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exhaust port (Read 2609 times)
james may
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Re: exhaust port
Reply #120 - 06/11/10 at 21:11:24
 
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That would be nice if that worked but it doesn't,You lose low end torque,If it worked you would only need one size engine you just put on big carb big ports,big exhaust and you have the power you wanted,Being you lose low end power you have to go to a bigger engine to get more low and high end power.


If ya actually read the post you'd see that it was just annoying me that nobody explained why any of this happened just stating it did... I stated that because exhaust gasses are expelled more completely you lose power in the lower RPM band..
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Re: exhaust port
Reply #121 - 06/11/10 at 22:19:12
 
james may wrote on 06/11/10 at 21:11:24:
Quote:
That would be nice if that worked but it doesn't,You lose low end torque,If it worked you would only need one size engine you just put on big carb big ports,big exhaust and you have the power you wanted,Being you lose low end power you have to go to a bigger engine to get more low and high end power.


If ya actually read the post you'd see that it was just annoying me that nobody explained why any of this happened just stating it did... I stated that because exhaust gasses are expelled more completely you lose power in the lower RPM band..

I think this was talked a little about when there was a discussion about exhaust pipe length and size.
I'm gonna use 2stroke as an example cause it's the extreme...
When the piston opens the exhaust port, the hot gases rush out.  This flow creates a momentum and when the intake opens it pulls in a fresh charge.  You can get a lot of unburned gas in the tail pipe if it wasn't tuned.  Tuning causes the exhaust pressure wave to bounce back and push the unburned gases back into the chamber before the piston closes the port.  Now this bounce happens best at certain rpm's and not as well at others.
Now this does happen with 4strokes but in different ways.  If the pressure wave bounces on the exhaust port just as it opens, it's going to slow down the exhaust.  And also with the intake.  So that little ridge right where the header hooks up is called a torque cone (oh I picked that up from someone here, don't remember who)  And it works to bounce the pressure wave back before it hits the exhaust port.  I think the reduction a little bit into the header works at this too.  causing the pressure wave to slow down before impact.

Now all this has to be tuned to work over a broad range of rpm's.
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Re: exhaust port
Reply #122 - 06/12/10 at 05:20:57
 
There's nothing like a little hot exhaust talk first thing in the morning to get ya up and running !   Smiley
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Re: exhaust port
Reply #123 - 01/28/16 at 19:08:17
 
Now, I know this is a 5 year old thread, and resuscitating it might bring up old smells like my 5 year old dead cat buried in my back yard...
I was really interested in the exhaust port mod, and had to skip some posts concerning 2 strokes vs 4, and other people's ex- wives and what not...

My question would be,
Would it be possible, to open up the exhaust port, gain top speed hp, and to compensate, try to increase compression in an easy, cheap and quick way?

I remember back in the day, guys would take a gasket out of the cylinder, and replace it with a paper thin one, and it would increase compression.
This, to gain low end torque.

I always thought that by freeing the exhaust system up from back pressure, that the engine somewhat was running leaner, and needed a larger pilot jet?
But the posts before, mentioned it being loss of compression.

Is there any way to improve compression through a "tuned" air intake?
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Re: exhaust port
Reply #124 - 01/28/16 at 19:52:24
 
MeeLee wrote on 01/28/16 at 19:08:17:
Would it be possible, to open up the exhaust port, gain top speed hp, and to compensate, try to increase compression in an easy, cheap and quick way?

Why all the questions about top speed? This ain't the bike to set land speed records. But, yes, opening the exhaust port and increasing compression will help. It will neither be cheap nor easy. Wiseco offers a larger piston to increase engine size. The real benefit is higher compression.


MeeLee wrote on 01/28/16 at 19:08:17:
I remember back in the day, guys would take a gasket out of the cylinder, and replace it with a paper thin one, and it would increase compression.
This, to gain low end torque.

Certainly possible but you need to watch valve clearances to the piston.


MeeLee wrote on 01/28/16 at 19:08:17:
I always thought that by freeing the exhaust system up from back pressure, that the engine somewhat was running leaner, and needed a larger pilot jet?
But the posts before, mentioned it being loss of compression.

Compression won't change because you opened the exhaust. I dunno why but you will need larger jets if you open up the exhaust.


MeeLee wrote on 01/28/16 at 19:08:17:
Is there any way to improve compression through a "tuned" air intake?

That won't affect compression.


BTW, when you say "compression" or you really meaning "cylinder pressure"?
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Re: exhaust port
Reply #125 - 01/28/16 at 23:49:33
 
Kris01 wrote on 01/28/16 at 19:52:24:
MeeLee wrote on 01/28/16 at 19:08:17:
Would it be possible, to open up the exhaust port, gain top speed hp, and to compensate, try to increase compression in an easy, cheap and quick way?

