Air Flow / CFM's

[Chaz]

Scribed.....Interesting and good for my learning curve! Gary

Me too , I find it ..........................

[W2F a V-King]

Does anyone know anything about CFM's. for Mercs
Like whats the cfm rating for a 2.4 efi bridgeport horn or a 2.5 fishing,260,280 horn.
Or whats the air flow for a 4, 6 or 7 pedal reed cage
Or the air flow readings for a 4, 6 or 7 pedal intake
I never hear anything about how much cfm's the WH series carbs do flow or can flow or efi throttle plate plenums.
Back in my High performance car days, everything was about CFM's. then compression was second and then the cam, yada yada yada. I was a multiple carb/intake genius, still am.
But I never ever hear anyone bringing up CFM's on their mercs.
CFM's = Horsepower.
Porting is to boost CFM's
Opening reed cages is to boost CFM's
Higher compression is to spin your motor faster in turn to pull in more cfm's
Everything is about air flow and cfm's but there is never talk about it.
I'd be very interested in hearing how many cfm's can flow through a merc. I see Brucato and MAD have huge plenum openings especially MAD but thats only as good as your motor can inhale the cfm's. Carbs are all about air flow, in fact they require it to create vacuum in the venturi. But yet its never brought up.
I would never put an 850cfm carb on a 350 chevy because you would be stuffing it excessive fuel and they guy with the 600cfm carb would pull away from you even if the 850 is a cooler carb to have. Thats kind of a comparison to a Brucato and MAD front end. I'm not talking about the electronics of Brucato and MAD. I understand the electronics are there to control the fuel/air ratio. thats obvious. I'm not trying to compare to a top fuel power grid and throttle body injection system. Just saying that on a car with brodix, afr, whatever aluminum heads you can't build an engine without knowing what your intake and induction system flows or your just playing a dangerous guessing game at your own expense. So this is why I am curious about air flow and cfm's (cubic feet per minute) on these sweet little two strokes.
So what gets me curious is just pondering the thought of crazy things like modern technology for instance. You take your efi plenum off, weld up a tapered plate to mount a F.A.S.T. Holley, Edelbrock self learning fuel injection system. You can mount the carburetor looking throttle body sideways being you using your stock six injectors so there is no fuel in the unit anyways. Now you have a self learning system that will calibrate itself every time you change cylinder compression, octane and timing. Sounds simple. I'm sure there are hiccups to overcome. But to have an ecu that is constantly changing its variables to adapt to your driving habits seems like a lead foot and racers dream. Exhaust tuners and back pressure are a different story. Thats why I didn't bring up those subjects. Yet.
I may be a newbie around here and I haven't race boats in the past and I am still learning the simple yet highly delicate operations of multi cylinder 2 strokes. But I accell in the automotive field. I'm just trying to acclimate myself into this super cool boat engine world. Its pretty fun and interesting. Its just that its like learning a new language and I really appreciate everyones input, especially when I am wrong.

Good read up there ^^^ Just not agreeing with the 350 chevy and a carb bigger than a 600 cfm being to big.......maybe on a daily driven hot rod street car, maybe not even then.
Depends on cam, heads, intake design, valve-train, designated rpm, VE of said motor, rotating assembly capable of desired RPM, etc.... If I remeber correctly a 358 CI NASCAR motor is LIMITED to 830 CFM...

Me personally...I choose the cam I need to get me what I want, then build the motor around that.

But back on topic vv...

If you could come up with a waterproof, non oil fouling O2 sensor, preferably a wide band for better tuning accuracy. You could indeed build an ECU to do just that. MegaSquirt makes several combinations of ECU, PCMs to do this. Just no two stroke O2 sensors for long term closed loop operation like your suggesting. It would be great, indeed!

There is a member here that has a great deal of experience with this and has made and runs a MegaSquirt system on his 2.5 Merc.

Has a Youtube channel.... https://www.youtube.com/user/panronnie/videos

Just no feed back control, or closed loop control due to no O2 feedback. He has incorperated EGT into his though. Maybe he will chime in here somewhere....

As far as CFM's and outboard motors....good thought there, actually never have seen any numbers myself.

First question that comes to mind is does a two stroke need twice the air flow as a four stroke?
Two strokes cram the 4 strokes operations into 2 strokes. With 2 stroked leaking some flow to scavenging and bleed over losses. One would think so anyway, minus the slight loss mentioned.

