Take a look into what the atmospheric conditions (and therefore the correction factor) was during these runs. The SAE J607 equation is linear only to +/- 7% correction. Based on the differential between the BSFC figures reported and calculating the BSFC with the corrected horsepower, it would appear the correction factor was around 1.15x (+15%), which is too high.

In my long experience with dyno charts, typically if the correction factor is jacked, the BSFC's are lower. When ever I see BSFC's into the .300's, especially well into the .300's, 99.9% of the time the correction factor is way higher than it should be.

I'll have to look at this sheet more carefully and see what my brain can wrap around. I'm not too excited/interested to do so though. So maybe I won't. LOL.

Edit in: again, I can't remember if I ever see a four stroke into the .900's. Even on a first pull with no tuning. 2 strokes, yeh.

This data suggests that the measured horsepower is lower than the corrected horsepower. Looking at 4500 rpm on Engine #2, 268.8 lb/hr of fuel flow divided by 436.1 Hp gives a BSFC of 0.616 lb/Hp-hr off of the corrected horsepower number. To arrive at the reported BSFC, the measured Hp would have to be lower.

In response to what I said earlier, I do remember a few 'stupid' high BSFC's. Had to dig thru my slow cranium.

Fuel system on motor on dyno had return fuel system. They just used one flow meter instead of two.

With a return style system and one fuel flow meter, you are only readiing what the fuel system is flowing, not what the engine is using.

In a return system you want Meter 'A' measuring fuel flow going in and Meter 'B' on the return measuring what is returned. You simply set up software to be 'A' minus 'B' . This will tell you what engine is actually using for fuel.

Now, back to reading what you have posted....

I've seen the "A minus B" issue before as well. Maybe the OP can comment on how the fuel system was plumbed. Notable is there is a lot of variation in the fuel flow data, more than I'm accustomed to.

Just got off the phone with shop, the fuel system is not a return system so that's ruled out. I told him how everyone is ragging on the high BSFC numbers and he's going to look into his notes. He said they tried different jets but where he left it was where it was best. I also mentioned about the power valves and he was upset to hear that all four blocks had provisions but his guy somehow missed it. He'sl also going to pull the AFR charts once he gets a break.

We are not ragging on them (BSFC's).

They are brutal !

Point blank.

I have seen where there can be some variability in the fuel flow figures, as carburetors tend to fill the bowls in "gulps", rather than flowing a constant flowrate. That still doesn't explain the high BSFC's, though. Be sure to ask about the atmospheric conditions the engine was run at, and the correction factor applied to the measured data.

Here’s a 496 build I did a few years back and has the same guy dyno it. This seems a lot more along the “normal” side of the number readings so I know this guy knows how to get it right. NautiSouth, I’ll ask him these questions. Could these carbs actually be that bad but still run as good as they did on Sunday? Once I lower the primary jets they should clean up even more. Either way I’m stuck with these for the rest of this season.

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Lowest BSFC at Peak Torque.

Most all in the mid .400 range.

All good.

Did I mention your present Motors BSFC's are brutal ? LOL.

This 496 is flowing a lot less fuel, and making a lot more power than your first two engines!!

Compare your old motor's Fuel Use (located under Fuel A+B) vs your new motor's Fuel Use (again, located under Fuel A+B)

Either the fuel flow meter is f*k'd or your poor engine is absolutely drowning in fuel.

This needs to be found out.

Based on the aforementioned carb jetting, my bets are on the flow turbines being miscalibrated.

Remember 1st post. His carb was flooding out.

No one caught lack of PV's when tuning.

I do agree with you Naughty. Would be a good bet. However, we know where assumptions leave us #1, and #2 he paid for a dyno test. He did not get a fair one.

This should have been caught during dynoing. Very obvious ! Definately not a next day or later catch.

Sucks.

Agreed. He paid for a service he did not receive, it appears. I would push for another day on the dyno, gratis.

Yea, I can see the comparisons between my older build and the newer ones. The new ones are brutal. I'm heading to the shop this weekend to grab the other graphs and talk to the builder. he says he has other charts with lower BSFC numbers so I'll have more info after Saturday.

For now in my current set up I would like to ask what is the "proper" procedure for testing plugs? Is it WOT then shut down while at WOT and in gear or do you pull the sticks back and then shut down?

Pull the sticks back and then shut down as quickly and safely as possible - i.e., without getting swamped by your wake or run over by that cruiser that is trying to keep up with you. Crack a beer, let it cool for a few minutes, and then pull a couple plugs. Definitely do not shut it down while the sticks are at WFO. That would probably not end well. :eek:

Thanks, will do this weekend. That's exactly how I did it on Sunday after a run at 3800rpms. I posted a pic of the plugs a few pages ago. I will test again this weekend from WOT along with the other tests I need to do.

Installing an O2 sensor in the riser, and getting a wideband, would be the best money you can spend at this point IMO. I know pulling the engines to go back to the dyno is probably not a feasible option at this point.

