Porting information, specifically intake

[AK jonsered]

I have read many porting boards on here and know that trying stuff is the only true way to know answers like this but I have too many projects to do instead of seeking advice.

The Question--- are there definite signs that the intake is opened too far/too low?

Facts--
--I ported my 2159 very conservatively. Widened ports only, modified non cat muffler. Then I did a popup---~.030" with a squish about .022". Continued by raising exhaust a little. Advanced timing, different filter, polished the zama, then traded to a fixed walbro.... You guys get the picture. It is hard to stop.
--lastly I opened up the intake track some, lightened piston, and I lowered the cylinder intake. Had a few issues with the tuning (in another thread) so I went back to the Zama. Everything is good again--revs and torque---but I am second guessing the intake lowering. Lowering the jug and then a little on the port had me thinking.... Might be imagining it, maybe SLIGHTLY slower rev (still way better then stock).

I know I should open it up and get out the timing wheel, but it is summer in alaska and I am busy. Will run it this summer as is...

Anyone know any signs I should watch for from a low intake?

One person said rich at idle and lean at WOT?

 

[Big_Wood]

low intake will sometime makes a saw seem like the low is lean in extreme cases. signs of a lean lope when coming off high to low and an extremely smooth idle. very low intake saws seem to lose their pop in the exhaust note at idle. increasing blow down seems to help the lean lope off high. i've had a few saws with machined cylinders that were just turds in the torque department despite high compression and i blame it on the low intake losing case compression. after porting a bunch of saws i like trimming the intake skirt better and flattening it out on pistons that don't have a flat skirt bottom (corners of the port opening before the middle). lowering at the cylinder wall only IMO does more then working over the whole port. think it's got something to do with intake velocity. out of everything the skirt trimming is just easier and leaves the port same size leaving velocity alone. a little intake goes a long ways as it will open earlier and close later so a degree lowered is actually more like +2 added duration. i'm always learning and my saws are getting faster and faster. addicting porting these buggers especially when you use them alot.

 

[drf255]

Excellent question. I'm interested in the opinions to come. I don't think there is a straight answer on this topic.

It's multi factorial and will vary from saw to saw.

It depends on saw design, case volume, carb and boot size and transfer timing, to name a few.

I like at least 40* of case compression minimum.

The 360 I'm selling right now has an intake of 84*. I added an 038 boot to it which was much larger in area than the stocker (~60%). Because of the increased area (I postulate), the saw lost torque and had a ton of spitback with the 038 boot. It was able to push more back because of the low restriction. I swapped the stock boot back and the saw woke up.

So on the 036/360, at least for me, the small boot enables the saw to work with a low intake. This is just a recent example that's made me think about it.

 

[shorthunter]

My first experiment with intake port timing happened to be on a Sachs-Dolmar 116si. I put a degree wheel on the saw and found that the saw had 144* of duration (IIRC) on the intake side. Thinking I could increase duration to help performance I started to trim the piston skirt. Trim, bolt together, run, trim, bolt together, run........ The first few file strokes seemed to help the saw a bit but somewhere around 152* of duration I started to go backwards.

From this experiment I learned that too much intake duration can do several things. From my experience you will see a loss in throttle response and performance first. Keep going with the intake duration and you will get a bunch of spit back. As the previous poster mentioned, each saw will have its own characteristics. Some saws have quite a bit of spit back off idle by design and some saws can handle 180* of intake duration. If you feel like you have gone too far, grab some JB weld and raise the intake up a little

 

[AK jonsered]

Thanks guys.... It seems like I might be on the edge.... Perhaps a boot that isn't opened quite as much is the ticket. I have a feeling JB weld will be the final route.... Although I haven't messed with the upper transfers so I better degree wheel it first. Might just be a transfer flow issue. Funny, with the Walbro there was excellent torque but I lost my nice idle and insane acceleration after the last few mods. The zama gave the idle and acceleration back, but the torque decreased some I think....
Power loss, spit back, lost acceleration, decreased torque, decrease crankcase compression, smooth idle, lean after WOT are the signs.... Looks like it is time to run the saw again to be sure....

 

[Chris-PA]

You really need to do the degree measurements so you can see the whole picture. It's too interrelated to discuss just one measurement in isolation.

 

[blsnelling]

You're shooting in the dark without a degree wheel. I've never seen a saw that required significantly less than 80* on the intake port timing. There's not a saw I build that gets less than 78*. But, Intake port timing is only part of the equation. There's a LOT more to making a saw run really well. You can't just pick a port to work on.

