Primaries...what size to go for?

sidecar · Post by **sidecar** » Fri Sep 19, 2008 6:31 pm

Hi Guys,

I'm planing to make a set of 4 into 1 headers which will mate up with the sidepipes on my cob but I can't decide what size to go for.

I've read Des's book and it has left me wavering between 1+5/8 and 1+3/4. I have some scrap tube of both sizes and I must say the the larger stuff looks very large! I don't think that it would be easy to square it off to the right size to match the exhaust ports. Getting the exhaust bolts on and off would be tricky too. I was also planning to leave a step between the flange and the pipe at the top to aid the prevention of exhaust gas reversion. The step would be about 3mm when using the smaller pipe. It would end up being very big with the larger pipe.

My engine is a 4.6 with stage III heads, piper 285 cam and all the other usual stuff. I have a scrap head and the ports look very small, I know that the stage III heads will have bigger ports but I bet that there only a couple of mm wider and maybe 5mm taller.

I was planning to make all the primaries around 36 inches long but as it has taken me ALL day to rough up one primary using some scrap tubes I think that getting all four just to poke through the hole in the bodywork will be a big enough achievement, never mind getting the lengths the same! As a guess I reckon that they will all vary between 30 and 40 inches!

Anyway your thoughts guys?

Pete

kiwicar · Post by **kiwicar** » Fri Sep 19, 2008 9:32 pm

Hi
first thought is that the step at the header face is a good idea but it should be at the bottom as that is where the flow reverses first. At the top it should be a good match to the port. I would go for the 1 5/8 pipe as your cam is fairly mild. There are loads of primary pipe length calculators on tinternet. I would tune for peak torque revs not peak power for a road engine. put a ballance pipe after the collectors.
Mike

sidecar · Post by **sidecar** » Fri Sep 19, 2008 10:23 pm

kiwicar wrote:Hi
first thought is that the step at the header face is a good idea but it should be at the bottom as that is where the flow reverses first. At the top it should be a good match to the port. I would go for the 1 5/8 pipe as your cam is fairly mild. There are loads of primary pipe length calculators on tinternet. I would tune for peak torque revs not peak power for a road engine. put a ballance pipe after the collectors.
Mike

Cheers Mike,

I'll stick the step at the bottom! A.Bells book gives details on header lengths as well as what the difference is with a the step at the bottom or top. I'll have to read that chaptor again!

As to the lengths, God knows how anyone can make a set where all the lengths are the same let alone the length that you want them to be!

The headers that are one the car now are really bad, the longest is 18" and the shortest is 9" so hopefully what I make will be better than that!

I can't fit a balance pipe as the car has side pipes.

The diameter was worrying me, but I'll go for 1 5/8

Cheers,

Pete

kiwicar · Post by **kiwicar** » Sat Sep 20, 2008 6:05 am

Hi
I'm building a a set of headers for mine at the moment, the engine is a 383 chevy, the cam should give peak power at 7400k revs, and rev out to 7800 to 8000 ish, I am aiming at 570 to 600 bhp, I'm using 2" OD primaries into a 3" system. On the research I've done this is on the large size, Nascar headers are 2" and they are producing 820 ish BHP at 8400 revs, Many say I should be using 1 7/8 tube. I've chosen 2"as the system is a full cross over set up, it will be a little long and I want the engine to produce as much power beyond peak power as possable.
Mike

sidecar · Post by **sidecar** » Sat Sep 20, 2008 7:37 am

kiwicar wrote:the system is a full cross over set up

......and I thought I had problems!

Wotland · Post by **Wotland** » Sat Sep 20, 2008 3:51 pm

The primary diameter should be based upon what the exhaust port flows.
There is design equations from David Vizard for header and muffler. The equations are based what the exhaust port flows at your maximum exhaust valve lift:

OID = SQRT (Ex_CFM * 1.27/FD)
CDiam = 1.75 * OID

where:

OID = optimal internal diamter (inches)
Ex_CFM = exhaust port flow (CFM) at 28" H20
FD = flow density (around 80 CFM/square inch @ 28" H20)
CDiam = collector diameter (inches)

sidecar · Post by **sidecar** » Sat Sep 20, 2008 4:50 pm

Wotland wrote:The primary diameter should be based upon what the exhaust port flows.
There is design equations from David Vizard for header and muffler. The equations are based what the exhaust port flows at your maximum exhaust valve lift:

OID = SQRT (Ex_CFM * 1.27/FD)
CDiam = 1.75 * OID

where:

OID = optimal internal diamter (inches)
Ex_CFM = exhaust port flow (CFM) at 28" H20
FD = flow density (around 80 CFM/square inch @ 28" H20)
CDiam = collector diameter (inches)

Thanks for the info, I guess my prolem is that I don't have the numbers to pump into the equations! I was hoping that some forum members could just tell me what to go for based on experiance. I think A. Bells book states that the ID of the pipe should be round the same diameter as the exhaust valve for a road engine at least.

