Someone explain blowdown and performance

[JonCraig]

I *think* I understand that blowdown is the time after TDC until the upper transfers open. (Is that right?) EDIT: No, dummy.

What I don't understand is how that affects performance. Wouldn't the transfers opening early cause a loss in cylinder pressure?

Somebody smarter than me please 'splain it to me.

 

[blsnelling]

It's the number of degrees of crank rotation from when the exhaust opens until the transfers open.

 

[blsnelling]

It's a balancing act, as are all things. Yes, case compression stops once the transfers open. However, if they never open, no fuel gets to the top. Every saw is different. Some models require WAY more BD than others. Some need very little.

 

[JonCraig]

So when you guys are developing your preferences on a saw you've never done before, what makes you think "raise the transfers", etc.?

I totally understand if you'd prefer not to disclose any trade secrets.

Maybe I should sit down and actually read that 2-stroke tuners handbook?

 

[MustangMike]

I think they generally have "basic" #s in their head based on the size of the saw, then experiment to fine tune things after that.

For example, most builders seem to lower the exhaust more on larger saws then on smaller ones. Case capacity, 2 or 4 transfer ports, bore & stroke, etc all influence what a saw will like, in addition to port shape, etc.

Different configurations may work, but everything has to be in sync for it to work well. For example, things may be done a bit different if the operator wants more torque than speed, etc.

 

[Chainsaw Jim]

Some models require WAY more BD than others. Some need very little.

Do you have an example for each end of that spectrum?

 

[huskihl]

Do you have an example for each end of that spectrum?

I'm sure Brad knows a couple as well, but an early 046 likes maybe 14 or 16°, and a similar sized Dolmar 7900 likes 28-32°

 

[Chainsaw Jim]

I'm sure Brad knows a couple as well, but an early 046 likes maybe 14 or 16°, and a similar sized Dolmar 7900 likes 28-32°

Way more to me is like 40°+.

 

[Al Smith]

It all has to do with port time/area open .The faster it goes it either needs more area or more time or a combination of both .
I could never figure out why some raise the transfers to actually get less time open .Somebody explain that one .

 

[Lightning Performance]

It all has to do with port time/area open .The faster it goes it either needs more area or more time or a combination of both .
I could never figure out why some raise the transfers to actually get less time open .Somebody explain that one .

You mean lower the transfers, a higher number. More, "longer" time for case pressure and less chance of transfer back-flow with a choked up muffler. Raising them adds more time with a lower degree number. Think off it like ring and pinion gear ratios. Less is more.

 

[drf256]

Like Brad said, it's a concert-not a single instrument.

Through trial and error, some models have been figured out and some still haven't.

Ideally, you want the cylinder pressure to be less than case pressure when the transfers open.

Then you want the transfers open enough time to fill and purge the swept volume before the exhaust roof closes. And you want enough velocity in the transfers to keep the charge moving in the event the rpm drops suddenly by load-more important in a work saw. You want the tiniest transfers possible to fill the volume at peak rpm.

Many variables exist. With a physically lower exhaust roof, you can run less BD because in many cases the pressure in the cylinder will be less when the exhaust cracks open.

A physically higher intake floor, small case volume or multiple tight tunnels allows more BD-because the charge will move faster and with great pressure. If the case is really small, you want a physically higher exhaust roof because you have a lot of pressure but less volume. Small or large case volume being defined as the ratio of case volume vs. displacement.

Great question. The answer is "It depends" unfortunately.

 

[drf256]

I have an opinion question that I may as well ask in this thread. I apologize if this is a hijack, but it may help the OP have his question answered.

I'm currently working on a Stihl 361. 59cc and a 47mm bore. First one I've ever touched.

I have 2 jugs on my bench. One a "Stihl" branded jug (may be Gilardoni) and a Mahle.

The Mahle has smaller tunnels and smaller transfers. The Stihl are larger. I've been told the Stihl jug "is better" because of this-size matters.

I still haven't timed the jugs, that's happening tonight. The lower ring pin is on the flywheel side of the exhaust- so I will need to lose the bottom ring, not widen the exhaust port, or relieve one side only slightly.

I'd like a 104* exhaust roof and a 76* intake on this saw. My thoughts are for around 20* of BD for the primaries and 22* for the secondaries. So 104/124P/126S/76.

If I cut the band, I will likely need to raise the transfers up. If I cut a popup piston, I will need to raise them less, but still will need to raise them a bit.

If I use the smaller Stihl Jug, I should be able to run less BD because A) there should be greater pressure in the tunnels and B) they will need more time to fill/purge the column of swept volume.

So which jug would you use and why?

If I cut the chamber, I can raise the smaller transfers and they won't be as large as the bigger ones will be. If I cut a popup, the bigger transfers will only be raised a bit, so I can maintain the factory uppers with less grinding and the factory roof.

Even though I've been told then Stihl Jug is "better", I'm thinking that the smaller and tighter transfers might actually make more rpm and power.

Opinions?

