Actual cfm used vs carb size

[Steve.k]

I had the single 1050 but a update to a little better intake may have got me closer to 700. I think I'll update the cam in a year or so to compliment the dual setup. Thanks andyf and Af2

[F-BIRD'88]

If you have a single 4 bbl carbed 700 hp engine and you replace it with a tunnel ram and 2 carbs it will now make more than 700 hp..

If you have a 500 hp engine with a single 4 bbl and replace it with a tunnel ram and 2 carbs it will then make more than 500 hp. (And likely more torque too.
Do not use 2 small carbs. 2 small carbs are TOO small.

Further: if you have a single carb 700 hp angine and or a single carb 500 hp engine and you replace that single carb with 2 4bbl carbs on a adapter on that same manifold. BOTH engines will now make more horsepower.
Again , 2 x small carbs are TOO small.

You will go a bit faster and you will get lots of guff and blowback from other racers using 1 single carb and even more guff from those that $ell single ¢arbed ra¢e motor$. You will go a bit faster and they will not like it.

[Stan Weiss]

I have pondering alot of this,

So what am I missing here, making hp is about getting air and fuel into the engine. Airflow into the engine is based on the pressure differential between atmospheric pressure and the internal engine pressure.

So playing with some calculators on airflow. Theoretically at 100-110% volumetric efficiency,at 6000 rpm my engine should be able to consume between 708 and 772 cfm. My engine is showing approximately .5 inches of vacuum on track.

At 7000, my engine should be able to consume approximately 826-900. My engine shows approximately .75 on the track.

Now the kicker, assuming a 1050 dominator is flowed it 1.5 inches, using a wallace calc the calculated flow based on manifold vacuum, at 6000 rpms at .5 inches of vacuum, it's only moving approximately 606 cfm. At 7000 rpms at .75 inches, it says 742.

So based on this, with this carb, I need the manifold vacuum around 1.1 to get the 900 cfm at 7000.

I plan on testing this, I am going to try some skirted boosters. My question is, will the restriction created by the skirted boosters to get the manifold vacuum higher offset the gains created by the increased pressure differential? The air should be traveling faster into the engine.

So this would suggest to me that I am not moving enough air though the carb for max performance. The pressure differential is not enough to flow the air I need to support the engine. Is there something else that is in play here?

Do you have a dyno sheet that you can post?

Stan

[MadBill]

...
So based on this, with this carb, I need the manifold vacuum around 1.1 to get the 900 cfm at 7000....

Er...no. The only way to raise the manifold vacuum absent more displacement, RPM or component tuning is to slightly close the throttle and you know what effect that has.
Reducing the pressure drop to the lowest value that still supports adequate fuel delivery/conditioning at the low end of the required power band gives the highest possible charge density/mass and thus power.

Generally speaking, manifold vacuum less than say 0.6"Hg. at peak revs has almost no effect on power but makes low speed carburation a major challenge. Consider:

• At standard sea level air pressure of 29.92"Hg, an engine pulling 3.0" of vacuum at peak power will be operating with 29.92-3.0 = 26.92"Hg absolute internal pressure.

• With a bigger carb or carbs giving 1.5"Hg at peak revs, it will be 28.42", a gain of 5.6% pressure/mass and power.

• With 1.0" it will have 28.92", gaining another 1.8%.

• Dropping it down to 0.6" via better flowing carbs gives a further increase to 29.32", a theoretical increase of 1.4%

• With carbs big enough to give only 0.4" pressure drop (a 33% reduction in pressure drop, requiring way more carb capacity) and assuming carb metering was still adequate, the gain would be an inconsequential 0.7%

There are other factors at work, but this gives a useful overview of the situation.

[F-BIRD'88]

In this pursuit for more power thru lower wot manifold vacuum by using bigger carb capacity, you will run into fuel metering quality issues at the low end of the rpm range sooner by using 1 big 4 venturi carb than you will if using 2 carbs with 8 venturi total. 2 x4 carbs is better than 1x4 carb. Especially if the secondaries of the 2 carbs are progressive.

