82suped-up-coupe

does anyone know the cfm rating for stock camaro tbi??

thanks
Rich

nick harmon

between 400 and 500 cfm. lower than 500 im sure. not sure of the exact figure, maybe 430. try a search, im sure youll find some results.

ShiftyCapone

iTrader: (7)

I am pretty sure it is 450cfm.

dimented24x7

iTrader: (2)

not much, in 4bbl carb terms its around 375 cfm or so.

kdrolt

https://www.thirdgen.org/techbb2/sho...&highlight=cfm

82suped-up-coupe

thanks for the replies guys,

my hopes are to replace my carb with TBI
I have a 330hp 355 with just under 400lb/ft, is there anyway I can use TBI without spending lotsa cash

I have a 625 quadrabog on it right now, but the engine was designed around the 750 test carb we use for build up testing, It runs good enough with the 625 but I wouldn't want to go any lower I don't think


any ideas? I'm just trying to get the hang of all this VooDoo that you guys do


thanks in advance
Rich

vjo90RS8

iTrader: (1)

The best bet would be to get a 454 or Holley 2" TBI which are suppose to flow 670cfm. The only problem is that flows 670cfm at 3" of Hg while carbs are tested at 1.5" of Hg. So when compared to your 625cfm carb, the holley tbi might flow about the same but probably alittle less (closer to 600cfm).
Correct me if im wrong.

ShiftyCapone

iTrader: (7)

To maintain the power you have now with a TBI system you will need either a 454 TBI or holley 670 TBI both with larger injectors than what comes on the 305. Those two TBI's can supports the power you want but it will take a lot of chip tuning.

Dewey316

the 700+ 4bbl CFM units would really fit the bill here, but the price might damper that idea.

Ronny

iTrader: (1)

you can bore over 2.00 inch cant you? Or does that dual TB unit prevent that ? mine is a dual single TB bored to 2.00 but i can go larger with a sleve(Dan LionsDen)

vjo90RS8

iTrader: (1)

i think the largest you would want to go safely is 48mm.

brodyscamaro

I would think it flows a little less. It might not cost him a lot of HP, but I will never understand why people would choke an engine on purpose.....

25THRSS

in stock form it flows 520 cfm at 3 hg.

dimented24x7

iTrader: (2)

Well, for me its the fact that tbi is a no-brainer, super simple, dead reliable efi system that makes 500+ miles a week of commuting economical with my V8. Theres something to say about having a V8 thats faster then most of the stuff out on the roads, average 25 mpg, and be cake to fix when something goes wrong. TBI is definatly not the fastest induction system out there, theres no arguing that, but it delivers efi economy and performance in a super simple 'only what you need' package.

RBob

iTrader: (1)

This was posted on the DIY PROM board a while back (also see the top most sticky on this board):

The following airflow tests were performed on the University of
Northwestern Ohio's SuperFlow SF600 Flow Bench. All CFM values
are corrected for airflow at 28 inches of water.

Stock 4.3/5.0/5.7 2bbl TBI complete -- 574.1 cfm (dry)

28 " of water is ca. 2" of Hg.

RBob.

Dewey316

rbob,

what is 570 @ 1.5Hg or at 3Hg?

(yes it has been alot of years since i took any schooling )

kdrolt

And here:

https://www.thirdgen.org/techbb2/sho...&highlight=cfm

we have:

Stock single bore TB (42.8 mm or 1.68" inside diam bore) two injector tower gaskets @13.6" (or 1" of Hg)

286 CFM (each), 572 CFM together

which is the equivalent of 572 CFM together for a single 2-bore stock TBI unit (as used here). This shows that doubling the pressure differential across the TBI (base to top) results in no increase in the flow -- so the flow in the TBI is stalled at around 570 cfm.

Now let's calculate the Mach Index and see if Taylor is right again:

Total bore area = 2*(pi/4)*(1 and 11/16")^2 = 4.47 sq in

Now convert to sq feet: 4.47/144 = 0.03106 sq feet

Now solve for mean flow speed in ft/min:

570 cfm = area*speed = 0.03106*speed

so speed = 18349.7 ft/min

then convert to ft/sec:

18349.7/60 = 305.8 ft/sec

Remember that this is the mean, or average, flow speed through the throttle body bores. Since the flow speed goes to zero at the walls of the bores, the max flow speed (near the bore center axis) is significantly higher. More on that in a moment.

