Compression Ratios

#1
Are there any scooter blocks with a compression ratio around or above 17:1? If so what is the brand and what cc? Im looking for a minarelli block that has a compression ratio around 17:1
 
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Browni

B.A.T tuning
#2
MrBenelli said:
Are there any scooter blocks with a compression ratio around or above 17:1? If so what is the brand and what cc? Im looking for a minarelli block that has a compression ratio around 17:1
Malossi has max 15.8:1 or max 16:1, The Athena 80 cc kit has 16.3:1, and maybe the Hebo Manston has the biggest but I don't know how much!

Why do you want to know?
 
#3
Why would you need 17:1? Standard petrol won't resist that much compression, it will detonate. However you can modify your head to that compression.
We run an engine with CR 18:1, but it does run on pure alcohol.:)
 

Browni

B.A.T tuning
#4
roost said:
Why would you need 17:1? Standard petrol won't resist that much compression, it will detonate. However you can modify your head to that compression.
We run an engine with CR 18:1, but it does run on pure alcohol.:)
Yes, it's sure it's very high, but the real compression ratio is different from the theoretical! It depends from the exhaust port's duration! I made my old 3KJ cylinder head to a very high compr and I used 207 degrre!! exhaust port duration so I could use 95 gas.

However the 16:1 can go with the 98 gas too!
 
#5
Of course that is not the real CR, but the geometrical. Petrol 98 anyways does not compress much more than (real) 10:1.
There are two ways of measuring compression to 2T engines; the uncorrected (european method) and corrected (japanese method), and both are wrong:)
I don't like the practice that by rising the exhaust port you have to rise the geometrical compression ratio too. This practice is adapted if you measure the corrected CR method. In practice the real compression ratio will be the same at max torque rpm with an high exhaust port compared to a slihgtly lower, this because the filling efficiency of the cyl will be the same.
Nevertheless the real compression ratio is very much affected by the exhaust pipe.
The CR in 2T is tricky, and it does not payoff the effort to try find the highest CR an engine setup could take.
 
#6
The reason i ask is because i've done some research and in order to properly run nitro methane i need a block with a compression ratio around 17:1
 

Browni

B.A.T tuning
#7
roost said:
Of course that is not the real CR, but the geometrical. Petrol 98 anyways does not compress much more than (real) 10:1.
There are two ways of measuring compression to 2T engines; the uncorrected (european method) and corrected (japanese method), and both are wrong:)
I don't like the practice that by rising the exhaust port you have to rise the geometrical compression ratio too. This practice is adapted if you measure the corrected CR method. In practice the real compression ratio will be the same at max torque rpm with an high exhaust port compared to a slihgtly lower, this because the filling efficiency of the cyl will be the same.
Nevertheless the real compression ratio is very much affected by the exhaust pipe.
The CR in 2T is tricky, and it does not payoff the effort to try find the highest CR an engine setup could take.

However the practice is different. Look at the Malossi user Manual he recommends at least 95 octane gas for MHR, Athena recommends at least 96 octane gas for his 16.3:1 cylinder. So the 10:1 is totally irreal and this is my experience too!

You can't separate the "the filling efficiency " from the exhaust port!

The filling efficiency depends from the exhaust ports shape and size too. So your theory is not very good! Cause there are a lot of mixture loss at the exhaust port's for a standrad exhaust too, and if we talk about expansion chamber it has a lot of loss too at it's dead range, expansion chamber is only good with a little range! At two-stroke everything is everything is conncted with everything!
 
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#8
MrBenelli: then just modify the head. But are you going to run nitromethane+petrol or nitromethane+alcohol? Because nitromethane+petrol won't take that high CR.

Browni: The REAL compression in 2T petrol engines never exceeds 10:1, 11:1. It closes to that when the engine RPM are at max torque.
I never sayd that the filling efficiency isn't connected to the exhaust port. The filling efficiency is connected to everything.
I said that if you rise the exhaust port, you should not rise the compression ratio! The filling efficiency of a cilinder with 196° exhaust is practically the same as one with 200°. The difference is that the cyl with 200° will have this filling efficiency equal to the cyl with 196°, at higher RPM. There we gain power, with the higher RPM!
So if you rise the CR you may have detonations.

The corrected CR method teaches that you measure the volume of the part of the cyl from the top of the exhaust port up. So by consequence the higher the exhaust port is, the lower the calculated CR will result. But in reality the real CR will not be lower!
Because if it were lower, then the engine would make less torque! And power is a function of torque/time.

The exhaust pipe is also a supercharger, this means that when the pipe enters in resonance, we have a sharp rise of the real compression ratio.
 

Browni

B.A.T tuning
#9
roost said:
MrBenelli: then just modify the head. But are you going to run nitromethane+petrol or nitromethane+alcohol? Because nitromethane+petrol won't take that high CR.