Pete

spend · Post by **spend** » Tue Sep 23, 2008 1:29 pm

There is quite a lot of meat in the head & flange to make the transition from the rectangular port to round pipe.. The Pipe can also be welded into the flange at angle to make it broader in the direction of the flow turn, downwards in your case I guess (which is easier to mate with the tall ports..).

Here's a couple of pics of my latest grinding++, for TVR which go frontwards rather then downwards. Slotting the flanges on the bottom makes them much easier to fit as well. The only downside I have (?) is that I have to use silicone instead of gaskets.

http://album3.dspen.co.uk/2008/08/30/rimg0006.jpg
http://album3.dspen.co.uk/2008/08/31/rimg0001.jpg

I know they don't look regular, due to the TVR manifold having the pipe mating with the flange at odd angles...

sidecar · Post by **sidecar** » Tue Sep 23, 2008 3:39 pm

spend wrote:There is quite a lot of meat in the head & flange to make the transition from the rectangular port to round pipe.. The Pipe can also be welded into the flange at angle to make it broader in the direction of the flow turn, downwards in your case I guess (which is easier to mate with the tall ports..).

Here's a couple of pics of my latest grinding++, for TVR which go frontwards rather then downwards. Slotting the flanges on the bottom makes them much easier to fit as well. The only downside I have (?) is that I have to use silicone instead of gaskets.

http://album3.dspen.co.uk/2008/08/30/rimg0006.jpg
http://album3.dspen.co.uk/2008/08/31/rimg0001.jpg

I know they don't look regular, due to the TVR manifold having the pipe mating with the flange at odd angles...

Hi Dave,

Thanks for the info and the ideas. I have just spoken to PowerSpeed about the headers and they do reckon the 1 3/4 is the size that I should go for so I my well have to do some work to get them to fit!

Pete

HairbearTE · Post by **HairbearTE** » Mon Sep 29, 2008 6:05 pm

1,3/4" is theorecially too big for all but the lariest of rover V8s. I'm using it in my headers for my racing engine but i'm starting off the primaries at 1,5/8 at the port and stepping them to 1,3/4 shortly afterwards.

sidecar · Post by **sidecar** » Tue Sep 30, 2008 5:20 pm

HairbearTE wrote:1,3/4" is theorecially too big for all but the lariest of rover V8s. I'm using it in my headers for my racing engine but i'm starting off the primaries at 1,5/8 at the port and stepping them to 1,3/4 shortly afterwards.

The bod at PowerSpeed described 1 5/8 as "adequate" and that 1 3/4 is what I should go for. They still look bloody big to me!

Pete

kiwicar · Post by **kiwicar** » Tue Sep 30, 2008 6:29 pm

In last months hot rod was an article on headers, on a 540ci engine with a 250degree at .05 cam and some pretty good heads, it was producing around 700 bhp. they started with 1.75 primaries and ended on 2.25 primaries, the long and short of it was the 2.25's produced the most peak power but lost quite alot of bottom end power and less peak torque but hung on at the top (7000revs). best overall was the 2" set with the best peak torque and about 6 less BHP than the best.
What I am saying is that 1.75s may give you top peak power by a little bit and more power over peak but you will loose out on the bottom end and I still think your hydraulic cam and heads won't make use of the big pipes. But go 1.75 if you think you want the top end. I will admit that I am going larger than recommended (1 7/8 are meant to be ideal for mine) but I am aiming to make the enging hang on at the top end (and have chosen the exhaust cam timing to help), I am making the pipes for peak torque tuning and the cross over set up should give quite alot more torque than a normal set up and I have a pretty hairy set of heads and cam timing.
Mike

topcatcustom · Post by **topcatcustom** » Mon Dec 22, 2008 6:58 pm

Just to bring this up again, I wouldn't mind trying to find some off the shelf headers as it would take ages to make up a pair- the prob is that all the readily available ones are 1 1/2" primaries which I think may be a bit small for me, but that is still bigger than the exhaust valves and as soon as the gas is out of the cylinder it is cooling down which means it is rapidly decreasing in volume yes?

Sooooo, even if I fitted a pair to start with- could I get a huge drop in power or is it more classed as fine tuning? (4.0, SC cam with 4-71 and mild heads for reference btw)

Ian Anderson · Post by **Ian Anderson** » Mon Dec 22, 2008 7:57 pm

Th original GT40's ran 1.5 primaries on their 5l cars
I benieve 1 5/8 on the 7 litre ones

But they were both on crossover systems which also helps evacuate the gasses

Ian

sidecar · Post by **sidecar** » Mon Dec 22, 2008 9:44 pm

Ian Anderson wrote:Th original GT40's ran 1.5 primaries on their 5l cars
I benieve 1 5/8 on the 7 litre ones

But they were both on crossover systems which also helps evacuate the gasses

Ian

Hi Ian,

Cheers for the info, unfortunately its a bit too late!

I've now built the headers with 1+3/4 tubes. The headers have been camcoated but have not been run yet as I need the new sidepipes making.
(I was going to use the ones that I already have but have changed my mind as they are too narrow to get a decent sized baffle with wadding inside them)

I'll just have to see what happens when I run the car, I hope that all the low end torque has not gone down the pan!

Pete