As Big As It Will Go

Here is the ultimate displacement bump for the Virago series. Any larger on the bore would require welding up the cases and moving the studs outward and designing one's own cylinders. Any further elongation of the stroke would necessitate enlarging the crankcase internal space - welding material to the outside and losing ground clearance. It would also require very short pistons with clipped skirts to clear at BDC. And there is no simple fix for clearing the transmission gears.

So what we have is a 108mm bore and an 89mm stroke. The pistons will need ceramic crowns and treated skirts. They would be rather short. The conrods would need to be shorter as well. Shorter rods in combination with the longer stroke would have the pistons coming to a stop 21mm lower than stock at BDC. The rod/stroke ratio will be 1.66; what street/strip car guys consider optimal for high revs. The flyweights will need to be skimmed down to allow for longer sleeves to circumvent piston rock/slap.

At such a large displacement (just a hair under 100 cubic inches) in such a small package, it would be necessary to run plated bores, an oil cooler, and add water-cooling. It would be advisable to opt for the large valve heads. In any case eleven hundred heads only on any of our stroked setups. The breathing is far less restrictive than the 750/920/1000/TR1 heads.

Cost? You are looking at six thousand dollars and leaves it to you to enlarge the case spigots to accomodate the larger OD sleeves and to otherwise relieve the case crank "chamber." Due to variations between the various models, you may have to do a little fiddling to hang the ratiators.

Early inspection shows us that 108mm might represent too big of a bore, and 89mm too long of a stroke. The spindle that the cam gear/sprocket rotates on appears to be lubricated by oil scraped off the cylinder walls by the rings on the down stroke. A very large and/or long sleeve would close off the bore that houses this piece. It appears that a hole would need to be bored through the side of the lower sleeve and champfered to oil this critical part. This is not a unique idea. It would necessarily make each cylinder either a front or rear - unless a superfluous hole were put on the other side as well. And because the pistons pull partway out of the bottom of the sleeve at BDC, needed longer sleeves for strokers would come closer to slimmed down flyweights and this oil hole would be essential.

It appears that the low pressure transmission oil feed comes quite close to the pivot bore. A narrow, carefullly-executed drilling may do the trick. This disturbs me on a couple of counts: I have to trust people who's ability and attention to detail I cannot assess. And I need to develop some rather involved instructions. More as this develops. And a hold on this bore and this stroke.

I assembled a crank with rods and pistons to a case today to see what would work. A 106mm bore would work with an 84mm stroke. Maybe. I sent my design thoughts to Falicon to see if they thought that they would work. I run it backward and forward in my head and turn the crank to check clearances. The only machine work necessary would be the reaming of the case spigots to acompany the larger sleeves. The sleeves would need to come lower down and closer to the flyweights. Stock the pistons pull half way out of the sleeves at BDC. This causes piston slap and scoring on the thrust faces. We want to reduce and possibly eliminate this. At BDC the skirts would barely clear the flyweights. One of the front rod nuts comes very close to the back of the windage tray. Some filing may be necessary. Then again, we may get around this with the rod style we utilize. I am dithering between aluminum and a style where the bolts thread directly into the cap. No nuts.

How about 108mm bore by 89mm stroke? Sleeves large enough to guide these pistons would need to come very close together. Especially with a longer stroke. The flyweights will need to be reduced in diameter. Tungsten (or other heavy metal) plugs will be needed to restore balance. The windage tray will need to be dressed with a rotary grinder. Very carefully as a major oil passage moves from one case half to the other in the bottom.

Stroker kits will only be offered in 106mm by 84mm; balanced and complete. If I decide that it is possible, the same will be true of the 108mm by 89mm kits.

For some further analysis, see here.