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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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Why the ITB's so high on EFI builds?
For those of you who have put ITB's with EFI on your 911, why do you have them so high? Is it due to throttle geometry, clearances of something or other, ICV clearance, or what?
I can't get the thought out of my head of putting ITB's nearly right on the intake port with no intake tract like an E46 M3 or sportbikes for my setup. I'd imagine the throttle response would be unbelievable..... Last edited by Tippy; 02-23-2013 at 07:33 PM.. |
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Registered
Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 4,184
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Quote:
Another thing that is only useful on N/A motors is tapered bore. Both take advantage of tuned intake pulses (reversion?) to over-fill the intake at a specific range in the power curve - and are pretty much irrelevant for forced induction, to my way of thinking... Quote:
Together with a stout plenum that won't oil-can under boost, you'd be all set... Might need to re-tune the EFI slightly. Must.. Resist.... Spending.... More.... Money..... ![]()
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'77 S with '78 930 power and a few other things. |
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Registered
Join Date: Jul 2001
Location: Portland Oregon
Posts: 7,007
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Tall throttles help contain intake reversion in engines with long-duration camshafts.
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Steve Weiner Rennsport Systems Portland Oregon (503) 244-0990 porsche@rennsportsystems.com www.rennsportsystems.com |
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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Quote:
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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Quote:
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7.0:1 > 11.3:1 > 7.0:1
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Longer stack produces peak torque at a lower rpm, shorter stack produces peak torque at a higher rpm. Since ITBs are a fixed length, you are effectively optimized at a single rpm, and not optimized above or below that rpm. Variable length would be utopia - hence Varioram.
Read about intake reversion wave and ram tuning to understand what's going on inside the stack. |
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Quantum Mechanic
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Back in the day some can - am cars had velocity stacks of two different length, claimed to spread out the torque curve. Don't know about how much it helped - looks cool tho
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Mark Petry Bainbridge Island, WA 81 SC |
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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Quote:
So, do ITB's alter lopiness coming from a common plenumed intake or not, being's all other engine parameters unchanged? Last edited by Tippy; 02-24-2013 at 01:43 PM.. |
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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When I mean lopiness, I mean "pop-pit-tee....pop-pit-tee....pop-pit-tee" when an engine idles with long-duration cams.
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Registered
Join Date: May 2010
Location: atlanta
Posts: 1,979
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Quote:
Yes. Generally single throat per cylinder can handle much more cam overlap than a common plenum. An engine with a "lopey" cam will idle smoother with ITBs vs a common plenum. Also, it is common in the motorcycle world to have multiple velocity stack heights on the same engine to broaden the power curve. |
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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That was my theory; less lopiness since you're not cross contaminating cylinders like a common plenum.
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Registered
Join Date: Jul 2001
Location: Portland Oregon
Posts: 7,007
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Quote:
Common-plenum, single-throttle intakes are extremely sensitive to lobe center placements and will not tolerate narrow LC cams due to reversion issues. 112 degrees is MAX and 114 degrees is much preferred for these intake systems. ITB's tolerate this much better, even with a resonance-plenum intake so the limit is really about the engine's configuration and purpose. This is really a very complex subject and I've just barely touched on this,... ![]()
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Steve Weiner Rennsport Systems Portland Oregon (503) 244-0990 porsche@rennsportsystems.com www.rennsportsystems.com |
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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Ok, I have some studying to do then....
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Moderator
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As Steve says lots of overlap isn't handles well by street common plenum single throttle intakes because of exhaust reversion
The narrow lobe centers are one tool used used to get more valve overlap which is useful in a motor that has tuned headers to generate hp at specific rpms(usually high) another is lope shape/height ![]() street cams(dotted) vs race cams (solid), not the extra overlap in the race cams, this is where hp comes from. The overlap time where booth valves are oped at the same time is sometimes called the 5th cycle as the effects derived from the headers suck additional fual/air mix into the combustion chamber. Modern variable cam timing engines can have the best of both worlds, little overlap for low speed drive-ability/fuel economy/emissions and increased overlap for high rpm hp ![]() ![]() 993 vram used long tubes for low rpm and short tubes w/ plenum resonance for high speed hp, the difference between working and non working hp is ~50hp, ie when the tunes stay short at high rpm you lose ~50hp
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Bill Verburg '76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone) | Pelican Home |Rennlist Wheels |Rennlist Brakes | |
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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You mean stay long they lose ~50hp?
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Moderator
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yes
transitions are @ 5160 and 5920 ![]()
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Bill Verburg '76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone) | Pelican Home |Rennlist Wheels |Rennlist Brakes | |
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,126
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Gotcha
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