Advanced Engineering Tech - Question on what .050 duration will be after changing events
07-28-2011, 06:36 PM
Okay, I was playing with a engine simulator and changing valve events to see the effects on over all power. I started with a cam that has these spec's-
Avd. Duration- 270/276
.050 Duration- 218/224
Valve Overlap 51
Ramp Acc. Rate- 3.14
I changed the exhaust closing and LSA a bit, but I need to know what the .050 numbers are now...
here is the new specs...
the specs are comp grind that started life as a 218/224 @ .050...
270/278 adver. duration
Lift- .544 .544
Seat to Seat timing-
Intake Centerline- 107
Exhaust Centerline- 114
Acc. Rate- 3.14
07-29-2011, 10:03 AM
By delaying the exhaust closing by 2 degrees, you've increased the duration of the exhaust lobe 2 degrees at .050" and at the advertised value (usually where the valve is virtually fully closed at about .004 to .006" of lift). All of this assumes that you have same lobe series profile.
If your exhaust ports don't flow very well, this 2 degree increase would maybe help HP a tiny bit above 5,000 RPMs and hurt torque a tiny bit in the lower RPM register. I assume that you are looking at an engine dyno simulator software program and this is just a "what if" exercise. You can learn some basic principles from this sort of thing. However, If I already had the cam you're describing and are basically satisified with it, I wouldn't change a thing as the 2 degrees extra exhaust duration won't really do much in the end.
07-29-2011, 10:28 AM
Thank you so much Steve! Very informative! The engine simulator I have been running shows this as helping hp and trq about 3500rpm. It should a +2 increase in trq and a +3 in hp.
I do not have the cam yet, I was just playing with the sims to see "what if" as you say.
Oh and the motor is a stock L31, with stock OE vortec heads, 1.94 intake, 1.50 exhaust valves. 9.4:1 compression, Edelbrock Performer RPM Air Gap, 750 carb.
08-01-2011, 09:10 AM
I wouldn't change a thing as the 2 degrees extra exhaust duration won't really do much in the end.
Steve is correct, you are chasing something that is only possible to guesstimate in theory. Today's cams are ground so inconsistently, and often vary so significantly from claimed spec, that you need to consider much larger changes. More on that in the 'lobe dynamics' thread. Furthermore, ignoring differences in various pieces of software, the output will only be as good as the model. They are certainly entertaining, but when it comes to execution, you'll have a higher probability of success deferring to someone w/ practical experience & data. I hope you won't take that as my discouraging you from playing with the software to attempt to glean something. :)
08-01-2011, 09:21 AM
No Sir, I do not. I really appreciate all the time answering my questions. Thanks!
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