Target Air Fuel ration N/A engines.?
08-07-2016 | 09:35 PM
#51
Registered
Joined: Jul 2004
Posts: 11,332
Likes: 73
From: chicago
Have a box full of them here. Its funny when you look at them, some have the little retainer dohicky threaded on further than the next. Basically tightening up the spring more than the next
08-09-2016 | 04:16 PM
#55
Frank's Marine Service
Joined: Sep 2002
Posts: 11,959
Likes: 142
From: Elkton, MD
They stay open once your intake vacuum drops below the PV set point (6.5, 4.5, 2.5, etc).
In a car, most of the time, the PV is closed, unless, your are heavily accelerating. In a boat, they can be open, at a cruise rpm. Mainly, because boats almost never run in "high" vacuum conditions.
This is one of the reasons, I don't care for boost referenced PV's in roots blower engines in boats. Pretty much, you're PV is simply ALWAYS open, whether you have a 8.5, or 2.5. Unless you're running a really light cat or something like that. I just bought new "standard" 850 marine quick fuels for my twin carb roots setups. The carbs are basically identical in every way as the "SC" versions, except for the boost referencing. Also 200 per carb cheaper, and when you're buying 4 of them, it adds up.
With a roots, at 3500RPM, my boat is already at 0-2psi of positive pressure in the manifold. So any PV is gonna be open if it sees that, which it would if its "boost" referenced. However, being NON boost referenced, the PV sees the signal on top of the blower, which is different than what is going on in the intake. So, while I may have 2psi in the intake, the carbs are seeing 6-8 inches of vacuum lets say at that point. Now I am in PV tuning territory. Meaning, I at least have a window of fuel tuning, that you do not have, with boost referenced carbs in boats.
In a car, most of the time, the PV is closed, unless, your are heavily accelerating. In a boat, they can be open, at a cruise rpm. Mainly, because boats almost never run in "high" vacuum conditions.
This is one of the reasons, I don't care for boost referenced PV's in roots blower engines in boats. Pretty much, you're PV is simply ALWAYS open, whether you have a 8.5, or 2.5. Unless you're running a really light cat or something like that. I just bought new "standard" 850 marine quick fuels for my twin carb roots setups. The carbs are basically identical in every way as the "SC" versions, except for the boost referencing. Also 200 per carb cheaper, and when you're buying 4 of them, it adds up.
With a roots, at 3500RPM, my boat is already at 0-2psi of positive pressure in the manifold. So any PV is gonna be open if it sees that, which it would if its "boost" referenced. However, being NON boost referenced, the PV sees the signal on top of the blower, which is different than what is going on in the intake. So, while I may have 2psi in the intake, the carbs are seeing 6-8 inches of vacuum lets say at that point. Now I am in PV tuning territory. Meaning, I at least have a window of fuel tuning, that you do not have, with boost referenced carbs in boats.
I'm pulling a few inches of vacuum at 3500 and I run boost referenced carbs with 2.5 pv's for the last two seasons. I ran non-boost referenced carbs for 10 years but there were some part-throttle lean spots which is why I switched to boost referenced to get the signal below the blower. I don't see zero vacuum until about 4k rpm and 1-3 lbs boost about 4200 depending on fuel and humidity.
08-09-2016 | 05:02 PM
#56
Registered
Joined: Jul 2004
Posts: 11,332
Likes: 73
From: chicago
If I read this right, you're making zero vacuum to 2 psi boost at 3500 rpm? What is your max RPM? What is max RPM vs. where peak power and torque were on the dyno?
I'm pulling a few inches of vacuum at 3500 and I run boost referenced carbs with 2.5 pv's for the last two seasons. I ran non-boost referenced carbs for 10 years but there were some part-throttle lean spots which is why I switched to boost referenced to get the signal below the blower. I don't see zero vacuum until about 4k rpm and 1-3 lbs boost about 4200 depending on fuel and humidity.
I'm pulling a few inches of vacuum at 3500 and I run boost referenced carbs with 2.5 pv's for the last two seasons. I ran non-boost referenced carbs for 10 years but there were some part-throttle lean spots which is why I switched to boost referenced to get the signal below the blower. I don't see zero vacuum until about 4k rpm and 1-3 lbs boost about 4200 depending on fuel and humidity.
I am running 7.5lbs of max boost at the moment, 17x28 props, blower at 8% over, full throttle rpm around 5500-5700 currently.
Its rare that i boat in flat glass like water that allows you to trim the boat for max cruising speed.
Ive been running non boost referenced carbs forever. Never had a lean spot in the rpm band. Although, i can certainly see it, if you have too low of a pv opening point, or too small of carburetors.
08-10-2016 | 07:38 AM
#57
Frank's Marine Service
Joined: Sep 2002
Posts: 11,959
Likes: 142
From: Elkton, MD
That's interesting. I guess every boat is different.
