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MAX Compression on Pump Gas B4 BOOOOM!

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the following is sort of a mish-mash combination of posts i had put into a thread on the crazy azz topic of:

"How much compression can i run on pump gas and stay just shy of grenading my expensive race engine?"

long duration camshafts make a HUGE difference in octane tolerance....!!

long duration helps at the point where detonation will be the biggest problem.... accelerating from lower rpm ... where the load is the highest...

put in a big cam and there is more reversion ...and thus less complete cylinder fill at low rpm.... so there is much more compression/detonation tolerance with a long duration cam...such as the early HRC....

there are so many variables in the "whats safe" debate... proper jetting is another HUGE variable...consistant fuel quality another

so... what with things like:

outside temp

gearing

rider weight

riding sand hills or flatland

high elevation or sea level

cam duration

volumetric efficiency

and others....

volumetric efficiency is HUGE in this topic...and it is DIFFERENT for EVERY engine....

think of it like this.... if i don't get ANY air charge in a cylinder at all... how much pressure will a super high compression piston generate while compressing all that non existant mixture??

the answer is ZERO pressure!!

no charge in means you are compressing nothing.... the vacuum in the cylinder means you are compressing nothing...so its like like multiplying by zero....a zillion to one time zero still equals zero pressure

the other end of the volumetric efficiency scale: how much pressure will you see with a 14:1 piston compressing a charge that was force fed into a cylinder by a turbocharger running 80 pounds of boost??

the answer is: WAY TOO MUCKING FUCH..!! the excess pressure isn't so much a factor of the piston compression as it is the cylinder is packed with a ton of fresh pressurized charge before the piston even began to compress it... that's why blown or turbo supercharged engines dont want or need high compression... the dome of the piston would even take up space that could be packed with charge while the piston is at bottom dead center.....

so.... that's a huge reason why you need to know your Volumetric Efficiency BEFORE you ask what compression you can get away with... that is what affects your DYNAMIC COMPRESSION... which is the actual pumping compression YOUR particular engine combo is seeing during operation

no matter what, two engines of the same size..... whichever one is making more power is seeing a higher: volumetric efficiency....cylinder filling....dynamic compression....

a 12:1 engine making 55HP is seeing higher VE and pressures than a 15:1 engine making 50HP.... more charge is being lit in the 55HP setup because more charge is entering the cylinder.....

with most people not knowing anything about the detailed dynamics of their engine specifice.....if you are a manufacturer and want to tell people what is safe compression for pump gas... it had D@MN well better be safe for EVERYONE ....

splitting hairs over a point of compression when you cant control everyone's variables is like dancing in a mine field... lots of fun ...until it aint

another thing i like to tell my own kids to consider in any situation is:

"whats the worst thing that can happen?"

sometimes it's not much of a deal... put too much air in your bike tire and you pop it like a balloon.... not really a biggie.... put too little in and you wreck the tire AND bend a rim... little bit more of a problem...

if anyone has seen an engine that detonated...

cracked cylinder

broken piston

bent crank

sheared wrist pin

which also:

bends valves and trashes heads

that also plays into the "whats the worst thing that can happen if i get bad fuel or too much comp" etc... etc...

and for me personally... blowing a good perforamce thumper over a half point of compression or a couple octane points... is like playing russian roulette with something that can have dire consequences on my free time and bank account...

utterly not worth it for me....

i default to safety.... but that's my own paradigm... nobody has to buy into it...

just sharing some experience...

flat tire on road trip.... delay of trip for a bit...

so what??

bungee cord not inspected for cracks before jumping....

splat.. game over..

pick a cam/ pipe / whatever that doesnt fit your ride...

so what?? engine still runs

guess wrong on octane....

boom... game over

its all matter of what you feel is something that has an acceptable consequence....

they make pistons with LOTS of compression

they make fuels with LOTS of octane

they also make pistons that make pretty darn good power with a good safety margin...

it just comes down to comfort levels with what you are gambling with....

that's what detonation looks like...

it doesn't take long ...mere seconds or less....with the pressures that detonation creates...

sort of like using way too fast burning gunpowder behind too heavy a bullet....

gas...and gunpowder...are supposed to burn in a controlled, albeit very fast burn.... a push if you will...

detonation ignites the whole charge at one time...supernova style... and the pressure peak is enormous...

a huge component of detonation is radiant heating of the fuel charge...

