A Trio of 660’s

[GoBigBlue1984]

I'll sacrifice a little exhaust so I don't end up with a ton of intake and raising the trans a mile

A wise man once talked to me about induction stroke. By raising the transfers we are essentially losing power. I had never thought about it like that but he is right. However, are we negating the effects of shortening the induction stroke by making other changes to the engine? Let’s discuss shall we?

 

[lwhaples]

Good stuff. Glad you shared your work.

 

[smokey7]

I think i need to be led a little closer to the watering hole on this one. Someone please break it down for me a little bit more. Thank you abd very nice thread so far.

 

[Deets066]

A wise man once talked to me about induction stroke. By raising the transfers we are essentially losing power. I had never thought about it like that but he is right. However, are we negating the effects of shortening the induction stroke by making other changes to the engine? Let’s discuss shall we?

I really look at flow and velocity, raising the trans a lot makes a different flow pattern and most definitely reduces that crisp entry when they open

 

[GoBigBlue1984]

Ever notice the uppers shape and size on a lot of the newer quads? That tells me the engineers know exactly where they want the charge directed initially and secondarily. The transfers are small and tight and they travel a long way to get there.

Here’s another question I’ve always had. It may be a simple answer and I’ve just missed it. Why only open roughly half or in some instances more than half of the upper transfer at BDC? Do you not get wasted charge that travels along the tunnel and smacks into the top of the piston? Or is pressure so low at that point that it’s negligible?

 

[Deets066]

Ever notice the uppers shape and size on a lot of the newer quads? That tells me the engineers know exactly where they want the charge directed initially and secondarily. The transfers are small and tight and they travel a long way to get there.

Here’s another question I’ve always had. It may be a simple answer and I’ve just missed it. Why only open roughly half or in some instances more than half of the upper transfer at BDC? Do you not get wasted charge that travels along the tunnel and smacks into the top of the piston? Or is pressure so low at that point that it’s negligible?

Like this? Not sure why they do that. If the tunnel tapered up to opening then we'd have something.

BYW, TM has done this with JB

 

[GoBigBlue1984]

Yes sir Deeter. Just like that.

 

[drf256]

These are timed roughly the same in a Stihl 460 cylinder.

Which one traps more volume under it at intake floor closing?

Will that make any difference when the difference in volume will be the same at int close - volume at BDC?

The upper is open for longer under the piston lands on the single ring piston. So there’s more time for the tunnel to fill/mix through the upper and lower during induction cycle.

Does any of it matter?

 

[GoBigBlue1984]

View attachment 113236

These are timed roughly the same in a Stihl 460 cylinder.

Which one traps more volume under it at intake floor closing?

Will that make any difference when the difference in volume will be the same at int close - volume at BDC?

The upper is open for longer under the piston lands on the single ring piston. So there’s more time for the tunnel to fill/mix through the upper and lower during induction cycle.

Does any of it matter?

If they are timed roughly the same, then which one can trap more volume once the intake floor closes is irrelevant IMO. In that regard the 272 piston (I’m assuming that’s what it is) would make less actual case compression due to the greater volume under the rings.
Now we’re back to velocity/volume conundrum. If I’m off Doc please school me, you’ve got more smarts than me. Im here to learn

 

[dustinwilt68]

Good stuff here, even though you are all making my head hurt a little, learning and that is what it's all about.

 

[drf256]

If they are timed roughly the same, then which one can trap more volume once the intake floor closes is irrelevant IMO. In that regard the 272 piston (I’m assuming that’s what it is) would make less actual case compression due to the greater volume under the rings.
Now we’re back to velocity/volume conundrum. If I’m off Doc please school me, you’ve got more smarts than me. Im here to learn

I’m in no position to school anyone.

Yup, the 272 piston adds case volume. So less pressure, but more volume in case. Total case volume matters, even though the change in case volume between TDC and BDC remains unchanged.

The 272 piston is going in a 460 I’m building for low end grunt. I’ll see how it turns out.

 

[dustinwilt68]

Great info Doc

 

[GoBigBlue1984]

Degree’d the other two saws last night. Both jugs fairly close. 95/122-124/78 and 95/123-125/79.

Squish .030. Tighten that up to .020. The first one I’m going to cut a .035 pop up. Picture of that shortly. Taking .045 off the base and raising transfers to 120.

Going to epoxy the intake after machine work. Stay tuned.

 

[dustinwilt68]

 

[GoBigBlue1984]

I’m going to try less intake duration than I normally would. Start out at 145 and work upwards. Should make for a well behaved, tune-able saw. Widen exhaust/intake and square the corners with some arc in the roof. Beveling well.

 

[NPKenny]

Going to epoxy the intake after machine work. Stay tuned.

Please show a picture of your prepped port and finished epoxy. Thank you!

 

[GoBigBlue1984]

You got it

 

[GoBigBlue1984]

Exhaust before....


Exhaust ready for chamfer


From the outside. Still some arc to the roof. I didn’t go as square as originally intended. The ring land has been weakened by the pop up. Some curvature should help tuck those suckers back in.


Intake

 

[GoBigBlue1984]

I’ll rough up the floor with 80 grit before applying epoxy. I’ll detail this later.

 

[GoBigBlue1984]