port timing on my 371

[mark09]

i have a 371 with a bb kit.I widend the intake, exhaust and opened the tranfurs. I checked the squish and it was .053". I took the bace gasket out and the new squish is .027". I raised the exhaust port to match for the squish. droping the squish droped the intake? the saw is torky as hell but it needs more rpm. how high should I go with the exhaust port for the gain and how long should the blowdown be? i still don't understand the blowdown. I know its the time between the exh opening and the trans opening but what does it mean in the skeem of things?My numbers are exh. opens at 98*
exh./duration 164*
trans. opens 130*
trans./duration 100*
blowdown 32*
any info on what to do next would be helpful thanks!!!!!
also i have moded the mufler.

 

[mtngun]

32 blowdown sounds high. High blowdown helps mid-range a little, and helps fuel efficiency and emissions, but limits top end. Try for 26 blowdown, if you have the tools.

164 sounds fine for the exhaust duration.

What's your intake ? Intake has more influence on the powerband than the exhaust.

 

[mark09]

so the lower the blowdown the more rpm. is there a to low? havn't done the intake yet get that number tomorow and post it. thanks

 

[mtngun]

so the lower the blowdown the more rpm. is there a to low?

You hit diminishing returns beyond a certain point, where there are no gains.

 

[trappermike]

Mark,you are correct about what "blowdown" is, the difference in degrees between ex. opening and transfer opening. Blowdown time is not exactly one of the most critical numbers you are shooting for when porting a saw,more a result of your porting,but it is of significance,and an important figure too.
Blowdown time allows the high pressure of combustion and the exhaust gases to exit the cylinder before the transfers open to deliver the fresh fuel charge. Too little blowdown time would impede the incoming fresh fuel charge because there could be more pressure remaining in the cylinder,but since porting often involves raising the exhaust port then blowdown time is increased as a result. The most critical figures to pay attention to are the ports (Intake,transfer,exhaust) opening and closing times,and the "Total Duration" -time that the ports are open.

 

[mark09]

32 blowdown sounds high. High blowdown helps mid-range a little, and helps fuel efficiency and emissions, but limits top end. Try for 26 blowdown, if you have the tools.

164 sounds fine for the exhaust duration.

What's your intake ? Intake has more influence on the powerband than the exhaust.

int./open 105*
int./duration 150*

 

[mark09]

what does 1* on the degree wheel = in thousands of an inch in the jug?

 

[trappermike]

Mark,the way you are reading your degree wheel seems confusing...
Try taking your readings this way: (The way the events actually happen)

-Exhaust
Opens @ ? ATDC
Closes @ ? BTDC
=Equals total Dur. ?

-Transfer
Opens @ ? ATDC
Closes @ ? BTDC
=Equals total Dur. ?

-Intake
Opens @ ? BTDC
Closes @ ? ATDC
=Equals total Dur. ?

Blowdown = ? Degrees Difference from when ex. opens to trans, opens

Take new measurements by filling in my ? ,fill in the blanks and then lets see if they say the same thing...

 

[trappermike]

Be sure to turn engine in it's correct direction of rotation when taking readings.

 

[mark09]

-Exhaust
Opens @ 95* ATDC
Closes @ 95* BTDC
=Equals total Dur. 190*
-Transfer
Opens @ 129* ATDC
Closes @ 129* BTDC
=Equals total Dur. 258*
-Intake
Opens @ 72* BTDC
Closes @ 72* ATDC
=Equals total Dur. 144*
Blowdown = 34* Degrees Difference from when ex. opens to trans, opens



is this better? the degree wheel must have moved some the last time....

 

[komatsuvarna]

-Exhaust
Opens @ 95* ATDC
Closes @ 95* BTDC
=Equals total Dur. 190*
-Transfer
Opens @ 129* ATDC
Closes @ 129* BTDC
=Equals total Dur. 258*
-Intake
Opens @ 72* BTDC
Closes @ 72* ATDC
=Equals total Dur. 144*
Blowdown = 34* Degrees Difference from when ex. opens to trans, opens



is this better? the degree wheel must have moved some the last time....

Your durations are way out of wack.

If the exhaust opens at 95*, duration would be 170*
If transfers open at 129*, duration would be 102*
Intake would be correct at 144*, Blow down is correct too.

 

[mark09]

I just realized this myself...

 

[trappermike]

Yes,those numbers look better. I would say you need more intake duration,I would either lower the intake port about 2mm,or trim the intake skirt of the piston 2mm. Both methods do the same thing,and each has it's advantages.
In fact,put a pencil line across the piston intake skirt at 2mm(Where you would trim it off),then use your degree wheel and see what your new intake duration is,using the pencil mark,as if you had trimmed the intake skirt 2mm.

 

[mark09]

I droped the intake and the saw runs a lot better. thanks for all the info...

 

[powerking]

New at modding so i figured i'd ask a dumb question...How does one measure duration? i widened up the exhaust port on my 254 to the witness marks and it repsonded well, and opened up the muff too, butI'm thinking about experimenting a little more...
Thanks
Rich

 

[trappermike]

There is a Sticky thread here in this forum on "Reading a degree wheel" where you can learn how,please read thru it.
For your stock intake port I would suggest you lower it 2mm,this is a safe amount to alter most stock intake ports for more duration resulting in better power without much real negative affects. Alternately you can also trim the intake skirt of the piston for the same result. When doing this try to keep the same shape/curve of the bottom edge of the port as stock,it must be slightly rounded,not straight and flat across. Also it is vital to chamfer the edge of the port after you alter it,it must have a bevel to prevent the piston from contacting a sharp edge and to help guide the piston skirt past it,again look at the factory chamfer job on all the top and bottom port edges as a guide. Failure to chamfer the port edges can result in the damage of the piston or rings,in many cases I will make a better chamfer job on the cyl. ports than stock for reliability and engine life. You can use a round file to do this,take your time and use care,you can chip off the Nikasil plating of the bore if you are not gentle.

 

[powerking]

..Have some rep Trappermike!!:msp_thumbup:

 

[user6bx2x]

My first look in..

Hi guys, I'm new here and loving reading all the posts. I've been porting and doing performance mods to engines since 1977.
And just recently had someone ask me to do some port work on a saw. So out of interest started looking and found this site.
I was Just reading some of the posts regarding the intake port timing. If you remove 2mm or whatever amount you want from the piston skirt or the same amount from the bottom of the intake port. The timing will be increase by the same amount.

But the area will not increase if you remove it from the piston. But if you remove it from the port floor both the timing and the area will be increased. This something to think about as you may or may not need more port area.

Same goes for the old trick of removing some material from the piston where the exhaust and or transfer ports open and close the ports
to see how the engine likes or dislikes the timing increase.

This can be a handy trick to play with if you are thinking of replacing piston anyway. But just remember the area does not increase just the timing.