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Discussion Starter #1
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,
 

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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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Discussion Starter #4
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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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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Discussion Starter #6
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...
 

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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.
 

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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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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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Discussion Starter #10 (Edited)
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...
 

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As long as you don't go too restrictive you won't hurt much, and if it's too restrictive you'll know right away from the loss of power. As you were saying, the mufflers are from a bigger bike. I would bore the centers out though,, for sound enhancement.😳
 

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Discussion Starter #12
Well, my stock ebay cans came today... and they're perfect/virtually new... so rather than chop them up, I may just try the Harley cans for a while and see how that works out...

The centers of the HD mufflers have been suitably bored for sound enhancement.
 

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interesting what eric says. i've got an aprilia racing akrapovic muffler at work from an early rsv mille, the ones that had the 2-1. it has a 60mm inlet and 48mm ish outlet, with a tapered core. also, a lot of the late 996 chassis termi systems had muffler outlets noticeably smaller than the inlet.

i recall one of the now gone local exhaust guys (mark from madaz) saying his zx6 (?) race bike made more power with a reducing taper collector between 4-1 merge and muffler.

eric - did those motors have a honda car rod in them? if so, any idea which one it was? 999 crank?

i think tom has sent me some stuff he has done with 1x98 where he has run baffles with 35mm ish holes in them that is the smallest you can go without hurting the power, even though there is quite a good reduction in noise. all my dyno work with baffled mufflers has been noise based, with baffles having probably 20 - 25mm id holes in them. i think the old staintune ones would be about 20mm id?

the 750ssie made about 2-3 hp more with open pipes and airbox lid compared to all std, but they did feel quite a bit better from memory. i have runs showing no improvement in peak power when the only change is mufflers. so quite possibly the expansion from the collector into the two mufflers is enough there.

BikeBoy.org - Ducati750 SS IE : MUFFLER, AIRBOX AND ECU MODS

conversley, my hot 750 engine in the monster chassis with std monster mufflers as quite noticeable. from memory a similar result to the gt1000 dp kit with baffles in, which were almost std muffler quiet. amazing that a outright flow volume restriction can hurt low flow power, but not higher flow power.

http://www.bikeboy.org/graphs/750mg22.gif
http://www.bikeboy.org/graphs/750mg23.gif

BikeBoy.org - Ducati GT1000 with Ducati Performance Termi kit (first graph)

so it does depend on the muffler. i think the monster ones were not too flash as a std muffler, like the 900ss mufflers they gave a real midrange hole. the ssie ones were comparatively better. but i do also have some m750 runs with a straight through aftermarket muffler that made no more anywhere compared to std.

sorry about the rambling.
 

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Brad
I am not sure what rods Bruce used in the 1000/883 build But can find out when he gets back from Daytona. Not sure on the Honda but my 2v/ 853 motor used toyota rods which were longer than stock.

I seldom see much Hp increase from exhaust systems, most of the gains come from improved fueling. There are some exceptions of course but most manufacturers do not plan to make their exhaust systems down on power but must make them quiet so a lot of R&D goes into keeping much of the power. Opening an airbox can be the biggest gains though you have to be careful of messing up drive ability so though open often = Hp it can make the bike run poorer more often due to inconsistent air flow or rain messing the jetting up.

Doug Cook used to make some of his full systems with necked down venturies as he was an aerospace tubing welder and saw them used at work he tried with some success. He also found that most of the time the smaller motors worked well with a 2-1 where the 900's on up preferred 2-2 arrangements. I can tell you one area he never spent R&D time was making muffler volume a prioity, His systems were often all core and no packing, the louder the better. Both systems I have by him have Db killers I had to fit so I did not go deaf and yes they cost me a little Hp, more so on one than the other.
 
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