sredish Posted November 28, 2004 Report Share Posted November 28, 2004 Certain types of porting can cause dips like that as well, that can't be jetted out. Quote Link to comment Share on other sites More sharing options...
racer Posted November 29, 2004 Report Share Posted November 29, 2004 shit, tyler knows more about porting difficulties that cant be jetted out than the rest of us combined. i have heard that the stock ignition retards the spark a significant amount at higher RPM's though. maybe you would run better on a dyna setup, but i guess we all would. Quote Link to comment Share on other sites More sharing options...
RNBRAD Posted November 29, 2004 Report Share Posted November 29, 2004 Certain types of porting can cause dips like that as well, that can't be jetted out. 285957[/snapback] That statement is absolutely correct!! Most people think dips in the powerband can be corrected with jetting. This is not always the case. Quote Link to comment Share on other sites More sharing options...
Banchetta Posted November 30, 2004 Report Share Posted November 30, 2004 Certain types of porting can cause dips like that as well, that can't be jetted out. 285957[/snapback] That statement is absolutely correct!! Most people think dips in the powerband can be corrected with jetting. This is not always the case. 286347[/snapback] True,true, I got mine out w/ my stroke w/ a different head shape and higher compression...worked nicely for me...of course, higher compression alone will change port timing. Quote Link to comment Share on other sites More sharing options...
sredish Posted November 30, 2004 Report Share Posted November 30, 2004 (edited) True,true, I got mine out w/ my stroke w/ a different head shape and higher compression...worked nicely for me...of course, higher compression alone will change port timing. 286588[/snapback] Banchetta, at what rpm do your PT Mids sign off. I've got my ride going and broken in and am getting used to how they sign off a little earlier than my Fatties did, and was wondering what rpms you experience them signing off at. I'm taking my machine next week to the dyno and just wanted to compare with you. Edited November 30, 2004 by sredish Quote Link to comment Share on other sites More sharing options...
RNBRAD Posted November 30, 2004 Report Share Posted November 30, 2004 Certain types of porting can cause dips like that as well, that can't be jetted out. 285957[/snapback] That statement is absolutely correct!! Most people think dips in the powerband can be corrected with jetting. This is not always the case. 286347[/snapback] True,true, I got mine out w/ my stroke w/ a different head shape and higher compression...worked nicely for me...of course, higher compression alone will change port timing. 286588[/snapback] I was aware that port timing can change compression but didn't realize it could be the other way around. Maybe that's what you meant. Quote Link to comment Share on other sites More sharing options...
sredish Posted November 30, 2004 Report Share Posted November 30, 2004 In what way does compression affect port timing, as in higher compression does this to port timing.... and what benefit. I get a little out of my knowledge base when we get into port timing, so an opportunity for me to learn here.... Quote Link to comment Share on other sites More sharing options...
RNBRAD Posted November 30, 2004 Report Share Posted November 30, 2004 The top of the piston passes the top of exhaust port at a certain degree of crank rotation. Compression will not change this. Quote Link to comment Share on other sites More sharing options...
Redlined Posted November 30, 2004 Report Share Posted November 30, 2004 (edited) Certain types of porting can cause dips like that as well, that can't be jetted out. 285957[/snapback] Can you, or anyone, explain how a porting job can cause a dead spot in the RPM range that cannot be cured with proper jetting? I am not trying to be a smart ass, I honestly would like to know the mechanics of this phenomenon. What is happening? The engine has the right air fuel mix from low RPM to, say 6000 RPM and then the mix is not right and the engine drops off, then at, say 7000 RPM, it gets the right mix again and revs on out. Are you saying that this Edited November 30, 2004 by Redlined Quote Link to comment Share on other sites More sharing options...
RNBRAD Posted November 30, 2004 Report Share Posted November 30, 2004 Certain types of porting can cause dips like that as well, that can't be jetted out. 285957[/snapback] Can you, or anyone, explain how a porting job can cause a dead spot in the RPM range that cannot be cured with proper jetting? I am not trying to be a smart ass, I honestly would like to know the mechanics of this phenomenon. What is happening? The engine has the right air fuel mix from low RPM to, say 6000 RPM and then the mix is not right and the engine drops off, then at, say 7000 RPM, it gets the right mix again and revs on out. Are you saying that this Quote Link to comment Share on other sites More sharing options...
racer Posted November 30, 2004 Report Share Posted November 30, 2004 where do harmonics fall into porting? i know they play a key roll in pipe design but what about porting? i too, with sredish, would like to learn more about the mystery of porting. Quote Link to comment Share on other sites More sharing options...
