S4R Exhaust Header Comparison - Written Feb ‘05

We’ve done a couple of S4R now with reset cam timing and both have had different exhaust styles fitted. Plus we’ve tried another catalytic convertor replacement so I thought I’d show some results and have a bit of a talk about them.

The cam timing spec for these bikes is 114.5/112 centrelines. We’ve been resetting them to 113/110, which seems to work very well. Although, given all the S4R have adjustable cam pullies I’m somewhat surprised and disappointed with the accuracy of the std settings. I checked a ’05 749s the other day and it was within 0.5 degree of spec on all cams. I was actually suspicious of my measurements it was so close and consistant. By contrast, the latest S4R we’ve done had the inlets retarded 7 and 8 degrees from spec and the exhausts retarded 4 degrees. I checked a ’01 ST4S (non adj pulley type) a while ago and it was within maybe 4 degrees on all 4. We reset that bike to std specs, and it’s the bike featured in the ‘ST4S with Ultimap U59 ECU’ report. I suspect now the ’03 ST4S fitted with DP Power Kit (adj pulley type) that this bike is compared to may be suffering from a cam timing induced lack of midrange.

The two S4R we’ve done the cam timing on have had the following combinations:

  1. DP ‘Two mufflers on right’ Power Kit – std catalytic convertor still in place.
  2. Spareshack catalytic convertor replacement header pipe, Staintune mufflers, modified airbox lid with Pipercross foam filter and Ultimap U59 ECU.

We’ve also tested another cat replacement pipe made locally that didn’t work, and I’ll show some graphs for it later.

The first graphs show the two S4R with cam timing reset. This graph is one I’ve made up using a spreadsheet because I didn’t get a final run on this bike. Nowadays, if I’m using the dyno to do a complete map for a bike we don’t have one for already, I do runs at all the required throttle positions and then head back to work to develop the map using a spreadsheet I’ve set up. And with the Weber Marelli 1.5M and Ultimap U59 ECU you don’t have the luxury of real time adjustment, so it’s not like using the great old “hand held terminal” where you can just do run after run adjusting the fuel and then quickly set up some zones for a ‘final’ run.

I did do some +/- runs on the map I started with (a made up S4/ST4S combination) but it’s rather time consuming and ultimately just increases the cost of the dyno time. Those +/- runs are the basis of the following graphs – you can export the dyno run results out of the Dynojet software in a notepad style delimited file that is then very easy to play with in a spreadsheet. Of course this means you can make up anything you like, but that’s not really why I’m here.

The blue curve is the DP equipped bike with the idle trimmer raised to give max power. The red line is the U59 ECU equipped bike. The difference in character of the two bikes is quite clear from the torque curve, so we’ll start with that. The Spareshack header is 40mm tube and joins under the rear of the engine in an oval x-over (not Y or X shaped) that’s about 70mm long with both header pipes coming in side by side. The exits then head straight out from there. So the headers are longer than they are in the std catalytic convertor system, where the headers end with their entry into the cat box (with is an open space inside). There’s also a bit more length in the Staintune muffler entry pipes compared to the std or DP ones. I’d expect the headers make the biggest contribution to the variation of the two.

You can see how much stronger the red curve is under 7,000 RPM, and how it falls off after that. On the road it’s a rather grunty bike in the low and mid range, but feels flat over 8,000 RPM. The blue bike is not so strong through the middle, but pulls nicely all the way through and then finishes off with a noticeable top end rush. It will lift the front wheel in second quite easily depending on how you hit it. The red bike does that no matter how you do it and lifts it right up - it’s a very eager little camper indeed.

The power curve below shows the red bike going off to some extent around 7,500 RPM, whereas the blue bike doesn’t go off until 9,000 RPM.

Next another header set (cat replacement) made by a local company, running the std header stubs. They had one of our customer’s bikes for a few days, but in the end seemed a little defeated and made up a 45mm diameter system that had the individual header tubes joined by a short balance tube that entered each header at 90 degrees. The headers were then tapered down to 40mm to meet the muffler pipe inlets. Meaning there was negligible flow between the headers of each cylinder and a reduction in pipe diameter about half way along the system. I figured this wouldn’t work and we went to the dyno to prove it.

This bike had a DP power kit with the mufflers either side. Otherwise it was as delivered. Green is with the std catalytic convertor, red is with the cat replacement pipe. You can see the top end loss that comes from a non flowing x-over.

If we compare the above curve (with the cat replacement) to the curve with the 40mm Spareshack cat replacement fitted you can see that they’re almost the same shape. I assume this is to do with the header length – the joins are in about the same place distance from the head wise. Green is the local 45mm header, red is the Spareshack header. The fairly consistant difference between the two curves I would expect is due to the reset cam timing of the Spareshack header bike. It’s the same for all the non reset S4R and ST4S that I’ve dyno’d.

I would have thought the non flowing x-over (balance pipe) of the local system would influence it more compared to the Spareshack system which does have a flowing x-over. Maybe the short balance tube in the local system does give the tuned length effect for header length, and the top end is helped by the bigger 45mm diameter. It’d be nice to see the Spareshack header in 45mm. I don’t really understand what it is about the 45mm size that manufacturers don’t like. Usually if a company goes to the trouble of making a full system it’s 50mm (and they call it “race”). 45mm seems to work very well on most 4V I’ve tested, and in this case would provide a stepped header design that could really help. Maybe even a short piece of 42mm then into 45mm before heading into the x-over. Once both pipes are joined there’s no real problem with the 40mm muffler pipes – two 40mm pipes will easily handle the exhaust from there out. All in all though I think it’s rather funny that the cat replacement pipes are costing top end, the one thing the catalytic convertor is supposed to do.

The only difference between these bikes and the ST4S we fitted the Ultimap U59 ECU to is the exhaust style and size. The final graph compares them. Green is the DP kitted S4R, blue is the Spareshack/Staintune/air filter kit/U59 S4R and red is the Staintune/air filter kit/U59 ST4S. The ST4S is running 114/112 cam timing compared to 113/110 in both S4R, so I’d expect it to be a touch soft in the midrange by comparison if anything. But, as you can see, it still equals or out powers both S4R everywhere.

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