+4 crank question

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ricar · #1 Post by **ricar** » Mon Jul 28, 2014 2:53 pm

I need to rebuild my engine this winter and was toying with the idea of a +4 crank and 115 rods and am looking for opinions. I am not exactly concerned with peak HP numbers, what I want is as wide a power band as I can reasonably get and a wee bit more fun out of the stock lump. I currently have Spec2 pipes on the bike and PWK 28s in a box waiting for the winter rebuild.

Would I need to run a base spacer plate to raise the jugs or could I just machine the domes deeper into the head?

How much of a difference would this be from stock?

T.RexRacing · #2 Post by **T.RexRacing** » Mon Jul 28, 2014 3:04 pm

I believe there are pistons to accommodate the +4 and 110 rods. If you're just looking for some grunt on a daily rider street bike that would probably be the ticket with stock porting.

L.B · #3 Post by **L.B** » Mon Jul 28, 2014 7:27 pm

64 thou base gasket & a wicked head to suit is what I'm running. 115 rods + 4mm....can't remember what weisco's I'm using? Can look it up if need be.
Guess you could get stock head machined & o ring'd at same time.

slinger · #4 Post by **slinger** » Mon Jul 28, 2014 7:33 pm

For what it,s worth IMO you should never use a spacer for quite a few reasons. Yeah sure a spacer will work ......people sell them ........hell taking the pipes of and watch the flames shoot out of the exhaust duct the engine still works.

It will just work a whole lot better with out a spacer!

2smoke · #5 Post by **2smoke** » Tue Jul 29, 2014 1:27 pm

Ricar:
I have the same questions......
What stroker options are there for stock cylinders and powervalves?
Street octane fuel. And what parts needed for each option.

ricar · #6 Post by **ricar** » Tue Jul 29, 2014 11:07 pm

I believe you can go +7 but need to trench the cases. With a +4 you can keep your carbs and pipes, not sure about the +7 needing different pipes though I'm sure it would run. I hope to use my PWK28s, I like the smaller carbs to keep some sort of bottom end and the bike is thirsty enough as it is. I'm not looking for a rocket personally, I'm looking for something reasonable. You can bore to I believe 66 without interfering with the power valves but my cylinders have not yet been bored so I'd hate to do that to them or I'd be all over it. You can get pistons with a lowered pin to run with 110 rods and no change. I wonder how the lowered pins would work with 115mm rods and head work...

So what I personally am looking at is...

1) +4 crank
2) Pistons with raised wrist pin
3) Head work

If I am not mistaken timing will not change, lots of guys run advanced timing, so I figure stock should be safe.

I have been told this will give a fatter mid range but no one seems to comment on what it would do to top end power.

cookie · #7 Post by **cookie** » Wed Jul 30, 2014 1:10 am

I run a +4 crank, 115mm rods, Wiseco 795 series pistons, head machined.

This kit:

http://www.ebay.ca/itm/STROKER-CRANKSHA ... d4&vxp=mtr" onclick="window.open(this.href);return false;

Has everything you need but comes with a thick head gasket to allow for the piston travel above deck height instead of machining the head.

seahorse · #8 Post by **seahorse** » Wed Jul 30, 2014 5:26 am

That is bloody cheap for a crank gasket set and pistons.
Did you get the crank measured on a v block before installation Cookie.
The reason I ask is that I saw a brand new out of the box crank recently, measured up with a dial guage she was running outside manufacturer spec.

slinger · #9 Post by **slinger** » Wed Jul 30, 2014 11:05 am

Using a +4mm stroke crank will give you more "angled timed area" to work with, and not raising the barrels gives you the ability to change the transfer roof angles. This will allow the "turner" to create a much better scavenging loop, you really don't want the "B" looping over the "A" transfer port as is the case with stock cylinders.

Your still limited to what the single exhaust port can offer, but your gain around 10% over a stock stroke at the same RPM. If you want to rev the thing to 11500k the gains are more.

Simples!!

ricar · #10 Post by **ricar** » Thu Jul 31, 2014 9:46 pm

slinger wrote:Using a +4mm stroke crank will give you more "angled timed area" to work with, and not raising the barrels gives you the ability to change the transfer roof angles. This will allow the "turner" to create a much better scavenging loop, you really don't want the "B" looping over the "A" transfer port as is the case with stock cylinders.

Your still limited to what the single exhaust port can offer, but your gain around 10% over a stock stroke at the same RPM. If you want to rev the thing to 11500k the gains are more.

Simples!!

Hey Glyn, how would the SpecII pipes work with a +4? I hear they like higher revs, but a longer crank will like to spin slower.

slinger · #11 Post by **slinger** » Fri Aug 01, 2014 9:22 am

I'm sorry I really can't give you a truthful answer as I have never used them.

tankslap · #12 Post by **tankslap** » Sun Aug 03, 2014 9:14 am

Hey Gyln, how come you don't like the spacer route?

The Athena top end I did for a RS 421 hybrid was a better bike all round with
the cylinder bases machined to allow the barrels to be lowered.
It is runnig the Wossners with the higher gudgeon pin height (Blaster pistons) and a bit of porting mainly in the auxiliary exhaust ports.

I wouldn't consider the +4 crank if the budget didn't cover porting as well. Just seems like a half arsed job otherwise.

slinger · #13 Post by **slinger** » Mon Aug 04, 2014 10:42 am

tankslap wrote:Hey Gyln, how come you don't like the spacer route?

The Athena top end I did for a RS 421 hybrid was a better bike all round with
the cylinder bases machined to allow the barrels to be lowered.
It is runnig the Wossners with the higher gudgeon pin height (Blaster pistons) and a bit of porting mainly in the auxiliary exhaust ports.

I wouldn't consider the +4 crank if the budget didn't cover porting as well. Just seems like a half arsed job otherwise.

For the reasons I posted above^^^^^ Athena are different as the timings are so much lower than the RZ. Exhaust duration's don't dictate what RPM range the engine should run at.

ststrider · #14 Post by **ststrider** » Sat Aug 30, 2014 9:09 am

regarding the spacer vs no spacer question i have finally got my +4 rz motor back together and dialed in using one of slingers o ringed heads and standard base gaskets.previously i had used rogers spacer and while it worked fine it a little peaky for me.using slingers setup the motor is much more willing to pull from as low as 5000 rpm and comes on strong at 6000 ,it pulls 10000 with ease but its past its peak.the old motor was weak below 7000rpm hit hard and pulled past 10500.good fun but the new motor is easier to live with as i ride this old girl a lot.hope this helps.

aaronmvrider · #15 Post by **aaronmvrider** » Sat Aug 30, 2014 10:32 am

i think the reference is for base plate spacers eg.... 5mm thick .... not gasket thickness.... and using different pistons

??????? how thick was Rogers "spacer"... or was it a gasket... what pistons did you run with the said spacer/gasket.....

i have built both.... with a spacer plate and encountered air leaks as having 4 mating surfaces is a pain to seal... and after discussion with slinger at the time decided to go with gasket

and now run my 4mm and 7mm no spacer... just gaskets...

RZ/RD 350 & Misc. 2-Stroke Tech BBS

+4 crank question

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