Quick Question... Valve Overlap and Spool Times
12-10-2003, 12:55 AM
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Quick Question... Valve Overlap and Spool Times
Does valve overlap have any bearing on how quick a turbo will spool? I understand that tuning out valve overlap on a boosted engine has benefits, but exactly what benefits does it give and why? Thanks. 
12-10-2003, 01:44 AM
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Re: Quick Question... Valve Overlap and Spool Times (pissedoffsol)
Overlap deals with scavenging, dynamic compression, and many other factors besides powerband placement.
Any other ideas?
Any other ideas?
12-10-2003, 01:47 AM
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Re: Quick Question... Valve Overlap and Spool Times (Unsung EM1)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Unsung EM1 »</TD></TR><TR><TD CLASS="quote">Does valve overlap have any bearing on how quick a turbo will spool? I understand that tuning out valve overlap on a boosted engine has benefits, but exactly what benefits does it give and why? Thanks. </TD></TR></TABLE>
The momentum from overlap will help pull in the intake charge when you are NA. With boost you don't want it just leaving the cylinder through the exhaust valves.
The exhaust gas pressure can flow backwards into the cylinder once you build boost.
</TD></TR></TABLE>The momentum from overlap will help pull in the intake charge when you are NA. With boost you don't want it just leaving the cylinder through the exhaust valves.
The exhaust gas pressure can flow backwards into the cylinder once you build boost.
12-10-2003, 01:52 AM
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Re: Quick Question... Valve Overlap and Spool Times (vteg)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by vteg »</TD></TR><TR><TD CLASS="quote">
The momentum from overlap will help pull in the intake charge when you are NA.</TD></TR></TABLE>
Correct, from my understanding, the exhaust pulses cause a low pressure behind the flow, and in effect, creates vaccume on the intake stroke, pulling in more air.
Under boost with reduced valve overlap, are there benefits however? Like quickened spool, etc.? Or is the main purpose just to retain boost in the cylinder?
The momentum from overlap will help pull in the intake charge when you are NA.</TD></TR></TABLE>
Correct, from my understanding, the exhaust pulses cause a low pressure behind the flow, and in effect, creates vaccume on the intake stroke, pulling in more air.
Under boost with reduced valve overlap, are there benefits however? Like quickened spool, etc.? Or is the main purpose just to retain boost in the cylinder?
12-10-2003, 02:10 AM
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Re: Quick Question... Valve Overlap and Spool Times (Unsung EM1)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Unsung EM1 »</TD></TR><TR><TD CLASS="quote">Under boost with reduced valve overlap, are there benefits however? Like quickened spool, etc.? Or is the main purpose just to retain boost in the cylinder?</TD></TR></TABLE>
I don't know. Too much is bad. But I don't know what the power output difference would be between a little overlap, and none. Hopefully someone will chime in that has played with different cams/cam settings and noted the differences.
In theory, the exhaust gas pressure should always be higher than the boost pressure in the intake manifold. So any overlap should cause reverse flow. Theories don't always work on real cars though.
Subaru's adjustable cam system(avcs) on the jdm cars, reduces overlap at the rpm that the turbo spools. This helps give them better low end torque and power after boost comes in.
I don't know. Too much is bad. But I don't know what the power output difference would be between a little overlap, and none. Hopefully someone will chime in that has played with different cams/cam settings and noted the differences.
In theory, the exhaust gas pressure should always be higher than the boost pressure in the intake manifold. So any overlap should cause reverse flow. Theories don't always work on real cars though.
Subaru's adjustable cam system(avcs) on the jdm cars, reduces overlap at the rpm that the turbo spools. This helps give them better low end torque and power after boost comes in.
