Masochist

iTrader: (1)

Hi guys,

My M6 cammed LT1(w/ 24x) bucks and surges pretty hard below 2k rpm, especially when it's cold. It smooths out considerably when it warms up(not completely) but when it's cold, it just does not like light pedal pressure. I have had the car tuned which lessened the issue but did not remove it and the car performs well under WOT. My tuner seems to think that its due to vacuum, related to the size of the cam.

I was wondering if anyone else had some input on what might cause this. I doubt the vacuum theory simply because my brakes work fine all the time, my previous LT1 had issues with vacuum and braking ability at low rpm and did not surge even half this bad though the cam was nowhere near as "big".

The cam is 228/236 .624/.606 110 lsa

hrcslam

It's likely in the tune. Work on your VE tables below 2k. I have a 230/238 .565/.565 110+6 (m6) and it doesn't surge. It took a while to tune it out, but I did. I can post up my VE tables if you'd like to see what they look like. Also, my spark tables have a double hump in the curve to deal with driveability, it looks unusual but works great.

Vacuum is poor (65kpa at idle), but brakes work fine.

Masochist

iTrader: (1)

If you could post that I'd appreciate it.

I do intend to have the car retuned when the weather changes. I was just looking for any points of reference I could find so when I get the car to my next tuner I will get my money's worth lol.

hrcslam

Engine build details in sig, SCR is 11.8:1. Stock TB and Intake manifold. Hope this helps.

See here for my Spark Table.


Here's my Primary VE table.





And my Extended VE Table. You can see here that my VE is over 100%, I had to correct that with my PE tables (see below). I may change my cylinder volume constant to something larger then scale the VE tables accordingly, then adjust the PE tables to a constant. But this works for now. As I'm set up, my AFR (as of my last Dyno tune) is 11.9 to 12.9 averaging 12.5, so a bit rich.




Here's my PE tables for reference.



Your VE and PE tables may obviously be different. If you have a ported Intake manifold or larger TB, your VE below 2K may be lower. But above 5K may be higher. This should get you close though. I see you're running a 24x 411 PCM. I can send you a copy of my Tune, but it likely won't do you any good.

I'm going to be revisiting my VE tables (the range out side of PE) as I play with Steveo's $EE hack, so they may change a little bit in the near future. But nothing drastically I don't think as driveability with this tune is good.

steveo_

i bet it's inappropriate timing advance, not ve tables or AFR. lt1s tend to like a ton of timing advance, some builds need a lot of added timing for a particular RPM range.

can we see your timing table?

i also question why you'd really want to drive below 2000rpm on a cam which is going to be horribly inefficient in that range, but not saying you shouldn't be able to tune it out... just that driving there is odd.

hrcslam

I average 21mpg 65% highway with my tune, 1500 rpms all day at 65mph. All highway at 75mph, I'm pulling down over 24mpg. Best full tank I measured was 26mpg. Not too shabby for my cam.

Yes the cam is inefficient below 2k, but its inefficient at making power and you don't need a bunch of power cruising at 1500rpm. You can take advantage (somewhat) of that lower VE, by accounting for it with a drivability focused tune in that area.

My car runs smoother at 45mph in 6th now than it did stock. Although, it still doesn't like it (bogs, but doesn't lug or jerk).