I'm just not a big fan of reading plugs. If you can get someone to weld bungs in, and pick up a NGK wideband for about 250 bucks, dialing it in properly will probably much offset the cost, based on fuel savings, or worse, hurting the engines. I also think those GM dual planes are hurting you a bit. I'd like to think there is also a port mismatch between the Brodix heads and that intake port design, unless the heads were port matched.

yup.... I got an AEM wideband for about $175 from summit complete kit with gauge.... works like a charm and is the best way to accurately tune your boat as installed with your exhaust.....I have a warmed over 502 with ported aluminum heads and an airgap intake and 82/88 jets work perfectly for my 850 mighty demon...... forget pulling the plugs until you get it close using the wideband... I bounce around 13.5-14.5 at idle and 13.5 @ 3.500RPM cruise..... going to full throttle with everything wide open and power valve kicking in I am right at 12.5 and then 12.2 at 4,900rpm max..... I bet I am not too far EFI efficiency at cruise but the only way you will ever know is to get the wideband hooked up. I did get a tapped spacer plate to install in between the riser and manifold for $95 and so far so good! TBF

Without a wideband I go #2 sizes jet richer than best performance and check my plugs here and there for specs of aluminum. Without mag glass they usually appear like pepper specs.

With a new to me carb I jet richer for first jet change. #2 each time (if going bigger or smaller)while keeping the carb's factory prim to secondary jet spread the same.

Only change to this is if 1st or 2nd run is bad lean, then I jump a bunch. Doesn't happen much cause a properly chosen carb won't be too far off.

Wish I could run the O2 Wideband, but i have the dreaded Lightning O2 sensor killers.

I have put in and pulled out O2 sensors in Lightning headers once on plane. Sometimes the only way to tune.

That sounds like it may have been fun...

Maybe get some marshmallows on a stick and make s'mores. ;)

You know I never thought of that. Who needs a grill when you have exhaust...

Lol, 15mph on plane and hatch up a little, is pretty uneventful.

been reading you all threads on jets...Ive been running 78 and 86 on my 496 stroked, alm. heads, deamon 750 and there still black rich.6.5 valve primary only.. maybe I got something else going on .. any thoughts??

Jetting is more dependant on the carburetor itself. Do not follow jetting from another part# carb.
And then of course the motor build up and use itself.

Anyone notice that on most Holley's, most 600cfm+ carb will have jets in the 60's , most 700+cfm carbs will have jets in the 70's, 800+'s in the 80's, 900+'s in the 90's ???? LOL. Ka winky dinky huh ? LOL.

Smaller venturi's and throttle blade size with same booster pulls more fuel for same given amt of air pulled thru.

Edit in: I see you have a Barry Grant carb. You really can't compare jetting and other things with a BG vs an 'equivalent' Holley.

ok yea that makes sense so based on most others with simialer engines does my jetting seem reasonable

Even though BG's are rated CFM wet flow (air and flowing fuel) and Holley's dry (just air)...which means a 750 BG will outflow a 750 Holley when flow tested dry (again, with just air), you'll be hard pressed to find anybody using a 750cfm anything on a peformance 496.

So....you may be alone, or near alone, on that one.

Hey guys, So i went to the shop and got some more paperwork. I'll post up some specifics when i get the docs out of the car at lunch. Long/short of it is that my carbs are definitely just not efficient, which we already know by the fuel consumption numbers. The shop has a holley 750 which he uses to get a base line then he goes from there with the owner's carbs. Both engines ended up making just about the same power with his carb and my carbs but just my carbs use over 100lbs more fuel per hour to make that power according to the dyno pulls.

So now I'm looking into new carbs for the remainder of this season instead of waiting for offseason, figured they will almost pay for themselves in fuel cost savings. Should I stick with the Holley marine 800? Not knowing much about the internals of carbs I'm curious to know how these carbs can be so "out of whack", is it something that can be fixed or is it a lost cause and just trash them?

Is anybody running dual PV's? I've asked this before without any straight answers. I had a carb built by Pro Systems with dual PV's. I'm running 94 square on the jets and have been going off plug color. I get incredible fuel economy at cruise but wonder if I'm hurting the motor?

Without knowing the application, it is hard to say. Generally dual power valves are used in applications where there is a possibility of going lean when the throttle is hammered. Very common in blower applications.

Only way to know for sure would be with a Wideband O2 for A/F numbers, or possibly a pyrometer to measure EGT's. You could be seeing good plug color at WOT because you have plenty of fuel then, but have a lean spot at the top of your cruising speed, say 3800 RPM.

I don't believe that can be right. If you are making the same power, but using 100lbs/hr (~16.5gallons more) that can't add up. If you're richer you are not going to make the same power. If this was the case you would never have to tune an engine unless you were looking for efficiency.
Ramos I'm not busting on you or your engines. I just think your dyno guy has taken advantage of you and did not do what you expected and paid for.

Smarter members please correct me if i'm wrong.

I'm not smarter and I think you are correct. I wonder if they were actually dynoed... If they were then whoever dynoed them needs to find a different line of work maybe a flower shop or something along those lines.

That's why I scheduled a vacation day to be there for my dyno session - I could see first hand what was being done. Besides, what better way to spend a day than listening to your motor sing?!? :)

Ramos can you get a list number off your carbs? Should be on the choke horn ...let's see what they really are and what came shipped in them from holley

I for sure know at least one was dynoed as I went to the shop during a lunch and saw the pulls. The one I witnessed was the stronger out of the two. I just ran the boat on Sun with 6 on board and 1/4 fuel and magaged to get some more rpms out of them. 4700 stb side and 45 port. Phone gps said 53 mpg but from what i remember the prior year it sure felt faster than 53 so I'll have to get another gps unit to compare.

Carbs don't have choke horns at all so no list number. Then only number I see on the main body is 6R 3767B. Here is a pic of the weaker motor's A/F and so on. The one on left is my carb, on right is first pull with shop carb. I'm currently running 78 primary with 6.5 PV and 92 secondary. Thinking about lowering the secondaries but I'll wait until I get the O2 bungs in my risers for more data.

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