BOWTIROC

This may be a stupid question to some, but if your exhast valve is only 1.60, why go bigger than a 1.625 (1 5/8) primary header? Please Explain.

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Black 89 IROC...Mods: 355, 10.5:1 comp, Big Solid Crower Cam, TFS heads, Victor Jr, Holley, and a 5 spd...

Apeiron

Exhaust slows down as it cools so a larger pipe is necessary to maintain the same rate of flow.

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1984 Camaro Z28 Astral Silver w/ 2 1/2" cowl induction hood
Engine: Decked 385 cid 4 bolt w/ oil cooler, 9.6:1 compression, ARP fasteners, 600 cfm Performer carburetor, Torker II intake, Performer RPM Heads, XE268H cam, Magnum roller tip rockers, MSD6AL ignition, Blaster2 coil, recurved HEI distributor
Exhaust: Terminator headers, Custom 3" TIG welded stainless exhaust, Twister muffler
Transmission: 4L60 w/ Corvette servo, .500 boost valve, TransGo 7-CS clutch springs, 700 PKH pump rings, 700-2 reprogramming kit, Borg-Warner High-Energy frictions & bands, 2400 RPM lockup torque converter
Drivetrain: 2.73:1 limited-slip Dana 44 w/ disc brakes
Tires: Front P225/50VR15 Rear P265/50VR15 Eagles

Because you can always sleep in your car, but you can't drive your house.

88IROCs

Actually it's a bit more complicated than just the heat vs velocity equation. After all the stock manifolds do a real good job of keeping the exiting gases heated. Basically it comes down to timing the scavenging plug to arrive at the intake valve, as the intake valve is beginning to open, at a predetermined rpm(this is referred to as resonant tuning). The method used to time the plug's arrival is pipe volume(primary and collector). A large diameter pipe will slow down the plug, so you can use a shorter pipe length to time to resonant tuning. Conversely, a smaller diameter pipe will allow the plug to move at a higher velocity, thus you need to use a longer length to achieve the same resonant tuning rpm.

Now in a perfect world, you would use a primary pipe diameter the same size or slightly larger than the exit of the exhaust port. So, with a 1.5" exhaust port, the 1 5/8" pipe would suffice(keeping in mind that 1 5/8" pipe has an inside diameter of 1 1/2" when using 14 - 16 ga. exhaust tubing). However, to resonant tune that 1 5/8" pipe to 3500 rpm on a cylinder of 43.73 cid, you would need a pipe length of about 36". Additionally, the collector would need to be about 14" long to create the strongest scavenging signal. Unfortunately there just isn't room in or under our cars for this much pipe, so we compromise. Scale up the pipe diameter, and scale down the length.

[This message has been edited by 88IROCs (edited January 18, 2001).]

[This message has been edited by 88IROCs (edited January 18, 2001).]