Ram air mods and performance

[JWSmythe]

G'morning all.

A little while ago, there was a thread about ram air mods that I had mentioned that I had changed mine a little, but hadn't had a chance to do a practical evaluation of the differences.

Last weekend, I took a road trip. 500 miles each way to see an old friend.

My mods are pretty simple. Actually, really simple.

A long time ago, I drilled out the rivets that hold in the air deflectors inside the ram air tunnels (like that area behind the honeycomb in the hood).

Recently, I lined the inside with foil duct tape (the shiny stuff, not the regular duct tape), and put a pair of low profile fog lights in. That's the reason for the tape, not for performance. They don't get in the way, and make me a little more visible at night.

I used some foam strips from Home Depot, sold to seal the space around window air conditioners. It's about 1.25"x1.25"x8', attached it to the top of the air box, and the hood. It's attached with what appears to be 3m double sided molding tape, but I bought it at Home Depot, so it had another name on it. Same stuff though. With the ram air grills out, I verified that with the hood closed, it sealed the whole way around.

I've had a vac/boost gauge in for quite a few years, attached to the intake manifold port just behind the throttle body on the drivers side.

Now, possibly unrelated things are an SLP MAF sensor, SLP air box lid, K&N air filter, smooth bellows from the MAF sensor to the throttle body, and Borla Y pipe. Basically, make it easy for the air to move.

Historically, the best MPG I've ever gotten was 26, with the cruise set to 85mph, crossing I-10 from Florida to California. That best performance leg, I accelerated gracefully onto I-10, set the cruise to 85, and didn't change speed until I was almost out of gas. The highway was fairly empty that day. It was a perfect case for best MPG, since I didn't have to change speed the whole time. In the same area, I got 25.5mpg with the cruise set to 80mph.

This time, I did hit traffic on all legs, so my speeds weren't constant. I made a few comfort and phone call stops, and a little sight seeing.

  
date       time         milage  gal     cost    per gal miles   mpg
05.01.2009 15:41        90385   11.299  $27.07  $2.219  0       --
05.01.2009 18:59        90614   10.075  $20.95  $2.079  229     22.729  5th gear - 70mph - traffic
05.03.2009 09:54        90888   12.794  $29.67  $2.319  274     21.416  5th gear - 75mph - traffic/city
05.03.2009 17:18        91202   12.320  $29.06  $2.359  314     25.487  6th gear - 75-80mph+ - highway
05.03.2009 20:35        91419    8.393  $18.46  $2.199  217     25.854  6th gear - 75-85mph+ - highway
Totals                  1034    43.582  $98.68                  23.725

By watching the vac gauge, at normal cruise I should be at 18 in/hg in the best gear for that speed, regardless if it's 30mph or 60mph. I did the first two legs in 5th gear, which put me at 18in/hg and just over 2k RPM.

On the return trip, I did it against the will of the vac gauge. I put it in 6th, which lowered my RPM, and then I observed the gauge a lot. If there were no cars in front of me for a while, the vac gauge would indicate 14 in/hg. If there was a car within about 0.10 miles ahead of me, the vac gauge would indicate 18 in/hg. The modification actually worked, and it was increasing the air pressure by 4 in/hg!

Now, trying to hold a decent speed, with traffic, and stay without any vehicles in front of me was damned near impossible. If I slowed down to increase the gap, someone would change lanes in front of me. I didn't want to fight through traffic and burn up too much extra fuel. As you can see, traveling between 70 to 85mph, my mpg improved. I'm sure it would have been even better if I could have been on an empty highway and not have anyone disturb the airflow into the ram air.

Hopefully, I'll be able to reproduce my long cross country drive with no traffic eventually. I-10 at night is great for it, since there is so much driving in the middle of nowhere. At night, even the truckers stop eventually. 4am is beautiful, where I may not even see another car on the road for over an hour, much less have them disturb my airflow.

