Throttle Response

[srcarr52]

I've never had a problem with throttle response with larger than normal transfers. The stagger at which the port/ports open and aiming of the side walls makes more of a difference.

The slower the port velocity the more the flow stays attached to the port wall and such it follows the imperfections.

 

[huskihl]

There’s also a huge difference between “hogged out” and making passages larger to flow for higher rpm’s. Depends on which operation needs/wants more air.

 

[markds2]

Obviously there is a trade off, and too small of an intake tract will not allow enough charge through. Trial and error will be the best teachers here.. some may respond better to being ported, other may need to have some epoxy added for higher velocity..

That's exactly what Scott Kunz aka @tree monkey did to my 066, he filled in the top of the intake port with a considerable amount of epoxy, to REDUCE its size and therefore increase velocity. His theory was, that if you compare the 066 to the 064, the 064 had a much tighter intake and could oftentimes be made to run stronger than the 066, even with the handicap of 7 or so less CC. All I can say is that saw is a monster and puts a smile on my face every time I use it.

 

[drf256]

There’s also how fast a port opens, and that differs from cross sectional area. A port that’s nice and tight, maximum port opening in first few degrees, makes a big difference IMHO.

A tiny intake at 78* is opening a lot slower than a huge square one at 78*, and the same goes for closing. A small intake effectively opens later and closes earlier than a huge one. Shape matters too, square faster but tougher on rings if too wide.

We left out case compression, which I believe is the most important for throttle response. Case compression is how a saw recovers rpm after it’s lugged down (or goes from idle to WOT). So a small tight intake at 78 will create more case compression than a big square one with the same numerical case compression measurements. The velocity in the intake port needs to overcome inertia, so you’ll get more forward flow and less spitback with a tighter intake port.

An 036 is a good example. Add an 038 intake boot and match the intake port and one gets a slugginsh saw that will 4 stroke at a higher rpm.

Anyone remember JMS here? He copied Shaun’s ports, except he did so without Shaun’s finesse and understanding. His saws were high rpm monsters with a narrow powerband. Dawg the saw in, and you literally had to wait for it to recover. Fast in a cant, but unforgiving for actual work. When we did the initial Dyno testing at the CT GTG, a JMS 066 had the highest peak power there. I had a Masterminded 066 there that made 1.5 HP less than the JMS saw. But the story was in the curves. JMS was a tall peak, and the MMWS one looked like you could sleep on it, nice and flat across a big rpm range. So which one would you want?

Another “it all has to work together” rant. Sorry for my loquaciousness this am.

 

[Ketchup]

There’s also how fast a port opens, and that differs from cross sectional area. A port that’s nice and tight, maximum port opening in first few degrees, makes a big difference IMHO.

A tiny intake at 78* is opening a lot slower than a huge square one at 78*, and the same goes for closing. A small intake effectively opens later and closes earlier than a huge one. Shape matters too, square faster but tougher on rings if too wide.

We left out case compression, which I believe is the most important for throttle response. Case compression is how a saw recovers rpm after it’s lugged down (or goes from idle to WOT). So a small tight intake at 78 will create more case compression than a big square one with the same numerical case compression measurements. The velocity in the intake port needs to overcome inertia, so you’ll get more forward flow and less spitback with a tighter intake port.

An 036 is a good example. Add an 038 intake boot and match the intake port and one gets a slugginsh saw that will 4 stroke at a higher rpm.

Anyone remember JMS here? He copied Shaun’s ports, except he did so without Shaun’s finesse and understanding. His saws were high rpm monsters with a narrow powerband. Dawg the saw in, and you literally had to wait for it to recover. Fast in a cant, but unforgiving for actual work. When we did the initial Dyno testing at the CT GTG, a JMS 066 had the highest peak power there. I had a Masterminded 066 there that made 1.5 HP less than the JMS saw. But the story was in the curves. JMS was a tall peak, and the MMWS one looked like you could sleep on it, nice and flat across a big rpm range. So which one would you want?

Another “it all has to work together” rant. Sorry for my loquaciousness this am.

I see it slightly differently. Intake close is when case pressure begins to build, regardless of port width. I’m not convinced a square intake shape is what makes the saw sluggish. I find a wide intake with a high floor is very responsive.

I also sort of agree though. If your intake track is over sized there can be issues. I think the biggest one is usually a larger carb. The larger venturi doesn’t pull fuel as well at low speeds (or ever) and the saw sucks too much air on throttle up. Any place in the intake line that has a dramatically larger cross sectional area (mismatched air horn, carb, or boot) will also allow the charge to expand and slow down. Rough edges and bad flow geometry also cause constrictions that reduce intake efficiency. If the case doesn’t fill completely, then case pressure and transfer velocity will definitely suffer.

Dr. Al, your example is interesting. I’m curious what that intake track shape looked like. Does an 038 boot have a larger bore than the carb? Was the carb bore larger as well? How much do you think the spigot and port volume was increased by squaring it up?

