My exhaust is cobbled together from Stock manifolds, some EMT, and flexpipe. It's ugly, and right now it leaks. Leaks from a gasket here and there and even blew through a weld in one spot. My concern is that I used too small a tube and have too much backpressure. The primaries (if I can call them that) from the manifolds are the same size as a late flathead crossover - I think 1.25"? Is this too small? From there it flows into some big flex pipe (2 inch I think?) clamped to a parts store glasspack. (also 2"?) Likely won't have new exhaust by the drags but I will be building some headers soon from weldable steel gasline parts similar to the headers in one of C9's old posts. I'm thinking 1.5" minimum.
1-1/4" sounds small, but that's about what came stock off my manifold crossover... I was going to build some headers, but got lazy & used some center dumps instead. My plan was 1-3/4", but I know those are kinda big - I think 1-1/2" primaries would be about right. My center dumps are 2-1/2" collectors going to a 2-1/4" reducer. The rest of my exhaust is 2-1/4" - again, probably too big, but it's what I had...
Flatdog is resident exhaust guru. I'd think if building headers, you should size them with future plans in mind and perhaps slant them re Flatdog's porting method, even though that won't match perfectly with current stock block. Whateve FD is up to, it seems to work...
1.5625 is pretty standard for off the shelf SBC headers not likeley you're flattie will ever flow that much volumn. But I'd think that 1.5 to 1.5625 would work for primaries, best bet is to measure the ports and ad a smidge to the cross section. if you go huge it will fall flat on its face. Big tubes are for motors that rev no reason in the world to build your exhaust past of the capabilities of your motor. If you want to keep it crisp from your headers back build it about 1.875-2". Unless you're building a monster in which case you'll want to expand that a little. Your blown gaskets probably have more to do with a bad mounting surface or improper torque than back pressure. But that's just my thoughts on it.
Can't speak to really high-performance flatties and what's best for peak HP, but 1-1/2", or 1-3/4" at most, w/duals sound "right" for most street engines, and don't seem to be a restriction. Bigger pipes kind of "blat" and bark, not a nice flatty sound. I've seen several posts on the MSN Flatty board of people who are going down from 2" because of the sound. Whether your heat crossover (intake manifold) is plugged or not makes a difference in sound, too.
The one problem that seems to effect most flathead headers is using single port flanges. (I have never been able to get a good seal using them) My new headers are using a plate port mount (single plate between all three ports on each side) 3/8" thick with 1 3/4" diam. openings. Then a gentle "massage" of the block outlets to "blend" to the larger pipes.
Hey Kevin I used 1 7/8" on my primary's into 4" and it seems to run ok but I've got a 306 ci with a 180 crank, it has a funny sound to it
[QUOTE="Doc" Parsons]Hey Kevin I used 1 7/8" on my primary's into 4" and it seems to run ok but I've got a 306 ci with a 180 crank, it has a funny sound to it[/QUOTE] Doc, I have HEARD a 180 deg. crank at Bonneville. (sounds like two 4 cylinder engines having it out! ) BUT, I have never seen one up close and personal. Got a pic??
I'm running 1.5 off fenton headers with Cherry bomb mufflers and it sounds great and haven't had a problem. Paul
I fabbed my headers and exhaust using 1 3/4" for the ehaders and 2" OD for the rest of the system. I went with 2" for the lung thats being bolted on now, so I wanted good flow that the 2" would give me. On the street, anything bigger than 2" is overkill IMHO. Rat
From my experience with headers on a flat motor overcoming the poor exhaust design is more of a problem. The exhaust exit port on a flathead is 1 1/2" diameter and coupled with the fact that the center ports are siamesed create an air flow nightmare. Therfore building an optimum header poses a degree of difficulty not necessarily worth the aggravation unless it is a fullout race engine. You might just want to go store bought, if you want to build think about this. 1 1/2 " tubes are more than adequate considering the engine rpm at cruising and hiway speeds. By going to a tube style header you will free up some of the exhaust overlap of stock headers and allow the engine to breathe somewhat easier. This allows you to transition to a 2" collector and a 2" exhaust for a good sound. The choice of muffler will also become important as the design of this unit will determine the efficency of the exhaust flow as well as the purr. If you are building your own headers, purchase a set of prefabricated manifold flanges, they save a lot of time and are heavy enough not warp after welding. If you scrounge the tubing section of your auto parts store or even the local muffler shop you might find all the bent tubing you need. Another issue is to create a port divider for the center port similar to those old cast iron dividers only out of 16 gauge and extend the port divider into the block and center tube as far as you can. This buffers the center two cylinders from exhaust overlap. If you should decide to store buy a set of headers, remember if you have a stock steering box or a box located near the block it would be best to determine the clearence between the exhaust port and the box to ensure that you do not have a fit problem. I have success with most manufactured tube headers but have found some to be very tight to stock steering and require a slight hammer modification as well as some hi temp wrap. Post a pic when your done.
I have the info you need.Will measure headers tomorrow. We have been very busy.Wait we will post pics soon have to see to belive ,I am very proud..
Hey Digger Dave, I can't send pictures because the crank is in the engine and you are right it does sound like 2 four cyl's having a fight
If sounds are a concern, go with 1 3/4 tail pipes. I did several flatties using 2" duals. Bigger is better! Right? They never had the classic flatty sound. Actually pretty quiet with Smittys or glasspacks. Disappointing to me and my customers. Until the guy with his 51 Merc insisted he wanted 1 3/4" pipes just as he had in 1955. Bingo! it sounded just like 1955. The original single exhaust was 1 3/4" so with duals you have doubled the capacity. I build my own 2" headers and run the 2" header pipe directly out the side to lakes plugs for the least resistance on race day. 2" out the side back to 10" glasspacks with 1 3/4" tail pipes. Obviously I'm not talking race cars here. If you love that classic sound... bigger isn't better. No dyno pulls to prove it but I personally don't think dual 1 3/4" tail pipes will limit the average flatty at realistic street RPMs. The exhaust comes off the side to go back to the muffler.
Grim,here is the dope,! 3/8 end tubes ,! 3/4 middle tube.Collectors size is 2 inch.Blower set is 1 1/2 end tubes anb 2 inch middle tube going to 2 1/4 inch collector .Exhaust pipe size is 2 inch to flow master muff.Restricted exhaust wii really slow a flatmotor down.
"I am very proud.." Flatdog Oh-oh...either his son was just appointed head engineer at Ford or we're going to be hearing about a very fast flathead... Either way, do what he says on your exhaust!
Done... that is when I can get to it. Very proud: I hope you're going to show what I think you're going to show.
The Jalopy Journal
