OK Tomcat, here is question for you.

I remember in an earlier thread you were talking about power valves & blow thru carbs. If I remember right you were saying that you did not think that a 4.5 power valve would open up until around 3 lbs boost in the intake, if inside the carb box the psi was 5. I understand the principle I think. Now the question, would a 8.5 power valve open with less intake pressure (boost) or a 4.5?

Yeah, we beat that topic to death. To answer your question, the power valve is held shut by the atmospheric pressure in the fuel bowl which is greater than the pressure in the intake manifold. As the throttle opens and the intake manifold pressure rises closer to atmospheric pressure, a spring opens the valve. A 8.5 valve has a stronger spring and will start to open when the pressure difference is still 8.5 " Hg. A 4.5 valve has a weaker spring and will only start to open when the pressure difference has been reduced to 4.5 " Hg. Since carbs were designed to run with atmospheric pressure which doesn't change much, the power valve does a good job of adding fuel when an opening throttle reduces intake manifold vacuum.

For cruising conditions with a blowthrough supercharger system, there may be vacuum in the intake and pressure in the carb box. The extra pressure in the carb box may have the effect of holding the power valve closed. Say you have a 4.5 valve with 4.0 " Hg intake vacuum; the valve should be open, but 2 psi in the carb box adds another 4.0 " Hg (1 psi = 2 " Hg). The power valve experiences 8.0 " Hg pressure difference and does not open. Obviously an 8.5 power valve would open.

Having said all that, the carb tuner has to make a choice. Does he richen the jets to allow cruising without opening the power valve, or choose the power valve to be open under the cruising conditions of pressure above and vacuum below the carb. I don't know. From the posts on previous threads it seems as though there are only a few carb tuners who have been able to get it right, Nickerson and Grimes to name two. Other owners have had problems.

Tell me that your dual carb setup came already tuned and you're not trying to get it right yourself.

Yes, working on it myself. I will say I have learned alot. I am not modifying carb, just jetting. After I started post last night did more thinking & decided that this this weekend I will take pressure reading at box & manifold at different RPM's

I came up with if 5 psi in box then it would take 3 psi in maifold for 4.5 pv to open & a 8.5 pv would only take .5 psi in maifold to open. Other problem is comming on plane with little boost in box & low vacuum in manifold.

It is good knowing if I fail I can always send them off.

Just fail safe! (rich)... failing lean means sending the engine out! Good luck with it. If you don't mind, post your setup, what carbs, what jets and valves. Did you buy the carbs (I'm assuming two small ones with that box)? While you're measuring pressure, have someone check it before and after the intercooler while you're driving. You may know this stuff already, but the thread has some good information. You can find it by searching.

I bought carbs from summit. Carbs are the holley 450 cfm mech. secondary. I installed the adapters that lets you use regular jets in the secondarys instead of the plates. The jetting that procharger told me to go with was so rich motor would not even run. I understand their point since this is not a standard package.

I have only been working on getting off idle & low speed cruise correct. Now I am past that point & working on secondarys. Last trip out sunday secondarys still so rich as soon I get there motor falls on face & plugs come out black. The ramp I was using was full of _____ & spent 3 hrs trying to get boat in & out of water twice.

Will keep you posted of results.

Sounds like fun. Check your email.

Tomcat,
Have you ever talked to anyone from Gale Banks about this stuff? They were THE place to go in the 80s before fuel injection got popular.

Hi bck - back when we were beating this topic to death I sent an email to Gale Banks about the whole blowthrough carb thing - no response. I guess they have moved on to EFI and diesels.

I suspect the manufacturers are not willing to invest the time to redesign the carb for this application, figuring that carbs are going the way of the dinosaur. So they leave it to the tuners to make it work. That's kind of what I decided to do, after hearing of successful installations using Nickerson or Grimes carbs.

It's the same kind of thinking that has produced the less than perfect intercooler configurations in these blowthrough kits. Not worth the time to design it better and/or maybe the customers won't pay more. It's easier to let the tuners and/or customers crank up the boost. Take a look at the air flow results I got with a better intercooler configuration. (Tech Q&A thread) I would value your opinion.

Tom

Gale banks was never a blow thru carb guy, he always used carbs before turbos. The way to go is EFI over blow thru carbs, I do not think anyone will argue that point. Problems with EFI's are upfront cost, few people with know how on non stock systems. I would have gone the EFI route before I started this project if not for no one in this area knowing anything about boats engines & setting them up. Everyone was either in CA. or AZ. & wanting me to tow boat out there or to pay for them to come here.

Before it is all over with I may be going that way.