ENGINE BRAKING

[Dave_zoom_zoom]

Please help the uninformed----

 

Why is my 06 R1200RT able to provide greater engine braking than a equally sized four cylinder motorcycle?

 

Thanks!

Dave

[plext]

Many things affect engine braking.

 

Compression.

Inertia, large slugs and rods == inertia.

Weight of machine.

Gearing.

Slipper clutch or lack thereof.

 

Could be any or all of them.

[Rich06FJR1300]

i would advise against it, especially with a dry clutch. Brakes are easier to repair than the clutch.

[RockBottom]

i would advise against it, especially with a dry clutch. Brakes are easier to repair than the clutch.

 

There's a time and a place. I use engine braking to control my speed coming down off of mountains because the brakes would heat badly if I used them for 5-10 minutes straight.

[Boffin]

i would advise against it, especially with a dry clutch. Brakes are easier to repair than the clutch.

 

I see a big difference between engine braking by closing the throttle and by slipping the clutch on down-changes. Closing the throttle will not harm the clutch any more than opening it will as long as the clutch is not slipping.

When I was training toward my Institute of Advanced Motorcycling test, I was given an exercise where I had to ride several miles at various speed limits, without using brakes at all. Throttle control sense is an important aspect of riding, and 'engine braking' a significant part of that.

 

I do however, have a pet, unproven, theory that slipping on down-changes as well as up changes magnifies wear significantly, as the fibres are being pulled in opposite directions. As a statistically totally invalid data point, I wore out one clutch in under 30K, but I am now have over 55,000 miles on its replacement having modified my down-change technique.

 

Andy

[upflying]

Emissions mapping could be part of it. Close throttle operation at road engine rpm results in shutting off all fuel to the injectors. Fuel is restored just above idle to avoid killing the engine. No fuel to an engine overridden by rolling inertia from the drive-line results in the feeling of strong engine braking.

 

[terryofperry]

I am sure we all downshift and engine brake somewhat as when we come to slow speed stop such as stop signs and red lights. But high rpm engine braking is a different story.

 

Both Keith Code and Nick Ie. say the engine is not a brake.

 

Terry

[Lmar]

I have used engine braking (close throttle operation) many times over the years. I stay away from downshifting in high rpm.

[notacop]

Everybody missed his question, why does the twin seem to be more effective than the 4 cylinder.

 

Because it has two pistons of great magnitude pushing air without flame producing fuel being applied so it has nowhere to go but slow. Well, that's what I sat and I'm sticking to it.

 

Down shifting doesn't make a squat of difference when you are attempting to stay in the power band. People only wear out clutches because they use them ineffectively and should be on a clutchless bike like the old Honda automatics. Or the new FJR or Mana. Just inability!

[ka5ysy]

i would advise against it, especially with a dry clutch. Brakes are easier to repair than the clutch.

 

There's a time and a place. I use engine braking to control my speed coming down off of mountains because the brakes would heat badly if I used them for 5-10 minutes straight.

 

+1. A fully engaged clutch is not going to have issues with engine braking.

[upflying]

Everybody missed his question, why does the twin seem to be more effective than the 4 cylinder.

 

Because it has two pistons of great magnitude pushing air without flame producing fuel being applied so it has nowhere to go but slow. Well, that's what I sat and I'm sticking to it.

 

Isn't the surface volume of a piston and/or combustion chamber the same whether you have 1200CC two cylinder or a 1200CC 4 cylinder?

Aren't all 4 stroke engines simply a vacuum pump when the throttle is closed? The Powertrain Control Module controls throttle position and fuel flow when engine RPM is being overridden by road speed. Whether an engine has strong braking or not depends on valve timing, compression ratio, ignition timing, fuel flow and throttle position commands given by the PCM during the time of override. All of it depends on the engineer who wrote the software in the PCM.

[Boffin]

The volume of space under the pistons of a straight-4 remains the same - it just shoves air from place to place. On a boxer engine the volume under the pistons varies by the capacity of the engine - on a R1200RT that is 1170cc on every stroke. That alone increases the braking effect though the principal mechanism comes from the pumping effect against the closed throttle plate - the reason that diesels have less engine braking, having a wide-open throttle.

 

Andy

[JayW]

i would advise against it, especially with a dry clutch. Brakes are easier to repair than the clutch.

 

There's a time and a place. I use engine braking to control my speed coming down off of mountains because the brakes would heat badly if I used them for 5-10 minutes straight.

