high compression is always good for more power and more toque....

BUT ...as with all things.... you do run into a point of diminishing returns....

there are several factors that put a limit on benefits of compression...

one of the one's that is often overlooked is combustion chamber shape.... but some of you didn't see that coming...! you see, we can't just go with the "all things being equal, more compression will....." because in order to get more compression, we do have to change the shape and volume of the combustion chamber...

one of the things that is VERY important for power production... as well as detonation resistance... is combustion chamber turbulance..... that is where the terms :

squish velocity.... the speed that gasses are pushed from the tight spaces between the flat perifery of the piston's surface and the flat areas of the cylinder head.. towards the center of the combustion chamber as the piston approaches the top of the bore

tumble..... during the intake stroke, the rolling wave of incoming fuel\air mix when flows across from the intake ports, across to the cylinder wall on the opposite side...and rolls over on itself like a wave as it reaches the top of the piston down in the bore... most commonly associated with cylinder heads with more than 2 valves

swirl.... intake motion in the manner of tumble....but instead it is more of a port angle that sends the incomming mixture in a circular motion down the bore...sort of like the downward threads of a very coarse screw.... mostly associated with 2 valve heads...

now...what's the point of all this emphasis on turbulent, swirling intake mixture?? it GREATLY increases the speed and efficiency of the combustion....

one way is that a turbulent mixture is better at making and keeping the fuel in finely atomized\ well mixed state...smaller fuel particles have a much better chance of combining with oxygen to take part in the combustion process... since fuel needs oxygen to burn, bigger droplets tend to only burn off their "skin" layer that has oxygen available to it... the rest of the droplet just tends to dampen the burn by absorbing heat..and adding to unburned hydrocarbons out the exhaust....

the other way that turbulence is better for combustion is that , comparatively, a non moving air fuel mix burns MUCH more slowly than a turbulent one.... a stagnant mixture burns from center to edges at a slow rate...much the same as if each radii was a fuse....burning from the spark of the spark plug, in a straight line to the outer edgess of the bore...a pretty slow\inefficient way to propogate a burn...

now...if there is a swirling\ tumbling torrent of a mixture vortex at the point of spark ignition.... that is sort of equivalent to a bunch of pieces of fuse being swirled & chopped in a blender ...each and every piece is going to be lighting a whole bunch of other pieces in a very short time! MUCH Faster\ More effiecient burn....

now.... back to compression..... a tighter\shorter\ flatter combustion chamber doesn't leave a lot of space for all of the swirl\sumble\squish motion to survive without being suppressed...partly by a higher (thicker) viscosity of a more tightly compressed mixture...and partly because there just isn't enough space to freely continue with motion....

and that's where compression meets one of its limits.... you need a tighter combustion chamber (with less space)... to get higher compression... at some point you are making the burn more stagnant and inefficient in its burn rate...

another point is that compression itself is a load on the engine.... you need to use HP (in the form of crankshaft inertia) to compress the mixture before each combustion stroke... there will be a point ( typically more pronounced at higher rpm) where the added load of compressing the charge is not compensated for by a better combustion.....

heat is yet another limiter... at some point heat from compression will not be effectively handled by the cooling system...and some of this extra heat will find its way into the intake port and pre-heat the intake charge... doing 2 undesirable things... a heated intake chage expands... so less fuel\air is taken in in the same space...so a less dense \ less powerful charge ...and not only will that expanded charge be less potent... but the higher temp will make it more prone to detonation....

so...after all that ...what's the answer for your engines...??

from all of my testing, the point of diminishing returns happens when you go above the 14's for compression.... sometimes you can see a few dyno runs where there is short term benefit above the 14's ...but then heat load kicks in on a fully up to temp engine..and those gains are short lived.....maybe OK for a drag race only setup....one run then a cool down sort of riding... but not good for sustained riding like GNCC\MX\Desert....

from my testing , you can get very good power form a properly designed pump gasser in the 12 comp range..and have good safety margin for high loads and not so good fuel or incorrect jetting....

from there...IME...the 13's are the "no man's land" for compression.... not always safe for pump...and not high enough to take full advantage of having to run race fuel....

so i either go poweful & safe with 12.25:1.... or go to 14.25:1 for best use of high octane fuels.....


compression itself is not a liability..... as long as you have enough octane and no detonation...you could literally run as high a comp as you want....

compression pressures will usually run in the 200's for psi..... but combustion pressures will usually be between 1200 to 1400 psi for a well built \ high specific output engine...

it's the pressure of combustion...that translates into the HP level of an engine...not the static rating of the piston.... a 55hp pump gasser is seeing more pressure and stress than a 54HP high comp engine...if they were both 55HP...they would be seeing the same combustion pressures....


the difference from 12 to 14:1 compression comes in at right about 1.5HP... as long as everything else is kept as equal as possible....

so sometimes it just isn't worth people wanting to be married to a race fuel budget for 1.5HP... and that's understandable...