[F-BIRD'88]

A 5% drop in pressure differential does not equal a 5% gain in mass flow in. Someone smart will know that formula.

[F-BIRD'88]

You gain some net mass airflow but in addition you also get a bit more power from this by a reduction in engine pumping loss. ( More air in with less work done.)

And because the intake/exhaust pressures (during overlap) is now a bit different going in and going out, the camshaft valve timing will need to be optimized to realize the full net power benefit. The fuel metering and fuel control and the fuel distribution will be better overall with 8 venturii vs 4 venturi.

[F-BIRD'88]

Re: in car performance gain.....
The more or longer time the engine spends its time running near, at and BEYOND THE max power rpm, (using the improved peak power. ↑↑→ AND improved "AFTERPOWER" the more performance benefit in a accelerating car.
(1/4 mile dtag racing) this equals using more gears and more rear gear ratio. (And optimizing the torque converter too). You want to use the extra power created and extended usable upper power band to gain car performance. Then you accelerate harder to get there quicker and at a higher trap speed. (1/4 mile)

[cjperformance]

Do not equate a 1350 single carb vs 1050 single carb test to a real dual carb test.
The shift recivery may be YOUr problem (with a big single carbvs a bigger single carb. But do not extend that to what a dual carb setup will do.
I bet you have NEVEr tested a 2x4 carb setup on adapter on a single 4 bbl intake at all… ever, of any type.

Further, on a boat the dual carb set up will cruise better run better and accelerste quicker. From a stop , especially with progressive secondaries like 2 eddy 650 avs2 carbs. As well as increase boat top speed.

"It violates pretty much every rule of induction tracts"
This is BS.

Actually yes i have , granted the only time on a V8 was using a twin impco lpg mixer setup on a single 4 barrel offy port o sonic (you did say of any type!) but yes this is a little different to the discussion at hand. That said I have done so many stupid carb setups in the quest of curiosity that I know whats going to happen. Mostly I learnt the hard way and am willing to accept that 98% of my expermental ideas were great in theory but a flop in reality! But valuable lessons learnt just the same.
If you are just in the prototype stage/theory stage I cangratulate your efforts in wanting to push on. What you are thinking and trying has been done a million times and there is a reason that it is not common place , and thats not because everyone is unwilling to accept the untapped magical HP !!
Randy's note on violating induction tract rules is sound advise.

[F-BIRD'88]

Check this out. Apparently this kid does not know Randy
And does not scare easy.

The 2x4 adapter is the general idea.
There is a youtube vid showing how to assemble it using
BDS parts and a few simple mods.
Credit: Slater Manifolds. God bless him.
https://www.thesamba.com/vw/gallery/pix/1447174.jpg

[banjo]

I have pondering alot of this,

So what am I missing here, making hp is about getting air and fuel into the engine. Airflow into the engine is based on the pressure differential between atmospheric pressure and the internal engine pressure.

So playing with some calculators on airflow. Theoretically at 100-110% volumetric efficiency,at 6000 rpm my engine should be able to consume between 708 and 772 cfm. My engine is showing approximately .5 inches of vacuum on track.

At 7000, my engine should be able to consume approximately 826-900. My engine shows approximately .75 on the track.

Now the kicker, assuming a 1050 dominator is flowed it 1.5 inches, using a wallace calc the calculated flow based on manifold vacuum, at 6000 rpms at .5 inches of vacuum, it's only moving approximately 606 cfm. At 7000 rpms at .75 inches, it says 742.

So based on this, with this carb, I need the manifold vacuum around 1.1 to get the 900 cfm at 7000.

I plan on testing this, I am going to try some skirted boosters. My question is, will the restriction created by the skirted boosters to get the manifold vacuum higher offset the gains created by the increased pressure differential? The air should be traveling faster into the engine.

So this would suggest to me that I am not moving enough air though the carb for max performance. The pressure differential is not enough to flow the air I need to support the engine. Is there something else that is in play here?

Do you have a dyno sheet that you can post?