 

+1. A fully engaged clutch is not going to have issues with engine braking.

 

...and matching revs as you downshift virtually elimates any wear on the clutch from this practice.

 

Consider tapping the rear brake anyway to let those behind know that you are slowing.

 

I cannot answer the original question, so will now stop encouraging this diversion.

 

Jay

[upflying]

The volume of space under the pistons of a straight-4 remains the same - it just shoves air from place to place. On a boxer engine the volume under the pistons varies by the capacity of the engine - on a R1200RT that is 1170cc on every stroke. That alone increases the braking effect though the principal mechanism comes from the pumping effect against the closed throttle plate - the reason that diesels have less engine braking, having a wide-open throttle.

 

Andy

Each stroke on a boxer is 585cc isn't it?

Or 300cc on an in-line 4. I'm still not seeing an engineering answer to the OP's question..

Not every 4-stroke gasoline engine has a closed throttle plate during compression or engine braking. Some engines shut off fuel and or/ignition and some don't. Some open the plate slightly for emission reasons. That open plate results in less vacuum and hence less braking.

Are we comparing apples and oranges? Does the boxer appear to have stronger engine braking due to the design of the intake, fuel system and cam timing?

Could the in-line 4 have similar engine braking if the intake, fuel delivery, cam timing, ignition timing and PCM mapping were the same as the boxer? What happens when the kill switch is activated on either engine design during engine braking?

[Dave_zoom_zoom]

My thanks to ALL that have responded! Specially

those who read my question. (top of page one)

 

To be a bit clearer---------- I'm not completely uninformed!

 

I've owned many bikes and ridden many, many more. Total approx. 1,000,000 miles under my butt.

 

My R1200RT is the 4th boxer I've owned. (I love it!!!)

 

It seems to me (maybe in error) that these boxers have a greater engine breaking under closed throttle than the 4 cyl. in line or V4 engines.

 

Am I wrong? If not, what causes that?

 

Another subject. Re: Clutch wear--This is dependent upon the amount of clutch slippage experienced. Not RPM or engine braking applied. When I'm on a boil, I ride this bike 5000 to 8000 RPM. I may bump into the rev limiter once or twice a year by error. NO PROBLEM! I've never in all my life worn out a clutch.

 

Back to engine braking. Is it my imagination that the boxer design provides better engine breaking?

 

THANKS TO ALL!

Dave

 

 

[Dave_zoom_zoom]

The volume of space under the pistons of a straight-4 remains the same - it just shoves air from place to place. On a boxer engine the volume under the pistons varies by the capacity of the engine - on a R1200RT that is 1170cc on every stroke. That alone increases the braking effect though the principal mechanism comes from the pumping effect against the closed throttle plate - the reason that diesels have less engine braking, having a wide-open throttle.

 

Andy

 

Thanks Andy!

I think you are onto something here!

 

Dave

[Peter Parts]

Total "flywheel" moment matters too. Monumental difference when I shaved 7 lbs from /2 flywheel. The Oilhead flywheels are large diameter but pretty vestigial and running at crankshaft rpm.

 

Some tricky issues with the primitive Motronic ECU because of the idle software switch. Hard to switch off the fueling but then modulate the braking, esp. in lower gears.

 

But the minimal cost and effort of replacing brake pads in a bike compared to a car makes the decision matrix (brakes versus engines) very different.

 

Ben

cool in Toronto

[Haynes]

Perhaps the relatively larger piston surface area on a 1200 twin will produce greater engine braking than a smaller bore 4 cylinder engines.

 

I avoid excessive use of engine braking. As it only acts on the rear wheel I feel over reliance could cause instabilty when entering a corner.

[Danny caddyshack Noonan]

I'm not out to offend but maybe this needs a critical eye. So, with all due respect to the OP, instead of drawing on conjecture, does anyone have any evidence that the engine braking is better? Maybe go out and do a test? Gearing may mess with data though.

 

Maybe it just feels like it is better and is somehow attributable to the twin exhaust note (yes a reach)? Maybe a physical characteristic like the suspension lack of dive?

 

Physics wise, the list is short.

Flywheel inertia is a function of not only mass but rotational inertia, a strong function of radius..as in squared. Inertia resists changes in velocity and is, therefore something that would tend to reduce engine braking effectiveness. However, it is possible that flywheel inertia of a four equals that or is greater than that of a boxer though.