60HP pulls an almost identical holeshot to 61.5HP.... the dyno picks that difference up more than the drag race ever will...

still, some want all they can get out of everything they've got... so for them it's high comp and high octane....

everyone has a goal... and budget that moderates it...lol...



hope that helps

Excellent read.

The swirl, tumble............or twirl has a huge effect on power. I did some good reads on this. Ive seen where they port each intake different on a 4 valve head. 1 port is ported to induce swirl, the air sorts shoots out to the side of the combustion area.......and the other port is more dedicated to tumble for the air in another direction, more directed towards the center of the combustion chamber. Togther they create twirl which, like you said really "mixxes" things up......lol.
This promotes a more complete burn...........good stuff!

so... run a huge gasket, gore out your combustion chamber, drop the compression as low as you can, and slap on a GT28RS "disco potato" running like 11 pounds of boost?

just givin ya crap. Good information, i concur on most of it, but i have no word so i'd say your good to go John.

twirl....

what about swumble...??

just putting up some stuff to think about for the fun of it.....

now you can add in the actual cylinder fill of "dynamic' compression to really muddy the waters.... LOL

if you lower the compression.......






you can add more NITROUS.......LOL





but alas....I love my HIGH comp........





good info by the way......thanx....

I've read that article........good read for sure. really makes you think!

Agreed, but how can we truly know that diminishing point, there are hundreds if not thousands of different combos we could run on these motors, without even getting into dome shape, how many possible shapes could we create in that head? I mean we could probably use some modern modeling programs to re-create what would "happen" but we really dont know til we try, right?





So when you say everything else is kept as equal as possible how does combustion chamber shape play into that statement? Same combustion shape? Different for each compression? Hard to put a HP number on something that "SHOULD" be different right? Just trying to understand here... those two lines seem to contradict each other to me, and I'm just trying to grasp this.

Good write up, Thanks!


Mike

mike,

there has been quite enough testing of compression ratios over the last several decades to say for sure that 15:1 definately the point of "negative gains" for our type of engines.....

now...if you had really restrictive ports....a terrible intake...terrible exhaust...terrible cam....or any combo thereof that would cut down on the cylinder filling efficiency..... you could possibly not have the problem of excessive heat load, since there isn't going to be a a compratively full charge to burn.... but static compression comes specifically from lessening cylinder volume at TDC.... so you will still have a crappy combustiuon chamber as far as burn promotion is concerned......

each engine has only so much room for combustion space per number of valves.... pretty finite set of variables there...


the differences between the 12's and the 14's are given from my own personal dyno tesing time with our 450 based engines..... not a guess.... actual testing.... about 1.5HP is everything else is the same, except of course for the compromise of area necessary to up the compression....

you gotta lose TDC volume if you want to up the static compression ratio..... pretty finite rule there also.....

just wondering why diesel engines last so long and stay running cool w/all that compression and a turbo?????

their cooling systems are, naturally, made to deal with all that compression.....

completely different engine design

just a note.
crank a diesel engine over 4000 rpm,then see how long it lasts!

I run 4250RPM daily.... I know a few pushing closer to 5000, but those are more high end builds. Mine is a street truck with nothing more than a programmer....

Like already said, completely different design, the rings on the diesel motors are designed to keep a tight seal even after they wear out. Diesels are made for longevity, but like anything else, start cranking em up and they go out quicker.

Could you post some of the reserach you've found about the 15:1 compression, I'd love to do some reading! Thanks for the reply! This is all very interesting!

I understand that in order for compression to increase, volume must go down. I think maybe I didn't word my question well.... lets say a 12.5:1 piston and a 14:1 piston, both can be used in a stock head. However I have seen builds with a 14:1 piston where there is significant dome shaping in the head. You can always change the shape of the head in the combustion chamber no? I guess I was wondering if changing shape really helps much, or if its just something that is usually done to coincide with other mods? My point was that a 14:1 vs. 12.5:1 isn't a straigh comparison if the two domes are shaped differently, and thats why I said there is an infinite number of ways to shape a head (combustion chamber).

Thanks again for your info.

there aren't infinate ways to shape a combustion chamber.....


there are things that dictate combustion chamber shape...such as all 4 valves.... the spark plug...and clearance around each....

now, the piston also needs pockets to clear the valves @ tdc....and also can't have a dome that will interfere with the spark plug...

so right there you have some serious deliniators on what the combustion chamber needs to conform to....

next up.... as compression goes up.... it goes up by limiting\decreasing the space of the combustion chamber....


so...if you re-read my first posting... you should start to understand that given the space limitations that copme hand in hand with high compression....along with it comes limitations on chamber shapes and clearances that are necessary to high performance..... since the clearances around valves and spark plug are a given constant, you really don't have infinate ability to hold a desired compression and not compromise something about the chamber.... as a matter of fact.... far from being unlimited.... it is actually very limited....

