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Sportster Motorcycle Engine Conversions Advice, questions, and tips for 1200, 1250, 1340, 1450 etc... for Sportster and Buell motorcycle engine conversions

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90" engine, angle, cams, daily

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  #1  
Old 26th September 2021
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Default Has there ever been a 120* vs 90" DD comparison ?

Anyone have a daily driver 90" with cases bored ? I heard some pros and cons about a 90" from different sources but thought would bring up here for one source.

2000 883 Cases are currently split, so why not think about it ?

Pistons have to be different and skirts cut different to clear crank ?

Not great to have above ?

Pistons are shorter ?

Not as much throttle as a 120* interstate riding ? More displacement.

Less vibration for highway cruising ?

Hot or low cam lobe angle ?

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Last edited by groovemeisterus; 26th September 2021 at 05:21.. Reason: More info...
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  #2  
Old 26th September 2021
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89” in my 2000”C”, probably 30,000+ on the upgrade. Pleasant ride if only a little throttle sensitive. The kit was from S&S, they bored the cases for me as well.
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  #3  
Old 26th September 2021
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I don't know what a "120*" is, but I have a chart you may find useful.



90 incher in blue, 1250 in red.

Same cams, same level of head work, same air cleaner, same exhaust. In other words, the breathing capacity is about the same between the two.

The one big difference here is that the 90 incher has a carb (HSR48) and the 1250 has a throttle body (ported stock piece). So it's not as good of a comparison as it could be. But it's the closest I've got.

Notice how the top end number came out almost the same. That's because at the top of the dyno sheet, it's the breathing that constrains the motor, not the displacement.

But through the rpm range, the big motor is making any given level of power at about 750rpm less. That's what you get for the additional time and expense and effort of going big.

Power is just torque times rpm. So the big motor is making it's power with relatively more torque and less rpm, as compared to the small motor.
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  #4  
Old 26th September 2021
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Quote:
Originally Posted by aswracing View Post
I don't know what a "120*" is, but I have a chart you may find useful.



90 incher in blue, 1250 in red.

Same cams, same level of head work, same air cleaner, same exhaust. In other words, the breathing capacity is about the same between the two.

The one big difference here is that the 90 incher has a carb (HSR48) and the 1250 has a throttle body (ported stock piece). So it's not as good of a comparison as it could be. But it's the closest I've got.

Notice how the top end number came out almost the same. That's because at the top of the dyno sheet, it's the breathing that constrains the motor, not the displacement.

But through the rpm range, the big motor is making any given level of power at about 750rpm less. That's what you get for the additional time and expense and effort of going big.

Power is just torque times rpm. So the big motor is making it's power with relatively more torque and less rpm, as compared to the small motor.

This is a great Dyno sheet that lets us see the comparison side by side . I personally don’t have much mechanical ability but with that being said I always enjoy being able to learn from everyone here . This chart is a perfect example of what the difference is when you go Big as opposed to going Big-bore . Thank you Arron for throwing this up here for everyone interested. Very informative.


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  #5  
Old 26th September 2021
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Quote:
Originally Posted by sportsterpaul View Post
89” in my 2000”C”, probably 30,000+ on the upgrade. Pleasant ride if only a little throttle sensitive. The kit was from S&S, they bored the cases for me as well.
Thanks for the feedback.

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Old 26th September 2021
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Quote:
Originally Posted by aswracing View Post
I don't know what a "120*" is, but I have a chart you may find useful.



90 incher in blue, 1250 in red.

Same cams, same level of head work, same air cleaner, same exhaust. In other words, the breathing capacity is about the same between the two.

The one big difference here is that the 90 incher has a carb (HSR48) and the 1250 has a throttle body (ported stock piece). So it's not as good of a comparison as it could be. But it's the closest I've got.

Notice how the top end number came out almost the same. That's because at the top of the dyno sheet, it's the breathing that constrains the motor, not the displacement.

But through the rpm range, the big motor is making any given level of power at about 750rpm less. That's what you get for the additional time and expense and effort of going big.