you know how you can feel the radiant heat from a campfire?!

well... the fuel charge is supposed to burn evenly from center to border of combustion chamber....

the center flame front moves fast... but it acts like a sun heating the the rest of the charge... and the radiant heat speed is of course the speed of light... worlds faster than the flame front moves...

soooooo ... the heated outer gas is WAY more volatile because of this radiant heating....and capable of self ignition without waiting for the flame front.... thats where the big boom happens... when it all self ignites... insane pressures...

wider combustion chambers are more prone to detonation because there is longer time for the flame center to pre-heat the outer gas charge from radiant heat.... smaller chambers are much more resistant to deto simply because the complete burn doesn't take as long...

tighter squish does lots to control this by speeding up burn rate with more swirl... faster burn chamber... and its also better at shielding (shading) the perifery charge from the radiant flame front...

deserves more discussion....

a few topics:

1) pre-ignition Vs detonation.... the difference in the signs... and consequences....

2) what can you do to minimize both of those above.... aside from lowering compression...

3) VE... volumetric efficeincy... Dynamic (actual) compression.... and BMEP.... what are they?... how are they related?... and what do they mean to octane requirements?

4) compression of gasses.... linear or exponential??... assuming same cylinder fill, difference in psi from 10:1 to 11:1.... and diff from 13.5:1 to 14:1.... come on engineering students, crack the books and chime in... Big Country Dude, you are an engineering student :smirk:

that should make for some good tech discussion on a rainy fall weekday ... :bonk:

as far as pressure rise from 10:1 vs 11:1 compared to 13:to 14:1 , it is exponential.... lots of info on the web with just a bit of typing...

quick explanation ... if you take a vol of air and cut the space it is in by half you will get a certain pressure rise....

if everything was equal , you would double the pressure if you cut the space in half again....but you dont.... you get a much greater pressure rise...

that is why going from 13:1 to 14:1 isn't as small a comparative rise in pressure as the change in static numbers would seem.... it's big jump in pressure...

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It all boils down to the package.

If you've got miss matched parts (cam with piston etc) you end up with a combo that is unmanageable. Not only is it unmanageable, it doesnt perform either.

Intake tract, port design, cam timing (lift/duration), piston design, exhaust port design, exhaust pipe diameter and length and all that when correctly matched can make a lot of usable power even when on the paper none of it looks all that dramatic when taken one piece at a time. But when in combination it performs at a very high level.

If there is room for a 15:1 compression piston that could make more power with the right cam, but the right cam for that compression may not work because there's not enough room for the necessary lift and duration for the CR to work at the necessary RPM range.

The reason I say room for CR is, when the CR gets that high, you need greater valve reliefs for the cam and the valve reliefs take away from the CR again.

F1 seems to defy some of this logic, but they can spin the things 25,000 RPM too and with that kind of velocity in the port your dynamic's change pretty dramatically. The inertial forces in the ports start working for you beyond what we ever even consider in a dirt bike engine.

that is why going from 13:1 to 14:1 isn't as small a comparative rise in pressure as the change in static numbers would seem.... it's big jump in pressure...

The static pressures are one thing. But that isnt really whats pushing the piston down is it? Its the heat pressure. You want to create as much heat volume as you can, as opposed to temperatures. The volatility of the fuels are what govern the cylinder temps.

All this is also why the correct fuel burn rate is important. Ideally you'd like your fuel charge to be completely spent at the end of the combustion cycle, but youve gotta manage the heat produced and since that perfect burn will typically melt down the material the engine is made of, youve gotta fudge that a little bit. We do that with mixture ratio's and ignition timing. The latent heat being removed in the unburned fuel charge is what saves the top of the piston, the exhaust port and valves and the pipe itself.

The idea of throwing a partzooka an engine and crossing your fingers and making big power with a modern thumper is a giant waste of cash if you ask me. The OEM's are producing pretty good engines these days for our dirt bikes.

Think about it. A 500cc industrial engine is rated usually about 13 hp. Our 450's make an honest 50 stock, start easy, run a long time and are generally pretty reliable. And they run on the same fuel.

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What is the answer! I don't see a combination listed.... What is safe at 14:1 with which cam, exhaust diameter and length, port cross section/curtain area, choke point etc... What is the best choke point area for the RPM range of a drag bike and MX bike?

It's all math and it has a formula!