I'm running about 6.5 psi boost (965hp). AED Super Mod 1000 carbs. My old carbs were smaller which is why I upgraded but I was also making roughly 75hp less back then then I am now with the new valvetrain setup.
To be honest, having done it both ways, I don't really see too much of a difference. In certain situations I can see where if you're running big cubes and spinning the blower quickly there can be more draw under the carb at part throttle making the carb think it's seeing a more vacuum than what is actually below the blower and the PV stays shut.
I'm running about 6.5 psi boost (965hp). AED Super Mod 1000 carbs. My old carbs were smaller which is why I upgraded but I was also making roughly 75hp less back then then I am now with the new valvetrain setup.
To be honest, having done it both ways, I don't really see too much of a difference. In certain situations I can see where if you're running big cubes and spinning the blower quickly there can be more draw under the carb at part throttle making the carb think it's seeing a more vacuum than what is actually below the blower and the PV stays shut.
08-10-2016 | 08:10 AM
#58
Registered
Joined: Jul 2004
Posts: 11,332
Likes: 73
From: chicago
My engines ride the short bus. They are only 468 cubic inches. To make any kind of power out of them, the small cams, small carbs, small heads, and all that, just wont give me the power numbers i want. They made 820ish hp in full dress, and ran out of pulley options on dyno. Hardly big power by todays standards, but hp per ci, they are up there. They werent gonna make that power at 5200rpm, with gobs of torque down low, like you can get with a big bore big stroke monster. But, they do start coming alive power wise in the upper rpm band, which allows them to turn the 17x28 to the speed goal. The boat does actually accelerate pretty dam good, as it has the help of boost. At 3500 cruise, they probably arent the most efficient making power on motor type of build.
Lets just say for a minute , i am cruising at 3500, and 2 inches of vacuum in the manifold, but say 7 inches under the carbs. Say i have a 6.5 pv. The pv should be closed , and my afr will be xxx. Now, if i wanted to either richen things up there, i could install a 8.5 pv. Or maybe i need to richen it up at 3000, i could install a 10.5 pv. Or, maybe its too rich at 3500, but good everywhere above that. I could now install a 4.5 pv.
If its boost referenced, the only way ill be able to richen or lean the mixture there, would be reducing jet, or power valve channel restrction. Which is fine, but now that reduced fuel , or added fuel, will now have an effect on the fuel tune in the upper rpm band , so, secondary jet will need changing as well now. Which also works, so i guess its all a matter of preference. And like you said, all boats are different.
My boat is extremely sensitive to tab and drive position at cruise, as well as fuel load. The water conditons are always changing where i boat. One minute it could be rough enough to have the drives out 1 notch, and tabs dragging, next minute, its calm enough to have drives out to nuetral and no tab. The load on the engine changes tremendously, and while the rpm stays constant, the throttle position varies quite a bit. Which sucks, because it can make for hard tuning when the condtions are always changing.
One of the reasons i dont put a lot of faith in a dyno pulls carb tune. That works much better for a drag car than a boat engine.
Lets just say for a minute , i am cruising at 3500, and 2 inches of vacuum in the manifold, but say 7 inches under the carbs. Say i have a 6.5 pv. The pv should be closed , and my afr will be xxx. Now, if i wanted to either richen things up there, i could install a 8.5 pv. Or maybe i need to richen it up at 3000, i could install a 10.5 pv. Or, maybe its too rich at 3500, but good everywhere above that. I could now install a 4.5 pv.
If its boost referenced, the only way ill be able to richen or lean the mixture there, would be reducing jet, or power valve channel restrction. Which is fine, but now that reduced fuel , or added fuel, will now have an effect on the fuel tune in the upper rpm band , so, secondary jet will need changing as well now. Which also works, so i guess its all a matter of preference. And like you said, all boats are different.
My boat is extremely sensitive to tab and drive position at cruise, as well as fuel load. The water conditons are always changing where i boat. One minute it could be rough enough to have the drives out 1 notch, and tabs dragging, next minute, its calm enough to have drives out to nuetral and no tab. The load on the engine changes tremendously, and while the rpm stays constant, the throttle position varies quite a bit. Which sucks, because it can make for hard tuning when the condtions are always changing.
One of the reasons i dont put a lot of faith in a dyno pulls carb tune. That works much better for a drag car than a boat engine.
08-11-2016 | 08:08 AM
#59
Frank's Marine Service
Joined: Sep 2002
Posts: 11,959
Likes: 142
From: Elkton, MD
I understand. When I lived in NJ the conditions would change in a moment's notice. You could leave in the morning and there were 1-2's, you eat lunch and the sea breeze kicked in and you were heading home in 4-6's. It's MUCH different here on the Chesapeake Bay. A rough day here is equivalent to a typical day on the ocean back home. BUT, it's been 5 years and I do miss swell jumping a LOT! https://youtu.be/ExbZfhvYlsA
