Part One: The Exhaust Port

[huskyboy]

I haven't seen this anywhere. Or missed it? With these custom exhaust sizes and shapes, what you guys using for gaskets? Modify the existing or some bulk roll?

@SOS Ridgerider has a good exhaust gasket material that he buys and uses. I forgot what it was.

 

[Mastermind]

I haven't seen this anywhere. Or missed it? With these custom exhaust sizes and shapes, what you guys using for gaskets? Modify the existing or some bulk roll?

I use this anytime I need a make a gasket for a muffler. Make the outside bigger than actually needed, and it will hold up better. High temp RTV coating it helps it stay in place too.

https://www.holley.com/products/gaskets/gasket_material/parts/77

 

[Leafy]

I just lap both faces with 220 grit on a flat plate of steel and skim with copper rtv.

 

[SOS Ridgerider]

@SOS Ridgerider has a good exhaust gasket material that he buys and uses. I forgot what it was.

This is what I use:

I tried a link, but it doesn't work.

 

[SOS Ridgerider]

Looking at what Randy posted and what I posted, it’s the same thing. The Holley one is made by Mr Gasket as well. Same thickness and sheet size.

 

[I Know Something]

I'm glad we are on the same page. I was assuming that. Used it a lot when I used to be a mechanic. I just didn't know if there was another offering I was unaware of for this type of application. Thank you everybody.

 

[heavy_oil_saw]

Been looking for similar stuff. Searched amazon, and low and behold, look at the UK price, and this is the cheapest.

Sent from my iPhone using Tapatalk

 

[ZukiRyder440]

A little late here but based on what I’ve read The flatter the roof, the harder it is on the ring or rings but be more efficient on getting the burned fuel out but the more curved the roof is (to a limit), would be a bit easier on the rings but wouldn’t be as effective getting the burned fuel out.
Width wise, I kept my saws right around 65% of bore besides my 2159 which is pushing 70%, I don’t remember.
Blowdown wise, I don’t think every saw likes 20* of blowdown. Some like a little less, some a bit more. Right now I’m running 25* of blowdown on my 281 and it feels freakin good.
Just tryin to get a grasp of things. I enjoy learning about this stuff

 

[Leafy]

What's even better is if you curve the top of the piston to match the curve at the top of the port. Then it's easier on the ring and you actually open a larger port area faster than a flat top.

 

[smokey7]

Only problem with curving the top of the piston is the pockets of dead space it creates where the piston meets the squish band. I think in a more high strung application deto would be a real issue.

 

[Leafy]

Maybe with an expansion chamber but I doubt anyone using a muffler is seeing high enough dynamic compression to have that issue. You can't exactly put a huge amount of curve before getting uncomfortably close to the top ring land.

 

[Terry Syd]

Only problem with curving the top of the piston is the pockets of dead space it creates where the piston meets the squish band. I think in a more high strung application deto would be a real issue.

There's a lot misinformation about 'squish' floating around. Check out Blair's work on tuning two-strokes. The maximum power occurs when 50% of the mixture has burned at about 10 degrees ATDC. In other words, the piston is already opening up the squish band and the mixture is only half burned.

Notching the piston on the exhaust or the transfers is a negligible issue regarding detonation.

 

[Ketchup]

There's a lot misinformation about 'squish' floating around. Check out Blair's work on tuning two-strokes. The maximum power occurs when 50% of the mixture has burned at about 10 degrees ATDC. In other words, the piston is already opening up the squish band and the mixture is only half burned.

Notching the piston on the exhaust or the transfers is a negligible issue regarding detonation.

Interesting.
So how do you think a tighter squish effects the 50% at 10•ATDC? Speeds up the burn? Seems more related to ignition timing and charge volume. And how do we gauge the fuel burn at 10•? Plug color? Wash pattern? Seems hard to know what changes we’re making there. Definitely something to think about though.

I agree that notched pistons aren’t a big detonation factor. Notching the exhaust side may change piston balance.

Also, a notched piston will increase port time area some. Can it be used to run a shorter blowdown?

Off in the weeds here...

 

[Terry Syd]

a notched piston will increase port time area some. Can it be used to run a shorter blowdown?

Shaping the piston crown to conform to the roof of the exhaust port effectively gives about 2 degrees of blowdown. So if you had an engine with 12 degrees of blowdown and you wanted to raise the engine speed by going to 14 degrees of blowdown, you could just shape the piston crown.

The port timing stays the same, but the port vents the exhaust more rapidly.

 

[splitpost]

Its good to be back ,just subbing in ,love these conversations

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[jurchybald]

Hello, I'm new to the forum. Really don't know where to start. This is my first post. I come from Slovenia, Europe.

So I've been reading this threads and I have a question about exhaust floor. Do you guys ever use JB weld on the floor when situation ask for it, like when there's danger from free porting?

 

[huskihl]

Hello, I'm new to the forum. Really don't know where to start. This is my first post. I come from Slovenia, Europe.

So I've been reading this threads and I have a question about exhaust floor. Do you guys ever use JB weld on the floor when situation ask for it, like when there's danger from free porting?

JB can’t take the heat inside the exhaust.

We mostly try to avoid free porting on the bottom of the exhaust but for the most part no one has seen any adverse effects from it

 

[Mastermind]

A small amount of free port at the exhaust floor really doesn't seem to hurt anything at all.

 

[Lightning Performance]

A small amount of free port at the exhaust floor really doesn't seem to hurt anything at all.

You guys been saying this for years but does it affect idol quality?

I ask because free porting the exhaust a tiny bit like 5-10 thou might be covered by an oil wedge. 30-40 seems like a problem at idol but I don't know for sure. Never went that far with a cylinder. Just had a 660 cut fifty and can go to seventy but the transfer roof will be thin and the compression will need to come back down some. Got the exhaust at 99.5 so my goals were accomplished shooting for 100 exhaust with 12 or more covering up the bottom nicely. It should go well when I get the intake back to 78 or 9 for long bars. Hot it should not have any free port happening.

 

[Mastermind]

You guys been saying this for years but does it affect idol quality?

I ask because free porting the exhaust a tiny bit like 5-10 thou might be covered by an oil wedge. 30-40 seems like a problem at idol but I don't know for sure. Never went that far with a cylinder. Just had a 660 cut fifty and can go to seventy but the transfer roof will be thin and the compression will need to come back down some. Got the exhaust at 99.5 so my goals were accomplished shooting for 100 exhaust with 12 or more covering up the bottom nicely. It should go well when I get the intake back to 78 or 9 for long bars. Hot it should not have any free port happening.

I like turtles?