"New" 750SS and exhaust question - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 16 (permalink) Old Mar 6th, 2017, 9:36 pm Thread Starter
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"New" 750SS and exhaust question

As I posted in the intro forum, I just picked up an inexpensive, slightly rough '99 750SS/CR that had been made naked by the PO (has all the plastics which I may or may not put back on). Anyway, has about 26k miles, Corbin seat, PC3, pod air filters and was being run with an open exhaust. I'm not quite that socially irresponsible and just won a set of cheap stock cans on ebay to tone it down (have been reading all the coring posts...). But in the meantime while I wait for them, I just threw some stock Sportster muffler take offs (with the baffle plate holed) on it to run around. Actually didn't sound bad, seemed to run well and I kinda like the look. Got me thinking...

Given that this is "only" a 750 (and these mufflers are used on HD 1200's), also given that with the factory pipes each exhaust pulse gets routed to both mufflers, are these really likely to be restrictive to the point of hurting performance? From what I've read about the design of the stock cans, they sound a lot more restrictive than these must be. These are certainly giving some back pressure based on the reduction in the noise, but still...

Any thoughts would be appreciated. Since this bike is a little rough, I'm not all that obsessed with keeping it stock or original or anything like that.

Thanks,

Steve
1999 750SS/CR (and many non-Ducati's...)
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post #2 of 16 (permalink) Old Mar 7th, 2017, 2:39 am
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any non open muffler will hurt the performance, and mostly it will hurt peak torque and response. late model std exhausts usually make comparatively good peak power.

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post #3 of 16 (permalink) Old Mar 7th, 2017, 8:25 am
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If you bought the bike not caring if you have the most powerful bike known to man guess the power lost may not matter at all. The worst exhaust I saw was a customers home built can on a 1000 monster and it was so restrictive it killed hp at relatively low rpms. Given that the exhaust you are running was designed to work at great expense by a major manufacturer it is likely to not kill to much Hp. Bottom line is the only way to know will be a Dyno.

Stock supersport mufflers can be properly cored and will equal most aftermarket cans for performance. Note this is not just removing 1/2 the baffles but actually putting a full length core inside and making them work as an aftermarket can. If you like the sound and performance with the Harley mufflers give it a go they are probably an improvement power wise over the open exhaust you had. Any pictures, before and after?
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post #4 of 16 (permalink) Old Mar 7th, 2017, 8:46 am Thread Starter
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Yeah, I guess my question is - what constitutes an open exhaust? Looks like the ID of the stock header is about 1 3/8" so area seen by exhaust pulse at port is about 1.5 sq.". The ID of the sportster muffler, with the baffle plug removed is about 1 1/4" so area is .98 sq ". But due to the 2 into 1 into 2 header header design, each exhaust pulse sees 2 x .98 sq", right? So is this considered an open exhaust? Anyway, it takes the edge off the tone pretty well, so I may just fiddle with getting that properly mounted as a first attempt.

Here's what it looks like at the moment. Needs a lot of tidying up and sorting of some of the electrics but for the $1k I paid, I'm pretty happy... Think I'm just gonna put an old style (BMW R90S-like) fairing on it.
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post #5 of 16 (permalink) Old Mar 7th, 2017, 12:22 pm
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If you are looking to make maximum Hp then you might increase the canister volume and have a larger core, If you were interested in making the most Hp then you have lots more work ahead of you. I do not think those cans will hurt much it will be as much a matter of sound level . Now the important part is more a matter of what is inside those mufflers.

If they are a turbo style muffler and someone has drilled them to make them straight through then it is a matter of an untuned poorly made megaphone. This modified type often means backpressue is changed and can lead to un tuned back pressure causing poorly timed pressure pulses. If it is a "glasspack" design meaning a center tube made of perforated core material surrounded with a baffle material (packing) then I would expect the only downside will be noise level.
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post #6 of 16 (permalink) Old Mar 7th, 2017, 3:53 pm Thread Starter
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I'm really not too concerned about maximum HP, I guess I was just hoping it would be similar to a stock HP & driveability level. With a little more tonal character... I think stock HD mufflers are as you described as a glasspack design - although stock they welded a 1 1/4" round plate right in the middle of the baffle to force all the exhaust through the baffle material. This makes for an unacceptably quiet Harley of course, so the budget mod to get more sound out is to drill a hole through the central plate or knock it out completely - so much of the exhaust can pass straight through. That's the configuration I've got right now...

Steve
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post #7 of 16 (permalink) Old Mar 8th, 2017, 6:50 am
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I've bored out the center of those stock Harley cans several times. That mod is worth about 5 hp on a Harley and still be acceptably quiet but throaty. I think they should be fine on a 750.

'96 Ducati SS900CR, '07 S4RS, '06 Busa, '77 Bonny,
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post #8 of 16 (permalink) Old Mar 8th, 2017, 5:39 pm
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the larger 900 headers will make more power (a couple), but the 750 engine is sort of restricted everywhere power wise - valve sizes, cams, low comp and exhaust.

open exhaust is an exhaust of the same or increasing internal diameter from port exit to muffler tip. anything you put in there will generally knock it back to some extent.

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post #9 of 16 (permalink) Old Mar 9th, 2017, 8:32 am
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Years ago we were building a race bike for a series on the west coast of the usa. The rules were it had to be about 883 and air cooled so you can guess who it was aimed at. Bruce (BCM) came up with a combination to de-stroke a 1000ss to the displacement so it would be legal for the class.

The plan was to build the motor for maximum power and then Doug cook was going to restrict the end cap to lower the Hp because there was a 100hp maximum in the class checked on a dyno. All went to plan until Doug started to change out end caps with smaller and smaller outlet diameters and the Hp went up not down. Now this is no scientific test but It was a clear example that some times a motor will tell you things you did not expect, they are all different enough that all you can do is try and see what your results will be.
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post #10 of 16 (permalink) Old Mar 9th, 2017, 3:44 pm Thread Starter
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Quote:
Originally Posted by bradblack View Post
the larger 900 headers will make more power (a couple), but the 750 engine is sort of restricted everywhere power wise - valve sizes, cams, low comp and exhaust.

open exhaust is an exhaust of the same or increasing internal diameter from port exit to muffler tip. anything you put in there will generally knock it back to some extent.
So... given the 2 into 1 into 2 nature of the headers, the diameter of the muffler is effectively doubled, right? So if that sum is greater than the exit port diameter, shouldn't be overly restrictive?

Just to be clear here, I realize it's a 750 and I'm not trying to pray to the HP gods with it or anything - just wanted to make sure I wasn't doing anything really stupid and strangling it. Also, since it has PC3 and has been effectively running with open pipes for the last several years, I didn't want to introduce a dramatic map incompatibility. I have no idea what's in it at the moment, but based on my limited time so far, it seems to run pretty well.

If I really feel the need to go fast, I have a Buell 1125CR with Barker exhaust and tuned ECM. It's plenty spunky enough for me...

Steve
1999 750SS/CR (and many non-Ducati's...)

Last edited by swamp2; Mar 9th, 2017 at 4:04 pm.
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