[JohnnyBs98WS6Rag]

I'm not quite following what you were attempting to show. Effects of your ram air mods? Effects of 5th vs 6th gear on MPG? Effects of "drafting" other cars on manifold vacuum?

Having no baseline data from before you performed the ram air mods prevents any valid assessment of your mods.

It's not surprizing your MPG is better in 6th on the highway than in 5th.

It's also not surprizing the load on your engine is lower (manifold vacuum is higher) when a car is in front of you on the highway. Having your vacuum guage BEHIND your throttle body tells you nothing about what pressure is present IN FRONT of your throttle body. The difference in 14 vs 18 in/hg manifold vacuum is not the result of changes in airflow into your throttle body via your ram air intake tract, it's the result of less loading on your engine when your car is traveling in the draft of another vehicle in front of you.

To do what I think you are attempting to show is to measure the effect of airflow into the ram air tract, you need to move your pressure guage in FRONT of your thottle body, then make measurements. Even better would be to use at least 2 or 3 pressure guages, one in front of your air filter, one behind your air filter and in front of the throttle body, then a 3rd in your manifold behind the throttle body. Then take data before and after mods under as-identical conditions you can get (various gears at various steady-state speeds up to 120MPH if possible). THAT set of data would then be very useful to draw solid conclusions.

[JWSmythe]

I'm not quite following what you were attempting to show. Effects of your ram air mods? Effects of 5th vs 6th gear on MPG? Effects of "drafting" other cars on manifold vacuum?

Having no baseline data from before you performed the ram air mods prevents any valid assessment of your mods.

Well, we have a good pool of baseline here.

Normally, better gas mileage comes with the higher in/hg number.

It's not surprizing your MPG is better in 6th on the highway than in 5th.

It was a bit odd for the speed. The RPM's were a little low at 65 to be in 6th gear and get good fuel economy. If you always assume the highest gear is the best, I could be in 6th at 30mph, but that would definitely be wrong.

It's also not surprizing the load on your engine is lower (manifold vacuum is higher) when a car is in front of you on the highway.

I'm talking like 1/10 mile (like 500') not a car length or so. The air was disturbed, not drafting. I like my car too much to do that.

Having your vacuum guage BEHIND your throttle body tells you nothing about what pressure is present IN FRONT of your throttle body. The difference in 14 vs 18 in/hg manifold vacuum is not the result of changes in airflow into your throttle body via your ram air intake tract, it's the result of less loading on your engine when your car is traveling in the draft of another vehicle in front of you.

Well, with the TB partially open, it would seem to indicate more air is pushing through.

Then take data before and after mods under as-identical conditions you can get (various gears at various steady-state speeds up to 120MPH if possible). THAT set of data would then be very useful to draw solid conclusions.

I wish I had a test track to do that on. If anyone has a spare one to loan me, I'll be right over. Otherwise, I'm limited to highway speeds (more or less) and traffic. As it was, I wasn't the fastest car on the road, but most of the time when I did move over to let a car pass, I passed them again several minutes later when they were stopped by the Georgia State Patrol. Florida is a little more lax, as I-75 is informally known as one of the Florida State Speedways. 85 can frequently leave cars buzzing past, but then again when it's ticket day, lots of tickets are issued. I like my insurance rates where they are. I wouldn't mind holding 120+ for a few hours, but that'd pretty much guarantee me a ticket.

[c5z28]

Informative but it sucks you couldn't keep a steady pace.

[JohnnyBs98WS6Rag]

Not trying to be an arse or piss on your parade, but.......

Well, we have a good pool of baseline here.

Where? You only showed data from your latest trip AFTER you modified your ram-air intake tract. Again I'm presuming that is the "mod" you are talking about and not the difference between using 5th vs 6th or having/not having a car in front of you:

On the return trip, I did it against the will of the vac gauge. I put it in 6th, which lowered my RPM, and then I observed the gauge a lot. If there were no cars in front of me for a while, the vac gauge would indicate 14 in/hg. If there was a car within about 0.10 miles ahead of me, the vac gauge would indicate 18 in/hg. The modification actually worked, and it was increasing the air pressure by 4 in/hg!