Right now, I prefer to widen intake ports rather than lower them. I like how they aim cool charge at the transfer lowers and I like the longer case compression stage. The change in shape should be gradual, and the port*time*area has to be sufficient (usually larger than stock for a ported saw). That seems to be working very well recently.

 

[Loony661]

It’s amazing how simple 2 cycle engine theory is, yet how complex all of the timing events are to get the results we want.

 

[srcarr52]

There’s also how fast a port opens, and that differs from cross sectional area. A port that’s nice and tight, maximum port opening in first few degrees, makes a big difference IMHO.

A tiny intake at 78* is opening a lot slower than a huge square one at 78*, and the same goes for closing. A small intake effectively opens later and closes earlier than a huge one. Shape matters too, square faster but tougher on rings if too wide.

We left out case compression, which I believe is the most important for throttle response. Case compression is how a saw recovers rpm after it’s lugged down (or goes from idle to WOT). So a small tight intake at 78 will create more case compression than a big square one with the same numerical case compression measurements. The velocity in the intake port needs to overcome inertia, so you’ll get more forward flow and less spitback with a tighter intake port.

An 036 is a good example. Add an 038 intake boot and match the intake port and one gets a slugginsh saw that will 4 stroke at a higher rpm.

Anyone remember JMS here? He copied Shaun’s ports, except he did so without Shaun’s finesse and understanding. His saws were high rpm monsters with a narrow powerband. Dawg the saw in, and you literally had to wait for it to recover. Fast in a cant, but unforgiving for actual work. When we did the initial Dyno testing at the CT GTG, a JMS 066 had the highest peak power there. I had a Masterminded 066 there that made 1.5 HP less than the JMS saw. But the story was in the curves. JMS was a tall peak, and the MMWS one looked like you could sleep on it, nice and flat across a big rpm range. So which one would you want?

Another “it all has to work together” rant. Sorry for my loquaciousness this am.

He'd use more intake duration and open it up more than I would, he swore it would gain a little bit more. With a good numbers 066 cylinder I won't touch much of the intake except to straighten the curve out to the port edges, the machine work alone would get the numbers I wanted. The carb and boot are the largest restrictions, there is no sense making everything large after that and letting the incoming charge slow down. So I agree, crankcase compression is probably the key factor to width of the powerband and throttle response assuming your ignition timing is relatively close.

Now an 026 is another beast, I've tried long, short and everything in-between on them and all had slow throttle response in my opinion. They'd cut really well at high RPM but just didn't have what I wanted... which was the performance of a Husky 346!

 

[davidwyby]

@Benwa you might like this one

 

[drf256]

I see it slightly differently. Intake close is when case pressure begins to build, regardless of port width. I’m not convinced a square intake shape is what makes the saw sluggish. I find a wide intake with a high floor is very responsive.

I also sort of agree though. If your intake track is over sized there can be issues. I think the biggest one is usually a larger carb. The larger venturi doesn’t pull fuel as well at low speeds (or ever) and the saw sucks too much air on throttle up. Any place in the intake line that has a dramatically larger cross sectional area (mismatched air horn, carb, or boot) will also allow the charge to expand and slow down. Rough edges and bad flow geometry also cause constrictions that reduce intake efficiency. If the case doesn’t fill completely, then case pressure and transfer velocity will definitely suffer.

Dr. Al, your example is interesting. I’m curious what that intake track shape looked like. Does an 038 boot have a larger bore than the carb? Was the carb bore larger as well? How much do you think the spigot and port volume was increased by squaring it up?

Right now, I prefer to widen intake ports rather than lower them. I like how they aim cool charge at the transfer lowers and I like the longer case compression stage. The change in shape should be gradual, and the port*time*area has to be sufficient (usually larger than stock for a ported saw). That seems to be working very well recently.

It’s Time x Area. Then there is inertia to deal with.

Think of a 4 stroke car engine with a big cam and a tunnel ram. There is inertia of charge taking place, that’s why a tunnel ram works. There’s also intake valve closing time, which mechanically happens after BDC because air movement doesn’t stop after the creation of cylinder vacuum does. There’s a lag at different RPM’s.

So when a small intake vs large intake closes functionally vs literally is indeed different. At 90% closed, the area that remains open is much smaller on a rounded intake than a square one.

It’s all fluid and in constant motion. Easier to think of an an isolated event, but it isn’t.

I’m also convinced that saws that puddle a very small amount of fuel at the bottom of the intake port may have better responsiveness because that will slightly enrich the charge when the throttle butterfly suddenly opens. It acts like a poor man’s accelerator pump.

 

[Al Smith]

Since a 200T has been mentioned many people have not seen the insides of that engine .Considering the bore size that engine has a huge transfer area .If a large displacement saw, say 100 cc had that much transfer and fuel delivery you'd never be able to hold it .IMO pound for pound that's the hottest engine by design ever and the good news is you can get a tad more out of them .Which although a fun thing to do it's got enough with nothing done to it .As far as the accelerator pump on the Zama C1Q carb it has had problems early on but I think it's been improved .

