Hello,

A discussion over in the V6 forum about increasing the compression ratio by either installing thinner head gaskets, or by milling the heads down led to a question about octane and chips. If one were running a Superchip already, then increased the compression ratio by say 1/2 point or 1 full point, is there a need (or better yet. . .can your programming account for this kind of mod?) for a re-burn? If this is done before ordering a Superchip from you. . .is this something you should know about?

I would assume so, but I just gotta hear it directly from the man, you know.


Thanks in advance for your insight. . .

 

Hi WrongdayJ,

There really isn't a "set" answer to that, it just depends on the end result in terms of the actual amount of cylinder pressure after the increase in static compression compared to the quality of fuel being used.

The way to bring cylinder pressure back down (via tuning, I should say) is to reduce timing, basically.

If the engine pings (detonates) after the compression is raised, and if using higher octane & better quality fuel doesn't quell the detonation, then it's time to retune it by pulling out some timing, and depending on the A/F ratios, perhaps add a bit of fuel.

 

Hmm.

I would think that supplying you guys with a dyno pull (with the A/F ratio information) would be the best way to 'see' problems like this. . .and correct for them in the tuning. . .right?

---WrongdayJ

 

Hi WDJ,

Such a small change to static CR alone generally doesn't have a *significant* effect on A/F in these MAF (mass airflow meter/sensor equipped) F-150 motors, as a lose general answer relating specifically to these low-CR engines; remember, the incoming airflow volume & density is already being measured, as is oxygen content in the spent exhaust stream during closed loop operation, so up to the limits of the system's ability, that (A/F) is compensated for via the MAF voltage signal to the ECU & ongoing adaptive strategy adjustments, which are at work not only on closed loop, but also affect open-loop operation.

It's a matter of exactly what affect would an increase of .5-1.0 CR have, which is to physically decrease combustion chamber volume, hence an increase cylinder pressure.

When you get right down to it, it's *ALL* about cylinder pressure. When you install an intake kit and gain power, you have increased cylinder pressure. By allowing more atmosphere (air/fuel mixture) to enter the cylidners, you now can burn more A/F mixture, and that raises power. Install a supercharger, same thing, only to a much greater extent. Install a cat-back exhaust, and you get the same thing, you increase cylinder pressure via increasing the amount of air volume the engine can ingest & process **when demanded by the driver,** by virtue of the reduction in restriction.

Internal combustion engines are nothing more than a glorified air pump, so power is all about effective cylinder pressure, or in other words, just how much atmosphere can we pack into the cylinders, as that is what determines how much fuel can be *effectively* burned, thus the amount of power made on demand.

When you decrease combustion chamber volume, which is what happens when you shave a cylinder head or decrease head gasket thickness, you increase cylinder pressure *all* the time, under any and all conditions. So while the same thing is happening (an increase in cylinder pressure) as when you do external bolt-on mods, it is of a somewhat different nature in that it is an *internal* engine change, so it's effect is present always, whether at idle, WOT or anything in between. And generally speaking, timing is the primary adjustment available to compensate for that decrease in combustion chamber volume. Swept area of the piston doesn't change, but the combustion chamber *volume* has changed, and for *that*, timing is the *primary* adjusment. Now, if the motor also happens to have A/F's off, then sure, adjustment would be called for. Assuming that we have a vehicle with safe healthy A/F's to begin with, increasing CR by that relatively small amount on one of these low-CR F-150 motors is not going to have a significant impact on A/F in reality.

Now, a completely different question is, "How to we custom tune an engine?" That answer would be sure, get the dyno data to include both power plots as well as accurate A/F testing (which requires installation of a proper wide-band O2 sensor located *before* the catalytic converters (no tailpipe sniffing or using the vehicle's own factory narrow-band O2 sensors!) as *part* of the data needed to dial the motor in, you bet!

You can have perfectly safe A/F's but still have unsafe conditions in the cylinder, A/F's are not the be all and end all of tuning; while they are indeed important, that is just 1 reference point. You could be @ 10:1 A/F & still have too much or too little timing, etc.

Sorry to go on at length, I hope that answers your question/concern a bit better, & please feel free to give us a call if you'd like to really cover this properly over the phone, ok?

 

Thanks Mike. . .will do.


---WrongdayJ