Lifting your exhaust port

[mcbob]

Well fellas excuse my ignorance on this matter but i realise lifting the exhaust on the Mac's allows the engine to rev higher but there must be a limit to how far one can go .... on my Mac 91b/1 and Mac 92 and Mac 93 there exhaust ports have been moved up enough to allow the head stud bolts thread holes to open into the exhaust area making the engines a pain to pressure test.

Is there a ratio or rule appling to this procedure of lifting the exhaust ?
Quick timed pistons are in a fashion doing the same thing ? but in a milder manner ?

Mc Bob.

[Rob Voska]

"allows the engine to rev higher " is wrong. It raises the RPM the engine makes peak power and narrows the power band of the engine. As you raise the exhaust you also reduce the mechanical compression ratio of the engine.

If I were you I would study direct drive kart engines. Wide power bands and more RPM that anyone in the US runs with a heavy clutch hanging on the end of the crank.

Study port time / area. It's port open time along with the area of the port.

[jmendoza]

One easy thing to always remember is more power requires that you must burn more fuel. More dissplacement is an easy way to get more power, as is artificial aspiration(boost, either super or turbo) and then there is oxygen releasing power ingredients, like nitro-methane, and nitrouse oxide!
I always preffer to go the dissplacement route, as it avoids overstressing the engine with heavy modifications.

[steveohara]

Howdy Bob,

The practical limit for your 100cc Mac engines is .850" from the top of the block to the top of the exhaust port. Using a quick time piston is not a good idea as it raises the transfers by the same amount and you gain nothing in blowdown time. If you raise the top of the exhaust all the way up there and keep the top surface square to the bore you will expose the head bolt holes and it makes the pressure test a pain. I think they run just as good with the top of the port angled so that the original exhaust port gasket surface is maintained and the bolt holes are kept closed. I know a lot of people that like to carve up Macs will disagree but just keep in mind that the critcal work of the exhaust port and header occurs well before the piston exposes the transfers on the way down so look at the exhaust port area actually opened up by the piston when the top of the piston is at the top of the transfers and compare that with the opening in the header..... no worries mate
Keep the transfers at stock height and flatten out the tops while retaining a general flow direction to the rear. Do not make huge radius cuts on the leading edge of the transfers.... that just diminishes the control over the direction of flow. Also, don't narrow the rib between the exhaust.... it needs to be as thick as possible to dissipate heat. A narrow rib just messes up the rings and they end up cracked half the time.

Your statement about higher exhaust allowing the engine to rev higher is correct from a practical point of view.... assuming load increase with speed as is the case in virtually all applications we talk about. Raising the exhaust will move the peak HP point up in the rpm band and that will always translate to the engine running a higher peak rpm for any given load. The offset will be a decline in power at lower rpm that may decrease acceleration or in your case might cause a saw to bog badly if the operator applies too much pressure or binds the chain.
Regards,
Steve O'Hara

[mcbob]

Thanks Steve the engines i have have already been modifiied by some previous kind hearted Karter so i suppose playing with these is better than chopping up NOS and besides they need something to do as i've had these engines sitting on my shelves for more than 25 years and i feel the need for a visually stock appearing saw to run in our under 100cc visually standard races.

I have fitted a rather heavly ported open Mc 92 in to a Mac 250 chainsaw and except for the exhaust muffler looks really stock ( the exhaust is off a D-44 with a new end plate drilled to set backpressure ) if the powers to be kick up a stink about it i have a Mac 15 style exhaust stub which was fitted standard to some of the 250's

It runs a 1" tillerton as everything has to be inside the origional covers so no BDC's or HR Tillerton's.

Anyway the proofs in the eating so they say and i have a few Stihl's and Huskies to convert to road kill. We also run a Mac SP 125C in the over 100cc visually standard races i have setup two of them to run a sloper manifold and a BDC both have proven uncatchable so far and they have more potential left so at least out here the Mac's are making a come-back

And it's great to see a little Black and Gold back in the winners circle again

Mc Bob.