 

Other inertia drivers are within the engine. The pistons. How does the total weight of two compare to four? What are the velocity changes of two compared to four i.e. stroke length?

 

The vast majority of my miles are on KZ1000's and a V-45 and thought the boxer has similar engine braking but, that is just an opinion.

 

 

[Mister Tee]

I dunno. If I botch a high RPM downshift on my gsxr, the rear wheel will skid from braking (it does not have a slipper clutch like the newer Suzuki's have.) Seems like its engine braking power is at least as good as the RT's was. Then again, I don't use the engine for heavy braking either. I suspect that if I ever ran out of gas in the gsxr, the rear wheel would probably lock up. I ran out of gas in the RT once (long story, fuel strip issue) and the bike coasted fairly gently to a stop from freeway speed, more like reduction in power vs. a total loss.

 

But getting back to the OP's question, what inline fours are you comparing the boxer engine to? I suspect engine braking characteristics are primarily a function of engine displacement, vs. configuration. I would expect a 600 cc sportbike engine to have less turning resistance than a 1200 cc engine of any type.

[Survived-til-now]

I don't think it can be the greater capacity of compression because the two cylinder has a phase of no compression whilst the four cylinder always has one cylinder on compression.....

 

So it must be the fact that two cylinders achieve more torque at lower revs than a four cylinder that has to rev higher to achieve the same torque....

 

Somebody will come up with the technologically correct answer - let's hope us ordinary mortals can understand it when it comes....

[Pilgrim]

There's a tool that will permit measuring this effect if you have an iPhone.

 

Go to www.lightemup.com (a Bosch site.) There, you will find a program that uses the iPhone accelerometer to provide all sorts of data about the movement of an object to which the iPhone is firmly affixed. Most particularly, it will record the g-forces generated by acceleration, deceleration, and turns (yes, I know; they are all acceleration, but...)

 

Pilgrim

 

p.s. I just checked the link. Site won't open right now.

[Joe Frickin' Friday]

My theory:

 

BMW's boxer engine uses a separate throttle and intake tract on each cylinder. The airbox is upstream of the throttles, the result being that there is a relatively small volume contained between each throttle body and its cylinder. The result is that for 3/4 of an engine cycle, there's nothing happening to that trapped air, and then for 1/4 of the cycle (the intake stroke) that volume is subjected to very strong vacuum: the cylinder quicky evacuates that small volume, and for the remainder of its intake stroke is subjected to very strong vacuum (read: strong engine braking).

 

Contrast this with a twin (or four, or six) on which all cylinders are fed via a common manifold from one single throttle body. The manifold volume (downstream of the throttle) will be much larger in relation to any single cylinder, resulting in a smaller transient dip in manifold pressure during any single intake event, and so the vacuum developed above each piston during its intake stroke won't be as strong. Presto, less engine braking.

 

A test to confirm this would be to compare engine braking on four bikes of equal engine displacement:

 

-a twin with both cylinders breathing through a common throttle;

-a twin with each cylinder breathing through its own throttle (e.g. the BMW boxer);

-a four or six with all cylinders breathing through a common throttle;

-and a four or six with each cylinder breathing through its own throttle.

 

If throttle/intake configuration is the big issue, then a four or six with multiple throttle bodies should exhibit engine braking on par with the BMW boxer (assuming the same total engine displacement).

[Peter Parts]

There's a tool that will permit measuring this effect if you have an iPhone.

 

Go to www.lightemup.com (a Bosch site.) There, you will find a program that uses the iPhone accelerometer to provide all sorts of data about the movement of an object to which the iPhone is firmly affixed. Most particularly, it will record the g-forces generated by acceleration, deceleration, and turns (yes, I know; they are all acceleration, but...)

 

Pilgrim

 

p.s. I just checked the link. Site won't open right now.

 

That's a really interesting app and might have a lot of applications.

 

However, in working with the accelerometer in the Tesla G-Force meter, I found you have to be really careful with bikes due to the vibration (which you don't have on a car dashboard). The accelerometer treats it like any other motion. Good synching and careful mounting orientation of the iPhone might help.

 

Great tool.

 

Ben

[Dave_zoom_zoom]

WOW! You guys are good!!!

 

Some really good theory here. I like it! Maybe we won't get a definitive answer here. And maybe we already have it in one of these very good thories. Thank you all for sharing your thoughts.

