Octane requirements for my compression.

[AKheathen]

I never mentioned a thing about the CR going up with porting, I simply stated that it changes the game. Reasons are that without knowing the durations, we would have no clue as to the CCR thus the fuel requirement.

 

Why would a ported engine require more octane with the same CR? The effects of reverse wave gas flow in the exhaust of course.... This acts as more of a super charger at certain rpms (on the pipe) and stuffs A/F mixture back into the engine. Obviously if the window of time is open longer, more of the A/F mixture can be shoved back in there. This will NOT increase the static CR but WILL increase the "dynamic" CR which is something we look at very closely when designing engine components, heads, porting maps, etc.

 

The "Japanese" method of testing the CCR is called such because it was innovated in Japan and it is a valid test indeed and still has huge considerations when selecting fuels, spark timings, etc.

Hopefully that answers your questions.

 

Regarding the 19cc dome on a stock port engine. Yes, pump fuels have been well proven in our shop to work, even with 4* of timing. I am making a few assumptions of the OP's engine but I will spit ball a .040"" squish clearance, 50% squish, etc. I was not really looking at the pumping numbers because that is not very relative as mentioned and really should pump up 170-180psi at sea level but a junk gauge can cause you problems and still not not change the fuel requirements. I have no idea how many engines we have had here that were comp tested "low" by the owner only to find out that they are fine when tested correctly with a good gauge. I am not trying to beat on bike owners but rather stating how things can be thrown out of context by simple using the magic "comp" number as the rule for rebuilding an engine. Other factors must be considered. I have seen pistons that were cracked, falling apart, rings worn out, and they still pump up good numbers.

 

B

i was wondering how you figured that......i just add 1:0 to factor the pipe and .5:0 for agressive porting. last time i mentioned about "dynamic cr" i got a flogging, so i just don't bring it up anymore.

[AKheathen]

What am I guessing on?? You seem to want to ruffle my feathers with nearly every reply. Did you want to step outside..

 

 

I don't quite see what I am guessing on. Thousands of Banshees through the shop here, worked with plenty, yet to have one blow up on me. Personally owned a stocker wtih 19cc domes and 4* on pump. I beat the piss out of it. By doing the numbers, I am not really guessing.....

 

 

B

not you, whiteboy..............you just posted when i was replying.....

[blowit]

The word for the day is "density". Everyone is obsessed with CR, CCR, etc, and do not take into account that this is a compression ratio but "what" are we really compressing and does it really start at ambient pressures??

 

There is also the effects of latent heating that certainly must be considered and relate directly to the density..

 

 

B

[RILS]

This is a simple way I use to figure it. On a stock bike with 513 series pistons. 350/2=175+22=197/22-5.5=11.9:1

chamber=22 piston dome= 5.5cc

 

I misfigured it in my first post. This is not a exact figur but a commonly used one. If I remeber correctly the japanese method uses a simlar equation but they only figure the space above the exhaust port or somethimg like that I have a book that covers it if you insist I look it up.

Edited November 12, 2009 by RILS

[AKheathen]

This is a simple way I use to figure it. On a stock bike with 513 series pistons. 350/2=175+22=197/22-5.5=11.9:1

chamber=22 piston dome= 5.5cc

 

I misfigured it in my first post. This is not a exact figur but a commonly used one. If I remeber correctly the japanese method uses a simlar equation but they only figure the space above the exhaust port or somethimg like that I have a book that covers it if you insist I look it up.

well, you are forgetting to subtract the piston crown from the dome size on both sides of the equation, and factor the extra volume of the overbore and then you will have your uccr, which will be alot higher than conventional engines.

 

 

now, how are you calculating the density and heating?

[RILS]

well, you are forgetting to subtract the piston crown from the dome size on both sides of the equation, and factor the extra volume of the overbore and then you will have your uccr, which will be alot higher than conventional engines.

 

 

now, how are you calculating the density and heating?

 

No i am subtracting read the equation.175+22=197/22-5.5 you can figure any bore you choose with this method and you will not find a equation that will figure temps and such.

