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25 Jan 2010, 22:29 (Ref:2619427) | #1 | |
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Exhaust manifold primary diameter to port/valve size ratio
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?? |
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25 Jan 2010, 23:07 (Ref:2619445) | #2 | ||
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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.
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26 Jan 2010, 13:41 (Ref:2619664) | #3 | ||
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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... |
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26 Jan 2010, 15:35 (Ref:2619722) | #4 | ||
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Balls of steel (knob of butter) They're Asking For Larkins. ( Proper beer) not you're Eurofizz crap. Hace más calor en España. Me han conocido a hablar un montón cojones! Send any cheques and cash to PO box 1 Lagos Nigeria Africa ! |
26 Jan 2010, 16:25 (Ref:2619745) | #5 | ||
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Ah, ok. If he's putting a bigger engine in, then chances are an increase in diameter would be appropriate...
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26 Jan 2010, 18:42 (Ref:2619833) | #6 | |
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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.
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26 Jan 2010, 19:42 (Ref:2619878) | #7 | ||
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steady on copper you will be needing a G suit in that little escort ...lol
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26 Jan 2010, 19:52 (Ref:2619892) | #8 | ||
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Quote:
Sadly I won't be doing much racing in it this year though Woody Last edited by Copperbottom; 26 Jan 2010 at 19:58. |
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26 Jan 2010, 20:10 (Ref:2619907) | #9 | ||
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Quote:
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? |
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26 Jan 2010, 20:12 (Ref:2619909) | #10 | |
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Mass designed the 1300cc one with the small diameter and 1600+ with the larger diameter.
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26 Jan 2010, 21:07 (Ref:2619945) | #11 | ||
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Quote:
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..... |
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26 Jan 2010, 21:55 (Ref:2619977) | #12 | |
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Ah huh, the one that's on there now is for a 1300 and hence smaller bore probably shorter primaries etc.
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26 Jan 2010, 23:18 (Ref:2620022) | #13 | ||
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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... |
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27 Jan 2010, 10:23 (Ref:2620216) | #14 | ||
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A full race 1700 x-flow ( Classic Clubmans ) I have worked on used 41mm ( 1 5/8" ) primaries. Made about 170 bhp @ 8000 rpm.
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27 Jan 2010, 17:36 (Ref:2620446) | #15 | |
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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 |
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27 Jan 2010, 17:59 (Ref:2620462) | #16 | ||
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27 Jan 2010, 23:12 (Ref:2620674) | #17 | |
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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.
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28 Jan 2010, 08:19 (Ref:2620820) | #18 | |
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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? |
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28 Jan 2010, 08:32 (Ref:2620834) | #19 | ||
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Balls of steel (knob of butter) They're Asking For Larkins. ( Proper beer) not you're Eurofizz crap. Hace más calor en España. Me han conocido a hablar un montón cojones! Send any cheques and cash to PO box 1 Lagos Nigeria Africa ! |
28 Jan 2010, 10:05 (Ref:2620883) | #20 | ||
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Quote:
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) Last edited by phoenix; 28 Jan 2010 at 10:28. |
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28 Jan 2010, 10:13 (Ref:2620887) | #21 | |
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28 Jan 2010, 22:27 (Ref:2621326) | #22 | ||
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30 Jan 2010, 11:08 (Ref:2622187) | #23 | ||
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30 Jan 2010, 14:25 (Ref:2622269) | #24 | ||
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Balls of steel (knob of butter) They're Asking For Larkins. ( Proper beer) not you're Eurofizz crap. Hace más calor en España. Me han conocido a hablar un montón cojones! Send any cheques and cash to PO box 1 Lagos Nigeria Africa ! |
30 Jan 2010, 15:07 (Ref:2622285) | #25 | ||
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Gordon, my wife just saw your picture and was disgusted, she says you should have cut the peppers up smaller.
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