Since you, Mixer, seem to know the theory and have built some engines for the TRX share your experiences of what works.

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What is the answer! I don't see a combination listed.... What is safe at 14:1 with which cam, exhaust diameter and length, port cross section/curtain area, choke point etc... What is the best choke point area for the RPM range of a drag bike and MX bike?

It's all math and it has a formula!

Since you, Mixer, seem to know the theory and have built some engines for the TRX share your experiences of what works.

Oh come on.. and old hotrod dog like you knows what needs what.

At 14.1:1, you need overlap or a ton of octane. At 14+ a short duration cam like a stage one is going to detonate because of low RPM cylinder pressures exceeding the fuels stability threshold. So to combat this youd have to back the timing off. In reality, based on the fuel you'd be better off running less CR and more timing.

Back in the 80's, I built a VW engine. I spared very little expense. Billet Chromoly crank ($1800), Carrillo Rods($600), Forged Barrels, Custom ForgedTrue pistons, Custom AutoCraft heads, 48IDA's on thick Scat trak port matched manifolds that still had to be welded up for more meat to port on, cam that had lift and duration numbers you see in ProStock apps. It was 90.5 x 74mm. (1906cc) This stupid little engine wouldn't idle below 2000 rpm. It'd spit fuel out the tops of the carbs and die.

Originally I set the CR at 13.5:1 and ran the 96 octane pump gas I could get. On the dyno, it made 186hp while detonating like it was going to die instantly. It only had about 28 degrees of timing in it. We backed the timing off until it stopped pinging and I was left with 136 hp with only 19 degree's of timing :lol::smirk:...

So I tore it back down and dropped the CR to 9.5:1 by machining a dish into the pistons. The next time it went I started at 24 degrees and got 160hp with no detonation at all. Bumped the timing to 30 and it made 192 hp with no detonation at all (at 9400 RPM :bonk:) We put some more timing in it, but the power fell off but still didnt detonate.

Now I knew that if I went out and got some kick ass race gas, some nice leaded 108 that at 13.5:1 it would probably make well over 200 hp safely. But this thing was my daily driver and dropping 6 bucks a gallon for Trick race fuel to drive back and forth to work seemed incredibly stupid.

Its all about managing the heat and getting the dynamics to run in harmony. When your pipe and intake tracts match your cam, and the piston is on the same page with the selected fuel, with the proper ignition timing, all kinds of neat shit happens. :lol:

The CRF head is pretty damn good. The combustion chamber and short stroke lend themselves to our stupid shit pump gas. With that said, you can run 13.5:1 in our little engines on pump gas because;

1. we've got a 3D ignition map

2. The short stroke and good combustion chamber helps to prevent detonation

3. The cam seems to balance the cylinder pressures down low to avoid detonation (the 3D ignition map is working hand in hand here)

4. The carb does a really good job of mixing the fuel. That helps a lot

5. These silly engines run really cold. If your moving along on a 70 degree day, the water temps rarely get over 180F. Our pump gas is looking for 210-220F. The temps being off helps with detonation.

6. In general, most CRF's are running rich. Which again holds temps down in the cylinder.

The low temps dont help with raw HP though. As a rule of thumb on Gasoline engines, best power is usually made around 205F on the water temps. Managing the 205F is the key.

I could go on all damn night :lol:

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:smirk:

I wasn't asking for me.

Granted, the post is interesting but does not have allot of information on what to do to run X compression. It mostly talks of physics and the effects of too much compression.

To sum it up I would think dynamic compression is what were after.

A limiting factor for most of us is the ignition system. We don't have the ability to change timing that has been set up for a 12:1 motor. This will force the playing field to change hardware to reduce the dynamic compression and seek funds to buy programming tools.

It was a little irritating to see a post that said the sky is falling and offered no real insight to what will work in building a high compression CRF450 motor, safely, when the OP clearly knows the answer.

I can say from experience 13:8 is no problem with the X1 camshaft on pump gas and a lobe center of 102/106 running 9.67:1 dynamic CR

http://www.wallaceracing.com/Calculators.htm

perfcrf450.jpg

Edited by Rocksalt

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A limiting factor for most of us is the ignition system. We don't have the ability to change timing that has been set up for a 12:1 motor. This will force the playing field to change hardware to reduce the dynamic compression and seek funds to buy programming tools.

