Reply With QuotePick up a copy of "Street Supercharging" PAW has it in their catalog. It has a section on blowerr referencing carbs that I found very helpfull. There's more to it than just jetting, the author suggests that as a last resort
OK.... I know this subject has been beat to death but lets go over it one more time.
I just switched from dual Carter 600's to dual 4150 Holley 750's. I plan to block the power valves as they will not be boost referenced. I am at 2800 ft. elevation the Engine is 468 Chev. with 8/71 BDS running very mild boost(about 7 PSI). The question is what is a good range of jets(#'s) to try? Squared or stepped up? These carbs sit bowl to bowl not side ways so I want to narrow it down as close as I can because the whole carb(s) has to come off to rejet. This is not any sort of race boat Its a big old(76)Howard daycruiser(but if I do whoop up on a certain 18 ft. Couger jet that would be just fine.) http://free.***boat.net/ubb/biggrin.gif
Pick up a copy of "Street Supercharging" PAW has it in their catalog. It has a section on blowerr referencing carbs that I found very helpfull. There's more to it than just jetting, the author suggests that as a last resort
CUSTOM CRUISER- YOU GOT IT BACKWARDS! YOU NEED POWER VALVES AND BOOST REFERENCED CARBS! THE POWER VALVES ARE FOR PART THROTTLE FUEL ENRICHMENT WHEN THE ENGINE IS STILL IN VACCUM AND UNDER A LOAD (CRUISING SPEEDS). THIS IS THE POINT WHERE IT LEANS OUT!
I'M POSITIVE THAT YOU CAN FIND PEOPLE TO ARGUE THE POINT AND THAT IT'S THE OLD WAY OF DOING THINGS, BUT YOU WILL NEVER HURT YOUR ENGINE BY BEING LEAN WHEN YOU ARE ON THE WATER HAVING FUN!
BOOSTDADDY: No wonder you will find people who will argue that. Power valves are part of the "power enrichment system" Power does not occur at cruise. It is to make sure there is sufficient fuel at high power settings.
This is why they are vacuum operated. vacuum keeps them closed (cruise) (part throttle) when you open the throttles for power, vacuum drops and they open providing the additional fuel needed for higher power. Not old school just basic physics.
If you need to boost reference your carbs. The real problem is not enough carb for the blower. If the blower is sucking enough vacuum under the carb at high power settings to keep the power valve closed. The carbs are not allowing enough airflow to begin with.
Granted boat engines are loaded harder than in a car. But on the other hand a blower is providing the carb a stronger signal and as a result more fuel is being metered. The same jetting is running richer than it would be if installed on a non-blown application.
It is much easier to block off the power valves (and jet up when you do so), than go to the added expense to boost ref the carbs. I am sure the engine builder and carb builder prefer boost reference. Good for the revenue.
custom-cruiser: If you are running a very large boat and require a lot of throttle at cruise, you may consider boost ref carbs. I am pretty sure this is not the case.
An easy way to tell if you need to boost reference the powervalves. Take a vaccuum reading at the carburetor base at WOT. If the vaccuum reading is less than the power valve number no boost refencing required.
Example, my engine 540ci, 871 blower, 14 psi boost, Two 850 carbs. Power valve size 4.5 My carb base vaccuum reading at WOT was 1 in mercury. So I concluded that no boost referencing was required,the power valves were still open at WOT. As Info stated, vaccuum closes the power valve, lack of vaccuum opens the power valve. I think that you would at least want power valves in the primarys or you will have a major flat spot. The power valves make for a smooth transition from idle circuit to main jets.
Boost Referencing the power valves is the proper way to set up carburetors for a blown application, not just a way of generating some $ as some would have you believe. There are also other changes to carburetors for blown apllications to help with performmance, and distrabution. The later can save you lots of money on rebuids. Even if your engine doesn't produce enough vacuum at WOT to shut your PV doesn't mean it won't work better referenced. While your at part throttle your engine may be under enough load to use the additonal fuel if you're making boost, but if you PV isn't referenced you can be lean. Taking the PV's out will make the engine run rich at idle, and part throttle since you have to go up on your main jetting to compensate for this !
Impossible for the main jet to have any effect on idle. The idle jet and idle mixture screws are the only thing metering fuel at idle!
Improper position of the throttle valves in relation to the transfer slot can really make one run rich at idle. Blown (leaking) power valves also.
What does Boostdaddy mean by "BUT YOU WILL NEVER HURT YOUR ENGINE BY BEING LEAN WHEN YOU ARE ON THE WATER HAVING FUN!"? Made no sense to me, unless he's kidding.
Snowboat, When you are just driving your boat around and not in boost, the engine is under a load. This is the point where boats lean out and hurt the engine.This really shows up in bigger boats, guys with lighter and smaller boat don't seem to have too many problems with this.
OK Boostdaddy, I think I see what you are talking about. My boat is light and I blocked off the power valves because it was easy and I kept the jets rich. The Biesemeyer is quite light and I checked the plugs at various extended rpms. They are fine, but a little rich and thirsty. I can play around a litle more this summer so I have a couple of questions. I see by your profile that your opinion has weight. When I dig my books out it says that I should block the vacuum passage on the underside of the throttle plate, and drill from the outside, into the hole above the place where I put the plug. Insert a tube and run a vacuum hose into where, an threaded hole in the blower manifold? Do you Tee the carbs together or run a separate line for each carb or two lines for each carb? Potentially there can be four vacuum lines. The other thought is if your cam is .700 won't the vacuum signal be so low that you might as well just block off the power valves?
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