The saga continues !
As offered previously, I needed to rebuild the engine on my blue 1976A GT750 but rather than just do a stock rebuild, I decided to try out a few common modifications in an effort to see what they actually contributed to performance increases. Comparing performance increases is tricky – looking at the original period sales brochures shows these engines producing a claimed 70 HP, but they don’t say whether that is BHP or WHP, and that makes a big difference. I’m not suggesting that manufacturers lie, but they do obfuscate. To make it even worse, both terms are often shortened to just ‘HP’ which doesn’t help at all. Add to this differences in readings from different dynamometers (dynos for short) and real life comparisons become tricky.
BHP is ‘brake horse power’ and is measured at the flywheel of the engine and will always be much higher than a measurement taken at the drive wheel due to losses in the transmission and drive train. A dynamometer will measure power at the drive wheel – WHP, and is typically around 13% to 18% less than BHP. Using 18% as a worst case number, this would indicate that 70 BHP would be around 58 WHP, which actually matches several measured test results I’ve seen. Recall in my original test (see this link) , the reading I got was 55 HP measured at the wheel.
So I added a lift plate (just click on any image for a larger version) and had the top of the barrels milled by the amount of rise. I also had the head milled to raise the compression and used a thinner head gasket having a 72 mm fire ring. If you measure the stock Suzuki head gaskets, the head gasket fire ring is a 73 mm diameter – that is fine if you are at maximum oversize on the cylinder bores, but wastes available compression otherwise. I ended up with 150 PSI across all three cylinders once the engine was assembled.
I blanked off the SRIS ports, and as mentioned before did not change the porting, but did spend some time cleaning up rough spots and misalignments in the passageways. For example, with the barrels fitted to the upper crankcase half, I made sure that any edges were smoothed so that they matched.
At the same time, I also matched the inlet boot inside diameter to the block inlet as the factory casting has a large step. You can see the size of the step and the amount of metal to be removed in the photo to the right.
I also decided to use the aluminium water pump gear in place of the stock nylon one – only because I happened to have one as I’m not actually convinced it will make any real difference to performance. It does look nice though !
I did use Suzuki pistons, seals, rods and bearings for the crankshaft rather than after market products. The lift plate was supplied by Cometic Gaskets in the USA along with the special order head gaskets. The machining and crankshaft rebuild was done for me by Joe at RPM Services just south of Calgary.
And on to Part 3 !