Say "Hello" to the 09- to current CRF. You can manipulate the timing and the fuel curves throughout the 3D map with the Honda Tool.

Most guys dont want to have to deal with the ignition timing so a 3 button tuner gives the FI a carb style tuning capability that simply adds or removes fuel from the map in roughly the same areas where you think of a pilot, needle and Main jet circuit.

From my own experience, I can tell you that a 13.5:1 piston with a Stage1 Hotcam makes buttloads of power down low but its kinda tough to start because of the short duration cam and the tall piston. Youve gotta play with the decompressor lash to get it easy to fire. On the later engines where you dont get to mess with the decompressor lash it might be tougher, I dont know because I havent messed with them.

I know with my own bike at 13.5:1 with the stock cam, it starts easy. I can flail at it like a 10 year old girl and it fires right up. But my carb is dead nuts too. I think getting the pilot circuit set properly is the key to getting them to start easy regardless of the temps. There's no set procedure I have to do to get the bike fired up even after I kill it by blowing a corner or whatever. Just flip out the kicker and hack at it once.

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Shawn you are awesome....handier thatn a shirt pocket! I ordered a 13.5 for my 07 crf as it had a stage two hotcam...my first 4 stroke....i would have kept it stock, but it had the big cam already so i figured it would like the high compression piston. Did I mess up? ... what do you think? I figured the high compression piston was just more power throughout the curve...so why not?

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Shawn you are awesome....handier thatn a shirt pocket! I ordered a 13.5 for my 07 crf as it had a stage two hotcam...my first 4 stroke....i would have kept it stock, but it had the big cam already so i figured it would like the high compression piston. Did I mess up? ... what do you think? I figured the high compression piston was just more power throughout the curve...so why not?

Mess up? pfffftt...

I think your motorcycle is going to haul ass :bonk:

That stage two is very similar to the 02 OEM cam. And that motorcycle loved tall pistons.

When you have the head off, blend the bowls into the ports and clean up the castings and valve guide bosses. You wont be sorry....unless you dont like bikes that never sign off clear to the rev limiter :smirk:

What pipe are you running?

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awesome add to the 450 forum... !!

i put this in the 250 forum too and everyone there is hoping if they ignore it, it will go away.... lol

i just wanted to throw this thead up as food for thought because i was a little tired of seeing people in ALL forums just answering straight up without taking ANYTHING into account from bore size to power level to cam timing to altitude to whatever...not even what octane? or measured how?

just:

Q: "what is the absolute most compression i can possibly run on pump fuel, and not grenade my expensive race engine?"

invariably nobody will ask for a shred of additional info, yet an answer will apear

A: "13:1 is totally safe bro dude"

maybe if you wear your Monster Energy drink hat on backwards when walking around the pits it makes that statement true.....

or maybe it works like an octane rheostat.... bill forward 12:1... sideways 12.5:1.... backwards 13:1

i haven't tested that theory, so i can't say for sure :smirk:

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I have a question, not to get off the topic but, me and my friend have two bikes. Same port work and stage 2 cams, mrd pipe on one bike and ti4 on the other. Mine is an 06 with an 07 head and 07carb. His bike is an 08. My bike makes more power than his. I weigh more than him. When we swap the pipe it helps him to have the MRD, but i still beat him pretty bad, and i out weigh him by 80 pounds! Does the 08 ignition limit the 08 bike in any gears other than 1st and 2nd? Also with a stock cam i can get 95mph and he could only get 91mph. We can't understand why the 08 does not have the acceleration and top speed as my 07 headed bike. We usually start in 2nd or third gear and do roll ons till top speed is attained.

Moderation edit:

Im sorry, but I had to edit this. I almost sprained an eye trying to read it.

S

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I have a question, not to get off the topic but, me and my friend have two bikes. Same port work and stage 2 cams, mrd pipe on one bike and ti4 on the other. Mine is an 06 with an 07 head and 07carb. His bike is an 08. My bike makes more power than his. I weigh more than him. When we swap the pipe it helps him to have the MRD, but i still beat him pretty bad, and i out weigh him by 80 pounds! Does the 08 ignition limit the 08 bike in any gears other than 1st and 2nd? Also with a stock cam i can get 95mph and he could only get 91mph. We can't understand why the 08 does not have the acceleration and top speed as my 07 headed bike. We usually start in 2nd or third gear and do roll ons till top speed is attained.

