Understanding port timing numbers

[jonsered 535]

I'm new and not really 100 percent but I think it's your port timing minus your transfer timing example if you're exhaust timing is say 102 minus your transfer timing is 120 your blow down is 18 . correct me if I'm wrong here guys I'm just learning too

 

[Rx7man]

Mdavelee is of course right, it's the amount of time in degrees the pressure from combustion can 'blow down' before another part of the cycle (transfer ports opening) happens.. I still don't have a grasp on how it affects the powerband of the saw

 

[Rx7man]

What I'd like someone to explain to me is how blowdown timing affects power...

Just going from intuition, I'd think that long blowdowns due to low transfers would make a saw that might not be very rev happy due to the lack of time to transfer the mix to the top end, but I'd like to know more about this.. my saws have about 21*, which is probably more than necessary, but that's because I raised the exhaust and lowered the jug, and don't really have the tooling needed to raise the transfers

 

[Terry Syd]

Here's a quick and dirty way to mod your saw. If you want to raise the powerband say 10% (from 10,000 rpm to 11,000 rpm) then you need to have 10% more transfer time/area. (Transfer time/area determines where the maximum delivery ratio from the crankcase occurs, roughly peak torque)

The formula for a 10% increase in time (by raising the transfers) is rather complicated (rod angle and all). However, if you simply WIDEN the transfer ports 10%, that will increase the time/area by 10%.

ALSO, is you widen instead of raising the transfers you retain the 'swept volume' of the piston. - Just because the saw may have 60cc from BDC to TDC, doesn't mean the piston is actually sucking in that volume of mixture. The piston can only start sucking the mixture in when the TRANSFERS CLOSE. So when you raise the transfers you actually reduce the 'displacement' of the engine. The actual 'displacement' is from transfer closing to TDC.

After widening the transfers you need to sort out the blowdown. Start with the stock blowdown and see how the engine is performing. Then take a couple of degrees off the front of the piston at the exhaust port and try it again. You can keep nipping a bit off the edge of the piston until you GO TOO FAR. After you go too far, you can go back to your best exhaust timing and cut that into the jug. Then you install a new piston and ring/s in your saw and you KNOW your blowdown is correct.

 

[Rx7man]

OK... that's some good information, thanks.. I have a spreadsheet with all the rod angle calculations, etc on it, so I can figure out what 1mm in transfer port height comes to in degrees duration, etc and figure the time from there... I think my saws would benefit from getting a little more transfer time (21* right now), especially considering I've already ported them to respond better in the top end... I've nipped off the top of the piston by the transfers for now, otherwise I'd have about 24*... Once I figure out how to get the tooling into the jug to do it and to do it neatly and symmetrically, I think the saw will run better.

 

[056kid]

I just had a 660 done. All of the changes, (besides widening intake & exhaust) where timing, and not via cutting the base so no rais in comp. The saw didnt gain any rpm, but it gained much torque. I don't know what the #s are or where the metal was taken from exactly. The builder has been at it for over 30 years...

 

[smokey7]

Has anyone seen or done this? Remove the tranfer divider completely? They are split at the top the front is pointing strait at each other. The rear is aimed at the intake. It pulls good high revs in cut and tons of torque. Im told transfers facing eachother is a torque thing towards intake and raising them is a rpm rasing mod and scavanging thing. I wonder if that what the reason for the dual upper is for best of both. Buy why remove the divider in the transfer?

 

[Cj genever]

Popcorn

 

[jmssaws]

I think 20 deg of blow down on a 660 is to much, I prefer 17-18.
I use to always use 20 deg but I was leaving power on the table.

 

[K-techcowboy]

I've been around here for a little while now and have noticed something. A lot of members (myself included) ask questions about port timing and they often go unanswered.

Is this because the modders here want to keep the things they have learned (through trial and error no doubt) to themselves. If that's the case who could blame them. I for one am a little apprehensive to alter port timing on an expensive jug without knowing what these changes are going to accomplish.

I have tried different timing numbers on several low dollar saws, and have had a great time attempting to learn all I can. For me, the learning to do a new skill is the best part.

I reached a point in playing with saws that I have more questions than I had when I started. The basic things I needed to know were easy to find using the search function. It's the details that I wonder about these days. And I'm sure I'm not alone.

For instance, will a longer intake duration raise rpm but cause a lose of torque? If I have amble blowdown time could I give up some of this by raising the transfer roof, in turn increasing torque and rpm? And this is the one I wonder about the most, If I have more exhaust duration than inlet should I increase intake timing to match? If I do this will I lose torque? Do certain numbers work better on a given size saw?

Some of these questions I've got pretty well figured out but know others are wondering about. What I'm hoping for is a good discussion on the pros and cons of altering port timing. Of course I'm trying to learn something here, but I'm also looking for a thread we can all use to trade info in for the good of our members.

I have a secret thing I bet no one else is doing actually there's a few things I'm surprised aren't brought up more often starting with lighting the piston by taking unless material out of the inside. Then there's knife edging the crank weights but that requires a rebalance which can be a big pain sometimes. What about homemade pistons and coatings, ceramic bearings ext. Any who if you actually see this respond and ill share a neat thing I do to custom shape part of the cylinder to anything you can imagine.