Help understanding VVT...
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From: Portland, Oregon
Help understanding VVT... I am trying to gain a better understanding of the LS VVT system, and its affects of engine performance...
For the sake of simplicity, let's use the L92 VVT cam as an example. It's a 198/209 .500/.500 115...
IVO 16 ATDC
IVC 34 ABDC
EVO 39.5 BBDC
EVC 10.5 BTDC
From my limited understanding, it starts with up to ~7* advanced, and can retard up to ~27*. Would this effectively make the cam equal to these (respectively)?
(ICL 108*) 198/209 .500/.500 115+7
IVO 9 ATDC
IVC 27 ABDC
EVO 46.5 BBDC
EVC 17.5 BTDC
(ICL 142*) 198/209 .500/.500 115-27
IVO 43 ATDC
IVC 61 ABDC
EVO 12.5 BBDC
EVC 16.5 ATDC
Overlap remains constant at -26.5*, which seems to indicate that retarding the cam can only provide limited results. Which makes it seem wildly excessive to offer 27* of retard.
And, since the VVT moves both intake and exhaust lobes together, is there a point of dimishining returns retarding the cam that far?
I am no expert on non-VVT cams, either, so the difference in events does not immediately mean anything to me... and I could use some help understanding what the diffrrences are. And what those differences do.
For the sake of simplicity, let's use the L92 VVT cam as an example. It's a 198/209 .500/.500 115...
IVO 16 ATDC
IVC 34 ABDC
EVO 39.5 BBDC
EVC 10.5 BTDC
From my limited understanding, it starts with up to ~7* advanced, and can retard up to ~27*. Would this effectively make the cam equal to these (respectively)?
(ICL 108*) 198/209 .500/.500 115+7
IVO 9 ATDC
IVC 27 ABDC
EVO 46.5 BBDC
EVC 17.5 BTDC
(ICL 142*) 198/209 .500/.500 115-27
IVO 43 ATDC
IVC 61 ABDC
EVO 12.5 BBDC
EVC 16.5 ATDC
Overlap remains constant at -26.5*, which seems to indicate that retarding the cam can only provide limited results. Which makes it seem wildly excessive to offer 27* of retard.
And, since the VVT moves both intake and exhaust lobes together, is there a point of dimishining returns retarding the cam that far?
I am no expert on non-VVT cams, either, so the difference in events does not immediately mean anything to me... and I could use some help understanding what the diffrrences are. And what those differences do.
The IVC has a HUGE influence on engine output, so closing the intake valve sooner or later depending on needs of the driver is where the benefit is. Sure, phasing the lobes separately like a DOHC engine would be better, but this is what we're stuck with.
If I remember correctly, I'm pretty sure the VVT camshafts are ground with a lot of advance in them already compared to a typical OEM LS cam, so when the phaser is at full retard, it really isn't that crazy. As a side note, there are two different LS phasers and cam cores as well. The two phasers have different amounts of cam retard capacity and the cores have different centerlines ground in. You have to match the cam core to the phaser.
If I remember correctly, I'm pretty sure the VVT camshafts are ground with a lot of advance in them already compared to a typical OEM LS cam, so when the phaser is at full retard, it really isn't that crazy. As a side note, there are two different LS phasers and cam cores as well. The two phasers have different amounts of cam retard capacity and the cores have different centerlines ground in. You have to match the cam core to the phaser.
The LS VVT single cam only retards so you get your cam ground with as much advance as you want ( or can fit ) and using the phaser you command the amount of cam retard as RPM rises. The OEM use is mostly used the reduce pumping losses during low throttle of off throttle coast. The engine spins easier with reduced compression which increases fuel mileage and thats what the huge swings in ICL are used for. That said there are some performance gains to be had much the same as any cam grind you set your ICL which helps optimize airflow at given RPM and intake air speeds. The general consensus over the years is VVT cams done right are good for 10hp or so more than a static grind of the same values and some torque gains down low respectively to cam specs. Of course the issue is you can go bigger with cam on static and make more and the bigger the cam gets the less you can move the cam around because of the duration and PTV and SCR and all that you start to use it all anyways so VVT becomes moot.
In a boost situation I have yet to see any solid reason to use a VVT grind. Once you enter boost land just add more boost to make more power or upgrade your boost volume with a bigger turbo/SC etc will yield considerably more gains than tweaking ICL with the cam in the tune up.
the GM single VVT is cool, it doesn't hurt anything and has proven extremely reliable and Texas Speed had ( has? ) the fastest stock cube cam only 6.2L that is a VVT grind.
In a boost situation I have yet to see any solid reason to use a VVT grind. Once you enter boost land just add more boost to make more power or upgrade your boost volume with a bigger turbo/SC etc will yield considerably more gains than tweaking ICL with the cam in the tune up.
the GM single VVT is cool, it doesn't hurt anything and has proven extremely reliable and Texas Speed had ( has? ) the fastest stock cube cam only 6.2L that is a VVT grind.