 

[Al Smith]

Now then comments about a dead spot between idle and wide open throttle . If the low speed jet cannot deliver enough fuel on a spool up it will falter a tad bit .If it's too rich it will load up .I think IMO that's where most complaints are . I don't think timing advance comes into the picture until the higher RPM ranges .Then again that's merely an opinion and what might work for me might not work for somebody else and visa versa .

 

[Ketchup]

It’s Time x Area. Then there is inertia to deal with.

Think of a 4 stroke car engine with a big cam and a tunnel ram. There is inertia of charge taking place, that’s why a tunnel ram works. There’s also intake valve closing time, which mechanically happens after BDC because air movement doesn’t stop after the creation of cylinder vacuum does. There’s a lag at different RPM’s.

So when a small intake vs large intake closes functionally vs literally is indeed different. At 90% closed, the area that remains open is much smaller on a rounded intake than a square one.

It’s all fluid and in constant motion. Easier to think of an an isolated event, but it isn’t.

I’m also convinced that saws that puddle a very small amount of fuel at the bottom of the intake port may have better responsiveness because that will slightly enrich the charge when the throttle butterfly suddenly opens. It acts like a poor man’s accelerator pump.

Now you have me thinking about inertia. I think the majority of the charge is still moving forward during the closed period of a port. All the charge behind what’s contacting the piston is still crashing forward like a highway pile-up. With enough time that directional inertia would bounce back, but the port is only closed for a fraction of a second. The little bit of charge that has stopped at the piston wall should be easily overcome by the mass behind it. Additionally, if the intake port opens later there is more negative pressure in the case. Assuming that’s how the inertia is behaving, it seems a wider, flatter and higher intake port that opens more area would allow the backed up charge to break through faster.

I think a squared intake slows the charge when it’s also low. Or just over sized. I think the sudden large opening is a benefit as long as it doesn’t hurt case compression.

But I’ve never thought about charge inertia in this way. Great discussion.

 

[Ketchup]

Now then comments about a dead spot between idle and wide open throttle . If the low speed jet cannot deliver enough fuel on a spool up it will falter a tad bit .If it's too rich it will load up .I think IMO that's where most complaints are . I don't think timing advance comes into the picture until the higher RPM ranges .Then again that's merely an opinion and what might work for me might not work for somebody else and visa versa .

I agree there is a direct relationship to rpm and ignition timing.

 

[Dolkitafreak]

I


I agree there is a direct relationship to rpm and ignition timing.

There definitely is, and some coils have built in advance, it can screw with things! Ask me how I know lol

 

[slackinoff]

I’m also convinced that saws that puddle a very small amount of fuel at the bottom of the intake port may have better responsiveness because that will slightly enrich the charge when the throttle butterfly suddenly opens. It acts like a poor man’s accelerator pump.

I wonder if this is why I always seem to prefer my low jet to be on the rich side (for my saws....I don't have any accelerator pump carb saws). On mine, they have better snap off idle when set just lean enough from rich to not load up at idle.

I actually took it further a few times and bumped up the idle a touch to give it a bit more low jet without loading down, if that makes sense.

 

[Al Smith]

I'll fess up a little bit .Although most of my saws spool up rather quickly I have a BG 85 Stihl blower that doesn't .The machine is a salvage that lost a war with a 7,000 pound brush chipper that I resurrected from the bones .It will falter on a start up cold and needs some choke to keep it running until it warms up .Then at idle for any amount of time it will die .I know it's just a partial turn with a little screw driver I've neglected to it for several years so it's just laziness on my part . Once it's going it does good but I use it so seldom I've over looked an obvious problem that should been been corrected . Guilty as charged .

 

[drf256]

There definitely is, and some coils have built in advance, it can screw with things! Ask me how I know lol

For clarity sake only (not trying to be a dick), no coil can advance timing. They can retard timing and then go back to a normal non-retard timing. So they are in fact only “advancing” timing by not retarding it at higher than idle rpm.

It’s effectively the same as advance for arguments sake, but for clarity with engine theory it’s actually as described above.

 

[huskyboy]

In my experiences lightening the piston a tad improves response and smooths out vibes. In theory it should improve bottom end longevity as well. I only remove casting flash, clean up all the edges to get rid of stress raisers (sharp corners, grooves, notches) and clean up windows if it has em. Anything more than that you might weaken the piston imo.

 

[huskihl]

For clarity sake only (not trying to be a dick), no coil can advance timing. They can retard timing and then go back to a normal non-retard timing. So they are in fact only “advancing” timing by not retarding it at higher than idle rpm.

It’s effectively the same as advance for arguments sake, but for clarity with engine theory it’s actually as described above.

Which is why you can’t always go to tdc and see if the magnets line up when checking for a sheared fw key. Some are off 30-40°

 

[Al Smith]

There's always been a debate about coils .Two I'm somewhat familiar with are the coils used on 042/048 Stihls which if you read the theory of operation on the Stihl manual they appear to advance according to the text .Another is the "magnetron" coil used on Briggs and Stratton engines .Those BTW if they go to advance on start they fire so far advanced you'll burn the starters out, all the smoke leaks out and they never work again .So does it advance or does it retard ?Depends on how you look at it .