same development led to a 128% increase in peak torque. This confirms, with reasonable accuracy, the published figures that show a 118% in peak horsepower and a 131% increase in torque. (And from the ’75/77 GL1000 to the published data on the 2001 GL1800? – a 48% increase in horsepower and a 183% increase in torque!)

MODIFICATIONS

If you remember, apart from the fun of exploring this, for me, unknown territory and the challenge of finding out just how little I knew or falsely preconceived, I wanted to get some basis for judging the consequence of modifications. I am not really interested in turning our 76s into drag-strippers (that is for private clubs only! J ) I wanted to know what could happen if I ever had to fit non-OEM parts.

There is a tip on where to look first in the specifications – that modest intake-exhaust valve overlap of only 10°. With so little time to bring any advantages, it looks like scavenging by the exhaust is not a very important aspect of the GL1000 performance. Without spending several hundred dollars to try this, the simulation program can be called up. I set up that ‘mild’ ‘75/77 model and then ‘fitted’ the most radical exhaust system offered – large stepped-tube race headers, that are probably highly street illegal in reality. The simulated peak HP went up from 78 @ 6500 to 93 @ 7500 – an increase of only 19%. Torque (leaving out the units) only increased from 68 @ 5500 to 74 @ 5500 – not quite 9%. So, maybe it wouldn’t matter too much what sort of non-OEM exhaust I fitted, so long as it wasn’t as constricted as a cat’s posterior.

If, however, I could replace the individual intake tracts with a single plane manifold that had a 30% better air-flow, the peak torque would only rise by 10% to 75 @ 6500. Horsepower however would jump by 33% to 104 @ 8500. That is considerable. Therefore, any replacement of the carburettor system could have striking consequences.

A quite different modification - if I could run a GL1100 camshaft in the ’75/77 (and not have any problems with the 0.3 mm extra valve lift), then simulated performance changes only a little, with both HP and torque increasing by 5-6%. Here I think the only notable effect of this modification would be to lighten my wallet.

There is a final reservation to make about the foregoing. I only referred to peak performance values – either real or simulated. No engine performance, no suggestion of the riding experience can be captured by these alone. The power curves over the range of engine speeds provide a fuller picture. Official data are rarely available. To add those from the simulations would turn make this thin book into a tome. Nonetheless, I can invent a tweak that would make knowledge of those curves crucial. Take that ‘75/77 GL1000 producing 80 N.m (59 ft.lbs) of torque at 6’500 rpm. Now imagine that after my tweak it produced only 75 N.m – but 75 N.m now available from 2000 to 8000 rpm. The peak has dropped a fraction, but is available over the whole of the useful rpm range – forming a very high plateau. I could probable save weight by throwing away the gear box and having a direct drive! However, it would only be the power curve that could confirm the advisability of that measure.

All in all, then, the major profit from this simulation was to jolt me into thinking about things I didn’t know – or thought I knew, falsely. Also important is the fact that it helps point me in the right direction and to pose some relevant questions. I don’t deceive myself that I now understand these processes that influence performance. For the moment it is enough for me to know that they exist. Lastly, the fact that simulation results are not absolutely exact numerically is not the issue. The program helps me adhere to that quotation from the famous American statistician, John Tukey, who I have cited before:

Far better an approximate answer to the right question, which is often vague, than an exact answer to the wrong question, which can always be made exact.

Trevor White
CH-3173 Oberwangen
9^th December 2000
1976 Honda GL1000 K1 *
1976 Honda GL1000 K1 *
1980 Honda GL1100 Std *
1981 Honda GL1100 Std *
1982 Honda GL1100 DX *
1982 Honda GL1100 DX+ EML GT1
1987 Honda GL1200 AH *
1994 Honda CB500 *
1964 Norton 650 SS

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