Hey Tomcat,
Just a suggestion--try calling Dustin to see if he has another way to mount a carb off to the side or some thing like that. Sometimes you need to think outside the box and get creative!!!

Dave

I talked to Dustin about this a while ago. He suggested going to an EFI system if hatch clearance was a problem. Once again, I have no problem with EFI but it seems like an expensive solution to a simple(?) metering problem.

I have also given some thought to making the secondary power valve respond to boost only. This could be done by reversing the valve and seat and referencing the power valve to atmospheric pressure. Then, whenever air pressure in the fuel bowl rises, the valve will open. The right spring, a deeper diaphragm chamber and a tapered valve stem might even add a measure of modulation to this valve.

Don't mind me, I'm just talking to myself.

Yea, mount that carb outside the box and on the floor. No really, your thoughts are great. What about using a nitrous nozzle for fuel it has removable jets and the fuel line can have a selenoid to open at 0 vacumm. Plumb it from your fuel line to the selenoid to the nozzel plumbed under or above the carb. You could have a 2 and 3 stage with 2nd and 3rd nozzles which open at 3psi and 7psi. Sort of an external adjustable fuel supplying device. Does this make sense?

I actually did something like that on my twin turbo 455 Firebird. With just one stage it was a crude on/off thing. With several stages or even better a pressure modulated valve it could work, but it doesn't control fuel flow in accordance with air flow the way a venturi does. A pressure modulated power valve in line with the air flow modulating venturi would be better if it could be made to work. Fun thing to think about, isn't it?

OK use a fuel injector and an auxillary injector controller. Oh yea EFI, would need a high pressure fuel pump. Allways thinking.

You're eventually going to have me buying EFI! Then I'll be on the board *****ing about programming the damn thing!

I am going to buy blowthrough systems for my carbed engines and I am going to beat this boost sensitive metering problem if it takes all winter!!!

Just to remind everyone what we are talking about fixing on blowthrough carbs. This is an excerpt from "Super Tuning and Modifying Holley Carburetors" by Dave Emanuel. I have edited it a little for clarity.

"Blowthrough systems are limited to relatively modest boost because no carburetor is able to cope with the higher density air delivered to the carb inlet. All standard carbs are designed with the understanding that atmospheric pressure will deliver normal density air into the venturi. Fuel circuits are subsequently designed to deliver the correct proportion of fuel into this air for proper combustion."

"However, under full or partial boost, the air delivered to the carb is denser than normal air. The carb has no way of telling that the incoming air is denser and requires considerably more fuel for proper combustion. It is possible to counter this problem somewhat by "over-richening" the secondary circuit of the carb, but this is really not a solution."

"The only real solution for boost is a third delivery circuit, but this involves a complete re-engineering of the entire carburetor. For practical purposes, the only currently available solution is to restrict maximum boost and carefully adjust secondary fuel delivery as best you can."

Those words were written in 1988, and they are still true today. The use of marine intercoolers actually makes the density of the incoming air even higher, so the problem is worse, even at modest boost levels.

Many people who worked with turbochargers on cars twenty years ago (myself included)just gave up on getting blowthrough systems to work. Due to the many different combinations of RPM and load in a multi-geared vehicle that can coast sometimes, EFI really was the only answer. But for the single-geared boat that can never coast, the problem is much more manageable.

I don't think a complete re-engineering of the carb is necessary for the performance boat application. As I have described above, a modified PVCR and power valve might do the trick.

Thanks for listening, and tune in tomorrow night when Professor Numbnuts will explain the difference between sucking and blowing.

[ 12-04-2001: Message edited by: tomcat ]

Tomcat, did you ever talk to Gale Banks in CA. In the late 70's and 80's he was the premier guy doing blow thru in cars and boats. He is still around and maybe has an old carb that worked.

Yeah, back then Gale Banks was God. As Wayne and Garth would say, I'm not sure I'm "worthy" to talk to him. But if anybody tried these ideas and got them to work it was him. I think I'll give him a call.

Hey tomcat, a friend of mine bought a Nickerson carb, a Turbonetics turbo, an air to air intercooler In a chevy Nova on a 350 and it runs 10.8 and its not tuned yet! Reguarding your question: My boost guage is sampling the box. I put it up there because of keeping an eye on the fuel pressure differential. In the manifold was about 1-2 psi less than the box. As for as the pistons go the low water pickup filled the block and intercooler with mud and leaf particals. The block couldnt even be drained, and the intercooler took about 1.5 hours of flushing and shaking to get everything out. I may have had bad gas also who knows. I am going to check out your theory on part throttle vacuum though. My carb is a nickerson also. overall it ran good. I think there was some tuning still to be done. Greg