Why all the questions about top speed? This ain't the bike to set land speed records. But, yes, opening the exhaust port and increasing compression will help. It will neither be cheap nor easy. Wiseco offers a larger piston to increase engine size. The real benefit is higher compression.


MeeLee wrote on 01/28/16 at 19:08:17:
I remember back in the day, guys would take a gasket out of the cylinder, and replace it with a paper thin one, and it would increase compression.
This, to gain low end torque.

Certainly possible but you need to watch valve clearances to the piston.


MeeLee wrote on 01/28/16 at 19:08:17:
I always thought that by freeing the exhaust system up from back pressure, that the engine somewhat was running leaner, and needed a larger pilot jet?
But the posts before, mentioned it being loss of compression.

Compression won't change because you opened the exhaust. I dunno why but you will need larger jets if you open up the exhaust.


MeeLee wrote on 01/28/16 at 19:08:17:
Is there any way to improve compression through a "tuned" air intake?

That won't affect compression.


BTW, when you say "compression" or you really meaning "cylinder pressure"?




You realize that what you're saying is the opposite of what people have been saying in this thread?
That freeing the exhaust port, results in lower compression. You say this isn't so; I'd be interested in hearing why not?

The valve clearances usually are ok, even when removing a cylinder gasket (which, without ever seeing one of an S40, I would guess be like 1 to 2 mm in thickness).
What's worse is pinging due to a too high compression ratio.

Just like a tuned exhaust, there is a tuned in. The air that gets drawn in, gets drawn in in pulses. With the right tune, the air at the valves will be under pressure, and more air can enter in than a non tuned air input, thus increasing the compression ratio, by allowing more fresh air in.
Not sure if this is the case with the S40 though...
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Re: exhaust port
Reply #126 - 01/29/16 at 03:57:43
 
MeeLee wrote on 01/28/16 at 23:49:33:
You realize that what you're saying is the opposite of what people have been saying in this thread?
That freeing the exhaust port, results in lower compression. You say this isn't so; I'd be interested in hearing why not?

The valve clearances usually are ok, even when removing a cylinder gasket (which, without ever seeing one of an S40, I would guess be like 1 to 2 mm in thickness).
What's worse is pinging due to a too high compression ratio.

Just like a tuned exhaust, there is a tuned in. The air that gets drawn in, gets drawn in in pulses. With the right tune, the air at the valves will be under pressure, and more air can enter in than a non tuned air input, thus increasing the compression ratio, by allowing more fresh air in.
Not sure if this is the case with the S40 though...


You seem to be focused on the exhaust port.  With the stock cam and piston and muffler...there is very little to be gained by fussing with the ring in the exhaust port.  It is not a big restriction to the engine in stock form...as the cam is very mild, the compression is low, and there just isn't a lot of HP looking for a way to get out of the engine.  The big restriction in getting more top speed and engine rpm....is the piston speed.  The Savage engine has a pretty long stroke - and the piston is just moving too fast to make any additional power once you get the engine spinning really fast.  That is one of the reasons the HP numbers drop off when the engine rpm passes 5,000 rpm.

When you lower the head on the Savage to get a higher compression with the stock piston - you affect the length of the cam chain - and you also retard the timing on the cam chain.....and the late timing will get worse as the chain stretches/wears.  Whatever distance you lower the head....the cam chain length is affected by double that amount.  The Savage engine uses up the available cam chain life too soon already, and lowering the head will only make that worse.  The absolute best way to increase compression, and torque on this engine, is to install a Wiseco piston.  

When modifying this engine, the best approach is to increase the power that is made below 5,000 rpm....and gear the bike so that it operates in that range.  Big singles are famous for low rpm torque...not for making HP (or for being freeway flyers).

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Re: exhaust port
Reply #127 - 01/29/16 at 07:40:29
 
I remember when the first Honda Interceptors came out
a guy who had just bought one pulled up to my buddy and I at a red light, gave us 'the look' and gunned his engine a couple times
My buddy was on a Norton, and smoked the guy off the line, holding the lead until the 1-2 upshift, where the Interceptor, having hit it's sweet spot in 1st, blew by him
the guy couldn't believe That Norton had walked him off the line, he just did not get that a big single makes power differently than an I4
but that off topic
doing the exhaust port on this engine without adding compression is like doing a high end valve job on a late 70s smog mobile V8 without adding compression, it's doing 1/2 the job, but it won't net 1/8 the performance
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« Last Edit: 02/02/16 at 07:52:27 by Art Webb »  
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Re: exhaust port
Reply #128 - 01/29/16 at 11:29:45
 
I'm not going against what's been said already. If you open the exhaust port you are allowing the exhaust gases to exit faster. They don't build up behind the port as easy. This will effectively lower the cylinder pressure (not compression -- I'm picky about terminology.). There's absolutely nothing you can do to change the compression ratio without changing the "squish area" above the piston.

