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compression ratio

11K views 66 replies 14 participants last post by  norton74 
#1 ·
reading in my service manual for the 2012 bonnie's it SAYS compression ratio in models with electronic speedometer is 10.2, is this correct? i believe my 2012 bonneville has electronic speedometer from prior readings. with triumph lower fuel requirements other aspects such as timing advance or cam shaft etc is prolly altered to control final cylinder pressure which is the bottom line pretty much to control knock. input appreciated since its my first bonnie since 1970!!! i am in the USA, do models differ over the pond, excepting emmissions of course
 
#8 ·
The oldlder 865 motors the pistons are dished on top.So if they rased it they would have to of changed that .Iwould like to see the new piston to get 10.2 I would think they would have to have a small dome on top.The aftermaket hc pistons are around 10.8to 1 and they have a big dome.
 
#9 ·
I'm surprised they made a change of pistons... not saying I don't believe it, just shocked that Triumph actually made such a change. I know very little about the specifics of engine internal workings, but wouldn't a 10.9% increase in compression make considerably more increase in horsepower output? (I do know a little about math. :D)

Did they also change cams, or something else at the same time? And what was the reasoning behind the change(s)? Still confused. :confused:
 
#10 · (Edited)
The camshafts part numbers have remained the same.

Apparently there are other factors that influence how much power you get from a given CR increase. I've read somewhere that 3% increase in power per unit of CR increase is a rule of thumb, consistent, of course, with the right fuel grade being available and possibly the right ignition timing.

This site has a chart halfway down the page giving examples:

http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/viewall.html
 
#14 ·
in automotive application raising compression increases torque everything else equal. when long duration high lift cams are used aka sport bikes high compression is necessary to maintain cylinder pressure, as those cams loose efficiency for big power. bottom line is cylinder pressure, many things effect it, a very long discussion, water cooling changes everything, our bonnie's oil cooling helps for sure. nice to hear it was not a misprint, i never seen it anywhere until i was reading the manual, i am retired lots of time, as soon as my house roof is finished. i have been using midgrade gas, only 470 miles so far, cool weather too. wonder if premium would help performance, depends on tuning, maybe triumph backed off timing with the higher CR?
 
#22 ·
Raising the CR or increasing the CC are two mods that boost performance across the rev range. Mostly everything else we do to our bikes raises performance in some areas, but not in others.

Raising the CR is particually good for the low and mid ranges if you're using performance cams, where it raises the CR to compensate for the reduction of dynamic CR due to the cams opening the intake valves earlier.
 
#24 ·
That is true but the normal performance cams such as the 813s dont kill the dynmic CR much because they are not that racey and all so let the bore fill with more fuel.If you go over those cams it becomes a trade off and they do efect cr a bunch.I played with my 1087 with 01 790 cams once on the comp gauge it gained a bunch. bike pulled like a tractor at low end but was dead on top.The big sub cams pull all the way through the rpm range.So you must match the cams to the motor to get what you want out of the motor.
As far as fuel goes anyone that is to cheap to buy high test gas shouldnt be building a hot rod lol.
 
#32 ·
I got that the first time you posted it. Does it make sense to you that for there is an ideal air fuel mixture? Does it make sense that if there is an ideal mixture, that for a given RPM, you have to burn the same amount of fuel regardless of the CR? You would make more power (slightly) but you wouldn't be using the excess power. That excess energy is going out the exhaust pipe.

If all of that makes sense, then why would you burn less fuel (better mpg) at a given road speed just because you had raised the CR?
 
#33 ·
what you are missing is the fact the tighter you squeeze the right air fuel mix the more of its energy you can get out of it.the more you get out of the same amount of a/f the less fuel you have to burn .That has been a fact since the early 1900s.
The less comp the more of the a/f energy is going out the pipe.
 
#34 · (Edited)
I get what you are saying. What I think that you are missing is that for a given speed, the engine is operating at a corresponding RPM no matter what the CR is. Unless you're arguing that raising the CR causes the engine to run leaner (certainly, the engine cannot run at a lower RPM for a given speed), you must run the same amount of fuel at a given RPM no matter what the CR is.

