Header & Sidepipe
 

What is the primary size of the headers ( on average ) for small block and big block Ford ? Do the 4 pipes on the sidepipes match the primary pipe diameter ? I'm asking because most small block headers I've seen are 1 5/8 diameter and I really need 1 3/4.

We've used the 1 5/8" pipes with good results even with 550 bhp small-block Windsor engines. Our FE headers use 2" primary pipes.

One of the biggest mistakes I see people make on their small block is to bolt on a set of headers and sidepipes made for a big-block. 2" primaries, 3" collector. I did too, then hade my sidepipes re-made to my specs -- 1-3/4" pri, 2.5" collector. Didn't feel any high-end power loss with the blown 5.0, and picked up a lot of low-end torque.

fordracer I put 2in headers on my 351C 4 barrel heads but I have 2 in ports on my heads seems to run fine !!!! guys say that I dont have the back presure for torck but I like what I have ??????? side pipes match the headers there you go heads have to breath right ???????? chuck

If you are talking 289-351w idealy you would have a 1 5/8 header going into a 1 3/4 sidepipe collector. What you are doing is creating a set of step headers that are properly matched to your displacement. By using a larger header on this engine you will need to move the powerband up in RPM. This requires alot of camshaft, single plane race heads ect. The larger tube can be run but you will sacrifice power and torque below 3500-4000 rpm. Some af the headers I have seen come through the shop have been 1 5/8 primaries with a short 2" long section of 1 3/4 pipe to make it a slip fit on the 1 7/8 collector tubes. Unless you want to scrub off low end torque, use the 1 5/8 headers on a small block, i.e. 289-351W. moorepower

Primary Pipe Sizing
 

In ths 60's when the 289 Cobra and Daytona Coupes were running, header and camshaft science was in its infancy. Dual pattern cams were not being readily used on the small block even though the exaust port flow is pretty poor comparativly speaking and the headers used were 1 1/2" diameter at the time. An example of very poor exaust design was the short header (no primary tube length) dumping into 3" tubes used on the FIA 289 cars. The reason they didn't change was two fold. The homologation design that had been submitted by Belanger (designer) had to be retained once accepted for homologation.
Secondly there actually isn't alot of room for anything different, until the redesign prompted by the 427 SC with the collector outside the body. Thankfully the aftermarket for the smallblock was stimulated by the modern day hot rod, the 5.0 Mustang. Now we have an enormous selection of headers, cams and cylinder heads in iron and aluminum. The only really different heads available in the 60's were the Iron GT40 (not to be confused with the current production item) head that became available during development of the Ford GT40. It had a significantly oversized exaust port and flowed very well by todays standards, and the other was the Gurney/Westlake head both being rare as hens teeth!

Rick...........

What manufacturer cobra do you have? You can probubly send your header out to a good custom header manufacturer as samples and have new ones made in whatever size you need. I strongly recommend Doug's Headers in anaheim ca. Your engine builder should recommend the header diameter.(350 cid, 6000 rpm 1 5/8 : 400 cid, 7500 rpm 1 3/4)The sidepipe primaries should be larger, as they are asthetic as well and what's another 10 hp?