Why is this number, 305.8 ft/sec, useful?

Because C.F. Taylor showed by experiments years ago that when the flow speed through an orifice (like a throttle, or past a valve) gets near half the speed of sound, the flow resistance increases significantly.... and you won't get any more flow through it. Many people call this choked flow, because it's the maximum it will flow (unless you bore it out to a larger size).

The speed of sound in air is around 1100 feet/sec, so the average Mach Index is the flow speed divided by the speed of sound in air:

305.8/1100 = 0.278.

which isn't that close to 0.5, but it is if we recall that the peak flow speed is going to be higher than the average flow speed through the throttle. The peak flow speed might be as much as twice the average flow speed, so that would make the peak Mach Index 0.56, which is well into the region where the flow is stalled (numbers beyond 0.5).

How is this math useful?

It's useful because the math tells you that you are not going to flow anymore than 570 cfm through the dual 1 and 11/16 inch (or 42.6mm) diameter bores of the stock 2-bore TBI. From there if you know roughly what the cfm demand is per hp, it's possible to figure out what the max power can be.

dimented24x7

iTrader: (2)

alright, now Im confused... As for Rbobs input on the 574 cfm flow rating on the stock tbi, I would make around 2.7 inHg of vacuum at WOT @ 4k with the stock one w/o an air cleaner. This would indicate that my engine would need over 600 CFM at that rpm and load. How can this be so? Thats much higher then the theoretical maximum air requrement of 400 CFM at 100% VE. Does the fuel really displace or block that much air? Does the injector pod really impede airflow that much? In reference to kdrolt's linked thread, the 2" monobodies airflow corrected to 1.5 cfm and multiplied would give around 800CFM of airflow. Thats alot, more then the 780 cfm q-jet sitting on my parts shelf. That thing not only ahs primaries but large 2.25 inch seondaries. If this data was correct, then there would be no need to convert over to carb. To me it jsut sounds a little to optomistic. Id like to see flow ratings done with fully assembled tbis with injectors mocked up to flow water during the flow test. Not saying their data is wrong, but to me it just seems like blue sky numbers that dont seem to jive with what the actual flow really is.

kdrolt

How did you come up with that number?




engine_displacement_cid*rpm*VE/(1728*2) = cfm

so if you have a 305 and we assume 100% (1.00) for VE and let rpm=4000, then

305*4000*1.00/(3456) = 353 cfm

or we can use the 574 cfm number and solve for rpm:

305*rpm*1.00/(3456) = 574

so rpm = 6504 at 100% VE.

600 cfm with 100% VE would be proportionally higher. So now I'm still confused with what you wrote.

Also keep in mind that you can't just indiscriminantly use a sqrt(pressure ratio) to equate measured flow differences measured with two separate column heights. That is an assumption along a streamline and it further assumes that the flow isn't choked. If the flow is choked, you can keep increasing the water column height (or height of mercury in a manometer) but you won't observe a proportional increase in flow. That was one of the points Taylor showed.



No to both. See the thread I linked in the above -- it has measurements with and without the injector pods. The pods don't really impede the airflow as much as people assume, so the gain of using stacked pod washers is questionable.




Again assuming that you can get away with that conversion -- and you can't because the flow is too closed to being choked by the fact that the peak Mach number is too close to 0.5.



It is optimistic -- which means that the practice of using new_flow = old_flow * sqrt(pressure ratio) isn't valid if the peak Mach number on the old_flow is near 0.5.

It jives. The problem is that no one here ever bothered to mention that the flow around a valve (a throttle body with a throttle plate is a valve) has a limit to what it can flow based on the Mach Index. To get more flow past that point, you need more area -- and that means a bigger TB or carb.

82suped-up-coupe

the main reason I want to do this is for mileage, I can afford to lose some HP, this used to be my race motor, but now I just want a comfortable driver.
1) Can I get the 305 setup I have been offered and use this for now??
2) can I just bolt on the 350 or 454TB later with only chip changes?
3) will this give me better drivability and mileage?

thanks again guys, I feel smarter just from reading this thread

Rich

dimented24x7

iTrader: (2)

ok... the stupid computer wiped out so im going to go with the short, to the point reply since the first didnt go through...

kdrolt,

Ok, the 2.7 inHg of pressure drop that i referred to was recorded at 4000 rpm. This is on my 350, BTW. This is the pressure at the vacuum port of the tbi, it was recorded from the MAP using winaldl. If it is true that the tbi really does flow 575 cfm @ 2inHg as said above, then i should never see this pressure drop since its physically impossible for my engine to demand over 575 cfm of air at that rpm. I personally feel that the flow rate in the real world is closer to around 375 cfm at that pressure drop.