 

[Al Smith]

It's the great debate .I'm not sure who's right nor who's wrong.Results are the bottom line no matter how it's done .
I'm an old school pop up type myself.Not to "raise" the compression because that's not what a raised dome piston does,it changes the flame flow at ignition .With that in mind if the transfers are flowed right they sweep the area on the intake side by flowing in back of the dome improving purging the unburned products of combustion ,at least in theory .
So if you cut the cylinder base to improving compression and do nothing to the exhaust it and the transfers effectively are in the same degree relationship as factory settings although the exhaust in degrees rotation will be longer or later in degrees ,slightly .If you raise it a few degrees it will lead the transfers a few degrees effectively increasing the blow down slightly if you leave the transfer roof alone .
By cutting the exhaust loose a little early over factory design it would make sense the pressure will be less there fore on transfer the differential pressure between crankcase and cylinder pressure would be greater therefore alleviating reverse flow to the crankcase .
Of course in this quest one must bear in mind you can go ape chit and not have the desired results .Too much raised exhaust might build RPM's but kill torque .Cookie cutter .Not really a good pulling work saw .You have to reach a happy medium .It's just a thought .

 

[Keith Gandy]

It's the great debate .I'm not sure who's right nor who's wrong.Results are the bottom line no matter how it's done .
I'm an old school pop up type myself.Not to "raise" the compression because that's not what a raised dome piston does,it changes the flame flow at ignition .With that in mind if the transfers are flowed right they sweep the area on the intake side by flowing in back of the dome improving purging the unburned products of combustion ,at least in theory .
So if you cut the cylinder base to improving compression and do nothing to the exhaust it and the transfers effectively are in the same degree relationship as factory settings although the exhaust in degrees rotation will be longer or later in degrees ,slightly .If you raise it a few degrees it will lead the transfers a few degrees effectively increasing the blow down slightly if you leave the transfer roof alone .
By cutting the exhaust loose a little early over factory design it would make sense the pressure will be less there fore on transfer the differential pressure between crankcase and cylinder pressure would be greater therefore alleviating reverse flow to the crankcase .
Of course in this quest one must bear in mind you can go ape chit and not have the desired results .Too much raised exhaust might build RPM's but kill torque .Cookie cutter .Not really a good pulling work saw .You have to reach a happy medium .It's just a thought .

A MS440 & 460 works well like that. Just a deleted gasket and leave intake where it lands, widen exhaust and raise alil, timing advance with a MM make strong runners IMO. Ive raised the exhaust too much and transfers and made cookie cutters or turd cutters!!!

 

[drf256]

It's the great debate .I'm not sure who's right nor who's wrong.Results are the bottom line no matter how it's done .
I'm an old school pop up type myself.Not to "raise" the compression because that's not what a raised dome piston does,it changes the flame flow at ignition .With that in mind if the transfers are flowed right they sweep the area on the intake side by flowing in back of the dome improving purging the unburned products of combustion ,at least in theory .
So if you cut the cylinder base to improving compression and do nothing to the exhaust it and the transfers effectively are in the same degree relationship as factory settings although the exhaust in degrees rotation will be longer or later in degrees ,slightly .If you raise it a few degrees it will lead the transfers a few degrees effectively increasing the blow down slightly if you leave the transfer roof alone .
By cutting the exhaust loose a little early over factory design it would make sense the pressure will be less there fore on transfer the differential pressure between crankcase and cylinder pressure would be greater therefore alleviating reverse flow to the crankcase .
Of course in this quest one must bear in mind you can go ape chit and not have the desired results .Too much raised exhaust might build RPM's but kill torque .Cookie cutter .Not really a good pulling work saw .You have to reach a happy medium .It's just a thought .

I see what you're saying about the popup acting like a ramp for incoming charge. But wouldn't the popup be below the charge jet by the time it hit it?

I guess, even if its for part of the cycle, it may be worth it.

 

[Chainganger]

What about the shape and angle of the top of the transfer ports? In a closed port 4 transfer cylinder, what about having two of them slightly higher (for example, the two closest to exhaust port) than the other 2 to create a "better" or more balanced charge in the cylinder before compression?
Does angling the top of the transfer towards the top of the cylinder help or makes no difference?

 

[Al Smith]

IMO the quad port works better than a big open transfer .On that though and staggered transfers take a look at a 10 series McCulloch some time .
While we are talking about it take a look at a Stihl 200T .The open transfers are huge in relationship to the bore .

The transfers to me are the "cam" .Not only do they have to charge the cylinder they also have to purge it so the fuel and air are as devoid as possible from the last charge of unburned fuel .I do it with sweeps .Some prefer finger ports .Some go one more step and use externals but those are mainly race saws .Those modifications all do the same thing so it's more a preference than anything .

 

[Chainsaw Jim]

I believe different degrees of blowdown affect peak power at different rpm levels.

 

[JonCraig]

Dangit. You fellers are too smart for me.

Riddle me this, Batman... how would you "wade into" porting? Obviously there's going to be screw ups. Are those best committed on OEM pro jugs (sounds pricey, lol), AM jugs? Or on cheapo Poulan clamshells bought by the pound?

And y'all keep talking--you're not derailing me. I'm happy to listen to conversations that go over my head--that's how I learned where babies come from.

 

[Dub11]

Dangit. You fellers are too smart for me.

Riddle me this, Batman... how would you "wade into" porting? Obviously there's going to be screw ups. Are those best committed on OEM pro jugs (sounds pricey, lol), AM jugs? Or on cheapo Poulan clamshells bought by the pound?

And y'all keep talking--you're not derailing me. I'm happy to listen to conversations that go over my head--that's how I learned where babies come from.

Am jugs for a pro saw. It will be way easier to take it apart and reassemble.