Engine upgrade? - Harley Davidson Street Forum - Street 500 and 750
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post #1 of 9 Old 04-09-2017, 11:24 PM Thread Starter
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Engine upgrade?

With reworked heads, cams, new intake, throttlebody and higher compression. Harley managed to make street rod about the same whp as the sv650 or fz07.

Soooo would that mean aftermarket cams and throttlebodys will be available for the street as well? Maybe a stage 3 kit?

And how could have they increased the compression ratio without changing the cylinder dimensions? Just less valve overlap or new piston and combustion chamber?



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post #2 of 9 Old 04-11-2017, 12:01 AM
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Valve overlap will affect the cylinder pressure but not the compression ratio. CR is determined solely by the ratio of volume (cylinder and combustion chamber combined) when the piston is a BDC, and TDC. Increased CR can be accomplished by either of the two methods (Piston or Combustion chamber) that you mentioned. They did modify the heads, but if anything they opened up the combustion chamber to unshroud the valves, which would have lowered the CR. I haven't dug into it yet, but I believe they changed the crown on the pistons. Anybody checked a parts list? I also saw a chart that said that the bore was increased by .050 If that's true, then using the same head would have resulted in a higher CR. This is just speculation on my part, because, like I said, I haven't actually dug into one yet.

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post #3 of 9 Old 04-11-2017, 09:46 AM
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if i had to guess... domed pistons increased the compression ratio.

but until i get a street rod to tear into i have no idea.
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post #4 of 9 Old 04-11-2017, 02:55 PM
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Originally Posted by Dave Novotny View Post
I also saw a chart that said that the bore was increased by .050 If that's true, then using the same head would have resulted in a higher CR.
It's not true; the bore is the same 85mm as the Street 750.

The change listed is likely due to someone in marketing rounding off the 3.35" bore to 3.4" in the printed spec. If the bore had changed .05", then the metric specification would have to have changed to 86mm, and it didn't. The person writing the spec sheet for public release has to determine at what point to round off; 85mm isn't 3.35", it's actually 3.346456692913386". Clearly nobody's going to publish a spec of a bore of 3.346456692913386"! That'd be absurd, even if it is technically accurate. So they rounded to 3.35 for the Street 750. For some reason they decided to round it off another digit to 3.4" for the Street Rod. However, the Streets are metric bikes and the actual measurement is almost certainly done in mm, and the printed spec for the Street 750 and the Street Rod is both 85mm -- which indicates no change in bore.
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post #5 of 9 Old 04-12-2017, 08:03 PM
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Well I'm still waiting for Harley to make a 500 to 750 upgrade kit! Surely they already have all these parts just needs to be packaged as a kit. I reckon it would be pretty popular, especially in countries where they had only released the 500 up until the street rod this year.Anyone who's listening,if you make a kit, I'll be the first to buy it.

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post #6 of 9 Old 04-14-2017, 11:13 PM
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Well I'm still waiting for Harley to make a 500 to 750 upgrade kit! Surely they already have all these parts just needs to be packaged as a kit. I reckon it would be pretty popular, especially in countries where they had only released the 500 up until the street rod this year.Anyone who's listening,if you make a kit, I'll be the first to buy it.

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You'd have to take the 500 engine completely apart and have the cases cut to accommodate the 750 cylinders.

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post #7 of 9 Old 04-15-2017, 12:13 AM
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Are you talking about increasing the bore? Is it the whole engine to achieve this or just the top half,and if its the whole engine how much would you expect it would cost?just a guess is fine,cheers.

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post #8 of 9 Old 04-15-2017, 01:30 AM
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You might consider dropping a line to the folks at Revolution Performance. They make a 550cc big-bore kit for the Street 500, and an 800cc big-bore kit for the Street 750. As such, they are probably the folks most knowledgeable (outside of the Harley factory) as to the workings of the Revolution X top end and might be able to advise you if such a thing is even possible. I mean, is the engine control computer the same between the two, and would a Street 500's ECU be able to cope with a change to 750cc? I don't know, but maybe those folks might; maybe you can pitch 'em on the idea of making an 800cc conversion kit for the Street 500 for all those countries where the 750 isn't available? I'm thinking you're going to need the whole top end (cylinder head and all the associated innards). And your Harley dealer may not even be able to get those parts over in Australia... might need to scour ebay auctions for wrecked 750's to scavenge parts from?

In any case, the Revolution Performance folks may be able to fill you in on what it would take and whether their kits would be useful towards accomplishing it.
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post #9 of 9 Old 04-16-2017, 06:21 AM
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Good idea,I have seen those kits.50cc is just not much of a bump but maybe your right about them knowing what it would take to get to 750 or 800.

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