Transfer port back flow

Nc grease monkey · Dec 25, 2019

ive been reading up on transfers, porting etc. and keep running across where people are commenting about the exhaust back flowing down the transfers and leaving carbon build up all the way down the transfers. After some thinking I’ve come to the conclusion that it’s not exhaust backflow, instead it is when the saw is cut off the ignition is killed, but the saw still turns over a time or two or more pulling in fuel. And when the fuel and oil mix sits in a hot motor you’ve have carbon build up. Even on crank bearings there is black carbon build up. The piston, cylinder and crank bearings are the 3 hottest places(except muffler but that’s a given) and are also where you see carbon build up. This theory also explains why it is seen going all the way down the transfer tunnels. The rod and crank remain clean because they do not see the high temps and also because of washing effect of fresh intake charge. I could be wrong, just something was thinking of while counting sheep.MerryChristmas

XP_Slinger · Dec 25, 2019

I can see that. But it leaves the question of why do some saws have a lot of carbon build up in the transfers while others have none?

Nc grease monkey · Dec 25, 2019

I have run saws that definitely run at different operating tempatures. EVen when adjusted right. What gets me is I don’t see the exhaust back feeding all the way down the transfer ports. One thing could be how the operator runs them? Does it idle before being shutoff? Or after a long hard cut does the operator hit the kill switch with vertical little idle time?

Nc grease monkey · Dec 25, 2019

Still trying to wake up fully so thoughts may seem scattered

XP_Slinger · Dec 25, 2019

Nc grease monkey said:
I have run saws that definitely run at different operating tempatures. EVen when adjusted right. What gets me is I don’t see the exhaust back feeding all the way down the transfer ports. One thing could be how the operator runs them? Does it idle before being shutoff? Or after a long hard cut does the operator hit the kill switch with vertical little idle time?

Valid questions.

Nc grease monkey said:
Still trying to wake up fully so thoughts may seem scattered

Same here brother. Coffee hasn’t kicked in yet, could be better or worse once it does lol

Nc grease monkey · Dec 25, 2019

Also, crank bearings, I’ve seen the same black residual buildup on the cages for the crank bearings, and on the races. Not sure how hot the bearings get but I assume the crank bearings would generate quite a bit of heat at 13k. Also under the piston and on the side by the pin there is the same black residue. I just can’t see exhaust back flow causing this. Makes more sense to near least that it’s the unburned oil and gas that is being pulled in and sitting in a hot motor after shut down. Need more coffe

Deets066 · Dec 25, 2019

I’ve done some reading about this also.
At peak power you will have back flow in the transfers, it’s almost a given. This is why blowdown is so often looked at when porting. The wider you can make your exhaust without short circuiting, the less flow in the trans.

Nc grease monkey · Dec 25, 2019

I’m not segueing that, I’m sure some backdown is going to occur. I’m talking about the black residue that some in other posts years ago have mentioned being there basis on there being backflow. I can’t see backflow going all the way down to the lower transfers, especially at higher rpm. Or am I missing something?

XP_Slinger · Dec 25, 2019

Nc grease monkey said:
I’m not segueing that, I’m sure some backdown is going to occur. I’m talking about the black residue that some in other posts years ago have mentioned being there basis on there being backflow. I can’t see backflow going all the way down to the lower transfers, especially at higher rpm. Or am I missing something?

I agree. Especially if someone abuses a saw like running with a dull chain. Clutch side bearing gets coated pretty quick.

Nc grease monkey · Dec 25, 2019

XP_Slinger said:
I agree. Especially if someone abuses a saw like running with a dull chain. Clutch side bearing gets coated pretty quick.

So back to your point about some saws having residue in transfer ports and some don’t, is that a sign of too much heat? Improper tune? Or hard running? Restricted exhaust causing heat build up in cylinder?

huskihl · Dec 25, 2019

Some of the newer front fed saws are actually designed as a delayed scavenging system where exhaust is meant to travel down the transfers and then purge the spent exhaust when they flow. Some only have 11* blowdown

Bjorn · Dec 25, 2019

Hi. To avoid backflo in the transferports you need to put on an pipe- expensioncamber to the engine. Best is to calculate one on computer so it matches the engine. And you can gain up to 20% of power With the right pipe

drf256 · Dec 25, 2019

Think about strato saws, they wouldn’t work without reverse flow into the transfers.

I find the entire thing fascinating.

I do believe the carbon found down transfers is simply coking and not combustion byproduct.

Nutball · Dec 25, 2019

XP_Slinger said:
I can see that. But it leaves the question of why do some saws have a lot of carbon build up in the transfers while others have none?

It depends on the oil used.

Wonkydonkey · Dec 25, 2019

Nutball said:
It depends on the oil used.

Yes I was thinking g the same,

To the op @Nc grease monkey What oil you or other guys been using/seeing this.in ?.

Do you have same saw models that have shown differently amounts transfer darkness/ carbon/oil deposits...

andyshine77 · Dec 25, 2019

drf256 said:
Think about strato saws, they wouldn’t work without reverse flow into the transfers.

I find the entire thing fascinating.

I do believe the carbon found down transfers is simply coking and not combustion byproduct.

I too believe it's coking. I tend to see it on saws that are run a bit lean and hot. And some fuels/oils tend to do this more. One spot you can almost always find some is under the piston crown.

Bjorn · Dec 25, 2019

drf256 said:
Think about strato saws, they wouldn’t work without reverse flow into the transfers.

I find the entire thing fascinating.

I do believe the carbon found down transfers is simply coking and not combustion byproduct.

Hi. A strato cylinder and piston put a charge of air first in the chamber to flush the exhaust out, and then a charge of fuel mlx from the crankcase in the chamber. I dont understand what you meen with backflo in stratocylinder can you explain a little more.

andyshine77 · Dec 25, 2019

Bjorn said:
Hi. A strato cylinder and piston put a charge of air first in the chamber to flush the exhaust out, and then a charge of fuel mlx from the crankcase in the chamber. I dont understand what you meen with backflo in stratocylinder can you explain a little more.

I believe there is less backflow of exhaust gases in the transfers on strato engines, the backflow should be just air, and maybe some fresh charge. The strato design keeps the incoming charge from escaping before the piston has time to close off the exhaust port. It's basically an air buffer. This is how efficiency in increased along with lowering emissions.

RocketRoss57 · Dec 25, 2019

First thing that pops into my mind is blow by.

Deets066 · Dec 25, 2019

RocketRoss57 said:
First thing that pops into my mind is blow by.

The piston skirt doesn’t give enough clearance to burn down in the trans even if there is blow by

Transfer port back flow

Well-Known OPE Member

They’re Just Saws

Well-Known OPE Member

Well-Known OPE Member

They’re Just Saws

Well-Known OPE Member

AKA Deetsey

Well-Known OPE Member

They’re Just Saws

Well-Known OPE Member

Muh fingers look really big

Pinnacle OPE Member

Dr. Richard Cranium

Here For The Long Haul!

Plastic member

Pinnacle OPE Member

Pinnacle OPE Member

Pinnacle OPE Member

Taco Bell Aficionado

AKA Deetsey

Similar threads