Procharger/Blow through application question

[sdba069]

I'm running a 509 ina 24 Scarab with a Procharger blow thru setup.I've got a Nickerson carb set up for the Procharger and a boost referenced fuel pressure system which by the gauges is working perfectly.Problem is we've already burned a hole in # 4 piston.All the plugs ,if anything show a rich condition and all the other pistons show no sign of a lean or detonation situation.I'm running 28 degrees total advance with a MSD ignition system and the carb has 86 jets primary with and 154 jets in the secondary.ATI has not been any help to speak of and I haven't talked to Nickerson yet.Infomaniac has a friend who was having the same problem with a Procharged 496 blow thru deal and they never could get it fixed.Finally got rid of it.The motor is very responsive down low but when you try to take it much over about 3800 you can feel it laying down.I have static fuel pressure set at 7.5# and with the boost at 4# fuel pressure shows about 12 or 13 # .I would like to hear from anyone who may have experienced this problem...Thanks........
Garry

[Infomaniac]

Garry,
Watching my friend's problem from the sidelines I have a theory about what is happening. The boost referenced fuel pressure regulator only makes sure fuel can get into the bowl. I honestly believe the basic fuel metering function of the carb is being disrupted.
Since what causes fuel to be metered is the difference in air pressure between the air pressure acting on the fuel in the float chamber through the vent and the low pressure of the accelerated air in the venturi. The greater the difference of these two pressures the more fuel is metered. Normally high throttle position is where the most differential happens. We call this the "Fuel Metering Force" in aviation.
My Theory: Since the carb is enclosed in a box and the boost is being fed into this box. I think the pressure differential required to meter fuel is being severely reduced causing a reduced fuel flow. Jetting does not matter if there is not suffecient pressure differential to push fuel through the jets.
When boost rises, I believe the pressures in the manifold, box, and float chamber are very close and preventing proper fuel metering. I think the air velocity pressure drop in the venturi is being offset by the pressure in the enclosed carb box. It seems to me at high throttle position the boost presure is the same from the intake valve all the way back to the procharger compressor. This will mean the pressure in the float chamber is too close to the pressure in the venturi. The engine will lean out regardless of fuel pressure or jetting.
Whew -- Any comments?

[MAXIMUS]

Nobody mentioned prop size, timing curve, compression or fuel type? Out drives tend to load a motor pretty good down low. Also seems like a "huge" spread on jet sizes from mains to secondaries? I think the proper way to approach this would be to start with the primaries! Operate the boat using the primary circuit only! Use new plugs & pay close attention! Then move on to secondaries & so on. If the load condition is to severe for the motor jetting will not offset the end result! A smaller prop would be a good idea during "dial in" to make life easier for the engine. Also what about power valves if any?
That is my 2 cents!

[Hotcrusader76]

Whether the set-up is blow-through with a Bonnet or carburetor box, as long as the bowls are sealed correctly and the top carburetor surface has a satisfactory foam type gasket you should not have any sort of presure difference. The carburetor fuel bowl vents allow the surrounding box or bonnet air-pressure to enter, equalizing the pressure difference. The throttle-plates are also sealed, modifications required!
Fuel metering is not affected, but you will need to run a little richer with either set-up. The fuel pump needs to be 1:1 in fuel pressure to boost ratio. That's a must or she will lean destruct on ya.
As far as the carburetor goes, the amount of air-flow and velocity has increased ALOT!!!! So the carburetor booster will die alot earlier in the ball-game due to the air-velocity. Blow-through carburetors need Annular boosters (unless it's a 650 main-body) to deliver the required amount of fuel. The internal metering is also another area that needs alot of attention. The squirters need to be anti-pullover versions to prevent siphoning out the fuel from acc.circuit.
Alot of folks think that a stock carb works just fine, and most times it will. But when you start pushing more air-through the carburetor, this vacuum fuel metering type device starts to act real different.
I am finishing up a Blow-through carburetor right now for a Studebaker/Paxton supercharger which is a 650DP. It has turned out real nice. I will post pics this week.
Just some food for thought...
[ December 17, 2002, 10:50 AM: Message edited by: Hotcrusader76 ]

[FASTERDAMITT]

First is that carb set up for blow thru? If not have Nicherson modify it.The floats, metering plate ect. I think 154 secondary are way to big. I'm running 85's on my 454 at Havasu. Have been for a 3 yrs. Call Nicherson if it's his carb!

