The V8 Owners Forum -

Need to scratch build manifolds and pipes for my v8 engined Sceptre - what is the minimum diameter pipe I can use without strangling performance ( engine is standard tune Rover 3.5 with carbs)?

Hi Trev
Personally I would think that no smaller than 1 3/8ths is smallest you could go.
I think the standard MGBGTV8 were this size but am sure the mg guys on the forum could clarify this for you.
Ideally IMO you would go for about 1 1/2 as this will not strangle the breathing of your engine but you may be limited with the engine bay space.
I am sure there must be a formula for working out the max and min sizes for exhaust diameter and someone on here probably will tell you how to work it out and I stand to be corrected.
cheers
Paul

Hi Paul

Thanks for the response. As you might have guessed, space is the issue, but if I can use 1.5" then there should be enough room for that - possibly even a little larger.

Cheers

Trev

Hi Trev
Again IMO I would not go any bigger then 1.5.
If you are running a standard tune motor and you have big primarys the spent gasses have to expand to fill the header pipe and this will in effect slow the gas down as it exits the port.This in turn will slow down the rest of the gasses exiting as there will be less pull from the vacuum created in the exhaust system itself.
Have you matched the gaskets and the runners?
If no then this is a really good idea to do when you next have the top of the motor off and is fairly simple to accomplish with a small die grinder.
Please forgive me if I'm trying to teach my Grandmother how to suck eggs.
Regards
Paul

Paul, thanks again - your input is much appreciated.

Comparison
GT40s in the 1960s at Le Mans with a 5l motor used 1.5inch primaries

Ian

Ian Anderson wrote:Comparison
GT40s in the 1960s at Le Mans with a 5l motor ised 1.5inch primaries

Ian

So my 1 3/4" primaries are possibly a little too large then?

Richard
It depends on the state of tune you are running.
If you are running a mild set up then 1.5 is ok and I would say this would still be ok to run with a higher state of tune.
My engine is 4.3 and I am running 1.5/8 primaries and it breathes well.
1 3/4 does seem a little large to me but I am NOT an expert on exhaust systems by any means.
If it works for you then stay with it.At least if you go up in capacity then you have a cracking set of headers from the outset
Cheers
Paul

I had understood that headers need to be at least the same diameter as the exhaust valves, with the lengths being the deciding factor for where the power was in the rev range.

Am I more or less right there?

Richard P6 wrote:

Ian Anderson wrote:Comparison
GT40s in the 1960s at Le Mans with a 5l motor ised 1.5inch primaries

Ian

So my 1 3/4" primaries are possibly a little too large then?

Remember the GT40's also ran 180 degree cross overs on open magaphones (180 crossivers = take 2 and 3 from each side and put into other collector to get correct pulse separation and aid scavanging - even made the inlets into the collectors work in a circle - clockwise in one and anticlockwise in the other to spiral the gasses down the megaphones!)

If you have 1 3/4 they are big enough and will work fine - perhaps the airflow rate through them may be a little low but unless you are full race application you will probably never notice

Ian

More or less yes.
What you have to bear in mind is that engine bay space is nearly always a factor when making your own headers and whilst pipe =valve diameter is generally the rule of thumb it does'nt always work out that way because of tight bends and room to get everything in.Its a compromise as usual.
The length of the primaries will determine whether you have torque at the bottom/middle of the rev range or at the top.
Generally speaking short primaries give top end torque and long give bottom end.
As far as I'm aware the primaries will not determine exactly where the the power peaks although I stand to be corrected again.
Ideally a set of headers for top end will be 4 into 1 with equal length pipes but as a general rule of thumb 4 into 2 into 1 gives good all round performance.
Hope this helps
cheers
Paul

gelmonkey wrote:Richard
It depends on the state of tune you are running.
If you are running a mild set up then 1.5 is ok and I would say this would still be ok to run with a higher state of tune.
My engine is 4.3 and I am running 1.5/8 primaries and it breathes well.
1 3/4 does seem a little large to me but I am NOT an expert on exhaust systems by any means.
If it works for you then stay with it.At least if you go up in capacity then you have a cracking set of headers from the outset
Cheers
Paul

Its a stage one, 4L with an RP4 cam, 500 weber, and a few other bits.

It does run a lot better with the new exhaust system on - but I always thought that these headers were too big. Can't change them now, they have been ceramic coated and cost me a fortune

Hi Richard
IMO I don't think that you will lose any noticeable performance with your headers.I doubt that anyone would tell the difference out on the road or the track for that matter.
As a point of interest could you take some photos for me to show my mate Tony?
He wants to build some headers for his P6 and any help he can get shape wise is good.
Are you pleased with the ceramic coating and did you have inside and out done?
Cheers
P

gelmonkey wrote:Hi Richard
IMO I don't think that you will lose any noticeable performance with your headers.I doubt that anyone would tell the difference out on the road or the track for that matter.
As a point of interest could you take some photos for me to show my mate Tony?
He wants to build some headers for his P6 and any help he can get shape wise is good.
Are you pleased with the ceramic coating and did you have inside and out done?
Cheers
P

Hi Paul

They were apparently coated yesterday, inside and out, and the front 'Y' piece outside only. £320.00 I haven't got them back yet so don't know what they look like but will let you know when they get here.

The manifolds were made by JP exhausts and are sold through RPI who tell me that they fit well and don't foul anything. They also told me how to modify my OS engine mount to get them to fit

Here are the pics I sent to Camcoat to get the quote

Hi Richard
Thanks for those.
Tony will now have something to go with when he builds his own.
Are you going to the RETRO do next year?
P