CAI’s...do they actually work?

[travis]

I am extremely skeptical of things like this, but I keep reading so many positive reviews I can’t help but wonder if there is some truth there.

In a couple of months I’m going on a 4000 mile trip in my 1999 Suburban C2500. It is a stock L31 with a fresh tune up, only “mods” are the upgraded GM injector spider assembly and a custom Spintech dual 3” in/single 3” out muffler. I run stock weight full synthetic oil.

I’m trying to up the highway fuel economy a bit. While investigating different options one of the big things that keeps coming up is CAI’s. Claims are typically .75-1 mpg, up to as much as 1.5mpg gain on these suburbans. Is that even realistic? The stock air filter is huge, and it draws air out of the inner fender, so it’s not like it is breathing hot under hood air. Plus highway cruising speeds are only in the 2200-2400 rpm range so it’s not pulling any crazy rpms.

Any suggestions to help the current 12.7mpg highway average would be appreciated. 1.5mpg at this point is a pretty significant gain. A little extra power too is always a good thing, a stock 350 can only do so much in a nearly 6000 pound tank.

[pdq67]

What trans and rear end gears are in it now?

Plus rear tire height?

I have no idea at all, but due to just size and weight alone, maybe a fresh, stock type, 454 and a gear venders OD might help here?

Drop a 210 to maybe a 218 hy-cam in her to help up the low speed T. And get her up to at least 9 to 9.5 to 1 CR...

pdq67

PS., and I have a feeling that a stock low compression 500" Cad will beat the 350 here as far as mileage goes IF geared and tired right! Those big suckers only run from off idle to maybe 2400 rpm or so..

[travis]

4L80 trans, 3.73’s, stock sized 245/75-16’s that are roughly 30” tall.

I’m not swapping engines...it’s up for sale right now so I don’t want to put much $$$’s into it. I really wish I would have held out for one with the L29 or a slightly newer one with the 8.1. This thing is just too under powered to tow much of anything.

We bought a 2017 Expedition EL that does EVERYTHING better than this suburban anyway, so I don’t need another 7+ passenger tow vehicle any more

[hoffman900]

Focusing on driving - preserving momentum, minimizing deceleration / acceleration. That'll do more than any CAI could do.

[427dart]

This is the setup I run on my 2014 Ram with 5.7 Hemi. Pulls in the air from front grill area. Plus it looks like a Hemi should!

[Schurkey]

Inflate the tires.

Check the brakes for dragging.

Verify the O2 sensors are responding properly.

Check the data stream for short- and long-term fuel trims, proper Coolant Temp Sensor input, "all the usual suspects".

Compression test?

[ijames]

Biggest thing I can think of is good highway tires at maximum sidewall rated pressure. If you can go taller and narrower that would lower rolling resistance and lower the overall gear ratio a bit. Do some browsing at tirerack.com to see what sizes are available and what the reviews and ratings say about mpg. You have synthetic oil in the engine, how about in the differential and trans? Probably wouldn't break even on the savings in just 4000 miles. Do you have a roof rack? Removing that would help a bit. You don't use a rooftop carrier by any chance, do you? Driving style matters a lot, too. On hills and east coast mountains (Maryland to Ohio or Alabama) I go from 17-18 mpg at a cruise-controlled 75 mph to 22-24 mpg if I run 55-60 mph and hold my foot steady on uphills so the speed drops away (I get over in the slow truck lane when available) and lift to keep under 60 mph on downhills, on my 05 2500HD duramax. Trading time for fuel money, you decide what's important.

[travis]

Tires are E rated, for the weight. 80 psi front/50 rear as per manufacture recommendation. I tried the rears at 80 and it didn’t change mileage, but definitely rode a LOT worse.

I always try to keep the momentum up when driving this...it helps with mileage and helps keep up with traffic.

I’ll be going over the brakes next week.

All 4 O2 sensors are responsive and are seem to be operating within range. Exhaust back pressure test shows only about 1/2 psi on either bank.