 

I understand brake pads are relatively inexpensive. My main motovation for engine braking, is to always being the best gear to apply the most effective power when required. Such as comming out of that curve. Or if slowing for a stop, be in the best gear to provide immediate power to avoid a problem. If road speed and engine speed are closely matched at time of clutch engagement, I don't think clutch wear is an issue during a downshift.

 

Thanks for all for your insights. Some of which I hadn't thought of. More insights are welcomed.

 

Thank You

Dave

[Peter Parts]

My main motovation for engine braking, is to always being the best gear to apply the most effective power when required. Such as comming out of that curve.

 

As I said earlier, Motronic is balky for various reasons in providing smooth, controllable, predictable braking, at least compared to other fueling systems. Add to that, the big cubes and you are increasingly on to the disk brakes.

 

Varies with models, but BMW lost their sense as they pushed for upper rpm HP instead of usable low rpm grunt (the Ss being worst, of course).

 

I've done some tricks with mine to broaden the power band and that is quite helpful in making a bike that is "user friendly." I think others should consider that - even though most sup-up advice is aimed at the opposite.

 

Ben

[nrp]

The ABS control system has to really be screwed up when it has to deal with the added inertia of the engine instead of just the wheel..........

[Joe Frickin' Friday]

The ABS control system has to really be screwed up when it has to deal with the added inertia of the engine instead of just the wheel..........

 

If you're at threshold, then the front wheel is doing the lion's share of the braking, and it won't know/care what you're doing with the driveline.

 

If the engine is providing some braking, then the rear brake system will just modulate around a lower pressure level than it would if the driveline were disengaged. I would not expect driveline inertia to be a huge issue since the ABS is watching for pretty small changes in wheel acceleration.

If it weren't December, we could go outside and test this.

 

Maybe next spring? Try a max-effort stop with and without engine braking, and see whether the results are any different.

[artig]

One of the best ways to test engine braking would presumably be to reach a certain speed in top gear, say 150km/h, and then close the throttle and measure how long it takes to reduce the speed to, say, 50km/h. Obviously gearing, tyres, wind resistance, and other factors play a part when you are doing this to compare different bikes.

 

So I decided to try this on the same bike, once with just the two cylinders and once with two extra cylinders. The only spare cylinders I had available were two 45kg LPG cylinders. Strapping one of these to each side of the bike, and repeating the run where only the two original cylinders of the F650GS were used, I can confirm that there is more engine braking on a 2-cylinder bike than on the same bike with 4 cylinders. It took much longer for the 100km/h reduction in speed, even with the extra wind resistance from the two extra cylinders. Perhaps not surprisingly it also took much longer to reach 150km/h.

 

Seriously though, although the bikes are not directly comparable, I think the 800cc F650GS has more engine braking than a Honda CB1000R. One of these days I'll go out and try it. Both bikes are doing about 4000rpm at 100km/h in top gear, so the gearing is very similar. Without any panniers their wind resistance should also be similar, as are their weights.

 

[James Clark]

Please help the uninformed----

 

Why is my 06 R1200RT able to provide greater engine braking than a equally sized four cylinder motorcycle?

 

Thanks!

Dave

 

Piston ringworms.

 

 

[Deek]

....

 

Back to engine braking. Is it my imagination that the boxer design provides better engine breaking?

 

I wonder if the motorcycles weight has anything to do with the perceived difference in "engine braking"? Most about all four cylinder bikes weigh a lot more than two cylinder one I'd think. I'm not engineer, but that seems reasonable to me.

 

 

 

[AllenW]

....

 

Back to engine braking. Is it my imagination that the boxer design provides better engine breaking?

 

I wonder if the motorcycles weight has anything to do with the perceived difference in "engine braking"? Most about all four cylinder bikes weigh a lot more than two cylinder one I'd think. I'm not engineer, but that seems reasonable to me.

 

 

 

I'd think weight of the bike and or rider, gearing, engine timing and fuel management along with the list of stuff others have posted all come into play, seems like far to many variables to give an accurate answer to in this case..??

 

Al

[Haynes]

Mitch's description of the relatively small volume of air in the intake affecting high induction vacuum is very plausible.

[Little_Brit]

Having done a fair few miles on boxers (about 90,000 and counting) I think Andy is on the right track regarding total engine displacement. Every rev the pistons try to compresses 1200cc of air inside the crankcases. An inline four has no change in crankcase pressure due to piston movement because as one piston descends another rises out.

