Author Topic: Port timing. (Read 12849 times)

Al Smith · « **Reply #50 on:** February 19, 2013, 08:40:32 pm »

You can down load Jennings if you want to .I'm not sure if you can Bell or not .

There's a lot of good stuff in both of them that I can't remember off the top of my head .They'll tell like how to figure time area open at certain perspective speeds and stuff like that .What good ports should look like and not so good of ports too and why they aren't so good .

What you have to watch is both of them as well as McDizzy get into motorcycle and snowmobile engines which have a larger crankcase volume than a saw engine .If you just stick to the basics though won't have any problems .

660magnum · « **Reply #51 on:** February 19, 2013, 09:13:00 pm »

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Al Smith · « **Reply #52 on:** February 20, 2013, 06:54:43 am »

On that wheel deal they really aren't that hard to fabricate .McMaster -Carr sells the transfer which is heavy mylar resonabley priced in both left hand rotation or right hand available in several diameters .

I've made several . You need to make an arbor to screw on the crankshaft whatever size that may be and I use sheet aluminum cut to the size of the wheel .I'd have to measure but if memory serves they are 8" .

jmester · « **Reply #53 on:** February 20, 2013, 07:03:19 am »

I have a degree wheel and an arbor/ hub to attach it. I was reading in bells book and he talks about filing some of the top of the piston off , too give you some more exhaust duration. Is that practical in saw mods or mainly motor cross. Have you guys also found his exhaust and transfer duration charts to work with saws.

Al Smith · « **Reply #54 on:** February 20, 2013, 07:10:54 am »

That's one method some use .You either remove metal on the port or from the piston .I'm not so certain most chainsaw engine builders use that method though .

aclarke · « **Reply #55 on:** February 20, 2013, 01:08:00 pm »

The duration charts are really not the best suited for saw motors. Piston speed on a square bore/stroke bike motor tuning 9500 rpm is going to be much higher than a saw motor with their over square (short stroke) motors. The time (duration) needed on a saw motor will be less until you start tuning very high rpm and need the extra time/area to support the higher Hp @ rpm.

Blowdown requirements on a bike motor might be 30+ degrees and many saws get away with half that. Other factors weigh in but in general saws require less duration with their lower hp outputs & piston speed

Al Smith · « **Reply #56 on:** February 20, 2013, 01:41:10 pm »

Interesting subject .On the few I have taken the port time opening measurements on ,both McCulloch and Stihl and one Poulan the exhausts were set anywhere from 96 to 101 after TDC and the transfers usually around 20 after stock regardless of who made the things .

jmester · « **Reply #57 on:** February 20, 2013, 04:47:38 pm »

Those reads have a lot of use full information in them. I do have one question can you change the transfer duration and leave the exhaust duration alone. I will have to study them for a while. I am thinking of raising the ex,trans back to stock height leave the intake alone and widen the ports. Any thing you guys would suggest, in regards to timing numbers. I think I might do what I mentioned above,see how it does. And go back in if I want more or something different.

Al Smith · « **Reply #58 on:** February 20, 2013, 06:38:13 pm »

You lost me .Let me get this straight .You want to raise the upper transfers but leave the exhaust port low after you have lowered the cylinder ?

jmester · « **Reply #59 on:** February 20, 2013, 07:21:27 pm »

Yes, you are correct. I want to provide more fuel at mid range, I think I can do that you more transfer timing. I don't want move the exhaust, because I don't want to raise the peak rpm. May be I am lost with this stuff ? My exhaust is @102* and the transfers are @126*. I was thinking of raising the exhaust back to 101* and raise the transfers to 120*.

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Author Topic: Port timing. (Read 12849 times)

Al Smith

Re: Port timing.

660magnum

Re: Port timing.

Al Smith

Re: Port timing.

jmester

Re: Port timing.

Al Smith

Re: Port timing.

aclarke

Re: Port timing.

Al Smith

Re: Port timing.

jmester

Re: Port timing.

Al Smith

Re: Port timing.

jmester

Re: Port timing.

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