port timing

[novaracr70]

ok my brother put a +4 crank in his 86 250r, I know he has to use a 5 mil spacer, and have the head machined for the stroke. But will it screw up his port timing?

[wilkin250r]

Unfortunately, yes.

A stroker crank isn't really a "bolt-on" modification. It changes all your port timing, beyond just adding duration to your exhaust, it also changes the relationship between your exhaust and your transfers. All these things are dependent upon the piston moving up and down, and any time you change that motion (like a stroker crank), you change those relationships.

As you probably already know, your port timing is REALLY critical in determining your power characteristics, which in turn trickle down the line and influence your pipe effect. A stroker crank can certainly be effectively used, but the ENTIRE engine must be designed with that stroker in mind, most notably the port timing. It's not something you can just throw in with no other modifications.

[novaracr70]

Thats what i thought , well i guess he will have to have it re-ported if possible .

[zedicus00]

not a perfect solution but ive noticed if you have a top end ported jug and through a stroker crank in it and dont change any porting it becomes a somewhat midrange setup based on the new port timings... so you could work around that and have an o.k. running bike....

least i think thas how it worked? maybe neil will chime in on this one?

[wilkin250r]

Originally posted by novaracr70
Thats what i thought , well i guess he will have to have it re-ported if possible .

Even that may not work, depending what type of port work he has had done. Most port jobs change many different aspects, including the exhaust/transfer port relationships. Those relationships change even further when you add a stroker crank. You may not be able to get them back where they need to be.

You might be able to use the current cylinder, if it wasn't altered too much. But it's possible you may need to start with an un-ported cylinder to alter it to work with the stroker correctly to full potential.

[novaracr70]

well we sent his motor and cylinder in. They told him that the port timing was stock and whoever ported it just cleaned up the ports,