750SS ie With Bigger Exhaust

This is a short (no, really) report on a 750SS ie with some bigger headers, in this case the std 40mm OD headers from a ’03 model 620Sport ie. The bike used is the one that got the 900SS carb cams. All we were after was an indication of the result, so no tuning was done, just some runs to get a graph.

I have wanted to try this for some time, but couldn’t work out where I’d find some headers to use. 900 headers are a bit too long, as the cylinders are longer, so you need to cut them down to fit. Them it struck me that we had just what I needed on the demo 620Sport ie. So we ripped them off one Saturday morning and got the owner of the 750SS ie to come in for a play. We swapped the headers (a pretty easy job, with the 750/900 DP slip on mufflers fitting straight up) and headed for the dyno. This was a particularly cold and rainy Melbourne spring morning, and we got plenty wet on the way.

The 750SS headers have been 35mm OD from the ’91 model onwards. The late ’80s 750Sport ran 40mm OD headers, but they were reduced for whatever reason for the next model. All the Monster range – 600/750/900 - have always used the 40mm OD headers, as have all the 900SS from ’89 on, both carb and ie. The 620/800/1000 SS and M models all run 40mm as well. I thought they may have gone bigger for the 1000, but, like the S4 Monster, they must figure 40mm is big enough. Given the results I’ve had with the 851 and 888, I’d like to try 45mm on a 1000DS.

The 750 headers are also very small where they meet the heads. The flange welds are on the inside of the pipe, reducing the actual opening at the port to around 30ish mm, certainly smaller than the port opening.

The graphs below show the result. The first is power and air/fuel, the second torque. The bike wasn’t really that warm even when we arrived at the dyno, and the air/fuel trace, compared to the same mapping with the smaller pipe, doesn’t really make that much sense. It is quite a bit richer, which I wouldn’t expect it to be. So I suspect the engine temp was a bit low (under 65 degrees), meaning the cold start enrichment was still present to some extent. That’s the only thing I can think of, anyway. The shape of the air/fuel curve hasn’t changed either. Having it a little leaner may contribute another hp or two, but the actual number, as such, is not what we were after.

The bigger pipe improves the output after the torque peak, helping keep the volumetric efficiency up as the revs rise. Whether a bigger pipe again would offer more top end I don’t know. The improvement isn’t that big, but nothing we’ve done to this engine has really added that much individually. It is more an indicator of where to head next, and what we need to address along the way. And it was a very easy test, which always helps. It also gets us 3 hp closer to the equivalent 800 output (another indicator of where to head), although we still have 9 or so more to find there.

If this bike gets the high compression pistons and bigger valves the owner is contemplating, it will definitely need some bigger headers to make the job worthwhile. At that point, he’ll probably get a second hand 900 set and have them shortened to suit, an easy job. And much cheaper than a new 620/800 set.

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