Is my Holley 390CFM not enough?

C

CobraSix

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I'm starting to wonder if my 390CFM Holley 4V is enough or if I just need more tuning.

Here is my list of mods in case you don't know:

Aussie head
Enlarged valves
Comp Cam 260 with .44 lift
CR at 9.5:1
Dual out headers

The reason I wonder if its enough, is that I'm really having to rely on my vacuum secondaries in order to get decent acceleration, especially at highway speeds (I'm running a T5). I'm not talking about driving super fast or break neck acceleration. I usually drive the 65 at 65MPH on the highway, and at that speed, if I want to try to gain a few MPH, the secondaries have to kick in.

I know I have some tuning issues to finish fixing on the carb. The 390 is still jetted a little too lean on the primaries (need to go up 2). I thought about changing the vacuum diaphram to one geared more towards mileage, but then I'm afraid of having so little acceleration. I also think my power valve may be set too low since I'm getting a decent sized bog at 25% throttle.

To add more fuel to the fire, I'm only getting about 21-22 MPG on mostly highway and open road driving.

Any thoughts, comments, experience?

Slade
 
The only definitive way to check it out is with a wideband O2 sensor in the tailpipe. You could do it on the road, but the best way is on a dyno. Do a baseline run, then jet up to see what happens.

You may find you're too rich at some loads, too lean at others.
 
Well, IMO that's the best option, but a carb can be tuned to work pretty well. It's jut that without some way of getting feedback on what your engine's needs are, tuning is a crapshoot based on the 'ol Butt-o-Meter.

You think it's lean. How do you know? And is it lean at idle, 1000, 3000, or 5000? Or is it rich and at what rpm and what load? And if you change a jet, did it kill your power somewhere else? Just know way to know unless you can monitor and record a few things.
 
Idle is fine...

it is leaning out at 2000RPM cruising at 65 MPH. I know this because of checking my plugs after doing an extended highway run and then actually shutting it off at speed, and coasting to a stop on the side of the road.

Here is why my plugs looked like:

http://www.kastang.net/sparkplugtuning.html

I'm not running lean by too much, but just enough I think to get my car to run warmer at highway speeds (yes timing is all set).

I'm thinking of ordering a Rich/Lean indicator for my car, and just have the exhaust shop weld in the O2 sensor that way I know for sure.

Slade
 
I would suspect a jetting issue then rather than a carb flow capacity issue. At 2000 rpm, you aren't pumping that much air. Let me qualify my calculations below by the fact that I have no idea how well carb air flow ratings and engine displacement values correlate. I don't know if the pressure differential that is used to rate the carb's flow is something that you would normally be able to achieve. I also do not account for volumetric efficiency, compressibility etc etc.

200 cu in x 1/(12 in/ft)³ x 5000 rpm = 578 cfm
However, the intake and exhaust valves are only open every other revolution. So, the engine is pumping something on the order of 289 cfm at 5000 rpm.

At 2000 rpm, it is pumping only about 116 cfm
 
Well, I figured something out on the ride home today. I didn't pick up on it yesterday when it happened. When I first left work, I heard it back fire just a tad (barely audible). So I figured I should check my timing. The thing that struck me odd, that even though I've been running lean, I had been getting pretty good performance, especially on medium-light loads. A couple of weeks ago I parked the car for some work and then when I started driving it again a few days ago, I was getting really bad performance.

So, the backfire made me think to recheck my timing, mainly to make sure I had tightened the timing bolt down (I've been known to forget that) and see if my timing was still set. Well, it was still at 10* BTDC at idle. Okay, no biggy there. I started doing some poking around on the carb, and then I saw it. My vacuum advance line was disconnected from the carb. Big problems. That means I was definitely leaning out above 1000 RPM with a nice vacuum leak. Also meant I wasn't getting the right advance. I fixed that...car runs much better.

BTW, the spark plug pictures above were taken with the vacuum advance hooked up. They were taken before I parked the car to work on it (which is when I disconnected the vacuum advance).

I still need to rejet...but at least my performance is back to what it was.

It's amazing what timing can do for you.

:oops:

Slade
 
stick around long enough...you'll find out just how mortal...

It's usually the little things that get me, like hooking things back up.

Slade
 
Funny thing, The diffrance between ur engine and mine is compression and cam( i have 8.1:1 compression where u have 9.5:1, I have the stock cam and i think you have the 264 cam) I am running the same carb and it seems to be running rich on my engine.
 
I actually got approve from the CFO (Chief Financial Officer, aka wife) to go ahead with my EFI when I sell my old Offy set up. I also have her 60% sold on a forced air project too...

I guess all the hard work around the house lately is paying off.

Slade
 
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