Yes I guess my term "infinite" wasn't properly used, obviously there will be limiting factors. Higher compression would mean less options for combustion shape.



Obviously, correct. Simple pressure and volume equation.



This is what everyone should take from this discussion. That sums it up pretty well.

Thanks for the input, got the wheels turning in my head!

Thanks, Mike.

I just dont understand why it would be more pronounced at lower RPM's. At lower/accelleration RPM's most of the power generated by the combustion is used up before it can compress the next mixture. I would think it would be less pronounced at high RPM because there is more crankshaft inertia. Or is it that it takes more power to slowly accellerate a machine from a high speed to a higher speed than it takes to fastly accellerate from a low speed to a high speed. I have always heard people say they didnt want too much compression because it wouldnt rev out good.

P.S. Awesome write up! Youre a smart dude!

Thats exactly what I was thinking!

Here's a neat CFD model that illustrates some of that mixxing [sic] action....


(may take a sec to load and display smoothly)


It'd be sweet to get someone with Fluent to model a real 450r or YFZ cylinder.

Does this rule of thumb that 15:1 is the point of diminishing returns still hold true when running e85 fuel? I'm looking to convert to an e85 setup and run an ultra-high compression piston to take advantage of e85's properties. Could e85 handle 15:1?

I have a quick question for ya i have an 06ER with the hrc kit and it ran really good then i went back and swaped the hrc cam for the stage 2 hot cam and also installed a je 14.8:1 piston and to me it doesn't run as good as it when it had the stock piston and the hrc cam. Do you think that the hrc cam is better or maybe need to change the timming due to the higher compresion (i'm running 110 race gas ) ANY HELP WILL BE GREATLY APPRECIATED

You may need to fine tune your jetting for a real good comparision. The 06 HRC cam has a ton of bottom end, probably more than the h2, however the h2 may do a bit better on top so it all depends on where you want your power.

My suggestion is to try to make sure jetting is spot on and if that doesnt yeild the results you wanted, try swapping back to the hrc cam to make a comparison to the h2 in that setup.

I read some where that the hrc cam is all on top end, and the hc2 had more grunt. I run the hrc in my 08 and it runs damn good but I havent tried others. Does any one have good hp info? Im curious nince I here so many diferent opinions. hrc seems to keep my valves good and runs well.

gotta remember that there are 2 completely different HRC cams for the TRX's.... the radical\ top end biased one from the 04-05 HRC kits...

and the low to midrange profile HRC cam from the 06 & up HRC kits...that cam is actually an oem cam from the similar years CRF450....

good write up,i know this is an old p[ost but i found it pretty informative.I too find it funny how "know it alls" dont know much when it comes to specifics.Some will try to argue points they read in atv action or dirt rider but really have no clue when it comes to exhaust duration etc when it comes to 2 stroke performance,what im more familiar with.Always looking for new info..thats why i run 12to1 in my 4fiddy,im old skool as far as compression builds hp but not 15to 1!

well said on those who argue without understanding.... unlimited supply of those types..

thanx for the compliment... :)

Great read man!

When I was racing carts we used 5hp Briggs and Straton on methanol and were able to pull up around 35 hp on a decent motor I am sure some people got more but we were just poor smart folk. :) But with their engine design the head interior looked like mickey mouse ears so we were able to angle the valves, direct the ports and cut the head to create a cyclone effect which not only allowed us to twist much higher rpm without losing as much tq but but also gave the engine a little better power response... BUT it wasnt until we angled the carb and then the exaust that we started getting big gains.

So with my 450r its a different design I know the exhaust is angled and the intake is slightly centered but with the OHV setup how can we get more "twirl" in the chamber?

and if some one says add one of those stupid vortex booster carb attachments I am just going to start drinking... heavily.... :)

JD

Hey FLSwampRat- if you want more "twirl", all ya gotta do is add one of those vortex boosters for the carb, lol...

i noticed in your sig u say you have 17.1:1 compression?? how did you come up with that and what fuel you running??

well..... there are basically 2 types of "intake charge motion" .... tumble, which is sort of an across the chamber and forward roll of the charge

and swirl...where the intake port is angled away from the centerline of the bore...such that the intake charge come in angled towards a close portion of cylinder wall...and proceeds down the bore in a "screw thread-like" manner...

intake port height and angle are the main controllers of each of the above....

then there is charge motion created from the squish bands as the piston approaches TDC...

all it takes is lots time and $$ to make all the different combinations... and testing to find out what works best for your setup...