Service

SERVICE

JOE MINTON

No mas compression

I own a 1985 Kawasaki 750 Turbo, and I will soon be tearing it down to install helicoils in some stripped headbolt holes. I’m considering having the head milled while the engine is apart to increase the compression ratio, which currently is a modest 7.8:1.

How much can be milled off without requiring stronger valve springs, connecting rods, etc.? I’d like to keep the motor reliable and leave enough cylinder-head material to have it machined again should it ever warp.

Also, I’d like to know any performance data you can give me on this bike, such as its horsepower and its top speed. I know someone who drag races a 750 Turbo like mine, and he claims it has a top speed in excess of 150 mph.

Steven Kersting Akron, Ohio

I’m not sure exactly how fast your Kawasaki 750 Turbo will go, but I once watched one reach 149 mph as measured by a calibrated radar gun at an altitude of2500 feet. It probably would have cracked 150 mph fairly easily at sea level.

If you substantially raise your Turbo s compression ratio, the engine will be likely to suffer severe damage from the effects of detonation.

In itself an engine ’s compression ratio doesn’t mean all that much. Instead, the cranking pressure within the cylinder is what critical to per formance. An engine with a high com pression ratio but long-duration cam timing (the valves close very late in the compression stroke) may be less prone to detonation than one that has a lower compression ratio but milder (earlier-closing) cam timing.

What more, when everything else is equal (fuel octane rating, ignition timing, etc.), any given combustionchamber design has an upper limit to the cranking pressure it will tolerate without causing detonation. A Suzuki GS1150 combustion chamber, for ex ample, can tolerate about 195 psi of cranking pressure with good pump gas, whereas Harley-Davidson Big Twin Evolution motor will detonate at much over 170 psi.

Moreover, the compression ratio of a turbocharged or supercharged en gine is not directly comparable to that a normally aspirated motor be cause that ratio does not take into ac count the effects of the added charge forced into the combustion chamber by the blower. When your Kawasaki is on full boost, its effective cranking pressure might very well be as high as that developed by the same engine without the turbocharger but with an 11:1 or 12:1 compression ratio.

In other words, don t mess with your Turbo compression. You wouldn `t believe how ugly the inside of an engine can get when destroyed by severe detonation. Kawasaki `s en gineers built the 750 Turbo to outrun the other turbocharged bikes of its time. If they `d thought they could get away with higher cranking pressures, they would have done so.

Slippery business

I recently bought a l9~ Yamana FJ1 100 that has 9500 miles on it and seems in very good shape. The previ ous owner claims he used an aviation oil for a while, then switched to some thing else-possibly a Bel-Ray synthet ic blend, although he isn't quite sure. Is his use of different oils something I should be concerned about? What oil do you recommend? I live in Wiscon sin, where temperatures vary from the 40s at the start of the riding season to near 100 in the middle of summer.

Gregory Halverson Eau Claire, Wisconsin

I can `t tell you whether or not to worry about the previous owner use of different oils, because even you apparently don `t know exactly what those oils were or how often he changed them. So, unless you `re will ing to disassemble the engine and in spect it for oil-related pro blems, you don `t have much choice other than not to worry unless a problem arises.

I do, however, recommend that you use a premium synthetic oil. FJJJOO engines tend to run very hot and can benefit from the excellent temperature protection provided by synthetics.

.,uôtutrie~ rti~,i tefnpet istut c~ cit one of the most common causes of ac celerated wear and outright failure in motorcycle engines. But not every bike has a temperature problem ; I have yet to see a liquid-cooled motor cycle engine that had been damaged as a result of overheated oil. The pri mary reason Honda Gold Wings com monly last more than 100,000 miles without a rebuild is the excellent tem perature control provided by the liq uid-cooling system

Many air-cooled engines also run consistently cool. Evolution-series Harley motors rarely get their oil over 210 degrees, and although I've had scores of them apart, I've never seen one exhibit the tell-tale signs of overheated oil. BMW Boxer Twins of all sizes also maintain comparatively low oil temperatures.

On the other hand, most air-cooled Japanese Fours tend to overheat their oil. The American Petroleum Institute says that the operating temperature of petroleum oil should be kept between 180 and 210 degrees Fahrenheit. But Suzuki ilOOs of the early `80s,forexample, would routinely generate oil temperatures in excess of3lO de grees. Late`70s and early`80s Honda CB75OF Fours-the last dohc, air-cooled models-would heat their oil supplies up over 260 degrees just running down the interstates. I've never measured oil temperatures in an FJ1JOO, but I've seen the insides ofafew that have shown the unmis takable signs of oil-film failure due to excessive temperatures.

There are lots of synthetic oils on the market, but not all of them offer substantially better protection than the best petroleum oils. I've had ex tensive personal experience with Red Line Synthetic oil, so I can high/v recommend it. I also believe that the current version of Mobil 1 would be an excellent choice for your FJ