[Hotcrusader76]

FASTERDAMITT:
First is that carb set up for blow thru? If not have Nicherson modify it.The floats, metering plate ect. I think 154 secondary are way to big. I'm running 85's on my 454 at Havasu. Have been for a 3 yrs. Call Nicherson if it's his carb!I agree, let the man who created that set-up work on it. He knows the build specs better than any and it's probably cheaper to return back to him anyhow.
The floats just need to be Nitrohyl and balanced. That's simple, but have him over-look your metering which includes all pieces, main-body, metering block, and throttle body.
~Ty

[Unchained]

Infomaniac
[The boost referenced fuel pressure regulator only makes sure fuel can get into the bowl. I honestly believe the basic fuel metering function of the carb is being disrupted.
When boost rises, I believe the pressures in the manifold, box, and float chamber are very close and preventing proper fuel metering. I think the air velocity pressure drop in the venturi is being offset by the pressure in the enclosed carb box. It seems to me at high throttle position the boost presure is the same from the intake valve all the way back to the procharger compressor. This will mean the pressure in the float chamber is too close to the pressure in the venturi. The engine will lean out regardless of fuel pressure or jetting.
Whew -- Any comments?[/QB]I agree with Ron, Those "shoebox over the carb" setups always looked like a bandaid system to me.
Mark

[sdba069]

I did make mention in the first post that it is a Nickerson carb set up for the exact application it is being used on,that being the 509 with the blow thru deal.Ron,your deal makes sense,but there are a lot of these blow thru deals out there working.Understand that I build lots of blown motors and have a lot of experience tuning both the carb and injected motors,but this is my first blow thru application.I personally would not own one,but our customer wanted it.I put this thing together by the book and considerable input from Dean Nickerson.By the way, the advance is locked out in the distributor and the timing is at 28 degrees.The barb is about 900 cfm.I normally run a 26 BraveII 4 blade,but I've got a 23 pitch 3 blade Mirage on it while I'm testing so as not to lug the motor and get into a detonation scenario.Compression is 8.2 to 1 and while I'm testin I'm running 93 octane and Sunoco 112 mixed 70 percent pump and 30 percent 112.I've never stayed in it long enough to know what kind of boost it will make,but I think 5 - 7 pounds max.According to the 2 fuel pressure gauges,the boost referenced pressure system is working correctly.I need to hear from someone who may have actually had this same problem and figured it out.Thanks for the replies so far.
Garry

[502procharger]

i also had nickerson build me a carb for my procharger. i bolted it on and ran it. i did all my plug testing and everything was perfect. i run 7lb's on a 502 with no problems. dean at nickerson helped develope the carb setup along with procharger for there setup. i would be very suprised if he messed it up. he is a stand up guy and i am sure he will go through it again and make sure all is right. i would tend to want to look at the rest of the fuel system. make sure you are running the boost ref line from the box and not the manifold. make sure you have at least 1/2 fuel lines, and make sure if you are running a filter that it is non restrictive. also you may want to check your float level whith the boat in the water and not on the trailer and make sure they are not set to low.

[HPjunkie]

69..Had the same setup. Cooked #2 and #7. Heat related of course. Had stock 8.2 JE's and iron heads and 29 deg of timing. The oil temp always came up quick and I was running it hard for about 1.5 - 2 minutes. Then it let a piece of ring upstairs and it sounded like a pushrod hitting the valve cover. Had to hone the block and put in some new pistons and that was it. All of this after a full season of running about 45-50 hours. What I did find was a plugged intercooler. I had run in the mud about 3 weeks earlier with a low water bravo drive. I had everything in there including snail shells. New setup is same shortblock, Brodix bb2plus alum. heads, crane "741" roller cam, and nickerson refreshed the carb. The boat went from 81 to 90 mph!The oil temp is a lot lower now. My old pistons were black on the bottom from burning oil I think. I dunno but for sure Id check that intercooler. and be sure on the gas. I keep octane booster in my boat for strange gas stops. And always fill up at busy and reputable stations. Greg