Port timing question

bjorn773 · Mar 20, 2010

I am attempting to get a grasp on port timing. I printed out a template degree wheel that I found by searching here. I glued it to a clear cd and have it attached via a spark plug boot to my crank. Sounds hokie, but it seems to work fairly well. Anyway, here are my numbers.

Please be kind, I'm a virgin at this stuff.

Exh begins opening @85*ATDC
Exh fully open @18*BBDC or 162*ATDC
Int begins opening @82*BTDC
Int fully open @28*BTDC
Transfer opens @68*BBDC or 112 ATDC

Do these sound like logical numbers?
Am I interested in the opening numbers primarily or the duration?

Evan · Mar 20, 2010

i dont under stand port timeing so when i port a saw i never lower or rasie the port. ive produced good gains this way. i have no intrest in port timeing or changeing it. thats just my view on it.
i guess if i took the time to understand and learn it then maybe theyre would be some simple easy gains to get. but the simple fact is i dont like math so i stay away from pretty much all numbers that i can

luvsaws · Mar 20, 2010

opcorn:

Evan · Mar 20, 2010

nothing wrong with understanding it either its good thing to know especialy if you do plan to raise or lower the ports or maybe setting a saw up with pipe. youd then for sure need to know the durations and all the math that goes along with it

bjorn773

what are your plans for this saw?

bjorn773 · Mar 20, 2010

It's a 346 I'm porting. I have an aftermarket NE sized p&c that I'm about done with. I've shaved .030 off the base to bring my squish to .016" I am widening the intake, exhaust and transfers. Don't really plan to do much with timing, I just want to understand it. If there is a torque gain from say lowering the intake or raising the exhaust, I may try it. It's gonna be a work saw.

mdavlee · Mar 20, 2010

Raising the exhaust usually gains rpms and loses torque. You might want to raise it back to factory specs. I'm not sure about the intake part.

tdi-rick · Mar 20, 2010

I'd re-check TDC, the exhaust looks too early/high.
Having said that, I have no idea on 346 numbers but compared to larger saws it's early.

Rule of thumb.
Raising the exhaust moves the power band higher in the rev range/narrows the power band (lose dynamic compression)
Dropping the inlet will tend to push the power band higher too, losing bottom end power.

I've always found the best way to check TDC is use a solid stop in the plug hole (I have an adjustable one I made years ago, it's a piece of threaded brass rod that screws down through an old sparkplug, but anything in the plug hole will do) and very lightly wind the engine over till the piston stops.
Note the number on the wheel.
Wind the engine back the other way till the piston stops.
Note that number, then split the difference between the two numbers and move your pointer to TDC/0.
Re-check, just because I'm anal

That's a dead accurate TDC measurement without worrying if an offset gudgeon is used and how that affects piston dwell if using a dial indicator, etc.

AUSSIE1 · Mar 20, 2010

Your No's don't sound right. Did you get TDC correct for starters.
Don't worry about fully open figures.
Exhaust open.
Transfer open.
Inlet open.
When you have these No's you can then work out your durations without closing No's. and you blowdown.

parrisw · Mar 20, 2010

opcorn:

blsnelling · Mar 20, 2010

That's way high on the exhaust and transfers. Also lower than factory on the intake. Did you check it on both sides of TDC to see if they matched? That'll confirm if you properly found TDC when zeroing the degree wheel. How did you go about finding TDC?

bjorn773 · Mar 20, 2010

I must have messed up TDC. I simply watched the piston through the plug hole. When it ceased upward movement and just prior to it descending I zeroed the gauge. In other words, I eyeballed it through the plug hole. I'll try the piston stop technique in both directions and split the difference. Then I'll repost the numbers. Like I said, I'm a rookie. I guess my eyes aren't as keen as I thought. Thanks guys.

AUSSIE1 · Mar 21, 2010

bjorn773 said:
I must have messed up TDC. I simply watched the piston through the plug hole. When it ceased upward movement and just prior to it descending I zeroed the gauge. In other words, I eyeballed it through the plug hole. I'll try the piston stop technique in both directions and split the difference. Then I'll repost the numbers. Like I said, I'm a rookie. I guess my eyes aren't as keen as I thought. Thanks guys.

That's no problem.

Yeah you need a piston stop to determine TDC.
Make sure your wheel is spot on centre.
Don't worry about closing figures, only opening figures.
Check your transfer through the plug hole.

SawGarage · Mar 21, 2010

i'm watching n learning...EVERY thing I own is modded...time for the saw..

:rockn:

tree monkey · Mar 21, 2010

just do duration. zero degree wheel when the port just starts to open, then rotate till it just closes. read wheel. alot faster.

scott

J.W Younger · Mar 21, 2010

tree monkey said:
just do duration. zero degree wheel when the port just starts to open, then rotate till it just closes. read wheel. alot faster.

scott

May be a lot faster but not worth much..
IMO

troutfisher · Mar 21, 2010

tree monkey said:
just do duration. zero degree wheel when the port just starts to open, then rotate till it just closes. read wheel. alot faster.

scott

That's how I do it.

bjorn773 · Mar 21, 2010

AUSSIE1 said:
That's no problem.

Yeah you need a piston stop to determine TDC.
Make sure your wheel is spot on centre.
Don't worry about closing figures, only opening figures.
Check your transfer through the plug hole.

I have one, I guess I didn't realize how much margin for error there was here. I've been eyeballing TDC on cars for years for setting up timing chains and belts, but that's not as critical. On cars, you're lining up whole teeth, not measuring degrees. I'll use the stop and try again, hope to post better figures later today. Thanks everyone, the education I'm getting here is excellent and most appreciated.

Tzed250 · Mar 21, 2010

bjorn773 said:
I have one, I guess I didn't realize how much margin for error there was here. I've been eyeballing TDC on cars for years for setting up timing chains and belts, but that's not as critical. On cars, you're lining up whole teeth, not measuring degrees. I'll use the stop and try again, hope to post better figures later today. Thanks everyone, the education I'm getting here is excellent and most appreciated.

Please see the thread I made here.

.

bjorn773 · Mar 21, 2010

Tzed250 said:
Please see the thread I made here.

.

Excellent info. I like the sharpened tip, mine is too blunt. I probably need to find a more precise way of mounting the wheel, like the thread on type you have.

ronT2 · Mar 21, 2010

tree monkey said:
just do duration. zero degree wheel when the port just starts to open, then rotate till it just closes. read wheel. alot faster.

scott

troutfisher said:
That's how I do it.

Ok...so now you know the duration but what is it relative to?

Port timing question

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