Power is just torque times rpm. So the big motor is making it's power with relatively more torque and less rpm, as compared to the small motor.
Now, I'm tapping my foot to see the torque curve. Never enough info. LOL
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  #7  
Old 26th September 2021
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Default

Quote:
Originally Posted by aswracing View Post
I don't know what a "120*" is, but I have a chart you may find useful.



90 incher in blue, 1250 in red.

Same cams, same level of head work, same air cleaner, same exhaust. In other words, the breathing capacity is about the same between the two.

The one big difference here is that the 90 incher has a carb (HSR48) and the 1250 has a throttle body (ported stock piece). So it's not as good of a comparison as it could be. But it's the closest I've got.

Notice how the top end number came out almost the same. That's because at the top of the dyno sheet, it's the breathing that constrains the motor, not the displacement.

But through the rpm range, the big motor is making any given level of power at about 750rpm less. That's what you get for the additional time and expense and effort of going big.

Power is just torque times rpm. So the big motor is making it's power with relatively more torque and less rpm, as compared to the small motor.
I meant it to say 12**, which would cover 1200cc, 1250cc, 1275cc for common displacement for a 1200 under 90"

I have heard there was a restriction at one time about air flow and this shows and mentions it. But on a 2000 883 and 883 gearing, would vibrate my self apart at about 7500 RPM.

Looking for interstate riding up to 70-80 MPH without higher RPM's to be more sustainable for riding comfortably. Will be getting crank rebuilt, and balanced, so will be making a choice in using current custom 1200 pistons for balancing and current head work for pistons weight,
Or go the extra route and have to to get new wider pistons, and have to have more head work done $$$.

What about the 90" pistons. Are they shorter ?

Have to be cut a certain way for more clearance from crank ?

Will the piston skirts get more wear and tear ?

Not worried about top end HP as I am about low end TQ and city riding, curves, and interstate, and the interstates with speed limit at 75, while everyone is just flying by.

I've went thru two old stock Harley tach mounts from to high RPM doing interstate riding. And of course the occasional race with buddies.

I like my current gearing especially when off the line, or a rolling start.

Rather not change the final belt sprocket to a 29T 1200 from a 27T 883, without throwing the odometer off MPH and miles traveled also.

Rather change primary main engine gear, and or clutch teeth, or transmission final ratio before pickup comes into play for odometer.

But that is getting a little off topic.

Just figuring without seeing a TQ curve (low RPM) mixed with current HP curve what TQ would look like so no needing to rev higher RPM's to keep same interstate speed without vibrating too much and more comfortable to ride since higher displacement.

Quote:
Originally Posted by aswracing View Post
Power is just torque times rpm. So the big motor is making it's power with relatively more torque and less rpm, as compared to the small motor.
So I could use excel and calculate and plot what the TQ curves would be ? And plot the data on and get an idea of what the TQ curve looks like from HP = TQ * RPM ?

Meaning at any given RPM on the Dyno sheet just use
TQ = HP / RPM ?

Taking my time on this hopefully last ole school build, and bucket list building motor up from scratch.


Last edited by groovemeisterus; 26th September 2021 at 23:03.. Reason: mistake, bold text
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  #8  
Old 26th September 2021
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Quote:
Originally Posted by rejeanprimeau View Post
Now, I'm tapping my foot to see the torque curve. Never enough info. LOL
I am assuming from algebra that since HP = TQ * RPM,

that

TQ = HP / RPM.
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Old 26th September 2021
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Quote:
Originally Posted by groovemeisterus View Post
I am assuming from algebra that since HP = TQ * RPM,

that

TQ = HP / RPM.
Ok, can you please draw the curve, I've no time calculating all those squares.
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Old 26th September 2021
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Quote:
Originally Posted by groovemeisterus View Post
I am assuming from algebra that since HP = TQ * RPM,

that

TQ = HP / RPM.
Think might be a conversion issue on calculation from HP = TQ * RPM.

TQ = HP / RPM and at 47HP at 3000RPM is a very low number.

Maybe TQ equals RPM / HP ?
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