Once the piston is moving fast enough and bringing in more air faster than the newly opened up exhaust port can expel the exhaust gas you start making up the loss in cylinder pressure. That's why opening the exhaust up moves your power higher in RPMs. The engine has to rev higher in order to make the same cylinder pressure.
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Re: exhaust port
Reply #129 - 01/29/16 at 14:13:49
 
Kris01 wrote on 01/29/16 at 11:29:45:
I'm not going against what's been said already. If you open the exhaust port you are allowing the exhaust gases to exit faster. They don't build up behind the port as easy. This will effectively lower the cylinder pressure (not compression -- I'm picky about terminology.). There's absolutely nothing you can do to change the compression ratio without changing the "squish area" above the piston.

Once the piston is moving fast enough and bringing in more air faster than the newly opened up exhaust port can expel the exhaust gas you start making up the loss in cylinder pressure. That's why opening the exhaust up moves your power higher in RPMs. The engine has to rev higher in order to make the same cylinder pressure.


Terminology or not, if there are still pressured gasses in the cylinder before the intake valve opens up, you end up with more gas in the cylinder on the compression stroke than if the cylinder would completely empty out of gas.
Cold air fuel mixes with more hot gasses, causing the cold gasses to expand,
Thus, increasing compression as well.
Well, technicallyyou're right in that the compression ratio hasn't changed, since the stroke and chamber remain the same size,
But pressure on the compression stroke has; so if you had a pressure gauge, it would show higher compression.

Opening the exhaust port will exit more hot gasses, thus allowing more cold gas to enter in the cylinder, and less expansion of the cold gas through mixing with hot gasses, essentially lowering compression due to lower air temperature, and less gas in the cylinder.
There! I think I said it twice in one sentence!  Cheesy

This results in higher efficiency, as the cold gas only starts expanding once it ignites, rather than as soon as it enters the chamber.
This also should result in lower vibrations, as there is less resistance at the exhaust, and less resistance at the compression stroke, again causing higher efficiency.

But that'd be my take on it all...
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Re: exhaust port
Reply #130 - 01/29/16 at 18:56:23
 
I don't think that would cut down on vibration. This bike vibrates mostly due to the long stroke/long conrod/single cylinder design. There's a lot of rotating mass in the bottom end that's not offset by another piston moving in the opposite direction.  Wink
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Re: exhaust port
Reply #131 - 01/29/16 at 19:14:07
 
I think your idea is a little off. When the intake valves open, the exhaust valves are already open. It helps to pull in a fresh intake charge. It's called scavenging. If there are any residual exhaust gases left (and there are!), it would be minimal and not likely to cause any appreciable heating of the intake charge. Besides, we only have 8:1 compression. That's not really conducive to burning the air/fuel mixture very efficiently. At 8:1, cylinder pressure and temperature shouldn't really get very high.

BTW, please don't say compression when you mean cylinder pressure. It's not the same thing.  Wink

(I don't mean that to sound condescending.  Smiley)
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Re: exhaust port
Reply #132 - 01/29/16 at 19:14:16
 
Does increasing compression ratio help low end torque more or high end torque. It would increase the chances of knocking at high load/low rpm.

Do pressure spikes from detonation end at higher rpms, or are they present no matter what in the event of knock-prone conditions?
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Re: exhaust port
Reply #133 - 01/29/16 at 19:18:43
 
Pre-detonation can happen at any rpm. You just can't hear it when you're screaming down the road at WOT.

Higher comp ratios should increase torque at every rpm. It promotes a more complete burn of the air/fuel mixture plus, by reason of being compressed more, it explodes with a bigger bang making more power.
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Re: exhaust port
Reply #134 - 01/29/16 at 22:39:43
 
For that reason it's important to start researching about installing a newer piston (with higher compression) and camshaft.
At 8:1 there's nothing like pre-detonation.
That usually doesn't happen until 10.5:1 or 11:1.
If it does ping, you can always go mid-grade, or premium.
Premium with Shell or BP gives you an additional 5 to 10% on performance gain, because the gasoline is mixed with some other high chemical energy fluids (some say BP and Shell use nitrogen based chemicals in their premium blends).

How much for parts and labor to get a new camshaft and higher compression piston installed?
I presume the newer piston might be also a little lighter in weight compared to stock?

If it's a hundred bucks or two, it might be a well worth investment; especially combined with a larger front pulley!

My only concern would be if the clutch can handle the extra force.


Terminology isn't my best, since English isn't my first language.
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