I’ll have to look into the fuel trims. I’ve checked them before but don’t remember the numbers.

This may very well be one of those situations where it just is what it is.

A friend has a ‘96 K1500 Suburban that gets 17mpg highway, and he drives his way harder than I do. I figured mine would be close since his is 4 wheel drive...the weight couldn’t be much different. He does have a 4L60 in his versus my beefier 4L80...could that be part of the big difference?

[BOOT]

I'd look to see if you can maybe trim the factory opening, most oem air intake are designed for noise reduction. I've herd modern airboxes are not that bad but not as old as a 99. The one thing I H8 bout aftermarket CAI's(other than most are HAI) is when they use an alum tube but peeps like shinny.

[travis]

Biggest thing I can think of is good highway tires at maximum sidewall rated pressure. If you can go taller and narrower that would lower rolling resistance and lower the overall gear ratio a bit. Do some browsing at tirerack.com to see what sizes are available and what the reviews and ratings say about mpg. You have synthetic oil in the engine, how about in the differential and trans? Probably wouldn't break even on the savings in just 4000 miles. Do you have a roof rack? Removing that would help a bit. You don't use a rooftop carrier by any chance, do you? Driving style matters a lot, too. On hills and east coast mountains (Maryland to Ohio or Alabama) I go from 17-18 mpg at a cruise-controlled 75 mph to 22-24 mpg if I run 55-60 mph and hold my foot steady on uphills so the speed drops away (I get over in the slow truck lane when available) and lift to keep under 60 mph on downhills, on my 05 2500HD duramax. Trading time for fuel money, you decide what's important.

I will be changing to synthetic oil in the diff as it is due to be changed anyway. I may change to synthetic trans fluid too.

It used to have 2” taller/2” wider mild all terrain tires. I changed to a more highway oriented stock size tire which made absolutely no change in mileage but made a very noticeable difference in acceleration and braking...more than you would think with such a small change. This thing needed all the leverage it could get and it is able to maintain speed in the hills much better.

[travis]

I'd look to see if you can maybe trim the factory opening, most oem air intake are designed for noise reduction. I've herd modern airboxes are not that bad but not as old as a 99. The one thing I H8 bout aftermarket CAI's(other than most are HAI) is when they use an alum tube but peeps like shinny.

Seems to me that the shiny metal tubing would actually conduct heat better...which I couldn’t see that being a good thing.

I’ll investigate the air box for any baffling or restrictions. It could have some sort of silencing in it.

[ijames]

Sounds like you've got all the low hanging fruit covered. Last idea is the octane - does your tuneup require 91 or 93 octane, and does the gain (if any) in mpg cover the difference in price over 87 octane?

[travis]

Sounds like you've got all the low hanging fruit covered. Last idea is the octane - does your tuneup require 91 or 93 octane, and does the gain (if any) in mpg cover the difference in price over 87 octane?

I have seen 0 difference between 87 and 92 octane fuel. It doesn’t ping on 87 and I haven’t seen it pull back timing even when towing.

[Circlotron]

I’ve never tried to measure the difference, but I’d expect a CAI to make fuel mileage worse, not better. If the air is colder it is more dense, therefore you will need less throttle opening for the same mass of air to pass through. Less throttle opening means higher manifold vacuum which means higher pumping losses. Hot air needs a wider throttle opening so less pumping losses, and may also contribute to better fuel droplet vaporisation. As long as the ecu doesn’t decide to richen things up at this wider throttle, I’d say that’s the way to go.

YMMV

[MadBill]

2X! Cold air helps power but hurts economy. Adding a heated intake like the exhaust-warmed GM Thermac air cleaner system used in the seventies and eighties could measurably improve economy.

My buddy bought a new 1970 GTO and commuted daily ~ 80 miles. He kept close tabs on the fuel economy for years. One (Toronto) winter the cable-operated ram air stuck in the 'cold' position and he lost almost 2 MPG (~10%) until it was repaired.