 

Having said that my own view is that cam design, valve timing and compression ratio all have a part to play. My K75s certainly had plenty of engine braking, more than my old 75/6 and in a different league to my beloved old Honda CX500.

 

As for using engine braking; I don't change down to slow down per se, that's what brakes are for, but I do enjoy the way the engine braking on my R1200GS enables me to ride for mile after mile on undulating, twisting roads without having to use the brakes at all.

 

Derek

[Dave_zoom_zoom]

I don't change down to slow down per se, that's what brakes are for, but I do enjoy the way the engine braking on my R1200GS enables me to ride for mile after mile on undulating, twisting roads without having to use the brakes at all.

 

Derek

 

+1 Oh yeah!!!! Fun isn't it!!! :thumbsup:

[nrp]

The ABS control system has to really be screwed up when it has to deal with the added inertia of the engine instead of just the wheel..........

 

If you're at threshold, then the front wheel is doing the lion's share of the braking, and it won't know/care what you're doing with the driveline. If it weren't December, we could go outside and test this.

 

Maybe next spring? Try a max-effort stop with and without engine braking, and see whether the results are any different.

I think of the need for ABS to be greatest when on gravel - or even ice. In these cases there is little overturning moment & the braking capabilities of the front and rear wheel becomes more equal. But I won't volunteer my bike for a test here in MN!

[kmac]

I hadn't weighed in on this thread yet, but i was thinking about it.

 

Even though you dont have fuel in the cylinder when the throttle is closed, you do still have compression which is why you have engine braking at all. I think if in deed a 2 cylinder 1 liter+ bike has more engine braking than a 4 cylinder of similar cc has, then it would be because each cylinder is of smaller displacement. Each bump of compression is half of the size, albeit more often on the 4 cylinder.

 

To demonstrate, find some 50cc bike with 11:1ish compression and try to bump start it. Now do the same with 450cc MX bike and see what happens. Even try a CR/KX/RMZ/YZ 250f and then a 450f. The 250 will turn the motor over in 2nd gear. The 450...no way unless you have some serious speed, and you are in 3rd gear. The sheer volumn of each compression stroke is about double on the 250 to the 450 which is similar to the equal cc 2 cylinder to the 4 cylinder.

 

I would think the lighter the bike and rider, the MORE the engine braking effect would be. Power to weight ratio working similar on acceleration as it would on decelleration.

 

As usual i may be wrong, but i still throw it out there in hopes that someday i might get one right...lol

[Joe Frickin' Friday]

Even though you dont have fuel in the cylinder when the throttle is closed, you do still have compression which is why you have engine braking at all.

 

In the absence of combustion, whatever happens during the compression stroke happens in reverse during the expansion stroke: if you give up mechanical energy squeezing the air during the compression stroke, you get that energy back during the expansion stroke (minus a small amount due to blow-by and heat transfer).

 

Moreover, when the throttle is closed, there isn't much compression happening anyway because there's very little air in the combustion chamber to be compressed. With the throttle closed, the engine cycle looks like this:

 

• intake stroke: high vacuum, lots of braking effect on the crankshaft
  
• compression stroke: high vacuum trapped in combustion chamber actually tends to accelerate the crankshaft over the course of the compression stroke, but...
  
• power stroke (no combustion): the expansion here undoes the acceleration provided during the compression stroke, effectively canceling it out
  
• Exhaust stroke: at the beginning of this stroke the exhaust valve opens, air from the exhaust system rushes in and establishes near-atmospheric pressure. relatively little mechanical work gets done pushing this air back out into the exhaust system.

You can confirm this a couple of ways:

 

1. pull your injector wires, and try starting the engine. While it's cranking, snap the throttle wide-open: youll find that the starter motor is able to crank the engine over faster.

 

2. While cruising down the road (with no traffic anywhere nearby), take your hand off the throttle and hit the kill switch. Observe the engine braking effect, and then - with the engine still killed - twist the throttle wide-open. You should notice reduced engine braking at that point.

 

Note also that diesel engines offer relatively little engine braking because of the absence of a throttle. If you want any useful braking effect from a diesel, you need to play with the valvetrain or install a throttle plate in the exhaust system (in which case you can develop even more engine braking than you could with an intake throttle).

 

See Wikipedia article on engine braking for more info.

[Dave_zoom_zoom]

Thanks Mitch.