What modification? Shifting gears? Not following another car?

Normally, better gas mileage comes with the higher in/hg number.

True, a higher manifold vacuum means less load and better fuel mileage, assuming all else (RPMs and MPH) is equal. We agree on this. However, change either RPMs or MPH (load) and all bets are off.

It was a bit odd for the speed. The RPM's were a little low at 65 to be in 6th gear and get good fuel economy. If you always assume the highest gear is the best, I could be in 6th at 30mph, but that would definitely be wrong.

Certainly, engines have a "sweet spot" in thier RPM range where they are most efficient at converting fuel to horsepower. But again, the only data you presented showed highway speeds with the only variable being which gear you had selected (5th or 6th) and the amount of traffic / city / highway conditions, and unsurprizingly, you got better MPGs in 6th w/ only highway conditions than in 5th w/ mixed city/highway conditions. WHY??

A) Because you modified your ram-air intake? Can't tell because you did not provide any comparable data from BEFORE you modified the intake.
B) Pure highway driving? YES, everytime you apply brakes some of the energy burned in fuel to achieve the kenetic energy the car has at speed is burned-off in heat.
C) 6th gear instead of 5th? YES, because at a lower RPM there is less energy loss internal to the engine in friction.

I'm talking like 1/10 mile (like 500') not a car length or so. The air was disturbed, not drafting. I like my car too much to do that.

Understand. Disturbed air is generally easier to slip through than undisturbed air. Undisturbed air presents a constant force against objects moving through it, while disturbed air, especially if it is already moving in the same direction as the object moving through it, presents less of a force against that object.

Well, with the TB partially open, it would seem to indicate more air is pushing through.

Actually, even if you assume the engine's demand for air/fuel (load) remains constant, if you were to hold the TB cracked open a fixed amount and then INCREASE the air pressure on the outside of the TB blade, the result would be more air flow through the cracked TB blade and result in INCREASED air pressure / DECREASED engine vacuum on the manifold side of the TB blade. Remember, your guage is measuring engine VACUUM, and a higher number means MORE vacuum, not less. So your higher number (18 vs 14) means LESS air is pushing through, not more.

POSITIVE VACUUM = NEGATIVE PRESSURE

The only plausable explanation for why the manifold vacuum increased when following another car at a steady speed was the reduced load on the engine from pushing the car through disturbed / forward moving air. If anything, the DECREASE in manifold vacuum (18 to 14) when moving through un-disturbed air should cause the engine to work harder (like supercharging), increaseing power output and increaseing vehicle speed until the resulting increase in RPMs pulled the vacuum level back up towards 18. Doesn't sound like that is happening, probably due to the TB blade actually closing slightly to maintain constant vehicle speed.

[third_shift|studios]

I get 27 MPGs cruising. Get a tune.

I cannont imagine dealing with the drone of 5th gear for 300 miles

[Firebirdjones]

I get 27 MPGs cruising. Get a tune.

I cannont imagine dealing with the drone of 5th gear for 300 miles

The drone of 5th gear for 300 miles??? Man you guys are spoiled. The days before overdrive thats all we had.

Anyway,,,you are right. We got 28 mpg on our last highway trip cruising at 80-85 mph. With a baby cam, headers, and tune. Pretty easy to do when the car just loafs along at a measly 1900 rpms

I do appreciate the effort of the original poster and find some of it interesting. I run a vacuum gauge in one of my classic cars full time, and it will provide a wealth of information, helps tremendously in carb tuning, it's what I cut my teeth on 30 years ago.
Shucks, vacuum gauge's were optional on some cars back in the early and mid 60's. You would see them alot on the pontiacs.