If I remember correctly, the ideal air/fuel ratio is somewhere between 14:1 and 15:1. The CR is not a part of the combustion chemistry equation.
 
#37 ·
You need to go study auto history and design 101 lol.They learned this around the 1920s.It is one of the main reason they came out with high test gas. what burns less fuel a gas or a diesel in a truck?
back in the 70s they cut comp on cars gas milage and power both fell.Now that they have learned how to make a hc motor run clean comp ratios are going up and so are mpg.
 
#40 · (Edited)
Apples and oranges Mike. Diesel fuel and gasoline have different specific energy densities. The ideal air fuel ratio for diesel is higher than that for gasoline.

High compression engines were developed for power. Fuel consumption was of secondary importance.

Intersting that you mention the 20's. That was when General Mortors and several other companies started the Ethyl Corporation to develop a gasoline additive (tetra ethyl lead) that increased the octane rating of gasoline permitting higher compression engines. The compound was introduced in 1923.

The problem was that lead was emitted into the atmosphere to be absorbed by every living thing. And lead does not leave your body easily. The residial lead content in people that died before 1923 is less than 2% of that of people living after 1923. Once lead was banned from fuel in the 70's, the lead concentration in peoples' bodies has been dropping and the atmospheric lead concentration has dropped rapidly. Everyone alive to day has a higher concentration of lead in their bodies than anyone who was living before 1923.
 
#38 ·
Norton74, in simple terms raising the compression can result in more power using less fuel. In reality it's more then that, keeping ALL things the same and raising just the compression will most likely result in knock, lowered performance and MPG along with a lunched engine.
This topic can go long and deep so I think if you research thermal efficiency of an internal combustion engine you will get the information you're seeking. BOL Pete
 
#39 ·
You're right. Raising the CR does improve thermal efficiency a little and as a result, torque and horsepower. What I haven't figured out is how fuel consumption goes down. At a given engine speed, the amount of fuel being burned with each combustion cycle would have to be pretty much constant. The only way you could burn less fuel is for the air fuel mixture to become leaner. Running lean presents a whole new set of problems so let's assume that the air fuel ratio stays constant no matter what the CR is. I guess that it's time to hit the books.
 
#48 ·
As far as doing mods hurting gas milage(on these bikes)If they are tuned right the only 2 things that can hurt milage are the bigger cams (not talking 813 class cams) and 2 useing the right hand more ,that is hard not to do when you have power because it feels so good.Even going to the big bore stroker motor does not kill mpg much.I get upper 40s mpg on the 1087 with full race cams .I got around 50 stock.the 1087 makes 2 x the hp of a stock bike and around 50% more torque.going a steady 65 mph on the open road you barely have the throttle open on the 1087.It does not slow down on hills as much as a stock bike does so you dont have to open the throttle to keep the same 65mph speed.
 
#49 ·
You just said that your mileage was better with the stock configuration than when you modified it.

Do you have to use premium fuel? If you do, then your cost of operation is higher.

The average tourer would rather get better mileage and use less expensive fuel. If you are racing, then mileage is rarely of much concern.
 
#57 ·
It's only about 3% to raise the compression ratio to the point of needing to use premium fuel.

The equations that I referenced are applicable to any spark ignition internal combustion engine.

Clearly, the Norton engine, designed in the '40s, is not comparable to the modern Bonneville engines; but, the thermodynamics of combustion are the same for both.
 
#62 ·
It is up to the guy that rides the bike what he wants out of it.If a guy wants to leave it stock thats ok.But anyone that has not been on one of these bikes that has been built should not think that building them makes them less rideable ,dependable or makes your mpg go to hell.Its not true.30 miles on my bike and the same guy that wants his bike stock might be pulling his apart soon lol.They are just that much fun.I dont know about anyone but me i ride bikes for the fun of it.
 
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