Bobby's 853

Bobby Hopp read the report about Michael's 748 on my homepage and called me about making his 853 into a similar piece.

That's a fair amount of work, but I was willing to see what his did, and then we'd decide what to do about it.

He shipped it to Minnesota from Florida, and I soon tested it. He had been told that it made 100HP but we saw only 94HP.

We checked the compression and it seemed pretty low, so we pulled it apart to find that piston of the 853 kit which had been installed, was 1/2 mm below the deck. That is considerably shorter than a stock piston and losses a bunch of compression.

The after-market cylinders were almost 1.25mm shorter than the stock cylinders. I compared the whole kit with a pair of BCM pistons in stock 916 cylinders, and decided that we needed to replace the cylinders and pistons.

We had planned on big valves and porting the head, and the BCM 94mm pistons are very close to the same weight as the stock 748 pistons, so the best plan of attack would be to do the top-end and save the expense of taking the bottom-end apart and re-balancing.

Bobby had a Termignoni intermediate system sent here and it went together with that instead of the slip-ons that it arrived with. And he sent an aluminum flywheel too.

The chips that come with these 853 kits are, very often, slightly modified 916 chips. That's fine from a fuel map standpoint, but the ignition timing of the 916 is 8 degrees retarded from that of the 748.

It just doesn't make sense that the ignition timing that far from stock 748 would work correctly on an 853.

The bore is the same, so that could change the requirement a couple degrees in the advanced direction, and the increase of compression (in the few cases where that actually occurs) should require a little ignition retard. At any rate, a major part of the chip development for this combination would be the ignition map.

As you can see from the chart below, the changes we performed (no change in displacement) had a pretty good effect.

The torque curve is quite flat, and should make for a pretty boring riding experience, except for the fact that the speed will increase pretty quickly.

I really like these Strada cammed engines because they work well over such a broad RPM range.

When we put the big valves in these heads we have to unshroud the combustion chamber around them, and that looses a little compression. Also we had to add a little clearance in the inlet valve pockets.

The result is an engine with a marginal increase in compression over a stock 748, so there shouldn't be any problem finding adequate fuel.

Bobby should be riding this as soon as it gets back home. I'll have to wait until I build another one to try it.