 

[shorthunter]

Had another well known builder tell me that most 50cc saws like 150-156. The Dolmar 7900 that I am building does not like 160°. I have epoxied the intake back up to 157° and I'm not sure there isn't more to be had if I take it up even further

All depends on how you build a saw

 

[blsnelling]

All depends on how you build a saw

Exactly. However, the winner of the 346 build off had an intake at 80°. I've seen gains on a 7910 by increasing the intake from 80° to 84°. Same thing with the 661. I've honestly never understand the recent affinity to intake duration. Perhaps it has to do with how the transfers are being ported.

 

[blsnelling]

Keep in mind, my comments above are in the context of a saw that has been degreed and all ports modded to my liking, and would likely not apply to a stock saw.

 

[AK jonsered]

I hate taking the saw apart... Putting together and running it is my fun. Anyways.... I should've left the intake alone. The little loss of acceleration was not imagined.
Here are the numbers.... Please don't laugh too hard

Ex 148
Tr 117
Int 169
Bd 15

Thinking I will epoxy the intake back to 150-155 and maybe take the exhaust to 158.
What do you guys think? Does JB weld actually hold up?

Is 158 on the exhaust too high for torque?

Or is the intake better left and the exhaust tweaked first? Acceleration is less important than torque. If 84 is where a 7910 likes to be, maybe I am not too horribly aggressive?

No right angle grinder and the top end is a Hyway with no openable transfers....

 

[AK jonsered]

Those were in duration... The opening numbers would be
Ex 105
Tr 120
Int~84.5

 

[drf255]

What's your muffler like?

How many CC's is your saw? Quad or dual port? I'm just not familiar with that model.

Your numbers don't look bad at all for a dual port. That's a low exhaust.

Are you sure on those numbers? I may be doing the math wrong, but by your durations, I'm getting

Ex 106
Tr 121.5
In 84.5

I'd raise the intake, especially if it's a dual. You need more transfers pressure IMHO. I've been liking low exhausts lately, but you may wanna raise it to 103.

You only have 37* of crankcase compression.

 

[AK jonsered]

That is interesting... I just used the number the port opened subtracted by the number it closed at. Is there an easier way to calculate duration? How did you get crankcase compression without knowing the size of the crankcase? It is a 59cc saw with quad, closed port transfers. My muffler is dual port, non cat but not opened up huge. 75-80% of exit at cylinder wall I believe.

Engine compression is plenty high.... It is around 240. Raising the exhaust a little should bleed some of that off...

Maybe the discrepancy in durations is caused by the lowering of the jug for the popup. Not sure the piston ever fully opens the transfers or exhaust

 

[blsnelling]

My advice is to get your exhaust and transfers right before putting epoxy inside your engine. That's a last resort IMHO.

Next time you want to experiment with intake duration, simply trim the length of the intake skirt. Then of you don't like it, all you have to do is replace the piston.

I'm also second guessing your compression number. I've never seen a saw generate that kind of number with only a pop-up. Matter of fact, it's rare to see them over 200. Was that with assembly oil all over the p@c?

 

[blsnelling]

Another thing...have you majorly worked over the transfer ducts? There's a TON of work to be done inside 359 transfers, required to allow them to flow as needed.

 

[AK jonsered]

It is a hyway kit with smaller top chamber. Transfers are similar to 357xp. My 2139t is 160psi and ported 2095 is 170. 215 was what it was after running it initially, and it raised to 240 over time. No buildup or carbon of any kind.


Sent by dog sled

 

[blsnelling]

Ok. I didn't realize you were running an AM topend. In that case, I would put a new piston in it. That will drop your compression a little and raise your intake.

 

[AK jonsered]

Nope. Lowered cylinder to make the popup. New piston would just mess up what I have done. Besides, the piston was not modified on the skirt. Maybe start with exhaust raising a few degrees. Could try another stock intake like drf255 did on his 360. Otherwise, it is jb weld I think.

She still runs great, so I think a touch more on the exhaust is worth a try. Wish I could widen those transfers just a tad. Anyone grind transfers with special files or sandpaper?


Sent by dog sled

 

[AK jonsered]

Transfers were about right on measurement. After screwing around with that dumb wheel I think that my intake duration is 162 which means it opens at 81, right? I have raised the exhaust slightly to a duration of 158-160..... It is 158 looking through exhaust and 160 through the spark plug. I think that means it opens at 101-100, right? Will test some and see how things work. It was more fun just widening the ports.... I need a better wheel setup.

Really, acceleration just barely dropped off--so 81 is a more likely number on the intake. No excessive blowback---just a normal amount I think. Will see what compression is after cutting some wood.

Thanks for the input guys.... Was a huge encouragement