De cat headers

[fivestar]

I have just fitted a set of de-cat headers - I already have a remus silencer. The headers do not have a balance pipe like the original. Now whilst it has made a big difference to the pick up of the engine, it seems to have slightly more vibes - particularly noticeable around 4 - 5,000 revs.

Now I no I am going to need to get it re-mapped, so I guess part of this may be fuelling issue, but does the lack of balance pipe make a difference? Presumably BMW put it there for a reason. Could this explain the vibes? Any knowledgeable tossers out there like impart their wisdom.

 

[Deleted account 211025001]

Normally a balance pipe is there to help low end grunt - in theory at least. On several Guzzis and my GS I have fitted systems that do away with it and, TBH, have not noticed a difference.

I can't say I've ever noticed any difference in vibes either but I guess if your fueling has been altered its possible? It'll be interesting to see if a re-map makes a difference to yours.

Andres

 

[fivestar]

de cat

Well I guess I shall have to get it up to Hilltop and see Geoff and find out. Will post results on the "Hilltop" thread

 

[Bendy toy]

I asked Geoff some while ago if the Remus larger pipe bore and no balance pipe would affect performance compared to an Akraprovic that has smaller front headers tapering out and a balance pipe. He said it makes no difference. So I do wonder why we bother. Bling I guess.

 

[ebbo]

Now whilst it has made a big difference to the pick up of the engine

I'm thinking about decating my RT, can you give a little more info on the improvement to pickup, does it help much in low speed, low rpm running such as when riding in slow traffic?

Cheers

 

[Sgt Bilco]

Keihan and Akrapovic headers retain the balance pipes as per the OEM headers as they help the exhaust flow. Remus get round this by increasing the bore and in my opinion, they do it to cut production costs.

I've never heard of any real difference between the performance between the headers but having being involved with the Keihan developement, we figured BMW know best so the Keihan headers have the balance pipes and same bore as the OEM ones

 

[fivestar]

Improvement

Ebbo, I have not had chance yet to fully evaluate the differences - between cat and no-cat, although I am sure there are threads on here with that very subject. I suspect until I get it re-mapped there will not be any change to low speed stuff, although to be fair, my bike never seemed as bad as some on here at low speed.

 

[(RIP) Miff]

I'm thinking about decatting my RT, can you give a little more info on the improvement to pickup, does it help much in low speed, low rpm running such as when riding in slow traffic?

Cheers

My GS has been decatted. It certainly runs better and picks up quicker. Low speed pick up is certainly faster.

Once it gets to 5000rpm, it accelerates MUCH better as proved by how much easier it wheelies compared to my last GS that had the cat in place. BUT... I also have an accelerator module fitted to richen up the fuelling, so how much of the improvement is down to the decat and how much is down to the module I can't say.

BMWs run very lean, so once decatted, the fuelling needs sorting.

 

[Sgt Bilco]

Ebbo, I have not had chance yet to fully evaluate the differences - between cat and no-cat, although I am sure there are threads on here with that very subject. I suspect until I get it re-mapped there will not be any change to low speed stuff, although to be fair, my bike never seemed as bad as some on here at low speed.

Removing the CAT will give you a little more low down grunt and the engine will spin up a little quicker as the CAT tends to strangle the engine.

 

[ebbo]

Thanks for that, may give it a go, cheers,

 

[Ali-bear]

I don't think the balance pipe makes much difference, perhaps helps a little at low end like someone said. Having a free-flowing pipe probably is more important on the low-tune GS engine. May be a different story on the RT.

Tuning the exhaust pipe makes more of a difference on engines where there is a large valve overlap. This is the time when both the exhaust and inlet valves are open together around TDC between the exhaust and intake strokes. High-revving and high-performance engines use more valve overlap (through cam timing) because its possible to make the cylinder fill more effectively at certain engine speeds. A partial vacuum in the exhaust header, created by the gas pressure waves, helps pull the mixture through the inlet. Obviously this can make the engine peaky at certain rev ranges. Also more thirsty.

I don't think the GS engines have much valve overlap therefore exhaust tuning - eg removing the balance pipe - does not make much difference.

 

[Bendy toy]

I fitted a Remus header with the standard can. It made a really obvious improvement in low to mid range drivability and pick-up over the standard headers complete with cat.

I later fitted an Akraprovic full system with smaller front headers, balance pipe, larger back headers with a cone just under the cylinders and stacked twin cans. Performance feels much the same as the Remus with standard can though it might have a little more at high revs with the "race" cans. Fuel consumption is also about the same.

By the time the Akra is baffled to half civilised levels I cant tell any difference.

What's to say BMW fit a balance pipe because they always have done?

 

[Martyn H]

I have read so many times that I think there must be substance to it, that the balance pipe as fitted to British twins ( BSA & Triumph) & also Boxers, were an aid to silencing, allowing each exhaust stroke to enter both silencers. Saw that attributed to no less an engineering guru than the late Doug Hele.

 

[fivestar]

One thing is for sure, my de-catted headers and remus can weigh loads less than the standard kit

 

[Bendy toy]

The old Brit bikes had a right old mixture of exhausts balanced on a T 110 straight as on a cafe racer "siamesed" as on Trophy Trails. None had much thought for tuned lengths.

BMW used a balance pipe on the airheads with twin silencers but why bother on a 2:1 system that's joined anyway?

I ran the Yamaha 900 details thread header design program. It came out spot on for the primary and secondary lengths. For max power at 9,500, primaries were about 23". A slower revving engine would be even longer.