Power Piston Selection Approach

[makomark]

I have twin Big blocks with quadrajets on my boat.

was wondering if anyone had an opinion on selecting the power piston to maximize economy. The OEM delivered it with what i call the 8-4 spring; at 8" vacuum, the piston starts to rise and at 4" vacuum, the piston is full up.

Based on measurements made to date, I get 8" vacuum on both engines, while at my preferred cruise rpm of 3000. I have flo-scan meters, that read gallons (of fuel) per hour. and could see the jump when the power piston rises. since then, I swapped those springs out for the edelbrock gold ones, which my measurements show are 6-4 springs. I have ran the engines for over 200 hours, each, in this configuration and have 'tan' spark plug electrodes and normal operating temps so I'm positive I'm not lean.

FWIW, the only suggestions/recommendations I could find, in print and on the www, all targeted auto applications where a low-load cruise state exists....just wondering on the thoughts for marine use.

[Cliff Ruggles]

Looks like you are going about it the right way, controlling the test criteria with by using the same engine rpms and monitoring the fuel consumption.  I've never really had much success tuning with power piston springs on automotive applications, since they encounter such a wide variety of load conditions.  At least with a boat it's much easier, since the engine speed and load can be maintained for long periods of time.

We often set up Marine carburetors (and all of our racing carburetors) without using any metering rods at all, and pull straight off the jets.   The jet size must be reduced about 5-7 numbers, but we have had excellent success doing this.

My street driven 73 Ventura does not use primary metering rods in the carburetor, and runs flawlessy at all levels and gets excellent fuel economy.  At least for a car with a 600hp engine in place that runs low 11's at the track!......Cliff

[makomark]

I don't think I've got that much range left, going lean, with the primaries. The OEM config was 069 gets with 042 rods (non-apt type). One of the earlier overhauls incorporated two factory service bulletins - open the idle down channel restriction and go to 070 jets (ethanol blend fears). the theory I'm working on says run up thru cruise (3000 rpm) on the primaries, with rods down, and keep the power piston to add the fuel for higher rpms. The next step on the economizer plan is to go back to the 069 jets and then the 068's if not too lean; after reading the jetting areas in your book, I think I'll revisit my spreadsheet to see where I can go with the jets & rods I've scrounged up over the years. Do you offer primary rods & jets? are smaller primary jets available below 066?

To have the fuel capacity to have 600 hp engines and be able to make use of them.....I guess that's why they sell those go fast boats.

thanks for the reply      mark

[ss-truck]

  Cliff are you saying you do not use any primary metering rods ? If so how do you block off the hole where the power piston goes ?  I have not heard about this before and am very interested in trying it in a 494 Buick. 

[makomark]

pull the piston and the spring and melt a bit of solder until the port is covered is one way.

You could include tapping the hole and loc-titing a screw in it (very permanent, especially if 271 is used).

I'd say the ideal, temporary method would be to take the spring out and put the piston back in (yes, after removing the rods).

[ss-truck]

   Thanks I will give that a try in the near future . About how much should I drop down on the jet sizes when I do that ?

[Cliff Ruggles]

We use the aluminum APT plug that is found in the airhorn of the later model carburetors.  The aluminum plug is the perfect size to drop down in the power piston bore, and easily removed later if needed.

Reduce the jet size about 5-7 numbers when removing the power piston and primary metering rods, this is usually a good starting point......Cliff

[ss-truck]

   Thanks , can't wait to try that one.  If it changes the mid range driveability , not worried about that .

[Hotrodrobert]

Makomark, do you have good stainless props??  I have seen big improvements over alumionum props and in getting just the right prop.
I have several metering rods to fit your carbs if they are the Mercruiser version.  Bigger rods will lean the part throtle and still give you the mixture richness to get on plane.  If you go too small on the jets you will have more trouble getting on plane, load decreases after planing.

[makomark]

Robert:

I could probably get Cliff to silver plate both carbs for what a pair of stainless wheels go for sized for my boat. Aluminum isn't an option as these are inboards, not stern drives. The ones on there now are bronze; we've had nibral on and not noticed any performance difference. What did make a difference was the propscan process; essentially a computerized dial indicator hooked up to a turntable. Lets the prop mechanic measure thte prop and then accurately check it as he works it. Not cheap but one of the few items that 'performed as advertised'.

The engines were crusaders, as installed, but when overhauled, all the marine replacement parts came from the Merc warehouse. The carbs were swapped out for Merc versions even before that as we had saltwater in the fuel and it sat all winter.....very ugly inside. Used the crusadder sized jets and rods, which were richer than the Merc selections; best as I could tell, both had the 8"-4" power piston spring. I'm certain that all of the marine q-jet parts swap, at least the internals, and that they are the "pre-170" style, even though the casting numbers indicate otherwise. How "deep" is your primary rod collection?

[Hotrodrobert]

I am so accustomed to working on Mercruiser that I just thought you probably had IO.
I would be happy with your props and not change anything.  So many boaters not into going fast just leave the Al props on and beat them up and don't know the fuel they cost!!

[Hotrodrobert]

For primary rods, tell me what you need and I will look.  I have probably 20-25 pairs varying in size.

[makomark]

Robert: sent you a message via this board - pls check when you can