| ichiwan2 |
Posted
Sunday, Dec 17th 3:32pm [Edit] [Quote] [IMS] [View car] |
Member
Post nr. 251
Kuala Lumpur, Malaysia
BMW320/6(e21) |
Read this article:
http://www.chevyhiperformance.com/techarticles/53439_cam_basics/index.html
The gist of the issue is that the above article said that split duration cams with longer exhaust vs intake improve performance over equal exhaust/intake duration.
Based on the note here:
http://e21.tricord.be/e21/about/notes/schrick.php
the duration of the intake for the split duration cam is longer that that for the exhaust.
So who's right?
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| imaradiostar |
Posted
Sunday, Dec 17th 8:59pm [Edit] [Quote] [IMS] [View car] |
Member
Post nr. 971
Nashville, TN
81 323i, 82 525i, 85 524td, 90 535i |
I think there are different goals in mind. Efficiency would point to a longer intake duration (assuming the engine is designed for this and is flowing the whole time) and a shorter exhaust duration so that you can fully utilize all of the power stroke. It's also a lot easier for pressurized exhaust to get out than atmospheric air to get in. If the exhaust port is the limitation on an old chevy then the longer exhaust duration makes sense. On most engines I would think the opposite- you're throwing away efficiency and poluting your incoming fresh air charge with excessive exhaust duration.
Metric Mechanic, Schrick, and a number of other companies all run longer duration on intake that on exhaust. I'm sticking with it on BMW engines!
jamie |
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| KenDanielson |
Posted
Sunday, Dec 17th 11:49pm [Edit] [Quote] [IMS] [View car] |
Junior Member
Post nr. 192
Petaluma CA
'82 320is, '78 320i |
Chevy engines benefit from longer exhaust duration because their exhaust ports are not as efficent as they could be. Some aftermarket heads are much better with raised and larger ports.
M10 exhaust ports are quite large for the engines displacement. I have heard people say the m10 is just a m30 minus 2 cylinders but there is MUCH more to it. Compare the port size on a 2.0L m10 head to those on a 3.0L m30 or even a 3.5L m30 for that matter....
ken |
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| ichiwan2 |
Posted
Monday, Dec 18th 5:25pm [Edit] [Quote] [IMS] [View car] |
Member
Post nr. 252
Kuala Lumpur, Malaysia
BMW320/6(e21) |
Thanks for the input. Clears things up a little bit. I guess the exhaust port for the M20 must be pretty good on its own. Wonder what was the philosophy/intent for the shorter exhaust duration - efficiency? emissions? more power?
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| Tricord |
Posted
Sunday, Dec 24th 3:35pm [Edit] [Quote] [IMS] [View car] |
Administrator
Post nr. 1002
Belgium, Europe
323i Baur |
I think it's a story to maximise inlet opening time without overlapping with exhaust opening. Since it doesn't penalize the engine too much to limit exhaust opening time, Schrick only increased inlet opening time on that particular cam to increase breathing.
That's how I see it anyway. |
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| Marquis_Rex |
Posted
Saturday, Dec 30th 4:35pm [Edit] [Quote] [IMS] [View car] |
Member
Post nr. 500
UK
BMW 323i 2.7-as featured in Total BMW Nov 2002,Porsche 911/993TT |
You cant generalise about cam timing or duration. I have designed cams and have measured profiles for litterally hundreds of engines. Im currently on vacation (In Finland) but from the top of my head here are a few examples:
BMW M52 uses a 228 inlet duration and around a 240 exhaust period.
Aston Martin V8 Vantage uses an identical 256 for intake and 256 or exhaust.
Jaguar V8 uses 240 intake and 230 exhaust
Porsche 993 Turbo uses 275 intake period and 250 exhaust.
The thing is, that there are many effects specific to each individual case that you cant always carry philosophies over.
For instance the BMW M52 has variable cam phasing so the normal "fixed cam phasing" rules dont apply. Honda S2000 uses a 264 "short" inlet period but a 336 long inlet period ad a much smaller 248 exhaust however this has variable duration as well as variable phasing.
For instance the camshaft I did for a high revving prouction V6 woked very very well with a longer duration exhaust, because it was a relatively unrestricted exhaust, very little adverse tuning or interference. When these profiles were carried over to a V8 they worked very poorly because of the uneven firing per bank which caused interference and also due to the 270 degree seperation connectivity which isnt ideal.
Im in the process of doing alot of extensive flow rig work on M20 heads along with simultion and POSSIBLY dyno work (if I can sneak some engines in!) and will be designing my own cam shaft- current indications show that a longer inlet period is beneficial.
So dont believe all the "Mad-Hatter-type" know-it -alls 
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