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Twins Talk Discussion of Hinckley Triumph Twin related matters and topics.

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Old 11-04-2012, 09:16 PM   #51 (permalink)
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Originally Posted by mikeinva View Post
lol can you beleave it for once we both agree.
Ha, yeah, I noticed that too. What's more, I also agree that performance mods can result in higher engine efficiency, and hence better gas mileage.
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Old 11-05-2012, 09:28 AM   #52 (permalink)
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Originally Posted by mikeinva View Post
That is true but as long as you dont rase it to much on these motors you wont have any knock.I have run them up to 11.5 to 1 on 93 octane pump gas on the street for many miles.If you took a other wise stock motor you could prob run 10.5to1 on reg gas because stock bikes the timing is way slow to start with.These bikes are way under tuned to pass epa reg.
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Originally Posted by leftarmlogan View Post
That's really splitting hairs, no?

If Mike got 50 mpg average on a bone-stock bike and now gets "upper 40s" (let's say 47-49 mpg average) while getting twice the bhp and 50% more torque, I'd say that's pretty good.

I get about 50 mpg out of my very mildly modded Bonneville. If I could have power like a 1087 and only lose one-to-three, even five-to-seven, mpg I'd be bouncing off the walls.
This thread started out as a discussion about changing compression ratio.

I think that we can all agree that science (thermodynamics and the Otto combustion cycle) tells us that changing the CR results in an improvement in efficiency with the adverse side effect of requiring premium fuel if you raise the CR above a certain point. Science also tells us that the increase in efficiency obtained by increasing CR is small (3% or so).

I think that we all can agree that changing the CR alone is an expensive proposition for the power and efficiency gains.

We can also agree that if you modify your engine for more horsepower, you are unlikely to realize an increase in MPG was you will take advange of the increased power by twisting the throttle a little more. Understandable
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Old 11-05-2012, 09:29 AM   #53 (permalink)
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Originally Posted by Grubscrew View Post
Ha, yeah, I noticed that too. What's more, I also agree that performance mods can result in higher engine efficiency, and hence better gas mileage.
Yeah, it CAN happen; but, it usually doesn't.
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Old 11-05-2012, 01:35 PM   #54 (permalink)
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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.
I have very wild cams in my bike if I was to change cams my mpg would go way up.When I had the 904 motor on my bike with 813 cams it got about the same mpg as stock but it made 85 hp at the wheel.I used high test gas both stock and now .Why ? because I run more timing that is needed for the 10% eth fuel we have here .With stock timing i got less mpg even on the stock motor.
I think the reason you have not good mpg when you did mods is your motor was not tuned right.You cant just throw parts on you must first plan out what works good with what as far as parts go then you need to get it tuned right.MY bike bone stock like i got it got real close to the same mpg as it does now.I got the best mpg with it stock after i did the air box remove ,put free flowing pipes ,added timing and got it dyno tuned.That was back before any of us used a/f gauges .If I had had my a/f gauge then it might of even got more mpg.
I dont have a clue where you come up with the just 3% gain stuff from with HC it can be way more then that.The guys that have the hc 865 kits prove that wrong they get way more then 3%.
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Old 11-05-2012, 02:03 PM   #55 (permalink)
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I think the reason you have not good mpg when you did mods is your motor was not tuned right.
That was not my experience with two Nortons. There were both set up by professionals and ran well. The mpg was not appreciably different than stock as long as I took it easy on the throttle.


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Originally Posted by mikeinva View Post
I dont have a clue where you come up with the just 3% gain stuff from with HC it can be way more then that.The guys that have the hc 865 kits prove that wrong they get way more then 3%.
The 3% figure is if you ONLY increase the CR and that is straight out of thermodynamic calculations using the Otto cycle (all sparked internal combustions engines use this cycle).

http://en.wikipedia.org/wiki/Otto_cycle

The equations are there is you want to give it a go.

If you start playing with cams, carbs, exhaust and other mods, well, we have an apples to oranges comparison.
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Old 11-05-2012, 04:39 PM   #56 (permalink)
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3% for how much increase? Forget what you have done with the old 60s nortons those motors dont even come close to being like these motors.
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Old 11-05-2012, 05:14 PM   #57 (permalink)
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3% for how much increase? Forget what you have done with the old 60s nortons those motors dont even come close to being like these motors.
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.
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Old 11-05-2012, 05:31 PM   #58 (permalink)
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I think that we all can agree that changing the CR alone is an expensive proposition for the power and efficiency gains.
Agreed, and I seriously doubt that anyone on this site will recommend the change to the higher compression pistons (and necessary engine rebuild) as a logical singular upgrade. But, if someone is already in the process of rebuilding or replacing pistons, or in the case of Triumph themselves, building from scratch, popping in the already available HC pistons instead is a smart move. The HC pistons have proven themselves over the past 4 years in the Thruxtons, and the price is the same as the LC pistons, I'm actually surprised Triumph has waited so long to implement the change across the board.
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Old 11-05-2012, 05:42 PM   #59 (permalink)
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Oh, my Thruxton with the HC pistons runs just fine on regular petrol, but I've found that running premium gives better mileage, enough so that it actually works out cheaper per km, so that's what I use.
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Old 11-06-2012, 10:35 AM   #60 (permalink)
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Originally Posted by Grubscrew View Post
Agreed, and I seriously doubt that anyone on this site will recommend the change to the higher compression pistons (and necessary engine rebuild) as a logical singular upgrade. But, if someone is already in the process of rebuilding or replacing pistons, or in the case of Triumph themselves, building from scratch, popping in the already available HC pistons instead is a smart move. The HC pistons have proven themselves over the past 4 years in the Thruxtons, and the price is the same as the LC pistons, I'm actually surprised Triumph has waited so long to implement the change across the board.
If I ever feel the need to rebuild my T100, I'll. consider HC pistons. Right now, I have to ask the question, why bother? If would take a worn out or broken engine for me to do a rebuild or even just adding a big bore kit. The stock T00 seems to be enough for me. It is just such a sweet runner that I would hate to muck about with engine mods and destroy its current character.

Last edited by norton74; 11-06-2012 at 10:38 AM.
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