I too am an engineer and im all to familiar with the formulas and theory of fluid flow through an oraface or valve. However, I personally feel that bringing in the engineering when discussing topics on TGO is like driving home a finishing nail with a sledge hammer, its just overkill. If it seems like what i say is just pure speculation, well........ your right .

As for the claimed mean flow rate of 666 cfm that would eb possible with a 2bbl 454 tbi at 1 inHg, this too just seems like too high a value. Even my smogger can get over 1.2 in of pressure drop at WOT. It just seems to high for real life situations. I personally feel that its probably closer to around 500 cfm. If it really was 666 cfm at 1 inHg, which is less then what carbs are rated at, we'd probably see alot more tbi'ers in the low 13's. Up to now, the wall seems to be around the mid 13's for guys useing the 2bbl tbi's. What is really needed to get to the bottom of this, IMHO, is fully mocked up fully assembled tbi's that are tested instead of bear bones units. The numbers seem too misleading.

kdrolt

Ok, then it was a measurement logging with Joby's code. I don't know how accurate the MAP sensor is, as compared to measurements on a flow bench but I would guess that the upstream and downstream measurement "taps" on a flowbench might be a little more accurtate than on the car. Not only that, but the MAP sensor, on least my 93 Roadmaster wagon (LO5 TBI), is not located above the TBI -- it's mounted on the firewall, so using the MAP (in my case) doesn't tell you the true pressure difference across the TBI. Instead, it tells you the pressure difference between the manifold and engine bay (outside the air filter).

So the MAP does correctly measure the manifold air pressure but it doesn't accurately measure the pressure across the TBI. You would need to mount a second MAP sensor above the TBI and inside the air filter. IMO.



There's always risk in giving the right answers, using math & engineering, on a group like this. Byt then again, it's done all the time on the DIY ECM and EEPROM forums here, so why not do it elsewhere?

Yup, but you need to rethink the MAP data. Where is the MAP sensor in your car? I bet its not inside the air filter, above the TBI itself.

Think of it this way --- if you measure the airflow on a head, say an L31 Vortec, you might see 227 cfm on the intake side at 0.400" lift at the valve, and 158 cfm on the exhaust side at the same lift. Will the engine flow those numbers? No. The flow isn't steady, and the valve lift is sinusoidal, and the pressure differential isn't constant, and there are other flow losses (intake, air filtration). So if you want to estimate power output get get close to the right answer, you have to take all the parts of the chain into account. The same thing is true for looking at flow numbers on a TBI -- it's not a complete enough story.

Keep in mind that were talking about 8 cylinder 4-stroke engines, so at any one time you are only pulling from 2 cylinders. That means at best you can only feed each cylinder with 570/2 or 285 cfm.... and that's assuming that the heads and intake will allow that level of flow. With stock components, they won't.

I'll say it differently --- measure through one port on an intake intake manifold ()with the other's blocked off), with the TBI and complete air filtration attached. Then repeat for the rest of the other intake ports. Sum the numbers.

If you can find a spare MAP sensor, try locating it inside the air intake tract, above the TBI, and see what you get for measured values. Actually, it might be better to try using a true vacuum gauge with one port located near your OEM MAP sensor and the other measuring your TBI vac portTBI. Then you can take the one at the MAP sensor location and relocate to above the TBI. SO you can get two analog measurements without interfering with the MAP sensor. HTH.

dimented24x7

iTrader: (2)

There was no air cleaner on it at the time. I also cross checked it with a vacuum gauge to see of it was reasnably accurate, and it was. Probably not the same accuracy as a flow bench obviously, hence the reason i only gave two significant figures. But, however, the engine was well and truely gagged with it on there. It wouldnt rev past 4000 rpm.

I also didnt think of this before, but the map is in fact a filtered sensor. IIRC its actually a mean average of the pressure in the intake manifold since its filtered. Basically its an average that includes the peaks of the airflow demand. Im probably not seeing or would be able to see the steady flow rate but rather the tbi being overwhelmed by the intake cycles.

