2JZ VVTi - Stock VS. Aggressive Lift/Duration Cams VS. Cam Advance VS. Spool Up VS. VE

Hi all,

Preface:

For the past 3 years, I've had a stock long block 2JZ with a pretty well tuned VVTi tables mated with stock camshafts on ECUMasters Black.

Just recently, I finished my build with a 10:1 3.4L BC Stroker mated with 272 BC Cams.

This is mated with a Divided T4 1.06 GTX3582R Gen II.

With the stock displacement setup- I am most confident that back pressure was not an issue with this turbocharger, however with now the 3.4L it may present issues concerning back pressure vs overlap.

Issue:

I feel like I should have better transient response now with the 3.4L, and higher comp ratio with the same turbo- but I don't, with my current VVT settings. Perhaps too much back pressure and too much valve intake overlap in VVT table is the cause?

On my stock long block, I used to run 30-40 degrees of cam intake advance on spool to full boost transition, and have it down to 0-10 degrees by 4500 RPM.

Now with the new 272 cams, higher lift and duration- we are technically changing the valve overlap MECHANICALLY through the Cam Specs and introducing MORE static overlap.

Tuners-

Does it make sense to PULL some cam advance, and retard the intake cam because of the natural higher duration of the new cam?

Or at this point because it is such an aggressive cam- there is no need to advance the intake cam on spool/transient since it will only hurt low end power more?

Another thought is back pressure. I exchanged email with Jay at REALST and his thoughts were that my turbo is WAY too small for my engine, and that I should be running a 6870 (70mm turbine wheel) or a 7175 (75mm turbine wheel) as opposed to my current GTX3582R Gen II with a 62mm turbine wheel.

This introduces another factor in tuning for proper cam advance. Too much back pressure with too much overlap in transition will technically hurt spool up since it can reject some of the incoming fresh air charge.

I would go on the dyno and play around with VVT vs Overlap vs TQ vs VE but I am currently in Alaska for the Summer so I am stuck with theory and "virtual" tuning right now- so I made a couple of pulls on the road, and logged it before leaving my car back home.

Log is 4th gear pull on CD009 350z Transmission, 3.4L 2JZ, 272 BC Cams, 92 Octane with Sophisticated Progressive Direct Port Water Methanol Nozzles (95cc per nozzle, and 250cc pre throttle body)

We can see that from 3500-3800RPMS, we are at 35 deg Cam Advance, and as soon as the cam advance dips down, you can see that the MAP curve spikes up. Which makes me believe that indeed too much overlap is hurting the spool and the engine. From 3500-5500 RPM @ 35 deg cam advance, according to the log curve- it seems that the engine is "LAZY".

I have also attached my cam advance table. Top picture is the Cam tables when I did my pull in the log shown.

Bottom is theoretical cam advance that I might try out when I get back from Alaska in September.

I plan on renting a load bearing dyno when I get back and mess with all of this. Did not have much time after the build before I had to leave for Alaska.

Thoughts/Comments?

Cheers,

Johannes

Hi Johannes, I'd agree with jay that the 3582 is too small to get the most out of the engine, however it does all depend on what you want to achieve in terms of power vs response. It's easy to throw a larger turbo on the engine and see wholesale gains in power but for a street car the additional lag can make the car less enjoyable to drive. There is also the aspect of exhaust back pressure to consider and you're correct that back pressure and cam timing both go hand in hand. I certainly wouldn't be surprised if you found you needed less advance in the spool up regions with the larger engine and larger cam. Ultimately though I don't go into a dyno session with a set direction to move in with cam timing - instead I let the dyno show me what the correct cam timing should be. I think you'll find that once you get the opportunity to get back on a dyno you'll probably find that your optimal cam timing might end up somewhere between your current VVT map and your theoretical one.

It is also worth mentioning that even with VVT to help you, moving to a larger cam will usually sacrifice bottom end performance in order to improve VE at high rpm and this can of course affect your spool up. The larger capacity will help but you still may see a net deficit in spool.