Browni: The REAL compression in 2T petrol engines never exceeds 10:1, 11:1. It closes to that when the engine RPM are at max torque.
I never sayd that the filling efficiency isn't connected to the exhaust port. The filling efficiency is connected to everything.
I said that if you rise the exhaust port, you should not rise the compression ratio! The filling efficiency of a cilinder with 196° exhaust is practically the same as one with 200°. The difference is that the cyl with 200° will have this filling efficiency equal to the cyl with 196°, at higher RPM. There we gain power, with the higher RPM!
So if you rise the CR you may have detonations.

The corrected CR method teaches that you measure the volume of the part of the cyl from the top of the exhaust port up. So by consequence the higher the exhaust port is, the lower the calculated CR will result. But in reality the real CR will not be lower!
Because if it were lower, then the engine would make less torque! And power is a function of torque/time.

The exhaust pipe is also a supercharger, this means that when the pipe enters in resonance, we have a sharp rise of the real compression ratio.
Sorry but this is not true!! Take a little test with your cylinder, make it to a 1:16 and increase your compression ration, than you can use 95 octane gas, this is not theory! And you talk about maximum torque! Not the high torque is the target if somebody raise the compression ratio! High CR doesn't increase very much the peak power! I recommend you a good 2-stroke book!
 
#10
I can make it 16:1 and it will work good. But this does mean that the CR before wasn't at the limits of the fuel. And there is the squish phenomenon. A correctly done squish can rise the limit of the CR of a fuel. And there are also a 1000 more things to count.
I once had detonations with ~14:1 CR!

Maximum torque occurs at some rpm where the cylinder filling efficiency is at his best. At this RPM the highest ammount of mixture is trapped in to the combustion chamber, so burning it creates the higher temperature & pressure possible. This pressure then creates a force on the piston, this force is translated by the crankshaft into the torque. Only this is torque to me. F*l [N*m].

And I have read Blair's Design & Simulation of Two-Stroke Engines, Jennings book about 2T, and pratically all the disponible data about 2T on the net.
 
#11
The hell with All those ratio's. Everybody builds motors differently. Just buy a compression tester. Anything over 150psi, you will need race gas. Using your method I would modify the Head Incert.
I just got my Metrakit Engine Dyno tested at Metrakit Factory. 22bhp and I have the compression set at 165 psi. What the ratio is, I could not tell you.


Note: Mac Tools sell compression testers

Speed
 
#12
All methods of calculating compression are guidelines only. Compression testers too, they are normally used to check the state of the cylinder setup.
 

Browni

B.A.T tuning
#13
roost said:
All methods of calculating compression are guidelines only. Compression testers too, they are normally used to check the state of the cylinder setup.
roost

I took your opinion in a nutshell:

Every theoris are bad and all of the practical measurements are bad too:):):)

"So do what you wanna do:):):)"
 
#14
Exactly!:)
What an good engine builder has to do is try what works best. You have to rise the compression, if detonations occur, lower it. Anything simplyer?
All scooter cylinders (most racing too) use a safety margin, dont they want that their customers get destroyed cylinders from deto, do they?

With an compression tester just the opening/closing of the carburettor will result in different pressure readings.

Also latest software engine simulators work quite well.

Most of the today tuning info on 2T engines is empirical.
 

Browni

B.A.T tuning
#15
roost said:
Exactly!:)
What an good engine builder has to do is try what works best. You have to rise the compression, if detonations occur, lower it. Anything simplyer?
All scooter cylinders (most racing too) use a safety margin, dont they want that their customers get destroyed cylinders from deto, do they?

With an compression tester just the opening/closing of the carburettor will result in different pressure readings.

Also latest software engine simulators work quite well.

Most of the today tuning info on 2T engines is empirical.
Yes, manufactures build their cylinders for very ban gas, even the racing ones too, but the calculation is a little help for tuners. And it's sure I try and try different compressions-ignition advances etc.. So you are right in 50%:):)

Compression tester has a misnomer, cause it measures the biggest final-preasure and it depends from your leg's power at the kick starter too, but this gauge is not for compression test, it shows you only your engine's condition. So you test your new machine and then you can compare it with your used machine.
 
#19
head +squish 0.5 area=103.3mm square (draw on solidwork for have the good surface)=> combustion room= 103.3*2*3.141 = 6.49cc

(69+6.49)/6.49 = 11.63:1




 

Browni

B.A.T tuning
#20
Hmm that calculation was useless, the easiest way to check the accurate compression, is that you stand up your block and fill the combustion chamber with oil or something (the piston is at the TDC and the liquid catches the spark plug thread's bottom edge then you can measure it's volume! :) The 0.5 mm squish is not constant at the EVO cylinder!
 
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