SBC wideband carb tuning Q's for grumpyvette

[bigbore468]

Hey Gang,

 

Have been trying to do some fine tuning of my Holley carburetor fuel circuits; via IFR's, IAB's, HSAB's, etc. and would like any feedback from anyone who has also recently done some of this.

 

Perhaps grumpyvette or other experts could chime in and share some wisdom on their experience in this area.

 

The base set up is a SBC 331cid street engine, 268 Lunati hydraulic cam, Z-28 hi-rise, with a 750cfm demon carb running on pump gas.

Using the Innovate Motorsports LM-1 wideband tuning program; have been trying to get the motor to run with an air/fuel ratio of around 14:1 from idle through 2,900rpm.

 

With my baseline carb tune, I can achieve a 14:1 ratio and maintain this through the idle circuit, but have been unable to maintain this through the transition into the high speed circuit, which must be run on the rich side (11-12/1) or the engine will stumble part throttle at around 2,800 rpm.

Naturally if I mash the gas right off the idle circuit, the motor will pull strong.

 

I've recently noticed that the brand of pump gas (all 92 octane) will affect how the car runs; Shell Oil 92 runs the best.

 

Have also tried working with different power valves, which have made a slight improvement.

 

Guess my question is given the quality of pump gas today, is it possible to wideband tune a carburetor to run with an AFR in the 13.5-14:1 ratio.

I bleieve CHP did several tests on this a few years ago, and was able to achieve these numbers.

 

I know there are many other factors that will play into this, like ignition timing, manifold vacuum, exhaust flow,etc.

Howver, any feedback or advice would be much appreciated.

 

Cheers

[grumpyvette]

as Im sure your aware the best torque will usually be with a ratio of about 12.5:1-13:1 and the most fuel efficient ratio will be closer to 14.7-15:1

carbs react to CHANGES in air flow rates, to change the fuel feed rates so theres always a delayed reaction, EFI generally uses sensors to detect BOTH changes in AIR/FUEL RATIOS in the exhaust and changes in vacuum and airflow rates

carbs generally cover up sudden increases in air flow such as stomping on the throttle to WOT from idle with a long shot of fuel from the accellerator pump and power valves adding extra fuel, neither one is a precisely metered responce.

you can tell a good deal about the conditions in an engine if you know the vacuum readings (use a gauge), and know how to read spark plugs

now I use a vacuum gauge a timing light, and an IR temp gun on the headers along with the data from reading the plugs and the a/f ratio sensors, each adds to the indications of whats going on, yes you can acchieve a 14.:1 ratio and get the engine to run decently, but in most cases it will tend to run better in the low 13:1 ratio range durring transitions and about 14:1-14.7:1 durring cruising simply because theres always a tendency to go lean durring sudden transitions with most carbs.

Id keep in mind that going rich durring rapid transitions is prefered over going lean and getting the a/f ratio stable and predictable at cruise and idle and durring mild accelleration like on the freeway,where youll spend 90% PLUS of your time is far more important than temp swings durring throttle changes, or durring WOT where running slightly richer, durring high load high rpm conditions is prefered simply because it makes better power and has less tendency to run into detonation, especially since youll spend far less time under those conditions

 

 

BTW (lots of links and sub links on carb tuning, spark plug reading, etc.)

if you do some searching on the hybridz site,

but heres a bunch

 

http://www.digitalcorvettes.com/forums/showthread.php?t=77602

 

BTW use a 190F T-stat and an extra electric fan that comes on at 200F to keep the engine coolant temp stable, try to keep the oil temp in the 215F-220F range if you can,to burn off moisture that can form acids over time in the oil, and the fuel pressure stabile at about 5 PSI,, have no more than 1-1.5 psi of back pressure in the exhaust at WOT, it makes tunning far easier, make sure the ignition timing curve is smooth & consistant, and do a leak/down test and adjust the valves, do a vacuum leak test, check the oil pressure,and fuel filter, to make sure your not working with mechanical problems.

if your tunning an n/a engine and the IR gun shows header temps over 1250F or under about 1100f once its up to temp, youll generally find vacuum leaks, ignition curve problems or a/f ratio problems

[bigbore468]

Much thanks to grump for the reply and for all the tech advice.

 

Cheers all!

[grumpyvette]

 

I ran accrossed this chart that might prove useful as a rough guide, and don,t forget this thread, and ITS sub-links

 

http://forums.hybridz.org/showthread.php?t=131390&highlight=plugs

[dr_hunt]

Street carbs are more responsive to changes if you use the annular discharge boosters as opposed to downleg or straight. What ones do you have? What carb do you have?

 

The booster determines signal to the carb and dictates fuel curve. You may consider the C&S aerosol billet carb if your looking for a great fuel curve for max power.