Calculating manifold header size.

[The AJP Griff]

HarryW said:

............re teh griff, Joolz if you are going bespoke on this I would seriuosly condider Tuscan routing with a matched long primary 4-1 system .

I'm hoping it will look as much like the original manifolds it had,as possible,which fortunately are 4-1 (see my profile pic)
So what different effects will we get re bottom,middle and top end depending on whether we go large or small primaries?And how will longer or shorter header length affect things then?

[HarryW]

The AJP Griff said:

HarryW said:

............re teh griff, Joolz if you are going bespoke on this I would seriuosly condider Tuscan routing with a matched long primary 4-1 system .

I'm hoping it will look as much like the original manifolds it had,as possible,which fortunately are 4-1 (see my profile pic)
So what different effects will we get re bottom,middle and top end depending on whether we go large or small primaries?And how will longer or shorter header length affect things then?

Below are only general rules of thumb, only the dyno will show if its worked or not.
4-2-1 better overall delivery, spread of Torque some limitation to peak power.
4-1 best for peak power some loss of low to mid range torque.
Long primaries aid low to mid torque
short primaries aid higher up power/torque
Big bore primaries flow peak power but will lose torque and poss peak power too
Too small bore primaries will lose all round
Ideal primary bore will gain all round.

To get the best balanced for road it all depends on what you are doing on the intakes, if these are to aid peak power (shorter) then do the exhaust to aid peak torque (longer) and vica versa.

If money is an issue then don't stray too far away from the original TVR sizing, just match the lengths of the primaries and secondaries and it will work, you can do all the maths in the world but it will be different to what you expect, best of luck .

[The AJP Griff]

HarryW said:

The AJP Griff said:

HarryW said:

............re teh griff, Joolz if you are going bespoke on this I would seriuosly condider Tuscan routing with a matched long primary 4-1 system .

I'm hoping it will look as much like the original manifolds it had,as possible,which fortunately are 4-1 (see my profile pic)
So what different effects will we get re bottom,middle and top end depending on whether we go large or small primaries?And how will longer or shorter header length affect things then?

Below are only general rules of thumb, only the dyno will show if its worked or not.
4-2-1 better overall delivery, spread of Torque some limitation to peak power.
4-1 best for peak power some loss of low to mid range torque.
Long primaries aid low to mid torque
short primaries aid higher up power/torque
Big bore primaries flow peak power but will lose torque and poss peak power too
Too small bore primaries will lose all round
Ideal primary bore will gain all round.

To get the best balanced for road it all depends on what you are doing on the intakes, if these are to aid peak power (shorter) then do the exhaust to aid peak torque (longer) and vica versa.

If money is an issue then don't stray too far away from the original TVR sizing, just match the lengths of the primaries and secondaries and it will work, you can do all the maths in the world but it will be different to what you expect, best of luck .

Great stuff Harry,thats just the kind of info i was afterI can see were going to have to make two different sets after all,one for Joo,and one for Mel and I

[HarryW]

Don't forget they are rules of thumb, however my last statement is the best tip I can give you.

[Busa_Rush]

If anybody is going to spend enough money to have their manifolds made properly then they need to calculate the sizes properly. There may be rules of thumb that give you the rough direction to go in but you need to know things like cam timing, volumetric efficiency etc to make an exhaust system work properly, it's the outlet of a pump, not a discrete part that can be thought of as separate from the rest.

It's well worth buying a book by somebody like A Graham Bell, 4 stroke performance tuning, as he provides some basic calculations and starting points in there. Not perfect but far more readable than some of the other texts.

[HarryW]

Busa_Rush said:

If anybody is going to spend enough money to have their manifolds made properly then they need to calculate the sizes properly. There may be rules of thumb that give you the rough direction to go in but you need to know things like cam timing, volumetric efficiency etc to make an exhaust system work properly, it's the outlet of a pump, not a discrete part that can be thought of as separate from the rest.

It's well worth buying a book by somebody like A Graham Bell, 4 stroke performance tuning, as he provides some basic calculations and starting points in there. Not perfect but far more readable than some of the other texts.

Done all the sums more than once and the TVR standard system, apart from not being matched lengths is not a million miles away. Packaging is a major issue with any 'ideal' system and not should not be underestimated.
After the sums comes the dyno testing, theory is one thing that puts you in the right area, the dyno is for seeing if the theory works and fine tuning.
I stand by my previous statement for the AJP for 4.2/4.5 lumps, particularly if money is tight .

[knighty]

knighty said:

not helped?.....yes it has!......rule number 1.....stay well away from 1-3/4"......for 50bhp per cylinder, I honestly believe 1-1/2" will be fine.....if you go much above 50bhp per cyl go 1-5/8".....I'm not a great fan of 4-1 systems, as they are not kind to the mid-range performance...... on a 4cyl my experience has taught me go for 4-2-1, which give good mid and top end performance, couple cyls 1&2 then 3&4 then into 1, go for a long primary, and short secondary, look at a good K-series pipe as fitted to a highly tuned elise, they are pushing 200bhp on 1-1/2 primary OD.......you will struggle to find anyone who stocks 1-5/8" in the UK, as its a rare size, but someone like Hayward and Scott will probably do it......do not go too big on the primary pipe OD otherwise you will lose ALL mid-range performance, and this is where you use the engine the most......even on a race track - believe it or not.

oops.....my apologies on the coupling bum-steer typo.......I was supposed to say couple cyls 1&2 then 3&4......not what I said above, or 1&4 and 2&3......if a 4-2-1 system is done properly, it will not limit top end power.....all 4 cyl race engines that I have worked on follow this format.

Edited by knighty on Friday 8th June 20:13

[Pigeon]

You've still typoed it

You pair pots that fire an equal time apart. So for a 1-3-4-2 firing order you pair (1 and 4), and (2 and 3).

[stevesingo]

Just been reading my copy of Modern Engine Tuning by Graham Bell and he quotes the following...

Cylinder size 500cc Primary 1.375-1.5" Secondry 1.625-1.75" for a 4-2-1 system or 1.5-1.625" for a 4-1.
Cylinder size 600cc Primary 1.5-1.625" Secondry 1.75-1.875" for a 4-2-1 system or 1.625-1.75" for a 4-1.

Steve

[knighty]

no I didnt....couple cyls 1&2 then 3&4.....this is exactly how we did the 220bhp Super 1600 rally engines for Ford Motorsport in the Puma......and the 270bhp BTCC engines for Peugeot and Proton......this is what makes a 4-2-1 system work......if you dont believe me.....take a look at most 4cyl bike engines......the vast majority follow this trend......but some dont.

Pigeon said:

You've still typoed it

You pair pots that fire an equal time apart. So for a 1-3-4-2 firing order you pair (1 and 4), and (2 and 3).

Edited by knighty on Saturday 9th June 20:54

[Busa_Rush]

HarryW said:

Done all the sums more than once and the TVR standard system, apart from not being matched lengths is not a million miles away. Packaging is a major issue with any 'ideal' system and not should not be underestimated.

You're quite right, but when TVR manufacture an exhaust system they also have production cost and producttion/fitting time to consider too so most manifolds are made smaller, more compact and in some cases this is what's created those horrible log type manifolds on other cars.

If you plan to get one made then why not spend a few ££££ extra and get one that's as perfect as you can get. Not having the big budget of a sponsored race team we have a lot of compromises we need to accept but where we can make a difference doesn't it make sense to have a go ?

Then again I'm a great believer in the 80:20 rule too, 80% of the benefit for 20% of the cost