MillerModSaws Stares Down the 7310/7910

[MillerModSaws]

The transfers are idential. And i mean that by shape and size.

 

[MillerModSaws]

Here they are a bit different. The 7310 actually has a bit wider exhaust. By roughly .050-.060".

 

[MillerModSaws]

And the trans tunnels are also a bit different. The 7310 are larger at the base and funnel towards the upper trans. The 7910 has almost straight walls. However both of them are very close to the same size as they make the turn into the trans. I'm guessing this is due to the smaller bore of the 7310. I would say the way they are shaped they would flow almost identical amounts. But that said the 7310 does have lager lowers.

 

[MillerModSaws]

There's the initial comparison. Hit me with your thoughts boys.

 

[drf255]

Is it me, or does the 7310 have primary transfers that are wider towards the intake side? Could just be an illusion because of the contrast from cylinder color.

Question, why do you think the factory maintains that tiny narrow SB at the cylinder junction with a step up to the chamber? When we cut them we get it flat all the way to the combustion chamber, I'm sure that the factory could do the same.

Interested on where you take the transfers to.

 

[MillerModSaws]

The

Is it me, or does the 7310 have primary transfers that are wider towards the intake side? Could just be an illusion because of the contrast from cylinder color.

Question, why do you think the factory maintains that tiny narrow SB at the cylinder junction with a step up to the chamber? When we cut them we get it flat all the way to the combustion chamber, I'm sure that the factory could do the same.

Interested on where you take the transfers to.

Primary are just a tad wider but maybe .020. It's not much and that's eyeballing it. I don't have a real good tool to measure that. I need some 90° small calipers. Lol. The tiny ring I would guess but it would be wrong. Maybe someone smarter will jump in here.

 

[benp]

OP,

I am excited to see how this turns out. Keep it up!

Carl's got the 7910 hummin.... Did a saw of his last week that turned 13,2 in 26 inch pine with a 24 inch bar.....that's way over the 12,5 limited coil will let it do. The unlimited coil changed the personality of the saw......its no longer a reined back race horse, and it gives it, its head......boy does it run!!!!!!
The saw was running way rich when I got it. Mechanically everything was good other than the carb. It had a warn needle arm and diphram nipple causing the over rich condition. Got that fixed and the saw took to the unlimited coil. Dialed it in then modded the carb, going 30-40 on the jets and blocked the H bleed orfice to regsain Hi adjustability. Dialed in was was 14,4-14,6 unloaded and 13,2 in wood!!!! It was FUN!!

That's the range both of my 7900's are tuned in at. I've had the unlimited coils in both of them since I had them modded a couple years ago.

I ran the 28" today in red oak and I just giggled. Very speedy.

 

[Derf]

The

Primary are just a tad wider but maybe .020. It's not much and that's eyeballing it. I don't have a real good tool to measure that. I need some 90° small calipers. Lol. The tiny ring I would guess but it would be wrong. Maybe someone smarter will jump in here.

If you take a good picture and post it up here I can mark it up in Photoshop for you to see if it's larger.

 

[MillerModSaws]

Not much tonight boys. Been completing my apparel order. Lol.

 

[MillerModSaws]

Sorry to keep ya all waiting. Fresh off the lathr base and squish cut. Took .023 from the base and .020 from the suish band. And polished the squish band. The left cylinder is the 7910 I'm doing at the same time.

 

[MillerModSaws]

The exhaust takes quite a bit of time on these. My first step is to simply blend this lip in.

 

[MillerModSaws]

It looks something like this. Just blended. The bit hasn't even made it to the inside edge yet.

 

[mdavlee]

7910 cylinder is a lot more work on the exhaust port.

 

[MillerModSaws]

I should also head a warning not to touch the bottom of the exhaust port. This saw is right at free porting with .023 off the base.

 

[MillerModSaws]

And here's what the exhaust looks like roughed in. I still need to widen it a tad and raise it about 1/2 a degree. I mount the exhaust head shield up when I'm grinding so I can transfer the pattern to my muffler once done.

 

[MillerModSaws]

Here's the intake roughed in. I patterned this one after what I do one the 7910. I do not widen them other than cleaning then up. I did make the lower corners just a tad more square on this one as I'm gonna try and run the intake at 78°. That's where it's sitting right now. Then I also raise the intake on these in a dome fashion. So I raise it more in the center and dome it down. I'll raise it at least as much as I lowered it. The pic does not show it. Lol

 

[drf255]

Did you time it after the base and SB cut?

I have a lot of interest in this. Whenever I cut a jug like you did, it lowers the exhaust a few degrees but lowers the transfers much more. This is because of rod angle. So when you raise the transfers back to the "stock" numbers, you have a blowdown that's the same as stock, but a transfer that's much closer phupysically to the exhaust roof.

I think we are decreasing the blowdown much more than we think, and I think it unlocks a lot of the saws potential.

I just did a 262 jug. Took .039 off the SB and .016 off the base. .020 squish.

Without a drop of grinding on the ports, the exhaust went from 103 to 108, but the upper transfers moved from 118 to 128. So I went from 15* to 20* of BD without touching a port.

Sorry for the partial Hijack. I'm still trying to wrap my head around the physical port placement vs. degree thing.

 

[MillerModSaws]

Did you time it after the base and SB cut?

I have a lot of interest in this. Whenever I cut a jug like you did, it lowers the exhaust a few degrees but lowers the transfers much more. This is because of rod angle. So when you raise the transfers back to the "stock" numbers, you have a blowdown that's the same as stock, but a transfer that's much closer phupysically to the exhaust roof.

I think we are decreasing the blowdown much more than we think, and I think it unlocks a lot of the saws potential.

I just did a 262 jug. Took .039 off the SB and .016 off the base. .020 squish.

Without a drop of grinding on the ports, the exhaust went from 103 to 108, but the upper transfers moved from 118 to 128. So I went from 15* to 20* of BD without touching a port.

Sorry for the partial Hijack. I'm still trying to wrap my head around the physical port placement vs. degree thing.

Hijack awy. I took a quick peak and I want to say if was 110-111ish.

 

[Derf]

Whenever I cut a jug like you did, it lowers the exhaust a few degrees but lowers the transfers much more. This is because of rod angle.

This makes sense to me. I understand the relationship between rod angle and duration because it affects the degree changes in a non linear fashion. To clarify that, the changes in vertical piston movement per degree are greater as you approach 90* and smaller near TDC and BDC.

Blowdown being defined as the time between exhaust opening and transfers opening would at first seem to be a fixed duration since the roof of the ports are at the same distance from each other before and after cutting the base.

But, if the cylinder drops so that the ports are at different locations of the degree wheel, there might not be a linear relationship between the degree number that the exhaust opens and that the transfer opens. Which would mean that there may not be an identical blowdown number.

So when you raise the transfers back to the "stock" numbers, you have a blowdown that's the same as stock, but a transfer that's much closer phupysically to the exhaust roof.

This make sense. But remember that you're interested in timing durations, not the physical vertical distance between the port roofs.

Without a drop of grinding on the ports, the exhaust went from 103 to 108, but the upper transfers moved from 118 to 128. So I went from 15* to 20* of BD without touching a port.

Yes, the time for the piston to travel vertically increases as you move farther away from 90*. Since the exhaust is closer to 90* and the transfers are farther from 90*, the time for the piston to make up the vertical distance of the lowered port from the base cut is less than the time it takes the piston to make up the vertical distance to the transfer port. But this is true of all porting jobs.

 

[MillerModSaws]

Here's the finished port work. Where sitting at 103/130/78 at this point and time. Well put here back together and see how she does! Lol.