Where did my low end go?
#11
#12
Sorry, but you're wrong. Some amount of back pressure is needed in a good system (excluding drag racing) for proper scavenging depending on the desired optimal rpm operating range. Reversion is NEVER a good thing and is different from back pressure. Velocity is in the domain of high rpm, high horsepower engines. Too much velocity leads to more of the intake charge being sucked out of the cylinder during cam overlap. Proper back pressure prevents this.
Last edited by Miami_Son; 06-15-2009 at 01:41 PM.
#15
Hi all,
I've been slowly modding my '99 4x4 5.9 to make it a little less of a slug. So far the list of mods include
-14x3 filter
-Superchips 3815
-Flowmaster 50 muffler single in/out
-Magnaflow high flow cat
-Modified throttle body
-180* Tstat
I think that's it. My truck screams as soon as it reaches 3000rpms (seriously, it's fast) but getting there is another story. I have no low end power whatsoever. Tires (stock size) won't even chirp unless I'm making a sharp turn. I have trouble towing even the lightest loads, especially up hills. My plenum gasket is fine (hughes kit) so I know that isn't the problem. I have the factory 3.55 gears, I know 4.10s would help, but I don't have the money for that right now. How can I move my power band down to a more usable rpm? Thanks in advance.
I've been slowly modding my '99 4x4 5.9 to make it a little less of a slug. So far the list of mods include
-14x3 filter
-Superchips 3815
-Flowmaster 50 muffler single in/out
-Magnaflow high flow cat
-Modified throttle body
-180* Tstat
I think that's it. My truck screams as soon as it reaches 3000rpms (seriously, it's fast) but getting there is another story. I have no low end power whatsoever. Tires (stock size) won't even chirp unless I'm making a sharp turn. I have trouble towing even the lightest loads, especially up hills. My plenum gasket is fine (hughes kit) so I know that isn't the problem. I have the factory 3.55 gears, I know 4.10s would help, but I don't have the money for that right now. How can I move my power band down to a more usable rpm? Thanks in advance.
thats weird im able to smoke mine with the 3.55 and have same stuff as you. whens the last time you done a tune up ?? check you cat and plenum agian, and if you want more low get the harland sharps on there
#16
During cam overlap when the intake valve is opening to allow the fuel/air charge into the cylinder and the exhaust valve is not quite closed. If the exhaust velocity is too high it will suck some of that intake charge out the exhaust. By controlling back pressure you reduce the amount of intake charge that ends up going out the tailpipe during that overlap. That's what is referred to as a tuned exhaust. Reversion is when the exhaust pulse goes back towards the exhaust valve and inhibits the incoming intake charge from entering the cylinder. Ideally, the exhaust back pressure is enough to help draw the intake charge into the cylinder, but not so unrestricted that it pulls it past the exhaust valve. This is considered proper scavenging.
This is why freeing up the exhaust too much can hurt low rpm performance. Less of the fuel/air mix remains in the cylinder due to overscavenging, so less power is made. In a closed loop EFI system the 02 sensor will signal a rich condition to the ECM due to more unburned fuel in the exhaust and cut back fuel delivery even more, resulting in even less performance.
This is why freeing up the exhaust too much can hurt low rpm performance. Less of the fuel/air mix remains in the cylinder due to overscavenging, so less power is made. In a closed loop EFI system the 02 sensor will signal a rich condition to the ECM due to more unburned fuel in the exhaust and cut back fuel delivery even more, resulting in even less performance.
Last edited by Miami_Son; 06-15-2009 at 01:55 PM.
#17
During cam overlap when the intake valve is opening to allow the fuel/air charge into the cylinder and the exhaust valve is not quite closed. If the exhaust velocity is too high it will suck some of that intake charge out the exhaust. By controlling back pressure you reduce the amount of intake charge that ends up going out the tailpipe during that overlap. That's what is referred to as a tuned exhaust. Reversion is when the exhaust pulse goes back towards the exhaust valve and inhibits the incoming intake charge from entering the cylinder. Ideally, the exhaust back pressure is enough to help draw the intake charge into the cylinder, but not so unrestricted that it pulls it past the exhaust valve. This is considered proper scavenging.
This is why freeing up the exhaust too much can hurt low rpm performance. Less of the fuel/air mix remains in the cylinder due to overscavenging, so less power is made. In a closed loop EFI system the 02 sensor will signal a rich condition to the ECM and cut back fuel delivery even more, resulting in even less performance.
This is why freeing up the exhaust too much can hurt low rpm performance. Less of the fuel/air mix remains in the cylinder due to overscavenging, so less power is made. In a closed loop EFI system the 02 sensor will signal a rich condition to the ECM and cut back fuel delivery even more, resulting in even less performance.
In any event, you can't have your cake and eat it too, I'm happy with my 3" exhaust.
#18
I've always been taught that valve overlap is to help draw in the fuel/air charge, the escaping exhaust is helping draw the fuel/air charge in, creating creating a higher VE. I see your point about letting the fuel air charge into the exhaust could be a bad thing but I'm sure it greatly depends on the engine and how much overlap is in the cam..
#19
#20
Yes I'm using the 91 tune. Yes my plenum is good, and I'm pretty sure my cat is good considering it's a magnaflow that's only 6 months old. I probably need new spark plugs since I haven't changed them since I replaced the plenum gasket. Would that help with low end though?
I would like to get the Harland rockers, but again, money is an issue, and I'd like to get 4.10s first. Do they help that much with low end?
Do you think my exhaust could be too free flowing? I have the stock manifolds going to a magnaflow kit made specifically for my truck (includes y pipe), then 2.5" tubing into a SI/SO flowmaster 50 series muffler, then the stock tailpipe.
I would really like my power curve to be a lot lower, preferably in the 1500 to 2500rpm range. What can I do to achieve this?
I would like to get the Harland rockers, but again, money is an issue, and I'd like to get 4.10s first. Do they help that much with low end?
Do you think my exhaust could be too free flowing? I have the stock manifolds going to a magnaflow kit made specifically for my truck (includes y pipe), then 2.5" tubing into a SI/SO flowmaster 50 series muffler, then the stock tailpipe.
I would really like my power curve to be a lot lower, preferably in the 1500 to 2500rpm range. What can I do to achieve this?