Right, bit of an update here. Been very busy at the garage, so its hard trying to find time to do my own stuff, but i crammed some dyno runs and changes in, and here is where we are up to:
As per initial post, after the original dyno, the plan was to do a series of tests as each new component was fitted. So without any other changes, we fitted the new pipe and dynoed up. Straight away the new rev range was VERY apparent, the machine was now peaking at circa 8000rpm. My prediction was that the pipe would take the BHP from 21bhp @ 6900rpm, to circa 24bhp @ 8150rpm....and i wasnt far off, the only problem was that the graph was shaped like an 'M'......by that i mean it actually had a dip at peak power, so at around 7000rpm it was circa @ 23bhp and at 8500rpm it was at 21bhp, but at 7500-8000rpm it dipped to about 20bhp. Had the curve continued in the normal manner, it would have hit the predicted 24bhp slot......but something was amiss here?
This was an odd result, which at first i thought was down to jetting, so i played around with various settings, but was unable to 'Iron out the crease'. So bearing in mind we did not build this engine, i thought it best to do the honourable thing, and look inside to see what was going on......so i whipped off the top end and measured up, trans were circa 129 and exhaust 185. The trans figure was OK for where i wanted to go, but the exhaust a bit low. I wasn't convinced the lower exhaust duration would entirely account for the dip at peak power, so investigated further.....and checked IGN timing. The previous owner had oddly set it at 14 BTDC??
So...whilst i would have liked to tweak the timing back to 18, and then re-dyno, then strip and port......i just didn't have time, so i made two changes at once (sorry)! I ported the cylinder to 190 ex and kept 129 tr. I also widened the exhaust, but only to 67% of bore, so nothing manic. I just gave it a nice shape in the important areas. I then re-set the ign to 19BTDC......and ran it up on the dyno. The figure is a pleasing 27.5bhp @ 8150, with 19ft-lbs of torque. I'm also happy with the fact that it comes on pipe @ 4750rpm, and therefore gives a broad power band of nearly 3500rpm, with torque remaining relatively flat throughout its peak area. It a good result at this early stage.
Bear in mind also, this result is with a low-comp/street head, static ign @ 18 BTDC, pump fuel, and safe/street jetting on a 30mm PHBH Delly carb. So if i wanted to go further, then a 32, 34 or even 38mm carb might help things along nicely. I don't know what reeds are in the engine, but a quick peep done the manifold looks like they are standard, so i could try Boyessen dual stage, or the V-Force block i have here. Potentially i could increase ex to 192-194 without and major adverse effects low down, but with more peak at the top.
So.....early days still, but pleasing at the moment. New suspension still needs to go on, and then out for a GPS run to see how the gearing handles the new rev-range!!
In red BHP and Torque with the new pipe, ign timing and ex duration:
Image by
drunkmunkey6969, on Flickr
the new pipe in blue this time, to compare against the old pipe (red), see how the power has dropped and gone on the old pipe, and had to change into 4th to keep going....whilst the new set-up (blue) is still powering on!
Image2 by
drunkmunkey6969, on Flickr
Progress continues.....