Tuning Tips: Sequence
Klaus recently commented on the issue of tuning sequence. Here are his thoughts (extracted from the thread):
The usual tuning sequence is to tune AFR first. Then tune timing. Then modify AFR in specific areas to possibly get a little more torque in specific cells. Then adjust timing in those areas again to MBT.
For example, some combustion chambers like to produce max torque at 13.x AFR at some load number. But let’s say you can get there only with race gas. Playing around with the AFRs and timing at that area you get the best compromise between torque and octane when running pump gas. In the higher RPM areas, where you have lower VE, it might be possible to lean out more to the optimum for the chamber as the octane requirements change with rising RPM as well, depending on the chamber design. This of course assumes you are not cooling system limited.
Start the cam timing with conservative AFR and ignition timing. Changing cam timing changes the VE curve of the engine. AFR and ignition tuning depend on that. Always change the most independent variable first. Best cam timing depends on your goals and the intake and exhaust system of the engine. This changes VE.
Fuel flow is relative to VE and load.
So adjust AFR next. Ign. timing depends on VE, AFR and load. So do that last.
Then go back and forth to fine-tune AFR and ign. timing. With a turbo SOHC engine, adjustable cam gears are not needed unless you are experimenting with different cams. Cam timing changes how the engine breathes, and can maximize that breathing by taking advantage of natural resonances in the intake and exhaust. This is not applicable on a turbo. With compressed air (turbo) you can make a corpse breathe. There is some advantage with adjustable cams on a DOHC for a turbo.
Tune the cams (without turbo) as you would for a truck: low RPM torque peak. This reduces “apparent” turbo lag and helps initial spooling. With cam timing changes on n/a (not electronically controllable) you can have one of three mutually exclusive goals:
۱٫ Max Power (move torque peak up and maximize it at higher rpm’s)
۲٫ Max torque at low end (low rpm torque peak for towing)
۳٫ Economy (leave intake open partially into compression stroke, or open earlier during exhaust stroke. Makes compression stroke essentially shorter than expansion and rises efficiency, but costs power and torque).
Of course your intake and exhaust system and valve/port sizes determine what’s achievable. A truck intake/exhaust won’t make a truck engine into a high-rev race engine with only cam changes.
Cam tuning procedure:
Set a rich AFR of say 11 or 10.5 across the WOT row and keep low ign. advance.
With a speed density or alpha-n system you log AFR, RPM and TPS. Make WOT acceleration runs in high gear (at least 10 secs for a sweep, using brake as load if needed to stay at legal speeds).
Select the WOT part of the run only and look in LogWorks at an x-y plot of AFR over RPM of the selected data. Changes in cam timing that result in better breathing (or VE) will make AFR leaner because more air enters. Your goal is to make AFR as lean as possible in as many RPM ranges as possible, but especially in the area you are interested in, depending on your goal. With a MAF based system you log MAF voltage or inj. duty cycle instead of AFR.
Once you are done with that, don’t touch cam timing again for further tuning. After that you get your AFR into a more reasonable range.
Until next time… Keep On Tuning!