Stan

Unfortunately, it doesn't have vacuum numbers, the vacuum numbers come from my logger going down the track. But here it is.

https://flic.kr/p/xJwVrT

[banjo]

So would everyone agree that for any given carb, it flows more air at 1.5 inches of vacuum versus .75?

And that the closer it gets to zero, the less it flows?

If yes, could we agree, that there needs to be a certain amount of vacuum(pressure differential) to meet the demands of the engine?

If no, please explain? What else is in play here?

What kind of vacuum is run in a high end fuel injection engine?

From a theoretical standpoint, my carb doesnt flow enough air at .7 inches of vacuum to support max performance.

I think there are diminishing gains the closer you get to the actual demand of the engine.

Any ideas what the pressure differential would be at the intake valve?

This all see counterintuitive to me.

I am just trying to better understand what is happening inside the engine, so please dont take anything I say as telling anyone they are wrong. But I love these discussions getting down to the physics of things.

[MadBill]

You're looking at it backwards. At a given operating condition, say 7,000 RPM WOT, the engine will draw into the cylinders a fixed volume of air*. The more restrictive the carb, the greater the pressure drop into the manifold and the lower the resulting all-important mass of air/fuel ingested. *The air meter measures said volume at atmospheric pressure, so a greater carb restriction will draw in less mass and thus show lower atmospheric pressure CFM.

The pressure drop across the valve varies hugely with crank angle and valve lift but can be anywhere from a few inches of H2O to 100" or more. In fact good software programs like Dynomation show inflow continuing as cylinder pressure rises above port pressure as the intake valve closing event is approached, as charge inertia can for a short time overwhelm the rising negative pressure drop.

[Stan Weiss]

I have pondering alot of this,

So what am I missing here, making hp is about getting air and fuel into the engine. Airflow into the engine is based on the pressure differential between atmospheric pressure and the internal engine pressure.

So playing with some calculators on airflow. Theoretically at 100-110% volumetric efficiency,at 6000 rpm my engine should be able to consume between 708 and 772 cfm. My engine is showing approximately .5 inches of vacuum on track.

At 7000, my engine should be able to consume approximately 826-900. My engine shows approximately .75 on the track.

Now the kicker, assuming a 1050 dominator is flowed it 1.5 inches, using a wallace calc the calculated flow based on manifold vacuum, at 6000 rpms at .5 inches of vacuum, it's only moving approximately 606 cfm. At 7000 rpms at .75 inches, it says 742.

So based on this, with this carb, I need the manifold vacuum around 1.1 to get the 900 cfm at 7000.

I plan on testing this, I am going to try some skirted boosters. My question is, will the restriction created by the skirted boosters to get the manifold vacuum higher offset the gains created by the increased pressure differential? The air should be traveling faster into the engine.

So this would suggest to me that I am not moving enough air though the carb for max performance. The pressure differential is not enough to flow the air I need to support the engine. Is there something else that is in play here?

Do you have a dyno sheet that you can post?

Stan

Unfortunately, it doesn't have vacuum numbers, the vacuum numbers come from my logger going down the track. But here it is.

https://flic.kr/p/xJwVrT

Looking at the dyno sheet and the listed BSFC's. What fuel are you running? If other than gas was the carb modified?

Stan

[banjo]

Do you have a dyno sheet that you can post?

Stan

Unfortunately, it doesn't have vacuum numbers, the vacuum numbers come from my logger going down the track. But here it is.

https://flic.kr/p/xJwVrT

Looking at the dyno sheet and the listed BSFC's. What fuel are you running? If other than gas was the carb modified?

Stan

This a 1050 dominator methanol carb. Everything was set up for methanol. I think I may have hit some limit with fuel because continuing to go up on jet size doesnt seem to have an effect on egts/afrs. Top of the 1/8 mile I am sitting with egts at high 1200s/low 1300s. So I am going to try some skirted boosters to riches it up more. It could also be like jmark was saying that some engine benefit more from increased vacuum to help with fuel distribution and atomization. To me the car has always seem lazy off the shift. Car runs well, and the skirted boosters are an easy thing to try.

What do you see in the dyno logs?