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Old 15-12-2019, 03:18 PM   #1
Bitza
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45 or 50mm exhaust

Hi all, has anyone out there experience of tuned 1078 2v exhausts? While I understand that using a standard M1000s 40mm exhaust will be over-restrictive, so going to 45mm dia (+25%) is a no brainer, but its been suggested to me that 50mm (+50%) would be better for max power. As my bike is primarily a road bike I was wondering if 45 might be better for mid-range, even if there was a loss around max revs/power. All thoughts welcome Bitza.
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Old 15-12-2019, 03:44 PM   #2
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My highly tuned 839cc at 99bhp exhales well enough through 45mm....
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Old 15-12-2019, 03:54 PM   #3
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Whilst I've no experience of your situation exactly, I would say that its probably best to over size your exhaust. It certainly wouldn't be a bad idea to make it 50mm instead of 45mm but as I understand 45mm is the way to go for the 2v 900 at least with a spaghetti header. I'm currentlu running a 916 testabassa S4 motor stock with 45 headers and mid section into hi level 50mm link pipes with probably about a 10% increase in mid and top end. I think stock is 40mm all through. So restricted. But with a 2v you can teally feel it (head flow restriction).
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Old 15-12-2019, 04:55 PM   #4
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My highly tuned 839cc at 99bhp exhales well enough through 45mm....
More detail
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Old 15-12-2019, 05:19 PM   #5
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Have just found an exhaust calculator on-line, based on the formula from A.Graham Bell's book. Having fed in the capacity and cam(Vee-Two 300) details, it tells me that a 45mm dia header would be optimum at around 6000rpm and therefore I guess becoming progressively more restrictive as revs rise. A 50mm dia is optimum at around 7750rpm. On which basis I conclude that the advice to use 50mm is sound.

Thanks for the feedback people have given, but don't let this post stop anyone from saying their piece. Bitza
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Old 15-12-2019, 06:26 PM   #6
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Can you share more details from the book, sounds(?) interesting, how does it calculate the degree of back pressure, open pipe vs a standard restricted exhaust?, megaphone vs an open pipe vs a reverse cone megaphone?
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Old 15-12-2019, 08:11 PM   #7
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More detail
Open air box, high flow filter, port & polish, 900ie cams, larger 44 inlet / stock 38 exhaust valves, pistal HC (11:1) pistons, Carrillo rods, lightened and balanced crank, flywheel, Termignoni, dyno work.

Very close to a pure race tune. Probably missing larger exhaust valves and tiny bit of added compression to exceed 100 bhp. Would be useless as a street bike.....
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Old 16-12-2019, 03:22 PM   #8
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The book is called "Four-stroke performance tuning" IBSN 97818442531142 printed by Haynes I believe, but I simply used an on-line calculator using a formula from the book. Due to the endless variables referred to above, I can only take the results as a guide to what the actual situation in my case is likely to be. I find that you can often make pretty sound decisions if you understand the general "tendencies" of a particular situation and have some concept of what magnitude of change will make a "significant" difference. Even where you don't actually know all the science/engineering of the subject.

Having spouted all this I just wish that I had realised how restricting the standard 40mm headers were when I had the 944cc engine in the bike, I might have even saved myself the heartache of replacing it with a 1078 lump?! Bitza.
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Old 17-12-2019, 10:16 PM   #9
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http://www.ukmonster.co.uk/monster/s...ghlight=stroke

Read the whole thread
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Old 17-12-2019, 10:23 PM   #10
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50mm primaries on a 2 valve will kill mid range power & torque
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Old 17-12-2019, 10:41 PM   #11
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Quote:
Originally Posted by stopintime View Post
Open air box, high flow filter, port & polish, 900ie cams, larger 44 inlet / stock 38 exhaust valves, pistal HC (11:1) pistons, Carrillo rods, lightened and balanced crank, flywheel, Termignoni, dyno work.

Very close to a pure race tune. Probably missing larger exhaust valves and tiny bit of added compression to exceed 100 bhp. Would be useless as a street bike.....
Just out of interest, what is the balance factor? ie stock and your alterations.
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Old 18-12-2019, 08:14 AM   #12
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Just out of interest, what is the balance factor? ie stock and your alterations.
Balance factor?
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Old 18-12-2019, 10:45 AM   #13
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Balance factor?
Crankshaft counterweight size.

https://www.laverdaforum.com/forum/i...?topic=81409.0
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Old 18-12-2019, 10:51 AM   #14
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But for 90 degree L twins:

https://www.cycleworld.com/2016/02/0...sights/#page-2
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Old 19-12-2019, 11:26 PM   #15
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Hi Kato if you're still out there, can you give me /us a bit more on the "collector outlet sizing" bit?! Generally you seem to be strongly confirming my original scepticism about going as far as 50mm for primaries on a road bike, any comments on the fact that I am planning on using twin silencers, 45mm ID basically mufflers with a reverse cone exiting via a slash cut 50mm pipe Bitza.
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