Moderation edit:

Im sorry, but I had to edit this. I almost sprained an eye trying to read it.

S

Same piston? Same airfilter? Same jetting? Same gearing? Suspension setups? Lots of variables there beyond the engines.

The chain and sprockets can make a big difference regarding top speed. I know a guy that went to MMI. While he was dynoing his CRF, he lubed the chain and picked up 4hp. Thats almost 10% of the gross output.

Regarding the head work, a simple valve job could make that kind of difference too. Unless the heads were CNC machined, they arent the same. And it sounds like yours is better.

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Shawn....

off topic again... but on the 4HP with chain lube deal....

Same exact thing happened to me on my dynojet..... i absolutely hate non o-ring chains... what a waste of steel....

a friend of mine came over from the east coast with a TON of parts to test.... multiple pipes, camshafts, intake systems...

basline runs friday night... then saturday all the parts swapping began.... the dyno runs didnt make sense, we kept going to setups that should be making higher peak power, but all with little effect ...by the end of the day we said screw it, lets go back to the very first friday night combo and re-test.... down 4HP??!!??

then i happened to look at his chain and i was like "DAMMMMMIT Gary, is that a non o-ring?!!" of course it was a dryer and dryer as the day went on non o-ringer!! one shot of chain lube and magically 4HP came back....*****!!

so then sunday was spent doing all the same component swaps and subsequent dyno runs all over again... with chain lube rust proofing the left side of the dyno between each set of runs!! gary knew better, but i had to tell him if he came near the dyno with a non o-ringer again i was going to have to shoot him in the knee cap....

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we have the same gearing same jetting same air filter same porting by the same guy and we do 2nd gear roll on we even swap pipes to see if that would make a difference the mrd is the better pipe.but he still does not have the power we are testing on long flat dirt levys the only difference i can see is the ign. even if i had a better port job or something to give me a advantage the weight should be a major factor i looked for the timing curves to compare the 2 bikes but i am having trouble locating any.he has a mx chain and i have an x ring his chain should be an advantage to him also i tried his old cam and my bike ran 3 miles an hour faster than his bike with the same cam

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dial them both in on a dyno with a air/fuel ratio meter.....

until you do that, none of the comparisons are really valid or useful

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i have not tried the dyno but we did it the old fashion way with a gps the main jet that gives the most top speed is the one we use,i am just thinking the 08 has less timing advance up the rpm range to help with traction i know 1st and 2nd gear have their own curves i am just wondering if 3rd thru 5th are different for traction also.i cant find any info.on the timing curves for the 06 or 08 ignition.

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I actually did want to ask about pipe, but didn't want to risk getting yelled at for not searching about pipes. I did search and I have seen you are a big advocate for mrd pipes, and I am all for them, but I want to know how much my stock pipe is holding me back? As with all things, money vs gain is a factor for me...I will be running it stock for now, but next christmas or for my birthday I will get a pipe if it is needed. My question is can i get away with putting a bigger silencer end on my stock silencer or am I losing a lot with the stock pipe? And if so is the header a big holdup as well or just upgrade silencer? Any advice is greatly appreciated.

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the stock pipe is ok but a good aftermarket pipe is worth the money.a full system is better than a slip on and i would never pay 800 or more for a exhaust system.the mrd is one you will hear a lot about and for good reason it is priced right and it is an excellent exhaust.i had doubts until i tried one and i know why there are so many people to recommend the mrd.

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I actually did want to ask about pipe, but didn't want to risk getting yelled at for not searching about pipes. I did search and I have seen you are a big advocate for mrd pipes, and I am all for them, but I want to know how much my stock pipe is holding me back? As with all things, money vs gain is a factor for me...I will be running it stock for now, but next christmas or for my birthday I will get a pipe if it is needed. My question is can i get away with putting a bigger silencer end on my stock silencer or am I losing a lot with the stock pipe? And if so is the header a big holdup as well or just upgrade silencer? Any advice is greatly appreciated.

For you, with the setup you currently already have, a mid/high end pipe would blow your mind.

Id bet the OEM pipe is probably holding back 5 maybe 7 hp from you. The reason its so much is because youre already doing the right thing on the other side of the engine.

A pipe will make the engine run "Free-er". It'll just be happy :smirk:

Ya know how the dog gets when you dig out the puppy treats? Thats what your bike will do for a good pipe. :bonk:

Call Dave at MRD.

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