+4 crank on a mildly modified pv engine

[Yamavarna]

I am building a new bike and I am looking for some input on cranks, some may have seen my build aspirations in my SmartCarb post, for those who have not; mildly ported street engine with head mods, 28mm carbs, reeds (Boyeson or V-force),and Toomey pipes.
My motor guy was not enthusiastic about the hotrods crank but will accommodate me if that is what I decide to do, so those who have experience with both could you please help me out with the pros and cons. I’ve already heard that the power is a little flatter with no real hp gains from a member on my SmartCarb thread; which I think deserves to go in the pros column. I thought it would be a slam dunk but I just read through Ken’s build and he bought a brand new stock crank and V-Force reeds, my motor guy says the increased port durations may not work out favorably for my needs so I’m a little confused and I need some input on this subject, please

[ststrider]

I am on my second +4 crank now the first lasted 10 or 11 years and i would not go back to stock. even with unported cylinders the motor had more pull right off the bottom, and once i did some basic cleanup on the intake and transfers and port matched the cases to the cylinders the motor is a beast. fyi i am running spec 2 f1 pipes tdr reeds head cut and oringed and ignition advanced 2 degrees. also pwk 28 carbs were a solid upgrade.

[Yamavarna]

Hmmm... that’s what I was thinking (no brainer)? Why isn’t everyone doing this?

[MK]

Because nowadays people tend to discuss endlessly instead of real world garage work.

And because a +4 crank is not plug & play like a stock stroke would be.
You either need a base plate or you need to machine the head. The latter one would be my recommendation.
For optimal performance you'd like to match the lower edges of the transfer ports (which is good for around 1.5-2 hp in the sim).

[Yamavarna]

There must be a compatible piston with the wrist pin height -2mm?

[MK]

Nope.
Woessner has - 5 and - 7 Banshee pistons for long rod cranks which is the next technical issue (stock rod 110mm, long rod 115mm)

[Yamavarna]

So... if you run a +4 crank with a 115mm rod and a -7 piston would that not give you a zero deck height?

[MK]

That's why they sell them.
The catch is that there's only 4 sizes.
64, 65, 65.5 and 66mm.

Problem here is that in that case the relative port placement along the stroke is pretty high (piston BDC is 4 mm below lower port edges)

It'd be the same when using a 7mm plate under the cylinders (note that these "7 mm" include an additional gasket, so the sheet needs around 6.5mm thickness)

Both results in insanely high port timing and I could not recommend it.

That's why I prefer a head mod where the piston protrudes 2 mm into the head at TDC. This would require a 5mm baseplate or a +5 piston.
By this mod you "shift all ports 2mm down" and get the timing into the ballpark.
This is what I'm currently using.

[Yamavarna]

So it sounds like splitting the difference +2 lower and +2 out the top and the longer rod for better angularity is the way to go, then cut the head to compensate? My machine shop guy is very good with 2 strokes so I’ll see what he says, we could even run zero deck height and mill the bottom of the cylinder if he wants less than 2mm out the top. Lots of options and I can kind I see why it is not as popular as I would have expected, but the results so far have all been in favour of the +4 stroker.

[ststrider]

I should mention i am running weisco 795 pistons with the 115 long rods and a one piece 20 thou base gasket. I ran the 60 thou base gasket and the copper head gasket that roger from wicked atv sells and did not like the power delivery . The motor had little below 7 grand and then hit hard enough to spin the tire on cold pavement or pull unwanted wheelies in second gear.

[Yamavarna]

I should mention i am running weisco 795 pistons with the 115 long rods and a one piece 20 thou base gasket. I ran the 60 thou base gasket and the copper head gasket that roger from wicked atv sells and did not like the power delivery . The motor had little below 7 grand and then hit hard enough to spin the tire on cold pavement or pull unwanted wheelies in second gear.

Are the pistons +7 to give you zero deck height? And, have you checked or set your squish with either gasket setup? Because a lot of people are recommending 1.2mm squish and would like to know if less or more was giving you the light switch power delivery?

[ststrider]

the pistons protrude 2 mil but my head is cut to compensate. squish is .9mil most people run .9 to 1.1. the light switch power delivery was due to the 60 thou base gasket altering the port timings. with that setup i had to rev to 10500 or it would fall off the pipe when i would shift.the set i now run is strong off the bottom and peaks at 9000 with 700rpm of overrev. much nicer to ride and makes similar power on top. also i lowered the transfer ports to match up with the pistons at bdc .

[jumjum01]

Does anyone have dyno sheet on a +4mm stroker with relative stock bore and porting?

[jumjum01]

For those of you who has a +4mm stroker engine.
How does it feel compared to stock-ish bike power vice?
Are there any problems with reliability / overheating?
Better i every way?!, just order the damn +4mm crankshaft??

[edgefinder]

Yeah I had the same questions so I just ordered the dam +4 with 115 rods and need 795 pistons. More displacement, taller transfers and it brings the stroke closer to the bore size so it has to be better right?? And here I am figuring it all out

So how to locate cylinder to head to keep bore in head same as cylinder bore and piston not hit? 1 or 2 dowell sleeves each side?

Luke