MustangWorks.com : Ford Forums - View Single Post - is there a hp diffrence with tailpipes vs. turndowns

84LX89GT · 01-16-2003, 11:51 PM

This is the reason i've always heard that a little backpressure is good.
It's the same reason you don't throw a Tunnel Ram Intake on a stock engine or a 290* duration .550" lift cam in a stock non-HO F250.
Resonance tuning, and torque range is the key reason. That's why it's recommended to have a crossover pipe as well. It matches the tuning and scavanging frequency of the engine. While one engine may pick up top end with little to no back pressure, another one may not have enough cylinder pressure and airflow to make up for the scavanging losses from having scavanging pulses eliminated at low rpm's.
That's why you hear that on a stock engine it's better to have smaller primary headers. It changes the rate and strength of the exhaust pulses and also the retention of heat within the exhaust helps keep the exhaust moving as well (loses less energy through cooling the exhaust).
If you stick a restrictive single exhaust on a vehicle and design the rest of the engine around that, the operating range will be at a lower rpm band because of the rates of scavanging in both the intake and exhaust. The will provide a wide torque range, but with the top rpm's suffering. If you were to open one side up excessively and not the other, chances are you will kill any kind of lowend the vehicle has because it cannot keep up enough airflow even at WOT to have proper reverberation rates. That's why you hear of people throwing on 1 3/4" long tube headers with 3" pipes and having a SLOW car. Or why a stock honda civic with 3" single exhaust can't get out of it's own way.
This is also an argument for proper cam selection. if you have a long overlap, you're better off on an engine that can move lots of air with opened up intake system and exhaust. If you have little overlap, the torque band occurs earlier and you want to match the airflow on both the intake and exhaust to the camshaft.

There is no set rule for how much back pressure you need, but if you're operating your engine at 4,000+ rpm at the dragstrip, you really don't need the torque range provided by lower flow components.
I hope this makes sense and is accurate, can anyone back me up on this?

01-16-2003, 11:51 PM	#14
84LX89GT Mustangs Join Date: Jul 2000 Posts: 1,938	This is the reason i've always heard that a little backpressure is good. It's the same reason you don't throw a Tunnel Ram Intake on a stock engine or a 290* duration .550" lift cam in a stock non-HO F250. Resonance tuning, and torque range is the key reason. That's why it's recommended to have a crossover pipe as well. It matches the tuning and scavanging frequency of the engine. While one engine may pick up top end with little to no back pressure, another one may not have enough cylinder pressure and airflow to make up for the scavanging losses from having scavanging pulses eliminated at low rpm's. That's why you hear that on a stock engine it's better to have smaller primary headers. It changes the rate and strength of the exhaust pulses and also the retention of heat within the exhaust helps keep the exhaust moving as well (loses less energy through cooling the exhaust). If you stick a restrictive single exhaust on a vehicle and design the rest of the engine around that, the operating range will be at a lower rpm band because of the rates of scavanging in both the intake and exhaust. The will provide a wide torque range, but with the top rpm's suffering. If you were to open one side up excessively and not the other, chances are you will kill any kind of lowend the vehicle has because it cannot keep up enough airflow even at WOT to have proper reverberation rates. That's why you hear of people throwing on 1 3/4" long tube headers with 3" pipes and having a SLOW car. Or why a stock honda civic with 3" single exhaust can't get out of it's own way. This is also an argument for proper cam selection. if you have a long overlap, you're better off on an engine that can move lots of air with opened up intake system and exhaust. If you have little overlap, the torque band occurs earlier and you want to match the airflow on both the intake and exhaust to the camshaft. There is no set rule for how much back pressure you need, but if you're operating your engine at 4,000+ rpm at the dragstrip, you really don't need the torque range provided by lower flow components. I hope this makes sense and is accurate, can anyone back me up on this? __________________ 2005 Suzuki Hayabusa GSX1300-R 1980 Ford Thunderbird - 255 V8 ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain hi-po Mack Truck hood emblem 1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc. Sold 02/06/04 1989 Mustang GT ET: 13.304@102.29 mph (5-24-03) Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)