Big ass cam, no vacuum and now stock VVE appears to require a 50% cut????

[mowton]

No matter how long you do this tuning shit, there is always something new around the corner.

Working a 2008 Auto Vette with a MAST 246/260 on a 114 (25 deg overlap) and MSD Atomic Airforce intake. Overlap and duration are large and at idle we are only seeing like 6" of vacuum (80 kPa). At 2500 rpm, in neutral it is like 13" (57 kPa). Have smoked the setup with no issues but the cam was installed by the customer so we aren't sure about if it was initially degreed......

Tuned started in MAF only and had a hell of a time getting the idle and startup (especially hot restart) and found the issue to be the VE table in the idle area was twice what is should be with MAF at like 20 g/sec and Dynamic Airflow closer to 40 g/sec even though the MAF was dialed in and only required like a 5% tweaking (done in MAF only). This variance played havoc with the idle tuning and wasn't until I pulled 50% out of the VE in the idle area and I finally got it stabilize. Assumed this was due to the size of the cam and inefficiencies down low.

Set the tune into Speed Density to look at the rest of the table. We then ran the rpms up to 2500 (no load/neutral) and saw the same thing.....trims went to -50%+ and needed to cut the VVE values in half to bring them back in.....MAP was 57kPa at this point....again higher than normal.

So my question is has the lack of vacuum thrown off the VVE equations by a factor of 2 while the MAF values are within +/- 5% of stock.....just have never seen this big a change....

Thank you for your time and we are somewhat suspect of the cam install, push rod geometry or something else but would like to see if anyone else has experienced this large difference in VVE.

Ed M

[SultanHassanMasTuning]

-50%? thats crazy.

i have tuned several auto vettes and idle was around 75-80 kpa 800rpm

if your in doubt regards VVE, OS it and see how that works

[barum]

Ed, here's what a cam with 18 deg overlap did to LS7 VE. MAP at idle 67..68 kPa.



Sounds quite possible, especially if the engine in question is LS3, IMHO.

[mowton]

Ed, here's what a cam with 18 deg overlap did to LS7 VE. MAP at idle 67..68 kPa.



Sounds quite possible, especially if the engine in question is LS3, IMHO.

Thanks man...we finished it up with final changes in the -30 to -50% area and the motor "loved" it (as did the customer) so I always go with what "they" tell me :-) Just was very strange...I guess insufficient CR was partly to blame.

Ed M