Background: After a dismal finish at the April 25^th WERA races, the 526cc was torn down. The motor that had proven so reliable since it's fabrication in 1995 had over the 50 hours and 30 weekends of fun under it's valve cover, and now the engine was showing more than 15% leakdown in each cylinder.

Teardown revealed that three of the valve springs were broken, and mostly likely this lead to overheating and head gasket leakage that plagued me at the last race. The heads, cams and valves were to be reused but inspection showed that the cams, originally purchased in 1990, were scored and in need of replacement. New cams are being ordered but the existing ones will have to do for now.

Building a 554cc Motor: The new, replacement engine is now 4mm overbore for 554cc. This requires that new sleeves be used, and I purchased a set of stock ZX11 sleeves already installed in an EX500 block. CPE bored the cylinders to fit Wiseco 78mm pistons originally for a ZX11. New 'knife edge' rods were purchased from Falicon, and an undercut transmission was installed into the lower cases.

Fitting the block into the cases required a small amount of grinding on the lower cases, as the larger sleeves rubbed the high spots. In order to prevent leakage around the head gasket CPE installed an o-ring in the top of the sleeve and procured a solid copper head gasket. The gasket is .043" thick, so I originally installed the engine without a base gasket, hoping to make up for the extra thick head gasket.

However, when I clayed the pistons there was insufficient piston-valve clearance. If you've never built a superbike motor, claying is a process to check for head clearances. You put modeling clay atop the piston, assemble the engine, spin it by hand a few times, and then pull the head off. The valves will indent the clay and let you measure the running clearance. With my first setup I had only .03-.035" of clearance, which is too close for comfort. So final assembly was made with a base gasket installed to get closer to my goal of .04" piston-valve clearance.

But-wait-there's-more. The clay also revealed that the squish area is too loose. This means that the compression ratio will be lower than my target, and power will suffer. The ideal solution would have been to cut the valve pockets in the piston a little deeper and mill the head so that I could get the compression ratio I want while still maintaining piston-valve clearance, but I didn't have time to do this. It would also have been best to measure the compression ratio, but I didn't have time to do this either. I did take a few hours to lap the valves into the seats. For instructions on measuring your compression ratio, see http://www.sport-twin.com/compres.html

The next issue was the valve springs. I've tried three different manufactures over the last 10 years, and I've finally concluded that with .410" lift cams you just can't expect more than 25-30 hours from the valve springs. I currently use springs supplied be EDCO, and heavy duty caps from R/D. Mike recommends an installed height of 1.32-1.33" for the springs, but try as I might I just didn't have the right combo of bases and spacers to achieve this. So I installed the springs about .01-.02" loose. The danger of this is the possibility of valve float (or bounce) but on the up side if this works the springs should last a bit longer.

I smiled as the engine sparked to life with the first push. I ran the motor several evenings before the first race to get some heat cycles through the new components, and on Saturday's first practice it seemed to run well. It's very difficult to ride around a track that you love without running to redline! By the end of the second practice the engine was ready to be fully flogged. Before the first race I re-torqued the head bolts back to 40 ft-lbs.

Race Results: It's definitely faster than the last motor was during its prime. I had to gear up 3% (about 5mph) to keep the engine from over-revving, and I had no problem drafting past all but one of the 750cc Ducatis. Lower down the tach the motor seemed rough, and I tried to keep the revs between 8.5 and 11 grand for good drive.

Dyno Results: Special thanks to Fast Lane Cycles for more quality dyno time. They really know what they're doing. The old 526cc motor was dyno'd at 68hp after significant development work. Since then the dyno had the computer replaced and appears to run about 3hp less than it did before. I've never believed that DynoJet dynos give good numbers anyway, just a real good comparison of changes made when using the same dyno to compare before/after. That said, this particular dyno showed 68hp for the new 554cc motor. I would guess that the true number is around 71-72hp at the rear wheel, but we can't calibrate the dyno. This same dyno does report a stock F3 as about 80hp, and about 90hp with a pipe and jetting.

We played with the jetting on the EX and found that if the midrange was correct then the top end was a bit lean. With the Lectron carbs you only get one ajustment for all throttle control, although I could add powerjets to the carbs to boost top end fuel delivery.

Future Plans: The cams need to be replaced, and this time around I will use .410" intake but will opt for the .375" exhaust cam. This should reduce valve spring costs as the exhaust could then use stock springs shimmed up to 45# of seat pressure. The bigger cams also eat up rocker arms. If I ever build another head I'll try +2mm intake valves; +1mm are currently installed.

Compression needs to be measured, and probably could be increased. I will look into cutting the valve pockets deeper. I may also test the power delta of dropping the charging system, but then I'd have to go back to stock coils.

At the end of this season I should have 10-15 hours on the motor and I'll tear it down again for a complete inspection, and replace all the main bearings

Below are some calculations based on formula(s) gleaned from Gordon Jennings Two Stroke Tuning book & Sam Fleming's RRW&MT intake formula which he got from someone else. There are a number of assumptions made with this chart. Like for example that compression is actually 10.8. Also it is assumed that there is a 1 to 1 relationship between intake airflow & BMEP (Brake Mean Effective Pressure) as it relates to HP calculations. Of course there are not such a 1 to 1 relationship, but it makes interesting numbers to look at in any case. The 12.5m/s intake sweet spot is figured at one atmosphere. Add about 7% for each psi of airbox over pressure if ram air is used.

Please note this document is in a draft state. We are going to tweak it some more with real world measurements. We will also add similar tables on other motors for reference. Also, I'm not sure how to make good html tables, so if you have any ideas on how to make this look better let me know. Once done we will make the original *.xls file available. Thanks to racer557 (John) for providing the rod length center to center for the calculations.