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The stock 70 hp Yamaha would likely make 70 hp at 5500 and 60-62 hp at 6400. Our stock 70 CES dropped fast after 6800.
I believe it was 66 hp and over 7100 it was 61 as I recall. The good HP is only in a range of 600 to 700 rpm. Same as with most stock ported motors. Yamaha was really hurt by three factors. One the porting had to be very close to stock, they had to keep the small carbs again because they are on the stock motor, they also had to keep the stock ignition which is designed for 5000 to 6000 rpm. What Yamaha did was do everything they could in the way of add ons. This made the racer pay extra and offset the cost. Their Spec sheet made it manitory to run their add ons. They put in single plastic reeds [$300.] a piston with a little narrower top ring $350.00, A tuner $600.00 and a head $600.00. For all this money you got that small improvement. Just a cash cow because Yamaha did not want to be in boat racing and since they had to be, the racer was going to pay. I believe Hydro Tec tested both the 70CES and a stock 70 with Hydro Tec add on mods. Their motor was better. That explains why your yamaha runs equal to or maybe better than Jim's original stock 70CES. I am very interested to try a Gig head.

Please guys do not give up on the rice motor. If your thinking of buying one. Buy a 60C powerhead, They are a premix motor and they are a 70 hp. Yamaha just put smaller jets in them and used a different reed stop to limit the opening of the reed. Change the jets, change the reed stops, add 2 degrees of timing and it is a 70 hp. Rock solid platform for a T motor.

The UIM kilo speed record for S850 - 70 Yamaha is 101.93mph. A few teams in Rouen will switch from the Merc 60 4 stroke to the Etec 90, should be interesting if they can find a legal gearcase. BTW what is a 60C ?

If it would help future T850 rules to run a 70 yamaha. I have lots of information from Japan and original 6J2 parts that I can show you.

The Story if your interested.
When we got our new 6J2 Yamaha 70CES in 1996 we thought we had this wonderful race motor that pushed a tunnel at 102 mph in the kilo. When it arrived at Dryland Marine, Sal, Rob and I looked at it and wonder how it could be so special when it looked bone stock. On the Critchfield all our initial tests were very slow. 62 mph. We thought we had a lemon and thru Hydro Tec we got in contact with the manager in Japan who oversaw the building of the70CES motors. He provided us with all the manufacturing specs for the motor. That is when we found out, it only developed 70 hp and that was at 6400 rpm. We tested our motor on the dyno. It was dead on. Not one hp lower or 1 hp higher. We never ran a race in 1997 because we couldn't get any speed out of the boat. In 1998 we raced it. The competition objected. The motor got approved and we won several races with a 65 mph boat. We were not fast, they were just slow. At the Nationals we got beat very easy by Steve Wrong's 49 OMC. After that I decided to work on just getting the lead off the dock and see if the the competition could catch and pass us. 67 mph was all the top end I could get. The boat is still set up the same way. The T850 kilo record is 68 mph. We must be as good as it gets. In the later years we were good on the short courses like Sudbury where the Nationals were run. On the longer courses Craig Ferrier could run us down with his OMC 56'r. Same is true to-day the 56r has the edge, but maybe Gigs cylinder head will help. Maybe try some port work.

Phil. a 60C Yamaha is a commercial version sold in North America. Same motor as your 70B with no oil injection. It sold for 1/3 less from the dealer.

Thanks. The old T850 UIM kilo record was 76.52mph, how did they do it ?

Thats the 1995 one if my memory is correct. I have no idea. The fellows in England are the ones at 68 mph.

Up to the mid-90s 'Mono hulls' had a more open definition than the current rules. Most if not all of the straight line records were set with "wing" mono hulls. You can google some pics. Some look like Gt40s and some are pretty wild (likely specialty built kilo hulls ... I hope ... because I would not turn one). UIM has frozen all those records. The new rule set can be challenged. The boats run under that set are conventional monos.

Nice to see Jim post on the site for the first time. At his birthday dinner last night I made 2 suggestions. One was getting a Thunder Cat and run one of our SST60's on it. Jim and Dave would be the crew. I thought it would add a boat to the class. That idea didn't go over well. Something like...You get one of them and we'll strap you in, lock the throttle wide open and aim it across the bay!. My second idea to replace the Suburban with a Police prepared Tahoe was better received. Just like the Blues brothers Cop brakes, Cop tires etc...
Sorry to go off topic! I am just being a DAD.

if they want to start off slow i have a Mini Cat in stock with a 25hp lol

"That idea didn't go over well???????... please elaborate! I have raced both T and TC and I can tell you:
- TC is just as fast as T
- the TC ride is way more stable
- a win is way more fun, as you can high five with someone while you are still on the water
- you can take people for rides
- water conditions do not matter, the rougher the better.

Funny cuz it's true! lol.

I'll add one... left and right hand corner rock! haha.