 

Here is another example 4 u. 421 cub with 20cc domes.

421/2=210.5+20=230.5/20-5.5=15.89:1

This is simple math if you are not familar with this then do some reading and talking to people who build a few motors instead of arguing with me.

 

 

421cc

/2 because you have two cylinders

210.5+20cc for the dome

20cc-5.5cc for the piston dome

 

Does all that make sense to you?

Edited November 12, 2009 by RILS

[AKheathen]

no, you are forgetting the dome (-5.5) still exists at the bottom of the stroke. it would be 175+[22+(-5.5)]=175+16.5=191.5.......then, 191.5/16.5=11.6/1, or, 11.6:1, not 11.9:1. once again, you're not gonna have the cylinders @ 175, anyways, so using the "350cc" rating won't really work unless you are @ the bore that gives just the 1cc to bring it to a 350cc

[RILS]

no, you are forgetting the dome (-5.5) still exists at the bottom of the stroke. it would be 175+[22+(-5.5)]=175+16.5=191.5.......then, 191.5/16.5=11.6/1, or, 11.6:1, not 11.9:1. once again, you're not gonna have the cylinders @ 175, anyways, so using the "350cc" rating won't really work unless you are @ the bore that gives just the 1cc to bring it to a 350cc

You have a valid point. Im thinking of how to best word this. Ok lets say you start with a bike with a 513 series piston from the get go and you have a 22cc chamber and thus giving you 350cc for simple math. When you figure your compression ratio you have already measured and determined that this is your actual cc. So your starting figure is 175cc that is what you are changing to come up with a different number. This just gives you a good figure not exact if you want exact you will need to measure everything and assume nothing, starting with your true bore and stroke with the piston you will be using and base gasket installed then measure your true deck height, squish. By my formula using a 513 series piston and assuming you have a true 175cc using that piston. What you are doing is changing your overall cc of the motor on the bottom side that has already been confirmed with that piston doing this will give you a slightly different number not like it will on the top side but still a different number however if you are accurate on the true starting cc with the piston used my formula will hold true. I will stick with my way and you with yours I guess. By my figures using my bike I have 385/2=192.5+18=210.5/18-5.5=16.84:1 your numbers would yeild 385/2=192.5-5.5=187+18=205/18-5.5=16.4

I hope some one like blowit will chime in here but I feel pretty comfotable im right on this if im not Ill gladly be corrected.Either way you figure it yours or mine you will be in the ball park which is all your going to be anyway without getting down to the above measurments.

[AKheathen]

ok, look at it this way. when you move from tdc to bdc, you are adding thd displacement to the trapped or compressed volume, which is chamber, minus dome, put simply, so the fomula is the same on both sides. 16.5 compressed, plus 175 displacement. this is just the way iut is, not my own special way of doing it. it's the plain and simple math. taught to everyone in school, not word-of-mouth formula. it does, hoerver matter when you get down to the cusp of octane requirements. you could be holding back on that one degree of timing, or .010" of squish that could bring you a little more power that you are looking for, but are otherwise trying to stay safe.

[Sammy]

Would my results be skewed because of me testing on a cold engine? This top end probably has about 55-60 hours on it. It runs strong, but 155psi does seem low to me for sure. It ran good on mixture of 111/93 @ 50:1. I ran the mixture 2 gals 93/1 gal of 111.

[blowit]

Would my results be skewed because of me testing on a cold engine? This top end probably has about 55-60 hours on it. It runs strong, but 155psi does seem low to me for sure. It ran good on mixture of 111/93 @ 50:1. I ran the mixture 2 gals 93/1 gal of 111.

 

 

 

Before jumping to conclusions, you may want to try another tester or check your test method.

 

http://mullengineering.com/rt/files/2_tech_docs/THE%20TRUTH%20ABOUT%20COMPRESSION%20TESTING.pdf

 

Here is a linky to something I wrote a while back. I should probably add to it but have not had time.

[87sheerips]

Pump 93 will be just fine and dandy....