Great thread !! Subscribed....

[Magcat 62]

YES a 2stroke pumps twice as much a some rpm as 4 stroke my son runs a 406 sbc and 1050 carb its a animal 68 camaro 6 sec 1/8 mi! flow formula i found to be close for 2 cyc is horsepower x 2.5 = cfm needed.

[Magcat 62]

RAISING compression on a hipo motor just levels the playing field. the longer duration in port or valve timing decreases time ports are CLOSED. DYnamic cylinder pressure would decrease , power would suffer, its a trade -off same as big cam and low domprression in 4 cycle=DOG

[Krazymaan]

Good read up there ^^^ Just not agreeing with the 350 chevy and a carb bigger than a 600 cfm being to big.......maybe on a daily driven hot rod street car, maybe not even then.
Depends on cam, heads, intake design, valve-train, designated rpm, VE of said motor, rotating assembly capable of desired RPM, etc.... If I remeber correctly a 358 CI NASCAR motor is LIMITED to 830 CFM...

Me personally...I choose the cam I need to get me what I want, then build the motor around that.

But back on topic vv...

If you could come up with a waterproof, non oil fouling O2 sensor, preferably a wide band for better tuning accuracy. You could indeed build an ECU to do just that. MegaSquirt makes several combinations of ECU, PCMs to do this. Just no two stroke O2 sensors for long term closed loop operation like your suggesting. It would be great, indeed!

There is a member here that has a great deal of experience with this and has made and runs a MegaSquirt system on his 2.5 Merc.

Has a Youtube channel.... https://www.youtube.com/user/panronnie/videos

Just no feed back control, or closed loop control due to no O2 feedback. He has incorperated EGT into his though. Maybe he will chime in here somewhere....

As far as CFM's and outboard motors....good thought there, actually never have seen any numbers myself.

First question that comes to mind is does a two stroke need twice the air flow as a four stroke?
Two strokes cram the 4 strokes operations into 2 strokes. With 2 stroked leaking some flow to scavenging and bleed over losses. One would think so anyway, minus the slight loss mentioned.

Great thread !! Subscribed....

Basically i was making a reference to a high school kid (back in the 80s) taking off their quadrajunk and installing a huge 780 holley thinking its going to add 200hp without adding the appropriate changes you would need to go along with that. like you mentioned.

[FMP]

Nothing wrong with an 800 Quadrajet that has been properly built and tuned.

[dwilfong]

OK so let me ask this.
What is the choke point on a merc 2.5 V6 or put a different way what limits the flow?
Carb's?
Reed blocks?
Case?
Timing?
Sleeve ports?
Ex chest?
Adapter?

[RBT]

Exhaust chest

[dwilfong]

Exhaust chest

The chest it self or the opening to the adapter?

[BarryStrawn]

Basically i was making a reference to a high school kid (back in the 80s) taking off their quadrajunk and installing a huge 780 holley thinking its going to add 200hp without adding the appropriate changes you would need to go along with that. like you mentioned.

Excellent example of misunderstanding about CFM. Since even the smaller Quadrajet flowed 750 cfm the Holley 780 offered little or no performance advantage but a lot of opportunities to screw things up. Or perhaps easier tuning if other significant engine modification were also done along with the Holley.

Maybe the few folks doing outboard hard parts were smart enough to just not bother quoting CFMs.

[Krazymaan]

The chest it self or the opening to the adapter?

Here is an opened up exhaust chest with proper ramps

[Magcat 62]

I HAD a s/p drag race nova that wheel-stand running a q-jet (foot brake). i think a svs set up functions a like takes in only what engine needs.

[W2F a V-King]

Gonna be a screamer, high rpm motor. Fo Sho.
What you loose down low, you will get up top.

How about the cylinder port work?

[W2F a V-King]

I HAD a s/p drag race nova that wheel-stand running a q-jet (foot brake). i think a svs set up functions a like takes in only what engine needs.

You mean like the old Ford V V carburetors, 2 barrel Variable Venturi? Those were a real PIA to work on, but when it was right, it out performed some smaller 4 barrel carbs.

That was just before Ford switched to EFI motors.

[Krazymaan]

Gonna be a screamer, high rpm motor. Fo Sho.
What you loose down low, you will get up top.

How about the cylinder port work?

Port work is on the side of Extreme and Obnoxious.