 

You never cease to amaze me. I can see you hit the bulls eye on your first post and then futher clarified it in your second post.

 

In due respect to others that have posted, I'm clear that many other factors affect engine braking as have been addressed in your posts. However I think Mitch's point on the relatively small volume of air contained between each throttle body and its cylinder is most significant.

 

I realize the significant amount of H.P. required to operate a vacuum pump. Due to the rather unique configeration of the throttle body being so close to its cylinder on our boxers, I believe this is by far the most significant factor that applies to the stronger engine braking on the boxer.

 

VERY INTERESTING! I really hadn't considered that!

 

Some may not agree. I thank all of you for your thought provoking comments.

 

Thanks! Dave

 

[kmac]

Afternoon Mitch,

I do not completely disagree with your logic, but maybe you need to stop by your local KTM dealership and try to kick over a new 530. Even with the throttle closed, and tell me there is nothing to compress....you can barely kick one over.

 

Even without the throttle opened a motor makes compression. I believe that is the definition of engine braking. Yes a motor, like a diesel, will increase compression with the throttle plates open. You even HAVE to open the throttle plates when doing a Compression test to get an accurate test, but you will still get a reading with them closed, right?

 

 

Maybe I am misunderstanding the concept

[Boffin]

The thing about compression is that after the piston goes over the top, the compressed gas expands again, giving back most of the energy taken to compress it, except for that lost as heat.

 

Andy

[Joe Frickin' Friday]

...maybe you need to stop by your local KTM dealership and try to kick over a new 530. Even with the throttle closed, and tell me there is nothing to compress....you can barely kick one over.

 

If the engine has been sitting for any length of time with the piston at BDC, then the combustion chamber is no longer at vacuum; it's at atmospheric pressure, meaning yes, in this case there is something to compress, and it will be tough to kick over.

 

But here's the key point:

if you're strong enough to kick it all the way up through TDC, that compressed air will force the piston back down again. In other words, the compression stroke doesn't dissipate energy, it stores it in the compressed gas, to be given back to the crankshaft in the subsequent expansion stroke. The net effect on crank RPM of any pair of compress/expansion strokes is just about zero. As Andy noted, you get back pretty much what you put in.

 

I've had the following happen on my RT's (oilhead and hexhead) before when it's cold out, and the battery isn't quite up to the task: the starter struggles and takes a moment to get through the first compression event (since one of the cylinders was at BDC-after-intake, and the cylinder was full of air), but then the subsequent expansion stroke provides a helpful boost. The engine enters the next cycle at higher RPM, which means a couple of things:

 

• stronger vacuum induced during the intake stroke (because there's less time to pull air past that closed throttle), and
   
  
• less resistance during the compression stroke (because of that stronger vacuum in the combustion chamber), and of course less of a push during the following expansion stroke.

So once it gets past that first difficult compression event and builds up a bit of speed, the starter is able to crank through subsequent engine cycles with a steadier RPM.

[kmac]

Stop by KTM and give your theory a test. It never get easier to kick even after its first rotation. There is SOME kind of compression with the throttle shut and on the 2nd and 3rd + kicks.

 

My local KTM dealer will not even work on the earlier 620s that were kick only because the compression is so high, one broke their techs femur. I owned one of these bikes and i can personally tell you, they have massive compression and massive engine braking.

[Green RT]

So once it gets past that first difficult compression event and builds up a bit of speed, the starter is able to crank through subsequent engine cycles with a steadier RPM.

 

Just to add to what Mitch is saying...

Jake brakes on trucks allow them to open exhaust valves at the top of the compression stroke, bleeding off the compressed air, so that additional deceleration is obtained. I have seen lots of highway signs about Jake brakes and heard them in operation but never understood what they were. This thread prompted me to look it up. Jake Brake explanation

[Joe Frickin' Friday]

Stop by KTM and give your theory a test. It never get easier to kick even after its first rotation. There is SOME kind of compression with the throttle shut and on the 2nd and 3rd + kicks.

 

If you could kick the engine over repeatedly in very rapid succession, there wouldn't be time for air to leak past the throttle plate and fill the cylinder between kicks (after the first kick). In that case, the second and subsequent kickstarts would require considerably less effort than the first.

 

Unfortunately the reality is that if you're kick-starting, you've got maybe a second or two between attempts. That's plenty of time for air to leak past the throttle plate and equalize pressure in the cylinder.

 

You're basically starting over from scratch each time.