Exhaust manifold primary diameter to port/valve size ratio

[Copperbottom]

Is there a common rule of thumb regarding this? I currently use a mass racing small (34mm) bore 4-1 manifold on my 1700cc crossflow engine but they also produce one that is about 38mm diameter both with equal lengths.

I run a 2.25" system on the car with no reduction at joints etc.

Any info/ideas??

[GORDON STREETER]

I would certainly go up on the primary pipes, the lengths make a fair difference to the delivery and basically shorter ones produce power lower down and longer ones up the top end.

[tristancliffe]

Gordon - he's talking about tube diameter, not length.

I would say stick with the 34mm (assuming that that matches the diameter of the ports on the head/exhaust face). Using 2.25 pipes after the collector sounds about right as well.

Unless you port the engine further and fit bigger valves or wilder cams then 34mm won't be far off. 38mm would be too big I suspect.

Playing with the primary lengths would probably find you more power or torque, but it's not a simple task as you'd need to have several exhaust systems made (or bought) and then try them on a dyno...

[GORDON STREETER]

I know he was talking about diameter as he sent me a PM and I was answering him.
As I understand he is going for a bigger cc engine with bigger ports I personally would go up in size like my own.

[tristancliffe]

Ah, ok. If he's putting a bigger engine in, then chances are an increase in diameter would be appropriate...

[Copperbottom]

Still waiting for the pm Gordon,how do you find the 105speed manifold by the way G? I'm not necessarily going up in capacity just yet, just improving on what I've got, I actually got it slightly wrong in my 1st post (misread my notes) ; although the flanges on my current manifold are 34mm and match the ports in the head the external diameter is 41mm which is more like 38mm (1 1/2") internally, the big bore one is 45mm external and 42mm (1 3/4")internally.Tristan,Mass Racing have already done the work with lengths etc.

[bdwoody]

steady on copper you will be needing a G suit in that little escort ...lol

[Copperbottom]

Originally Posted by bdwoody

steady on copper you will be needing a G suit in that little escort ...lol

It's not fast enough.....................yet

Sadly I won't be doing much racing in it this year though Woody

[phoenix]

Originally Posted by Copperbottom

Still waiting for the pm Gordon,how do you find the 105speed manifold by the way G? I'm not necessarily going up in capacity just yet, just improving on what I've got, I actually got it slightly wrong in my 1st post (misread my notes) ; although the flanges on my current manifold are 34mm and match the ports in the head the external diameter is 41mm which is more like 38mm (1 1/2") internally, the big bore one is 45mm external and 42mm (1 3/4")internally.Tristan,Mass Racing have already done the work with lengths etc.

I would advise caution here.

If MASS have worked out the lengths and the bore of the primaries, then changing the bore could have an impact on the engine performance....

If you increase the bore of the primaries without considering the length of the primaries, it will increase the RPM at which peak scavenging occurs, and at the same time reduce the scavenging depression - which is likely to reduce peak torque.... although peak BHP may benefit a little.

If the engine is not suffering exhaust back pressure or excessive exhaust gas speeds, then I would not think "going large" would be of major benefit. Sometimes bigger is not always better and I think a 38mm/1.5" bore is about as far as you need go with your engine capacity and exhaust valve size. As an educated estimate you might lose 4%-5% from your peak torque for an increase of 1%-2% peak power if you went up to 42mm internally - maybe that system is designed for 1800cc + engines?

[Copperbottom]

Mass designed the 1300cc one with the small diameter and 1600+ with the larger diameter.

[phoenix]

Originally Posted by Copperbottom

Mass designed the 1300cc one with the small diameter and 1600+ with the larger diameter.

Well that makes sense....

However, if they are "tuned" for the same peak torque or peak power RPM (whichever) then I'll bet a pound to a bucket of doo-dah that the primaries are of different lengths also.... which is what I was trying to explain above.....

[Copperbottom]

Ah huh, the one that's on there now is for a 1300 and hence smaller bore probably shorter primaries etc.

