Does a cold air intake add power on a 4.0?

[srimes]

All PCMs have long term and short term fuel trim tables which it adjusts over time while you're driving. If you disconnect the battery or otherwise force the PCM into relearning, you might notice that it runs a bit different for a while as it rebuilds those tables. It's constantly reading the narrowband O2 sensor voltage in closed loop and adjusting the fuel to run at stoichimetric which is 14.7:1 for pump gas. It uses those tables to predict what is needed. Changing the restriction in the system might mean for 20 minutes of driving the PCM is "learning" a change. But when we go into wide open throttle, it kicks out of closed loop (after some time) and moves to the open loop tables and those are static MAP and RPM driven.

I thought LTFT applied to open loop. Apparently that's true for many cars, but not Chryslers.

A CAI really doesn't seem to have any performance effect at all in lower rpms in WOT and I would assume zero effect in part throttle into even higher rpms. No change in MAP, no change in rpm = no change in the PCM commands to the spark or fuel. So there is really nothing to learn or unlearn. Maybe up in the 4000rpm range you start to see some part throttle situations where the CAI is flowing better than stock and you see less pressure dip in the intake. Maybe it's a bit more efficient in certain situations.

A CAI makes a small peak change that is measurable but not really significantly noticeable. Off-peak the difference only gets smaller.

The main noticeable difference is sound. Like if your girl gains 3 pounds and asks if she's fat now. Yes you can reliably measure the difference. No you can't really see it, and you wouldn't be able to tell anything changed if she wasn't making new noises now.

It's kinda like the placebo effect of a 62mm throttle body. The stock motor can't pull more air into the cyclinder than the stock throttle body supplies, so it's not a restriction. But the larger bore of a 62mm throttle body means that the throttle cracks open more with less push of the pedal, making you "think" the throttle is sharper or more responsive. On the dyno they're ineffective on a stock 4.0L at wide open throttle.

"The stock motor can't pull more air into the cylinder than the stock throttle body supplies, so it's not a restriction" - This is bro science and goes against the basic principles of hot rodding and fluid dynamics. It's wrong, but practically speaking it's right. Like getting a 1% raise, yes it makes a difference but no it's not significant, practically.

 

[GoldenGorilla]

I believe LTFT isn't really learning YOUR driving though. It's learning operating conditions. And if you understand the limitations of narrowband O2 sensors, as soon as you go into power enrichment, it will peg the O2 sensor at max voltage (rich). You're only adding 10% or so to the fuel input and you're already outside of it's operating range. Fuel attributes vary from place to place across the country. The LTFT is actually driving toward lambda = 1. Pump gasoline lambda is said to be a 14.7:1 ratio, but it's actually in a narrow range and humidity and altitude factor as well and the fuel tables may not have that degree of precision. The O2 sensors above and below the cat allow you to calculate the efficiency of combustion in closed loop very accurately and fine tune it for local altitude/fuel/humidity. The Chrysler version is also doing this adjustment, but only in closed loop and I'm not saying this is a good thing.

The main noticeable difference is sound. Like if your girl gains 3 pounds and asks if she's fat now. Yes you can reliably measure the difference. No you can't really see it, and you wouldn't be able to tell anything changed if she wasn't making new noises now.

Yeah, I didn't really want to open that door, but as soon as you step on the throttle, even at RPMs where there is no effect, you're gonna hear that CAI "growl". The Windstar intake is pointed right at your face in the cowl doing the same. There is a definite emotional and cognitive placebo effect. To the extent that car makers know this, many of them actually port intake sound into the interior in performance models (but buyer's think it's exhaust). Is it having an effect on the physics of the car? No. Does it have an emotional affect when driving? Definitely. In most cars, CAIs rarely produce more than 0.5% added power. The fact the TJ consistently produces around 6-8% additional HP with any CAI is very unusual in modern cars. And yet... For the time being I've pulled the CAI off my car. We are doing some longer trips, I feel I'm not getting any real world benefit from it, and my copilot finds it annoying.

"The stock motor can't pull more air into the cylinder than the stock throttle body supplies, so it's not a restriction" - This is bro science and goes against the basic principles of hot rodding and fluid dynamics. It's wrong, but practically speaking it's right. Like getting a 1% raise, yes it makes a difference but no it's not significant, practically.

I know. It's specific to the stock 4.0L motors with stock cams, over-simplified,100% true and incorrect. You could pretty quickly change those dynamics and benefit, but rarely with bolt on mods. Specifically I would go for a cam/springs that allowed the motor to rev to 6000+ and immediately the 62mm throttle body would make a measureable difference on the dyno. Newcomer even did a video where they used a dual 70mm TB intake vs single 70mm TB and saw pretty significant gains with dual TBs on a big 5.0L stroker. A CAI + 62mm TB on a stock motor will do nothing but make your wallet lighter.

Note: if you're reading this and not familiar with the terms - power enrichment means adding more fuel to cool the combustion chamber under higher demand. Stoichimetric combustion is lambda =1 or 14.7:1 for gasoline and it means that all the fuel and all the oxygen reacted in the cylinder, providing the most efficient power, but usually not the most possible power. The base open loop map targets stoichimetric. The short term and long term fuel trims indicate variance from the base fuel map. The 4.0L is thought to make the most power around 12.8:1. Power enrichment can take you into better power production, but it can also go too far in favor of cooling the cylinder with excess fuel that goes unburned.

 

[ColoJeep]

Power enrichment can take you into better power production, but it can also go too far in favor of cooling the cylinder with excess fuel that goes unburned.

Which really means you need some boost !

 

[Jezza]

Wow, lots to unload here. In my experience with the JTEC and NGC long and short term trims have no effect on WOT. On the NGC with the turbo it was hanging onto the long term for a few seconds into WOT, but then would drop the trim and default to the base tables.

I still drive the '97 almost daily and nothing has changed how it drives at all. The PCM will work to drive the air/fuel ratio to 14.7 when in closed loop. If it feels like you are loosing power after a tune or a mod, it's a good indication that the tune wasn't very good or the mod made a wild change in the airflow without a tune. A good tuner will log the trims over a range of driving conditions and set the base fuel tables to reflect the engines efficiency. Ideally a good tune would have long and short term trims close to 0%.

While bolt-on mods are not going to yield huge gains on the 4.0 some do add power. While it's true the cylinder is a fixed volume the 4.0 doesn't even get close to filling that volume under stock conditions. Which is why removing restrictions can add power even on a stock engine. It's the basic theory of volumetric efficiency. I believe the stock 4.0 to be around 75-80% VE under the best conditions. The cylinder head and low compression have a lot to do with that.

Yes, boost

 

[SkylinesSuck]

I thought LTFT applied to open loop. Apparently that's true for many cars, but not Chryslers.

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A CAI makes a small peak change that is measurable but not really significantly noticeable. Off-peak the difference only gets smaller.

The main noticeable difference is sound. Like if your girl gains 3 pounds and asks if she's fat now. Yes you can reliably measure the difference. No you can't really see it, and you wouldn't be able to tell anything changed if she wasn't making new noises now.



"The stock motor can't pull more air into the cylinder than the stock throttle body supplies, so it's not a restriction" - This is bro science and goes against the basic principles of hot rodding and fluid dynamics. It's wrong, but practically speaking it's right. Like getting a 1% raise, yes it makes a difference but no it's not significant, practically.

I do not think you could have summed up my understanding of how this works any better.

I think I'm going to bookmark this thread and just post a link every time somebody starts in with the usual nonsense they "learned" from one of Jerry's old posts quoting the father of the Rubicon he's close personal Facebook friends with