RNBRAD Posted November 30, 2004 Report Share Posted November 30, 2004 where do harmonics fall into porting? i know they play a key roll in pipe design but what about porting? i too, with sredish, would like to learn more about the mystery of porting. 286722[/snapback] Harmonics is basically a pipe characteristic that needs matched by the porting or better match the pipe to port characteristics. For example increasing port timing by opening the top of the exhaust port. This is for higher rpm power. The exhaust gasses are released sooner on the downward stroke into the pipe, and the window will be open longer during the upward stroke. The exhaust (harmonics) need to be tuned to this port timing, at whatever degree it may be, so that the resultant suction of the piston and the returning gas/exhaust mixture will work collaberatively. The harmonics of the pipe will help force this additional mixture back into the cylinder and increase it's loading pressure when the port is open. Some pipes can bring the wave to early or too late depending on the porting. Thats why it's soooo important to have the correct pipe for your porting. If your porter doesn't know the characteristics of your pipe, you better buy one that he does. This is not even touching the jist of porting by any means. The transfers alone would be a book of itself. Quote Link to comment Share on other sites More sharing options...
SDD Posted November 30, 2004 Report Share Posted November 30, 2004 High compression can be had with an increase of bore size. You can compensate for the increase in compression by raising the exhaust port timing a tad higher so the compression ratio will still be in check. Harmonics play a role in pipe design but more importantly everything must be timed right to get the most out of an engine. If your pipe is sending the reversion wave back too soon it will short circuit the incoming fresh intake charge with spent gases. Your entire engine from the carb to the end of the silencer need to be tuned together. If you have a motor built to run at low rpms with lots of torque putting a set of drag pipes on it will not let the motor do what it was designed to do. Derek how did your machine do over T-day at glamis? Quote Link to comment Share on other sites More sharing options...
Banchetta Posted December 1, 2004 Report Share Posted December 1, 2004 True,true, I got mine out w/ my stroke w/ a different head shape and higher compression...worked nicely for me...of course, higher compression alone will change port timing. 286588[/snapback] Banchetta, at what rpm do your PT Mids sign off. I've got my ride going and broken in and am getting used to how they sign off a little earlier than my Fatties did, and was wondering what rpms you experience them signing off at. I'm taking my machine next week to the dyno and just wanted to compare with you. 286599[/snapback] She made peak hp at 7800rpms and peaked at 8600 before falling fast. I'd post it, but the site won't let me.... Quote Link to comment Share on other sites More sharing options...
Banchetta Posted December 1, 2004 Report Share Posted December 1, 2004 Certain types of porting can cause dips like that as well, that can't be jetted out. 285957[/snapback] That statement is absolutely correct!! Most people think dips in the powerband can be corrected with jetting. This is not always the case. 286347[/snapback] True,true, I got mine out w/ my stroke w/ a different head shape and higher compression...worked nicely for me...of course, higher compression alone will change port timing. 286588[/snapback] I was aware that port timing can change compression but didn't realize it could be the other way around. Maybe that's what you meant. 286608[/snapback] No, compression will affect port timing. Anything that will affect timing of a split second will affect port timing and pipe-port timing. Higher compression will speed up the combustion resulting in a faster timing. It WILL affect pipe-port timing. Its one of the reasons why higher compression helps the high rev pipes get back that bottom end...There are so many variables w/ timing, that is almost endless. Your talking about a motor thats turning almost 10,000times a minute and pumps air in and out in a split second ( about 150 times per second) so if anyone doubts that compression will not affect the timing, then........... as for jetting out all dips, sometimes its impossible. Due to port timing, you'll have a resonating wave coming back from the pipe to the exhaust port that might go against the air coming into the intake, so it'll interrupt air flow. Depending on rpms the pressure wave from the exhaust will vary on the timing and will cause a dip at certain rpms. When the timing of the exhaust wave and ports are perfect, the pipe wave will help pull the air out of the motor, acting as a turbo, but pulling instead of pushing... Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.