Reed Block Spacers

[mainline]

the other week I was looking at an old MXA (the issue with Hannah doing his Superman impersonation on the cover) and they had an article on some White Bros mods for the 79 YZ's, part of it was to do with the rear shock, the other was a reed spacer.

Yesterday I was talking to Trev (worms) at the QVMX practice day and he recommended pretty much the same reed spacer mod.

Now I've done a search for Reed spacers, and the great lord humongous Lozza mentioned in the post I found, that reed spacers are more aimed at top end power, while the MXA/White Bros Article were talking about torque gains.

So firstly, do the spacers effect different engines configurations in different ways? The '79 YZ125 was apparently rather peaky anyway so I don't want to add to my woes.

Secondly, does the spacer always go on the cylinder side of the reed block?

Turdly, does this mod effect my already not-quite-right jetting?

and lastly, just out of interests sake, what are the technical aspects of this mod, ie what is it doing that makes the change to power delivery

thanks ever so much

 

[oldyzman]

Hey Mainline, I have heard that it can give a better midrange, Do you currently have YZ250F? As I would like to know waht jetting you are running and pipe etc. I have just done a ground up rebuild on one... First race was a disaster
OLDYZMAN

[Lozza]

great lord humongous Lozza

Take a look at where the spacer goes in the article and get back to me, before the veins in my head start pulsing

[mainline]

I had another squiz at the article and the only info they give you is that it costs $12.95 , no pics unfortunately.

5mm was the width of the spacer that Trev was using on his 250.

BTW, the humungous thing was because I keep remembering Doc's pic of you and stu in the pits with the tm400 where you're referred to as Lozza the Giant and Stu the Brave

[oldmxracer]

I used aa Moose spacer on a 92 YZ125. Just moves the power down in the rpm range while you lose some rpm off the top. Makes it FEEL like a gain in midrange/bottom end. I liked it.

[Lozza]

Then the spacer goes between the manifold and the reed cage. Some aluminium plate would be enough to experiment with.

[mainline]

fair enough, I'l pick up an offcut tomorrow and make something up. Just for interests sake though, what is it that this mod is doing that changes the power characteristics?

Thanks for your help

[Lozza]

Your actually tuning the inlet tract length and moving it into phase when you want it to, at best you will be looking at a 1% gain.You maybe in fact lessening the good work the stuffers on the manifold do(if you have them).

[Freakshow]

IIRC we used to want to get the reeds as close to the cylinder as possible with the moded barrels, then again it was a 350 Rd cage into a 125 , but i do recall the go was to get it in there as close as we could.

question .....Wouldnt just adding a block create a dead space behind the petal stops and make a still air void as your not maintaining the same tapered profile ?  id save your time unless its a known fault of that model or barell and a proven fix for that specific problem.

[Lozza]

A lot depends on the crank case volume, I have used large spacers on Karts and Rgv's and raised the on-pipe rpm.

[Freakshow]

more horses or just rpm ?

[ChrisACT]

Here's how my feeble brain interprets it all.

If you space the reed block away from the cylinder, you have essentially increased your crankcase volume.  This lowers the crankcase compression ratio thereby reducing the loss in horsepower created by the pressure in the crankcase acting against the bottom of the piston on the downstroke.  BUT .... it also slightly reduces the positive transfer port velocity which means you get a slightly more diluted combustion mixture seeing as it won't necessarily scavenge the chamber as well at certain RPM.

So, in a nutshell, it should allow the engine to rev a little more freely, but the combustion chamber gets a slightly more diluted charge.  So, overall cylinder pressures go down (reducing torque and power) but powerband broadens making it a little friendlier to ride.

Is that kinda on the right track?

[Lozza]

Miles off.
A common fallacy is that the decending piston creates 'pressure' in the crankcase. It's dwarfed by the pressure above the ring.
Case volume is a bit like a trumpet players cheeks, larger volumes have more for the pipe to 'draw' on.
Both Freaky, the Kart rules are very strict so sneaky stuff must be employed, we needed more punch off turns, got that but it lost 500rpm off the top, add spacer we had our low end punch and restored the top end rpm.

[Freakshow]

Obviously not the rotax as they got that class locked down. you working on unlimited karts

[Lozza]

Yep was on a Rotax and is completely legal.