannot start while the intake valve is open. It may start slightly before the intake valve is fully seated. However, there is no easy way to determine this point so using the advertised duration number provided by the cam manufacture is the next best thing. Most cam grinders use .006" of tappet lift (hydraulic cam), although some use other values, with .004" being a common one. This duration is often referred to as the "seat timing". We will used advertised duration for calculating the DCR.

The special case of solid lifter cams. Solid cams are usually speced at an abitrary lift value (often .015" or .020") determined by the designer to be a good approximation of the cam's profile. This lift spec is not always correct for a particular cam. The correct lift point to determine the seat to seat timing of the cam is: Lash / rocker ratio + .004". This accounts for the lash. A cam with a .026" lash (given 1.5 rockers) should be measured at .02133" (.026/1.5+.004= .02133>"). This cam lash, with seat timing speced at .020", is actually a bit smaller than advertised since the valve has yet to actually lift off the seat. How much is the question (.024" lash is the only lash that is correct at .020" with 1.5 rockers). Without knowing the ramp rate, and doing some calculations, or measuring with a degree wheel, it is impossible to know. Again, we have to use the mfg's numbers. Here is some Chevy factory cam help.


Why it matters: A 355 engine with a 9:1 static CR using a 252 cam (110 LSA, 106 ICL) has an intake closing point of 52º ABDC and produces a running CR (DCR) of 7.93. The same 9:1 355 engine with a 292 cam (having an intake closing point of 72º ABDC) has a DCR of 6.87, over a full ratio lower. It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane. The larger cam's DCR falls outside this range. It would have markedly less torque at lower RPM primarily due to low cylinder pressures, and a substantial amount of reversion back into the intake track. Higher RPM power would be down also since the engine would not be able to fully utilize the extra A/F mixture provided by the ramming effect of the late intake closing. To bring the 292 cam's DCR up to the 7.5 to 8.5:1 desirable for a street engine, the static CR needs to be raised to around 10:1 to 11.25:1. Race engines, using high octane race gas, can tolerate higher DCR's with 8.8:1 to 9:1 a good DCR to shoot for. The static CR needed to reach 9:1 DCR, for the 292 cam mentioned above, is around 12:1.
This lowering of the compression ratio, due to the late closing of the intake valve, is the primary reason cam manufactures specify a higher static compression ratio for their larger cams: to get the running or dynamic CR into the proper range.


Caveats: Running an engine at the upper limit of the DCR range requires that the engine be well built, with the correct quench distance, and kept cool (170º). Hot intake air and hot coolant are an inducement to detonation. If you anticipate hot conditions, pulling some timing out might be needed. A good cooling system is wise. Staying below 8.25 DCR is probably best for trouble free motoring.

>>Unless you have actually measured the engine (CCed the chambers and pistons in the bores), these calculations are estimations, at best. Treat them as such. The published volumes for heads and pistons can, and do, vary (crankshafts and rods, too). It is best to err on the low side. When contemplating an engine of around 8.4 DCR or higher, measurments are essential, or you could be building another motor.<<


Details: Long duration cams delay the closing of the intake valve and substantially reduce the running compression ratio of an engine compared to the SCR. The cam spec we are interested in to determine the DCR is the intake closing time (or angle) in degrees. This is determined by the duration of the intake lobe, and the installed Intake CenterLine (ICL) (and indirectly by the Lobe Separation Angle (LSA)). Of these, the builder has direct control of the ICL. The others are ground into the camshaft by the grinder (custom grinds are available so the builder could specify the duration and LSA). Changing the ICL changes the DCR. Retarding the cam delays intake closing and decreases the DCR. Advancing the cam causes the intake valve to close earlier (while the pistons is lower in the cylinder, increasing the sweep volume) which increases the DCR. This can be used to manipulate the DCR as well as moving the torque peak up or down the rpm range.

It is necessary to determine the position of the piston at intake valve closing to calculate the DCR. This can be calculated or measured (using a dial indicator and degree wheel). Since compression cannot start until the intake valve is closed, it is necessary to use seat times when calculating the DCR. Using .050" timing will give an incorrect answer since the cylinder is not sealed. At .050" tappet lift, using 1.5 rockers, the valve is still off the seat .075" and .085" with 1.7 rockers. While the flow is nearing zero at this point, compression cannot start until the cylinder is sealed.

Another factor that influences DCR is rod length. It's length determines the piston location at intake closing, different rod lengths change the DCR. Longer rods position the piston slightly higher in the cylinder at intake closing. This decreases the DCR, possibility necessitating a different cam profile than a shorter rod would require. However, the effect is slight and might only be a major factor if the rod is substantially different than stock. Still it needs to be taken into account when calculating the DCR.


Calculating DCR: Calculating the DCR requires some basic information and several calculations. First off, the remaining stroke after the intake closes must be determined. This takes three inputs: intake valve closing point, rod length, and the actual crank stroke, plus a little trig. Here are the formulas: (See the bottom of the page for a way around doing all this math.)

