Here is a question for all the hardcore drag (sand)racers out there. I have a 86' 250R 4 trax. Now I have all the usual mods. porting, pipe, carb, reeds, etc. Now I want to change the port timing in my 86' short rod engine. How much would it change the port timing if I used the spacer plate and piston from an 87' to 89' long rod engine? Is this even possible to do? I do know that there is an adjustable timing plate for this engine but I wanted to try this to see what happens. Has any one even tried this? See ya!

 

Hmmm. You run into some interesting problems from the get-go. First and most easily addressed is that by effectively moving the cylinder up, let's say 5mm(which is the standard spacer for the long rod-'86 piston combo.), you are gaining 5mm more deck height, easily fixed by decking the cylinder back down 5mm to get the compression ratio right. But...you'll most likely create a hybrid cylinder that can't be used on anything else.

Next is by doing the above, you lessen the power stroke. The distance from TDC to the top of the exh has gotten 5mm shorter, the duration of the power stroke becomes drastically shorter. Less power stroke, less power created at any RPM.

I'm sitting here and thinking while I'm trying to type, so we have to jump back a little too. The difference in the rod length to the piston pin location will be 5mm, so you will have to deal with that also. Hmmm...So with that, from the get-go the piston will have a 5mm lower deck height, add a spacer and you create even more negative effect on the deck heigth and you really start to have to whack a huge chunk off of the cylinder to get the compression ratio back. The volume above the piston at TDC has become extremely large.

So...I won't work approaching it that way. However. You can add thickness, without really sitting down and doing the calculations, I'd say ballpark at least 1mm, maybe 2 or 3. Most likely only 2.

Herein lies another problem. If all you do is increase the port timings, you create a super-hi RPM powerband. At that these now higher RPM's where everything starts to jive, you've done nothing to address the port time area, that is, how long during the duration of the stroke that a particular port window is open and the area of it and how much it is able to flow at any given RPM. Mid range biased motors can be tuned that way by increasing the port time area, by widening the port windows, without increasing thier port duration(inc. in timing).

You can address this problem somewhat by grinding the port windows down. But the piston at BDC is going to have such little support it will eat pistons SUPER fast, if not get all funky and break something. There also lies the problem that you can only grind the exh. port down so far before creating a nice skylite. I guess you could weld it, but....

Increased port timing(durations) isn't always the key to increased power. RPM is a function of power, but you reach limitations of internal components and intake and exh. considerations. It would have to be a super-hybrid motor, and the end result in my opinion...isn't going to get you ahead.

I think that for all the hassle of trying to figure it out, I don't think that you'll be anymore ahead than if you built a larger displacement motor, either with a Pro-X cylinder or a like a Packard Pilot hybrid. Especially something on alky, with an overrider tranny, and drag pipe. You may get near the same power numbers, maybe, but bring a wheelbarrow load of cash. It will take alot of trips back to the bench to re-cut to maybe finally find the right combination that it will like.

I admire your desire to get faster with innovation, which takes ingenuity and creativity, but there are very proven ways of getting where(HP) you need to go.

 

Why don't you go to www.macdizzy.com/ and ask him. He knows a lot about 250R's. Or, let hime build your engine!
ERIC