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Vacuum guage vs pulling plugs for efficiency check

T

thelasthour

Member
"Can I use the vacuum guage re

"Can I use the vacuum guage readings at different RPM ranges to tune my edelbrock or do I have to pull the plugs and burn the ?X*& out of my fingers? My plugs are not easy to get to, I have limited access especially on a hot engine. I've made big gains in efficiency from your great input, especially you Jeff, but the edelbrock is still stock out of the box (worse case scenario right)?
Again boat is 29 phoenix, twin 350's, 10000 lbs, 18*22 props,1.88 borg warner. Current efficiency is exactly 1mpg. Optimal cruise 2750-3150.
Steve"
 
I dont see why not? I am in t

I dont see why not? I am in the process of doing the same thing right now. I have only just installed the hose fittings on the intake manifolds a few days ago. The 4 outboard plugs on each of my engines are a pain to get to and laying over the block to do so IS impossible when just even warm
 
"What an Idea, never thought o

"What an Idea, never thought of that. I have a vacume guage but how do you hook it up and what do you look for on the guage to tune the engine? What steps would you take and what would the guage reading be?

Barry"
 
"I've only heard this done

"I've only heard this done at idle, no load. Where, you tune the carb screws for max vacuum. At any further throttle and loads, people use an oxygen sensor in the exhaust, or, even an EGT. I doubt a vacuum gauge is usable at cruise power for mixture tuning."
 
"I will agree However, I am st

"I will agree However, I am still hoping to check vacuum at WOT to see if the CFM of the carb is on target"
 
"..."I am still hoping to

"..."I am still hoping to check vacuum at WOT to see if the CFM of the carb is on target"

A scientist! The vacuum at WOT is practically zero. Hell, it IS zero!

Jeff"
 
"Better have a really large in

"Better have a really large increment gage, like 0 to 10 inches of vacuum full scale.


Jeff"
 
"Close to but not zero. If it

"Close to but not zero. If it were zero, you'd have an ideal carb w/o any pressure drop.

The gauge is critical for establishing the different operating points for the subsystems within the carb but really won't tell you about the mixture with any detail.

There are three ways I know of to assess mixture: exhaust gas analyzer (very expensive and not always portable), HO2S driven A/F gauges (not practical for wet exhausts), and plug removal and inspection (not as easy as it use to be). You don't have to pull them hot (time consuming but usually burn free). If you forgo the checking and something runs lean, the result will be much more "painful" than the delayed plug check."
 
"I do a couple inboard side pl

"I do a couple inboard side plugs, while adrift, and using mechanics gloves. With all this done, and buying the strip kit, it resulted in jets and rods right out of the box.
In a piston aircraft, it's very easy with an exhaust gas temp gauge and the mixture control. Lean until max EGT, and richen 50 deg."
 
The #1 or #2 plugs are prime c

The #1 or #2 plugs are prime candidates for checking. One is easier to get to as the manifold only partial obscures it. They are usually "uphill" from the carb so leaness first.
 
"I'll let y'all in on

"I'll let y'all in on a little secret: In checking plugs to see how a motor is running, all you need to look at is two adjacent plugs. That's all! Only if the motor has a problem--like a bad plug or plug wire--do you need to pull 'em all.

Naturally, Mr. Marina Mechanic--at up to $100 and hour--has to pull them ALL out. Gotta be thorough!

Jeff"
 
"while we're on this topic

"while we're on this topic, what size is that intake manifold port? I thought a 1/4" drive would fit but it's too small and 3/8" is too big?

Is there a vac port I can thread in to connect the gauge?"
 
"For this test, you can use th

"For this test, you can use the vacuum on the carb that goes to the PCV valve.

Jeff"
 
"do you mean the the hose on e

"do you mean the the hose on each valve cover that goes up to the flame arrestor?

On my '88's, there's a an elbow but it's not actually a PVC valve (although that's where it would be)."
 
"Wow! You need to install a f

"Wow! You need to install a functioning PCV system to keep acid-eating, bearing-killing moisture out of your motors.

That other hose is not going to a vacuum source, unfortunately, but you might find a fitting on the carb, on the choke pull off. It doesn't take a large hose to measure vacuum.

Jeff"
 
"Jeff - is there a PVC upgrade

"Jeff - is there a PVC upgrade for my vintage Crusaders?

Both engine are stock with just a plastic elbow in each valve cover which attaches to a hose that goes up to the flame arrestor (both sides, ea engine).

All the Crusaders I looked at had this same setup....

In the meantime, I'll look at the choke vac line.

Bob"
 
""Wow! You need to install

""Wow! You need to install a functioning PCV system to keep acid-eating, bearing-killing moisture out of your motors."

Well, that's an interesting statement, since on my last major 454 rebuild, I took out functional 1976 mains out of my port engine! That rebuild was last year and a lot of hours on that engine. Anyway, these/those engines did not come with PCV, only the vents to the arrestor. Which, obviously, work just fine, thank you.
The PCV would not be a upgrade. It was put on road vehicles to reduce the oil stripe in the middle of the road."
 
"Yes, Jeff, do I need a PCV Va

"Yes, Jeff, do I need a PCV Valve also on my Crusader 454, 350hp 1987? All I have is like Bob a plastic elbow.

Henry G."
 
"Actually, I'd label it a

"Actually, I'd label it a downgrade. Ignoring all the NRE, You'd be adding "moving pieces" to replace a system with NO moving parts.

As long as both vent tubes stay connected, i see no reason why the scavengine effect they create isn't equivalent to the PCV's.

I too have accumulated serious hours and suffered NO ill effect from lack of a PCV valve."
 
"Bob:

As Jeff noted, the ch

"Bob:

As Jeff noted, the choke pull-off is the easiest point to tap to get a vacuum gauge hooked up."
 
"On mine, it was the ONLY vacu

"On mine, it was the ONLY vacuum source available.
Weird, for cars have a zillion points you can use.

Jeff"
 
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