Variables used:

RD = Rod horizontal Displacement in inches
ICA = advertised Intake Closing timing (Angle) in degrees ABDC
RR = Rod Distance in inches below crank CL
RL = Rod Length
PR1 = Piston Rise from RR in inches on crank CL.
PR2 = Piston Rise from crank CL
ST = STroke
1/2ST = one half the STroke
DST = Dynamic STroke length to use for DCR calcs
What's going on: First we need to find some of the above variables. We need to calculate RD and RR. Then, using these number, we find PR1 and PR2. Finally, we plug these number into a formula to find the Dynamic Stroke (DST).
Calcs:

RD = 1/2ST * (sine ICA)
RR = 1/2ST * (cosine ICA)
PR1 = sq root of ((RL*RL) - (RD*RD))
PR2 = PR1 - RR
DST = ST - ((PR2 + 1/2ST) - RL)
This result is what I call the Dynamic Stroke (DST), the distance remaining to TDC after the intake valve closes. This is the critical dimension needed to determine the Dynamic Compression Ratio. After calculating the DST, this dimension is used in place of the crankshaft stroke length for calculating the DCR. Most any CR calculator will work. Just enter the DST as the stroke and the result is the Dynamic CR. Of course, the more accurate the entries are the more accurate the results will be.
Using this information: DCR is only a tool, among others, that a builder has available. It is not the "end all" in cam or CR selection. However, the information provided is very useful for helping to match a cam to an engine or an engine to a cam. It is still necessary to match all the components in an engine and chassis for the best performance possible. Pairing a 305º cam with milled 882 heads just won't cut it even if the DCR is correct. The heads will never support the RPM capabilities of the cam.

A good approach when building an engine is to determine the duration and LSA needed for the desired RPM range. Once this is know, manipulate the chamber size and piston valve reliefs (and sometimes the cam advance) to provide a DCR around 8.2:1. Now that the correct piston volume and chamber size is know, enter the actual crankshaft stroke in your CR calculator to see what static CR to build to. Often the needed SCR is higher that you would expect. Note: The quench distance (piston/head clearance) should always be set between .035" and .045" with the lower limit giving the best performance and detonation resistance.

Alternatively, with the SCR known, manipulate the cam specs until a desirable DCR is found. When the best intake closing time is derived, look for a cam with that intake closing timing, that provides the other attributes desired (LSA and duration). Often times the best cam is smaller than one might expect. Sometimes a CR change is needed to run a cam with the desired attributes.

The information given here should be used as a guideline only. There are no hard and fast rules. It is up to you, the engine builder, to determine the correct build of your engine. And remember, unless accurate measurements are taken, these calculations are approximations.

Here is a link to a discussion in which Jim McFarland discusses some issues regarding compression ratios and combustion problems.

Here is an article on High Compression by David Vizard

I hope you find this information helpful and useful,

Pat Kelley



--------------------------------------------------------------------------------

Automation, ain't it great: I have written a Visual Basic program to automate the calculations. It includes the Dynamic Stroke Length Calculator, plus a Valve Timing Calculator (to determine the intake closing point from the advertised duration), and a Compression Ratio Calculator.
There are two version. The larger file contains the required Visual Basic 6 runtime files. If you don't have these files on your system, this is the one to download. It will install these files for you. These runtime files do not come with any version of Windows and can be downloaded from Microsoft's site, if you prefer. If you have the VB6 runtimes, download the smaller file. It does not have the runtimes. If you have successfully run VB6 programs before, you have these files. If you have never ran a VB6 program before, you need the larger version. Un-Zip with your favorite archive program and run "setup.exe". This will install the program and register it with Windows. These files were compressed using WinZip 7.0. You can download a free demo copy of WinZip at www.winzip.com. If you have any problems, email me (my address is on the Home page) and I will try to help. You can take a look the the

DCR FAQ's, the answer to your question could be here.

DCR Calculator with VB6 Runtime files 1.55 MB

DCR Calculator without VB6 Runtime files 423 KB

*A note to users outside the United States.* The DCR Calculator was written with the Regional Setting of Windows set to the "English (United States)" setting. To run properly, you may need to change the Regional Setting of your Windows operating system to "English (United States)". This is due to the way various regions use the "," and "." place and decimal separators (there may be other factors I'm not aware of, also). After running the DCR Calculator, you should return the setting to your original region settings to insure the proper operation of your system. The Regional Setting applet should be located in Control Panel.

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I have received several request regarding what is the best DCR for lower octane fuels. At this time, I don't know. If you are running 87 or 89 octane successfully and know what your DCR is, I'd be interested in hearing from you (email address is on my home page). This would help those that want the best performance on lower octanes for drivers and tow vehicles.
Hot, humid air is less dense than cooler, drier air. This can allow a golf ball to fly through the air with greater ease, as there won't be as much resistance on the ball.

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Post by wildthings » Sun Feb 25, 2007 11:15 pm

Well I just read something that could explain why head and piston temperatures might go up with higher compression but without detonation having occurred. Because the mixture is compressed more tightly and is therefore more dense there is more hot mass closer to the head and piston surfaces, this along with additional squench common with higher compression which moves the hot mass rapidly along these surface would likely transfer more heat to the head and piston.

Conversely, more squench allows for a more rapid burn which requires less timing advance and therefore there is less time for the heat to be transfered so you would get cooler surfaces.

As Tom in PA said there is probably not one single answer.

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