FiTech running negative vacuum

[ljjohn]

Should I have negative vacuum reading while idling?

 

[DirtDonk]

By "negative vacuum" do you mean it's got positive pressure? Vacuum by nature is negative (at least in my mind it is!) so want to make sure we're talking apples to apples.

Are you reading it on a gauge, or on a controller/programmer?
A good tight engine with stock-ish cam should be seeing 18-22 inches of vacuum (if that's the proper terminology) or hg is it?
With a mid-range cam seeing 15-18 would not be out of line. And a more radical cam would be down in the 11-14 basement and not be great with carburetors at idle.

What is the number you're seeing?

Paul

 

[ljjohn]

On my hand held controller im reading -14/-15. Is that what it should read.

 

[DirtDonk]

Well, initially it seems low-ish to me. What is the rest of the engine set up like?
In other words, what cam and heads and pistons and such.

All those things, but especially the cam can make a huge difference.
But also ignition timing. What is your timing set at currently, and is the computer controlling it or do you set it manually?

Paul

 

[ljjohn]

Here’s what I’m working with a 408 striker crate motor from BluePrint engines,
Alum heads 60cc, 2.02” intake/ 1.60” exhaust valve. 190cc intake/ 65cc exhaust runners:
Cam hydraulic roller .543 in/ .554 exhaust, 218 in/ 226 exhaust duration:
Msd Pro billet distributor locked out and adjustable rotor installed
Hypereutertic pistons, dual plane intake manifold, FiTech GoEfi 600hp it controls timing.
Timing currently set at 10*.

 

[DirtDonk]

Nice setup! What's the HP rating on that particular combination? Is it one of their standard builds, or is it custom for you? Just curious. Sounds like their 425/455 build? Seems pretty stout, but not too radical for a heavy truck I'm thinking.

The cam is not "mild" by normal standards, but not such a radical one that you would see lower compression numbers down in the 14 range I wouldn't think. But a lot depends on the actual profile and overlap and stuff like that. Does it have a lopey idle that says "I've got a cam in he'ah!" by any chance?

It's possible this is normal, or it's possible the engine is still new and breaking-in too. How many miles/hours do you have on it? Brand new? If so, perhaps the numbers will climb up a couple of points. Hard to say.
While it's low, I'm thinking you'd expect to see maybe 15-17 lbs, which really isn't that far off of what you have.
My reason for sounding concerned though is that when you're getting down into your area things like vacuum assist brakes and any other vacuum dependent items are not as effective.
Do you have a functioning PCV system plumbed in?

It's also possible that the sensor/meter is reading a tiny bit low. Don't know if computes do that very often, but I would expect it's possible because the sensor is a combination of electronic and mechanical and anything can go wrong with anything.

Is this just something you were checking on and curious about, or was the engine running funny and you're trying to track down the gremlin?

Hope it's running good and you're just checking. Could be a fun engine. Literally about 2.5 times the power your original engine had!

Paul

 

[DirtDonk]

Oh, I meant to ask, when watching the screen does your timing jump around, or does it stay at 10 degrees?

Paul

 

[Broncobowsher]

Vacuum is a negative pressure. So I think your are correct.

IF you look at a vacuum/boost gauge (or vacuum/fuel pressure test gauge) you will see vacuum is the opposite side of boost. The tricky thing to remember is the scales are different between vacuum and boost.

Aircraft is an exception, they just read manifold pressure above absolute zero. Easier to figure engine load without having to back calculate altitude.

 

[Apogee]

On my hand held controller im reading -14/-15. Is that what it should read.

Not to be a stickler for units, but numbers without units are worthless IMO. Engine vacuum is typically measured in "inches Hg", not PSI. Atmospheric pressure at sea level is 14.7 PSI, so -14 or -15 must be is something other than PSI, since no engine is going to pull an absolute vacuum (~30 inches Hg). My best guess is that the negative symbol indicates it's a vacuum, the magnitude would seem to imply inches Hg...but you would think it would specify that directly or in the manual.

You can use google to convert between inches Hg and PSI here.

Tobin

 

[ljjohn]

Timing does jump around

 

[Broncobowsher]

I think that EFI can take boost. So a negative number would be vacuum and a positive number would be boost.