When I made the ports equal diameter, I got all kind of surging problems with stock individual cyl trim settings. I had to rework the trims it all the way and now #1 gets much more air and needs more fuel. You get to the point where balancing cylinders with more fuel at #1 is not possible. The reason GM restrict the air flow at #1, and some fancy drilling sizes on the other ports. I also remember that not all ports were equal at 6mm.
Odd. All others in mine are 6mm exactly.
I went up two drill sizes on #1 to 3/16". Hopefully it wont wreck the tune. I didn't see any other reason the left side would be getting less air than the right.
Prepare to start all over with cyl trims.
I made some progress on the first cold start. Still needs some fine tunning. While experimenting it turns out that running around 20-35*C coolant at startup it idles best at 14* advance and a liitle fatter mixture.[Tuned by sound and feel.]
I will let you know the best setting I came up when it gets fine tuned.
You can monitor the rpm rise after startup to tell if it needs more or less fuel. If the rpm rise alot like upto 1500rpm it has more unburned fuel left on the walls from prime pulses.
Another vote for the adjustent cylinder fuel robbing.
The lt4 settings are
Cylinder Multiplier
1 1.05 3 0.96
2 1.05 4 0.97
5 1.01 7 0.98
6 1.02 8 0.96
Somehow my trims closely resembles this pattern with the exclusion of #2 1.039 and #4 1.024.
Next on the list is to test it on running engine and starts to cut #4 to around 0.98 and see what happens.
The fuel robbing feature seems to be more pronounced on cammed car than to be related with ports size and headers lenght and style. Interesting to know the stock size idle air feed holes on a GM production lt4 manifold.
Can you source some pics on the vette air induction system like filter box, passages, maf mount and so on. Are you running some form of CAI kit. I suspect that the air induction system is the reason there are different scaling in MAF tables across platforms.
Last edited by kur4o; 10-18-2019 at 09:33 PM.
I also suspect it, times ten. In fact it's widely published that OEMs generally mock up the entire front of the car on a flow bench to develop their MAF transfer curves. Google: "helmholtz resonator". OEMs have been using them for > 10 years to smooth airflow in the intake ductwork for this specific reason (more accurate MAF transfer curves).Originally Posted by kur4o
Here's the stock y-body intake. I believe all MAF equipped LT-1s got this intake setup. I'm also fairly sure the ZR-1/LT-5 (read: 450 hp off the showroom floor) also got an identical airbox and MAF but with a slightly different shaped throttlebody bellows.
This might be closest to a ram air Pontiacs. The only anomaly in this tract is that the Y-body engine is slightly offset towards the passenger side of the car. Anything that came with an elbow in the air ducting will likely have vastly different MAF curves.
The recommendation from GM is at least 4 inch of straight pipe before the maf sensor, to get reasonable air flow readings. That is for the newer style slot type of mafs but I guess it can be applied to older setups too.
Thanks for the pictures. It looks like a funnel, before and aftre the maf and is vastly different from the f-bodies. The vette maf curve reads lower from 2-3% at low airflow to 5-6% at mid airflow upto 10-11% at high airflows, compared to stock f-body settings.
I am running the slp version of CAI and there is some problem with lower map readings at high rpm, high load at WOT, especially on third and forth gear I am wondering if it is airflow restriction of some kind. Have you measured the map drop at wot pulls under high rpm and load. What are the max AFGS you have reached with the new setup.
I haven't been doing much logging or tuning at WOT with this setup so unfortunately not much to report. I will try to pick through what logs I have but as I recall the few times I've gotten into it hard while logging were mainly corner exits or two lane highway stops / takeoffs where I didn't have much WOT time due to traffic. The majority of the hard throttle passes I've made thus far were a few un-logged joy rides I owed a couple guys that borrowed me tools and such where it broke the tires loose around 5500 rpm in 2nd and immediately bounced off the rev limit.
What I've read on the subject of MAF tuning (primarily Banish's stuff) has me thinking it will be something I touch as a very last resort if the wideband is still showing lean during WOT passes after I've removed as much as I'm comfortable with from the injector constant. The accuracy of the instrumentation I have to work with is inferior to what GM was using in the 90s by orders of magnitude, and I have a lot of unknowns like the modded 42lb mystery injectors.
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