[phoenix]

Originally Posted by Copperbottom

Ah huh, the one that's on there now is for a 1300 and hence smaller bore probably shorter primaries etc.

Tell me the volume of one cylinder in your engine, what is the diameter of the exhaust valve and what is the peak valve lift, allowing for the rocker ratio, which I guess is 1.5:1 ? I'll let you know what I think would work.

But to give you something to consider, on a 2.1 litre engine (i.e. 525cc per pot) supercharged and producing around 325 bhp @ 8000 rpm, primaries with an OD of 42mm/ID of 39mm are adequate...

[ian_w]

A full race 1700 x-flow ( Classic Clubmans ) I have worked on used 41mm ( 1 5/8" ) primaries. Made about 170 bhp @ 8000 rpm.

[zefarelly]

1 1/2" primaries are adequate for a majority of Kent engines, in the past I've dyno'd a 1 3/8" primary manifold which lost no power but gained torque over 1 1/2", what is critical is manifold length as Gordon says, you need to consider cam timing in this as well.

Any off the shelf manifold will be a compromise, the 105 speed manifolds most definitely are, I have several, they work well to a point, I have modified 2 of them to suit specific engines

[GORDON STREETER]

This is the current one in more detail

[Copperbottom]

That looks about the same bore as the larger mass one. I don't think it would fit my engine though,it doesn't look like there's enough space for the starter motor.

[zefarelly]

your starter motors on the wrong side!

Gordon, nice clutch slave cylinder

If your going for a 1 5/8 primaries then I'd expect you to be building a 10k (£ & RPM) motor like Gordons, otherwise the smaller one will probably be more appropriate and likely give you a better spread of useful power/torque, what cam are you using?

[GORDON STREETER]

Originally Posted by zefarelly

Gordon, nice clutch slave cylinder

Yes one of my many fans gave it to me at Spa, although I can't remember who ?

[phoenix]

Originally Posted by Copperbottom

Is there a common rule of thumb regarding this? I currently use a mass racing small (34mm) bore 4-1 manifold on my 1700cc crossflow engine but they also produce one that is about 38mm diameter both with equal lengths.

I run a 2.25" system on the car with no reduction at joints etc.

Any info/ideas??

The ideal primary diameter is a function of cylinder volume and the revs at which you want peak scavenging, so a 2 litre engine will require larger primaries than a 1600 and a 1300 smaller than a 1600 - if they are all required to make peak torque at the same revs.

In reality with a 1300 (racing) engine peak torque is probably wanted at higher revs than with a 2 litre, so they might end up requiring the same primary pipe diameter - but it's no good guessing.

There is one 'magic' number involved in the calculations, and that is the ideal mean gas speed in the primaries for maximum inertia without drag, which would cause a rise in back pressure. This is around 90-92 metres per second. The only other variable is the speed at which you want peak scavenging, and this is largely determined by the choice of cam and the inlet tuning on a given engine.

So the area of the primary is given by the formula:

Piston area / optimum mean exhaust gas speed x mean piston speed at chosen engine speed.

The optimum internal diameter can then be calculated from the area this equation gives.


p.s. if the 34mm dimension you give is the ID, then reverse engineering your manifold tells me it is tuned for peak torque at 5850 rpm on a 1700 engine (83.5 mm bore and 77.62 mm stroke)

[Copperbottom]

Originally Posted by zefarelly

your starter motors on the wrong side!

No,my car's more modern so the clutch cable's on the other side

Originally Posted by zefarelly

what cam are you using?

Top secret but somewhat similar to a KC 254 and 264 combined. I use 8500 max rpm

[morninggents]

[QUOTE=GORDON STREETER;2619722]


Hope Gordon's noticed his water pump drive belt is missing!

[terence]

Originally Posted by GORDON STREETER

Yes one of my many fans gave it to me at Spa, although I can't remember who ?

Who are the other two??

[GORDON STREETER]

Behind the salad !

[Tim Falce]

Gordon, my wife just saw your picture and was disgusted, she says you should have cut the peppers up smaller.