8 petal Polaris reed intakes

[ja-moo]

Just wondering why you are using the lift plates? Any reason besides less transfer work?

[Kris Bernstein]

You can only raise the ports so high before you go into the water jacket and ruin the barrel....and start over again. With the spacer plate you gain 5mm of port height off the bat and also the advantage of facing the top of the barrel so piston height can be spot on and gasket mating surface can be flawless.

Short answer is so you don't hit water getting to full race spec porting.
If you are just going to do a mild porting job the plate is not necessary.

Kris

[ja-moo]

I figured there was another reason. Just trying to catch-up on how you guys work over a 750, and why stuff is done the way it is.

Thanks!

[Kris Bernstein]

Share with me, please. What was the other reason(s) you were thinking?
Always on the lookout for different ways to skins cats and dogs....

Kris

[Suzukidave]

John , as Kris said yhe big reason is to not cut into the water jacket while changeing to race spec porting . Here is a cross section of the cylinder showing how close the water jacket is .

[tz375]

Dave,

That's an interesting cross section. The exhaust side looks right but the intake on the two blocks I had machined were not quite like that. Mine didn't have that bulge in the water jacket, so I broke through trying to get the reed blocks in really tight because I figured those huge reeds would need a strong signal to flow properly.

BTW, I'm starting to wonder if your smaller 6 petal reeds might not be a better bet than my Polaris 8 petals. On the sled with crankcase reeds, the outlet side of the reed cage is huge of course, but on a barrel mounted reed, the intake necks back down to a very small area. With the vertical mount and 4 additional ports (like your first barrel), it might work out better but I need to rethink my set up - again.

[ja-moo]

Share with me, please. What was the other reason(s) you were thinking?
Always on the lookout for different ways to skins cats and dogs....

Kris

Trick of the "word". Usually the lift plates are to reduce transfer work, but then you have more work to lower the port bottoms. I'm sure you guys know the exhaust works best with the piston (edited out "not") covering the bottom portion. (as do the transfer in some cases)

And in H2 testing, a lift kit alone didn't do much of anything without window work.

But to the question, the "other" reason was the fact you will break into the water jackets, I didn't know that.

And a point a lot of guys don't know, the transfer port/s that open first, are the last to flow. So when adding boost ports off the intake, the boost should open after the transfers to start the cleaning "loop" in the back of the cylinder first.

[Suzukidave]

And a point a lot of guys don't know, the transfer port/s that open first, are the last to flow. So when adding boost ports off the intake, the boost should open after the transfers to start the cleaning "loop" in the back of the cylinder first.

I didnt know that , i setup my boost port to open at the same time as the trandfers .

[Suzukidave]

Dave,

That's an interesting cross section. The exhaust side looks right but the intake on the two blocks I had machined were not quite like that.

I didnt even see that Richard and your right , this must be a very early barrel like pre 72 ? but its the one shown in the factory engine manual .

[water cooled]

I'm going to start with the caviat that I am at the begining of the learning curve so if I mis-state something, please recognize that I am trying to piece this together myself...and I am open to assistance in understanding the dynamics correctly. That said...

I had thought that the lift plate raised the Exhaust and Transfer ports higher in the piston stroke which allowed the ports to remain open slightly longer due to the decelleration of the piston (independent from engine rpms) as it approaches TDC (same on the down-stroke) and that lowering the intake floors (in part) is to compensate for the raising of the block to get the intake port timing back. (I understand that by opening intakes, youre also trying to maximize intake volume) My understanding is that "time" is not the only key factor but the area of the port in combination with the time is what allows a specific volume at any given point to move through the port. The lift plate also increases the (lower) transfer window area and volume to get the mixture going through the transfers at a lower initial pressure...

Lifting the block alone may have some benefit on a GT750 engine since it was ported so mildly for touring. That may be a difference between the H2's (more aggessively ported) and GT's. So some gains might be had by lifting the block of a GT while still keeping the port-time-areas within narrowly defined guideline for exhaust, tranfer and intake but again, it is only part of the equation and should be considered as part of the correct combination of port area, time and cylinder volume.

[tz375]

Kevin,
you are right that raising the barrel raises teh tops of teh ports and therefore the timing, but the real advantage is the increase in time-area, It's really the extra area. Stock ports floors are usually below the piston crown at BDC, so slipping in a thin spacer is Ok without touching the transfer and exhaust floors.

With say a 4mm spacer plate the floors of both have to be lowered to gain the area.

The intake floor also has to be lowered to maintain the timing. On a stock MAB model there's so much intake timing that it's less necessary than the early blocks.

In fact the exhaust on a GT750 can be raised to around 36mm down (4mm) more than stock MAB without breaking through, but for a race motor it needs to get up to more like 32-33 and that needs a spacer plate.

JA-Moo,
I hadn't heard that comment about the opening sequence of transfer ports. How can the one that opend first start to flow last? That appears to defy logic, so something else must be happening there.

[Kris Bernstein]

"Lifting the block alone may have some benefit on a GT750 engine since it was ported so mildly for touring..."

Correct....about 35-40 HP.....in combination with good exhaust system, carburation and finish porting...

Kris

[Suzukidave]

John , if someone were to send you a GT750 block would you care to take a look at it to see what input you might have or you could come up with to improve its performance ?

[Suzukidave]

Dave,
BTW, I'm starting to wonder if your smaller 6 petal reeds might not be a better bet than my Polaris 8 petals. On the sled with crankcase reeds, the outlet side of the reed cage is huge of course, but on a barrel mounted reed, the intake necks back down to a very small area. With the vertical mount and 4 additional ports (like your first barrel), it might work out better but I need to rethink my set up - again.

Richard , now that i cut up the one block for the Polaris reed block i am also thinking that even in the vertical position the 6 petal Arctic Cat reeds & blocks would be the better choice .

[ja-moo]

JA-Moo,
I hadn't heard that comment about the opening sequence of transfer ports. How can the one that opend first start to flow last? That appears to defy logic, so something else must be happening there.

You just have to look at all the systems working at that point. When the transfer are about to open, the crankcase pressure is still pretty low. The exhaust extraction cycle is almost over, but the pressure is still higher than the crankcase pressure at that point in the stroke.

So the first ports to open, actually open into the pressure of the end of the combustion cycle, and the new mixture is actually pushed back into those transfers.