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Ecky 03-20-2014 12:29 PM

Need advice regarding vacuum when accelerating
I'm considering getting a vacuum gauge to help find the sweet spot when accelerating, specifically when using P&G. I have an MPGuino for measuring overall economy, but it's not all that helpful for staying in the sweetspot when accelerating. The other missing bit of information here is my engine's BSFC and its correlation with vacuum, but I suppose that can probably be worked out experimentally.

A bit of related info:

I have the very short-geared transmission from a B16A (undersquare) paired with a much longer stroked B18B engine. Peak torque is supposedly at 5200rpm, vs the two peaks around 4000 and 7500 that engines with two cam profiles (VTEC) have.

At 75mph I run close to 4500rpm in 5th. (I don't drive at this speed)

I can shift into 5th at around 15mph and still get up hills, but prefer to do it between 25 and 40.

I have no idea at what vacuum/throttle position/RPM AFR begins to enrich.

At constant speed, I get about the same fuel economy between 48mph and 65mph. Because of this, I suspect aerodynamic improvements would result in better economy at 65 than at lower speeds.

Around 3700rpm/65mph my fuel economy begins to drop sharply.


What do I need to know? What vacuum/load should I be shooting for? Any tips?

Cobb 03-20-2014 09:17 PM

2 schools of thought. You need to test to see what your car likes.

Some say keep the highest vacuum reading by pretending an egg between your foot and accelerator.

Others say vacuum equals pumping losses so you need to WOT it and short shift til desired speed, then maintain the highest vacuum.

Of course afr and timing effect vacuum. :eek:

cbaber 03-20-2014 10:06 PM

From what I have seen from other EM members, 70-90% engine load is ideal for pulsing. Unless you have your engines BSFC chart its trial and error on what is most effective.

Since you are going to use a vacuum gauge, you will eyeball the 70-90% range from the vacuum reading. 0 inHg vacuum is going to be 100% engine load, which would be WOT. Idle on most cars is ~19-24 inHg on the vacuum gauge, and idle for my Civic is about the same range in actual engine load. So assuming your engine is similar, to achieve the 70-90% engine load you will want the vacuum gauge to read ~5 inHg. Like I said you will need some trial and error over at least a few tanks to see what levels of vacuum on your pulses get you maximum MPG.

Now understand I am no P&G expert, nor do I P&G anymore. But I never bought into the shift point idea of peak torque. For example your motor makes peak torque at 5200 RPM, there is no way revving to 5200 RPM to shift is economical. What I do know is that at a given speed, the lower the RPM the better MPG. The way I drive is basically granny shifting, trying to quickly make it up into top gear. I never go past 2500 RPM before I shift up. I pick the highest gear I can maintain speed (or accelerate) without stalling.

spacemanspif 03-20-2014 11:40 PM

I'm not very familiar with MPGuino but when paired with the vacuum gauge I assume you could take a know trip (on the way to work?) accelerating at X vacuum and check to see what your trip MPG is; then try it again at a different vacuum reading and compare until you find your car's sweet spot. Shift RPM will come into play too so pick a reasonable RPM and try to shift at it every time in order to eliminate that factor as much as possible. This is all just a guess but it seems like it could work...

serialk11r 03-21-2014 12:12 AM

As someone with a car that's faster than most of the cars on Ecomodder as well, something to consider:

Your engine runs faster than my engine on the freeway with the same displacement. On the freeway my engine runs at ~20 in Hg vacuum while idle is something like 24. That means my engine is producing something like under 15% of its available torque, and yours even less, whereas peak efficiency arrives at ~80%. From idle up to 80-90% peak torque output, your efficiency is increasing at a slower and slower rate with respect to change in torque. 60% load might be >90% as efficient as 80%, but 40% load is probably only 75% as efficient as 80% load, typically.

If you step hard on the gas, you'll accelerate pretty quickly, and then end up running your engine at high rpm with low load for marginally longer if you are cruising. If you step less hard on the gas and only use 60% of the available power, your efficiency will be slightly lower but you will have much more time to reach your desired speed. If you're going to pulse and glide, using less power will mean less speed fluctuation to piss other drivers off, and if you have a hill it might be better to hold 50-60% load than to try to accelerate up it and increase your aerodynamic drag.

On the flip side the higher the engine rpm, the more friction and so the peak efficiency point moves higher, and especially on cars like yours and mine where running well over 3000rpm is normal, that could be a substantial difference.

My personal method is P&G when I am not feeling lazy, I pulse at 80-90% load over 3000rpm, ~70% at 2500rpm, 50% at 2000rpm, if it's a moderate grade I use ~60-70% at 3000 to only slightly accelerate. In gears lower than 5th, drop those numbers by 10-20%, since it's very easy to accidentally go too fast and then brake.

Mista Bone 03-21-2014 02:14 AM

Stay at 10 in Hg or higher, I prefer 14-15 in my Honda's. Also a simple ECU tune will REALLY help.

By watching the vac gauge you quickly learn what the car likes. Tap into the hose between the intake manifold and fuel pressure regulator.

Flat ground cruising is my old 93 DX Civic

Cobb 03-22-2014 08:31 PM

What about tapping the pcv hose?

Mista Bone 03-22-2014 11:31 PM


Originally Posted by Cobb (Post 416511)
What about tapping the pcv hose?

Why would you wanna tap that? It doesn't read manifold vacuum.

betasniper 03-22-2014 11:45 PM

Yeah it does Link (See post 2 then 5).

Anyway, For such a short geared transmission, I would suggest P&G with the Glide in neutral because attempting to run required horsepower at that vehicle speed would put you way out of the efficiency island. I would recommend pulsing at 3.5~5" Vacuum (depending on rpm, 3.5" at 1500, 5" at 2500, then less vac as you go higher again) This is based on my cars bsfc(see sig) so it may be different for yours.

Cobb 03-23-2014 07:39 PM

Is this for the honda? Maybe a good idea to stop before you hit vtec engagement. Another idea maybe to get some parts to advance the cam or disable vtec. I know it varies from engine to engine how and what vtec does on an engine.

I had a geo metro and now a suzukie sidekick.Both cars I purchased 3 tech cam advance gear to drop the power band. This allows more shirt shifting and less winding of the motor for when you need power. I get 30 mpg in the kick with 205/75r15 tires and driving how I want with traffic. Tops out at 75 mph in 5th gear. :eek:

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