TruxtonFox

My thought, mind you I don't know jack about carbs, but under load, doesn't the blower start producing boost at 3k? If it does, is the air coming through the carb being sucked harder and thereby letting the power valve open and dump to much fuel? A PV with a greater opening pressure would delay the onset of fuel until the higher rpm ranges.......spitballing

MILD THUNDER

iTrader: (3)

Depends on how much load , as to whether or not it is in boost.

Typically, at 3000 rpm, in a 30ft boat, I would not expect it to be making boost yet. Irregardless, when the carb is NON boost referenced , the power valve is only seeing vacuum, and never boost. With a 4.5" PV, it will not open, until the vacuum drops below 4.5 inches. Going to a 10.5, will make it open sooner, and a 2.5, open later.

On a typical 750 carb, the power valve channels are not huge, like say, a dominator . Even if it was in fact opening too early, it should not be injecting enough fuel to "flood" the engine. If your carb was "boost referenced", it likely would be opening at 3k rpm on plane. Reason being, I doubt you would be seeing 5-6+ inches of vacuum there.

installing a wideband setup, would enlighten things.

MILD THUNDER

iTrader: (3)

Blowers really arent "wayy to big" for 502's. The 10-71 is not crazy bigger than an 871. Im running 420 B&M blowers on 468ci engines, which are close to an 871 in size. The engines run fantastic, and makes plenty of power.

If your plugs are legit wet with fuel , after a 10 second 3k rpm run, I'm thinking you have a fuel delivery issue. Lets say, you have a cheapy fuel psi gauge. It shows 7psi, but there is actually 8 or 9 psi in the setup (seen them off many times). Maybe it will idle just fine with boat at rest, and not blow the needle off its seat. Now, you get on plane, and the float becomes very sensitive, and the addtl fuel psi , simply will not regulate the fuel entering the bowl like it should. Your typical holley float, does not play well with excessive fuel pressure.

What kind of fuel pressure gauge are you using to set fuel psi ?

What regulator are you using? I've seen guys fight issues like this, when using cheap dead head style regulators with elec pumps. Fuel psi varying all over, flooding out, etc.

F-2 Speedy

iTrader: (6)

Were talkin about a 1071 on a basic stock 502 mag mpi long block with stock exhaust, you dont think it to big ?? Im learning the blower side of things and I know you, ICDP and Mike and others have a bunch on seat time with them.

MILD THUNDER

iTrader: (3)

Some have ditched the 256 blower on the 575sci, for an 871, and saw some nice speed gains. The biggest issue with too large of a blower, is inefficiency due to leakage between the rotors, at low speeds. Unless hes running it at 30% underdrive, i dont see an issue. Heck, some guys run 871s on small block chevys.

Certainly nothing that is gonna cause plugs to get soaked in raw gasoline.

MILD THUNDER

iTrader: (3)

All things being equal, the difference between an 871 and 1071, is less than 10% overdrive, same engine, same boost, etc. So at 5500 engine rpm, and say 1:1 ratio, the blower is turning 5500. Going to 10% under, now the blower is at 5000rpm roughly. Nothing drastic.

Now, take a 177 blower, at 2:1, and install an 871, you may go from 11,000rpm rotor speed, to maybe 5000rpm rotor speed. Now theres something worth talking about

MILD THUNDER

iTrader: (3)

Off topic kinda, but since we are talking blowers and boost.

Buddy of mine bought a boat with blown 572s. 1071s, dart 345s, chillers, etc. Engine expired within a few hours of ownership. Cams it had were way small for a 572, duration wise, in my opinion. Hyd roller. They made 883hp with 10psi before he took delivery of boat.

Rebuilt engines over winter. Compression was lowered from 8.5:1 to 8:1. New cams i picked for him from the lunati catalog. Very mild 629 lift .

Redynoed. Engines made 1000hp with 10psi, 950ish ft lbs. Stump pullers as it had over 900ft lbs from 4000 and up. Anyhow, in regards to the blowers. With the old pulley combo that made 10psi , and 883hp , now made 8psi, and over 900hp. He dropped 2psi, but gained power. Overdriving back up to 10psi, and then crested 1000hp. The orignal cam was choking it.

So boost is not just relative to cubic inch, as we know

mike tkach

iTrader: (3)

it,s all about cylinder pressure,to little and power will be down,to much and detonation will kill the engine in short order.small cam&10 lbs boost won,t last long even with lower compression ratio.

mike tkach

iTrader: (3)

for me if i have to choose between a blower that is to small or one that is to big i,l take the bigger one any day of the week,just spin it a little slower.

tommymonza

And plenty of lift or duration on the exhaust side to make it all work?