Quick Power For Yamaha's Rd
QUICK POWER FOR YAMAHA'S RD
DG Heads and Expansion Chambers Make a Good Pocket Rocket Better
Len Vucci
Some combinations are impossible to resist. Yamaha’s RD350 was probably the most successful pro-
duction road racer ever made. There are still thousands of them running around, ridden by owners who like the power, and who’d like more power still.
With this comes news that DG Performance Specialties, famous for motocross equipment, is expanding into the street market. First step is sets of gold-anodized cylinder heads for the RD350.
Fike the motocross heads, DG's RD heads feature redesigned combustion chambers. The squish band is closer to the piston's contours, and compression is elevated slightly over the stock item. Besides
producing a power increase, the radial head’s increased surface area w ill maintain cooler operating temperatures. When asked about a test of the heads, the guvs at DG said Sure, and how about some road-going expansion chambers as well?
It’s a natural. Nothing easier than bolting exterior improvements onto a twostroke. We decided to test an RD350 stock, with the DG heads, then with the DG expansion chambers and finallv w ith both.
DG also has road-racing experience and the street exhaust system is patterned after what’s worked on the track, plus muffling, of course. DG’s expansion chambers are designed to increase pow er at the top of the engine's rev range. The social acceptability of a works-like appearance frees the aftermarket pipe manufacturer from the constraints of a shortest - distance - between - two - points - in - chrome - onlv muffler system. Although a bit louder than stock, the trade-off is. we think, a worthw hile one.
THE TEST BIKE
The 1975 RD 350 we used forevaluation was basically stock, but had several modifications. Accel ignition amplifiers and coils had been installed (CYCFE WORED. September 1977). ensuring a hot spark throughout the rev range. Cafe-style low bars from Dick's Cvcle West brought the rider down out of the wind, and Number 1 Products and Mulholland/Interpart kept the front and rear ends, respectively, on the pavement. Internally the RD's engine was stock, and had logged nearly 8000 indicated miles.
BASELINE RUNS
Before we made any passes on the strip, the RD was tuned to stock specs: timing at 2.0mm BTDC. carburetors inspected and synchronized, oil pump settings checked, and a fresh pair of spark plugs installed. The first drag strip runs were right at 15 seconds flat, with a terminal speed of 86 mph. The times dropped as hole-shot technique was improved, with a best run of 14.82 sec. at 86.35 mph.
HEADS UP
Back in the pits a head transplant was undertaken. The DG heads were installed in about 10 minutes—love them twostrokes—using new copper gaskets. A set of B8EV plugs was installed, their -Vinch reach meeting the new head requirements. Back on the strip, the RD was launched dow n the lane to a 14.66 (a 87.46 pass. The next run lit the clocks at 14.68 sec. with an identical trap speed. The DG heads had cut more than a tenth of a second off the ET. and added more than a mile an hour to the terminal speed. Several more runs were made, all w ithin a couple of hundredths of the first two.
The drag strip figures are somewhat misleading. Although the heads increase power across the rev range, the gains are more pronounced in the lower and mid ranges. The wide-open-throttle condition of drag strip runs measures only gains in the upper rev range. Seat-of-the-pants reaction indicated the biggest improvement was down low. where the bike felt more tractable and responsive. This tvpe of power increase would be most beneficial for the stop and go conditions of the street rider.
CHAMBER MUSIC
The radial heads were removed and the Stockers reinstalled. Four bolts and four nuts later, the chrome RD exhausts were placed in the rear of our van. They were soon joined bv the bike's centerstand, which must be removed in order that the new pipes be fitted.
Although the DG expansion chambers utilize the stock mounting locations, they are fastened to the cylinders in a different manner. Instead of using flanges to hold the pipes against the cylinders, a short nipple is bolted on instead. The pipes themselves then slide onto the nipples, where they are secured by exhaust springs.
To make the installation a bit neater, the wire foot tab on the sidestand should be hacksawed off, and the stand cleaned and painted.
Before starting the engine, we changed the main jets. The increased flow characteristics of expansion chambers w ill tend to make an engine run lean, so richer jets are good insurance. The test RD had — 105 jets as stock, so we installed a set of # 1 10s. One kick fired the engine, which nowproduced a racy, though not extremely loud exhaust note.
The first run w as declared a practice run. because the extra power came on with such force that the front wheel went skyward and the pilot shut oft'. Next time, the RD was back in business, with 13.91 sec. at 92.30 mph. Zowie. The pipes cut 0.9 sec. oft' the ET and added 6 mph to the trap speed.
The new exhaust also added to the engine’s rev range. The stock RD stops making useful torque around 8500. so there's no point in revving the engine beyond that. The RD wants to rev with the DG pipes, though, and we began shifting at 10.000 rpm.
(The stock engine is strong enough to handle 10 thou in short bursts. Sustained running at these speeds is another matter. Road-race fans would be well advised to give their little ring-dingers some additional bottom end work, or risk a case of the basket.)
HEADS BACK
After the way the DG pipes yielded such a rush, literally, we were anxious to try the heads and pipes together. Anticipation turned into anxiety as both ET and speeds were oft', typically 14.20s at 90 and change. The mid-range seemed a bit weak. and the engine just didn't seem right on top. No four-stroking or detonation, but the crispness was no longer there.
We twiddled and tweaked, changing needle position, main jet size and timing several times. The best we netted was a 13.90-sec. run. the quickest yet. but not nearly what we thought it should have been.
Pooling our collective tuning experiences, we retraced our steps but could think of no good reason for the disappointing performance.
When all else fails, start at the beginning and do it over. We did and what we found caught us red-faced: One of the carburetor cable adjusters had come loose, and its throttle slide was not opening completely. The carbs were again synchronized, w hereupon the throttle response throughout the rev range was restored.
A couple more runs brought the best figures of the day: 13.77 sec. (a 93.26 mph. (We were back to it 1 10 jets, needles were in the #4 position, one notch richer than stock, and timing was set at 2.2mm BTDC.)
The bike feels transformed. Some lowend was lost with the expansion chambers, but most of the loss was retrieved by the
new heads. The engine pulls unbelievably strongly above 8 thou, and feels great all the way to 10.000 rpm shift points. Around-town motoring is more enjoyable, as the bike is generally more willing and able to perform. A certain degree of satisfaction is derived from the attention the little scratcher receives. Parking it in a shopping center guarantees a group of spectators and a barrage of questions about performance and availability. The real answers come w hen it's fired up and wheelied away.
For this simple bolt-on the results were excellent. We have no doubt that with a little bit of port work and detailing, there's more power to be had in the little ringdinger.
ADDENDUM
The expansion chambers we tested were pre-production prototypes. Production units will differ in two areas: Pipe routing will differ slightly to provide more ground clearance when cornering, and the silencers will be larger and more effective. Power output is comparable, as the quieter pipes produce more on the bottom end with little sacrifice at high revs, and are better suited for street riding.
DG is producing heads and pipes for the RD350 and the RD400. The kits won't interchange between models but the prices are the same for each: $99.50 per pair for the heads, $134.50 per pair for the pipes.
If not available locally, contact DG Performance Specialties. Dept. CW. 1 170 Van Horne. Anaheim, Calif. 92806.
