Compression question
- Thread starter jura
- Start date
WAB
salty nuts
- Location
- coastal GA
Can somebody explain how compression is lowered after porting?
I don't get it. The piston is still compressing the same amount of air I would think...the dome's cc size does not change and neither does squish.
I don't get it. The piston is still compressing the same amount of air I would think...the dome's cc size does not change and neither does squish.
WaveDemon
Not Dead - Notable Member
- Location
- Hell, Florida
THIS DOES NOT ALWAYS WORK!
just because your porting is hiding some compression doesn't mean you can put smaller domes in and not worry about deto because your guage reads 180. I've been there and have melted pistons to prove it.
Frosty
New York Crew
- Location
- Western New York
How can the porting "hide" some compression?
I thought the reason was that there is a lesser amount of fresh gas in the cylinder to compress because of widening or raising the exhaust port. Isn't that 1 of the reasons after porting the engine uses more gas?
Frosty
New York Crew
- Location
- Western New York
i have two 61x cylinders
1 ported by Paul - 35cc domes, 185 psi consistently
1 ported by adrenoline racing - 33cc domes, 180 psi consistently
When you examine them, the Paul ported cylinder has widened ports, slightly raised. The adreniline ported cylinder has a little bit higher port to it.
1 ported by Paul - 35cc domes, 185 psi consistently
1 ported by adrenoline racing - 33cc domes, 180 psi consistently
When you examine them, the Paul ported cylinder has widened ports, slightly raised. The adreniline ported cylinder has a little bit higher port to it.
Okay....Demon is correct, this does not always work. That's why I stated my uncertainty about it.
It works great in my application - I will not "guesstimate" beyond that.
Squish and cranking compression need to be verified. A physical compression ratio calculation would be a good idea, too.
Cranking compression is not the same as running compression. The sonic wave from the pipe will increase running compression, so it will be higher.
As for the question why porting lowers the compression: When you raise the ports, the piston will start compressing air later in the crank revolution than before. That means there will be less volume of uncompressed air to compress - resulting in lower cranking compression.
The pipe will stuff some air back in, which is why running compression is higher.
You might ask why FS porting would lower the compression...after all, that's not supposed to raise ports, right?
True, but I cut my exhaust port corners a bit more square than stock (I believe Paul does this, too). Changing the shape of that port from oval to rounded rectangle will lower compression, because parts of the port is being raised.
It works great in my application - I will not "guesstimate" beyond that.
Squish and cranking compression need to be verified. A physical compression ratio calculation would be a good idea, too.
Cranking compression is not the same as running compression. The sonic wave from the pipe will increase running compression, so it will be higher.
As for the question why porting lowers the compression: When you raise the ports, the piston will start compressing air later in the crank revolution than before. That means there will be less volume of uncompressed air to compress - resulting in lower cranking compression.
The pipe will stuff some air back in, which is why running compression is higher.
You might ask why FS porting would lower the compression...after all, that's not supposed to raise ports, right?
True, but I cut my exhaust port corners a bit more square than stock (I believe Paul does this, too). Changing the shape of that port from oval to rounded rectangle will lower compression, because parts of the port is being raised.
WaveDemon
Not Dead - Notable Member
- Location
- Hell, Florida
even on a good gague don't trust what it says on a ported motor. they play by diff rules. if you want to run 180 psi on a stock motor and then after it gets ported the pressure drops to 160 don't be fooled into thinking you can safely put in smaller domes to get 180 psi without deto (your domes didn't magicly get bigger).
here is what 180 psi looks like on a very ported motor...
http://www.pwctoday.com/attachment.php?attachmentid=16761&d=1130108299
http://www.pwctoday.com/attachment.php?attachmentid=16760&d=1130108314
http://www.pwctoday.com/attachment.php?attachmentid=16524&d=1129680295
http://www.pwctoday.com/attachment.php?attachmentid=16525&d=1129680155
Last edited:
Superjetjim
X-
Yeah... its official..... that's fuc*ed!!:biggrin:
Looks like the timing was fairly advanced and the mixture lean, causing the EGT to get very high and superheat the exhaust side of the pistons. I broke one off like that some years ago when I first built my 760 and had it tuned too lean with a bit too much compression on 93 octane gas. I was using a cheap compression guage and it was reading low. I now have a Snap On guage and it reads true. You can,t just assume that 180psi will always be ok on pump gas, several factors contribute to how it will behave at different loads and RPMs. Thats why the motor gurus' make the big bucks to setup an engine properly.
Superjet3
Waterlogged
- Location
- Jersey Shore
Those pics you posted were on a motor that was tuned for ********. You cna run High compression safely on ported cyls.
I run 33cc domes...those normally yield 210psi. I run pumpgas, and WOT all day long.
My cranking compression is 175-180psi. I run digital TL, so I can retard my timing up top quite a bit.
Demon, I am guessing lean mix and advanced timing contributed.
The point about compression still stands, though....but keep in mind that it's not the only factor.
My cranking compression is 175-180psi. I run digital TL, so I can retard my timing up top quite a bit.
Demon, I am guessing lean mix and advanced timing contributed.
The point about compression still stands, though....but keep in mind that it's not the only factor.
WaveDemon
Not Dead - Notable Member
- Location
- Hell, Florida
no advance. if it was lean it felt good. it happened after a long WOT.
chart102
I wish I thought of that
- Location
- Ruston, LA
those pics make me hurt
WaveDemon
Not Dead - Notable Member
- Location
- Hell, Florida
it was an enhacer with the stator not advanced.