Interesting that you mentioned the vortec heads... The main reason the swap never materialized is for the afore mentioned pressure losses during the intake strokes. I had the money for the heads but i didnt have the money for the pricey victor junior single plane intake and cowl induction hood to accomodate it. I just felt that id never properly feed the heads w/o a good ammount of intake volume to help buffer out the airflow demands.

dimented24x7

iTrader: (2)

I'll say it differently --- measure through one port on an intake intake manifold ()with the other's blocked off), with the TBI and complete air filtration attached. Then repeat for the rest of the other intake ports. Sum the numbers.


huh? I assume your implying to do the flow test with the tbi and intake manifold, but sum them? I dont follow... The flow rates would be significant to that intake port and cyl only and cannot be summed to give the maximum flow rate of the system since the flow at the tbi might become the governing restriction during high airflow demand.

kdrolt

Since only 2 cylinders pull at any one time, you can separately measure one port pulling through a head+intake+TBI+air_cleaner (with all the other ports blocked off), and then repeat the measurement on a different post. Then sum the numbers for flow. Since you know the firing order, you know what the pair-wise cylinders are (when they both go through an intake stroke).

It's a nice idea, but it's more than cumbersome to perform because (1) you need a flow bench, and (2) you need the space to have practically the entire intake tract bolted together.

As far the MAP is conerned, there is always pulsations within the intake tract. FWIW you can see the pulsations on the data coming from a MAF, on a MAF-equipped car, and these would be pulses travelling away from the throttle body and toward the air filter. The pulsations are a nuisance of acoustics within the intake manifold, but the hope is that they average out somewhat vs rpm, AND they can be mechanically filtered somewhat by taking the pressure measurement in a small port (small cross sectional area). Not only that, but the MAP itself probably doesn't have the dynamic range to measure the pulsations to begin with, so both effects combine to filter the pressure signal measured by the MAP sensor.

But that has nothing to do with the fact that the MAP is almost always mounted outside the intake tract and not close to the throttle (or TBI in this case). So you aren't getting a true measurement for the pressure difference across the TBI. The pressure differential you get (via MAP) is between the intake manifold and the engine bay, so you are missing the pressure drop associated with the air filter and the duct work that goes with it.

Pablo

iTrader: (1)

im no engineer but it seems to me you are missing the point about the TBI flowing 575? cfm at 2"? (so many numbers tossed around i lost track)

now correct me if i am wrong which I probably am since i have no credentials but to flow that 575 cfm of air it had to have 2" of vaccum on the other side of it

So if your engine is pulling 2" of vacuum through said tbi then its pulling in roughly what that TBI is advertised to flow, 575 cfm. Pulling a little more vacuum then its pulling in roughly a little more air than that. When you get down into the range of tenths can you be sure that your equipment is that accurate too? Maybe you are pulling 575 cfm. That doesnt sound unbelievable to me.

Not too hard for me to understand maybe I'm missing something.

Pablo

iTrader: (1)

and AWESOME INFO kdrolt on the other post you linked to. I cant believe I missed that one. That info excites me, my tbi unit outperforms my carb in airflow. I didnt think that it did

RBob

iTrader: (1)

Good info in this thread. I'd like to add a little more including the flow testing on the stock TBI. As above with the flowbench the TBI was flowed w/o the injector pod. Here are both values:

Stock 4.3/5.0/5.7 2bbl TBI complete -- 574.1 cfm (dry)

Stock 4.3/5.0/5.7 2bbl TBI w/o injectors -- 584.7 cfm

The following airflow tests were performed on the University of Northwestern Ohio's SuperFlow SF600 Flow Bench. All CFM values are corrected for airflow at 28 inches of water.

For the TBI setup on my 331 ci with the 2-1/16" 2bbl TBI. At 6,000 RPM the drop is 3 KPa. That is less then 0.9" of Hg. Not too bad at that. This is with a Torker manifold and a 5/8" open spacer.

RBob.

dimented24x7

iTrader: (2)

As i said above, there was NO air cleaner or any sort of impediment on the tbi. It was just sitting out there all by its lonesome. I did this for the same reason as mentioned above, so that i wouldnt see any additional pressure losses.

More Precisely

....and for my own info, i went out and checked the sensor with a mechanical vacuum gauge and a vacuum pump. I went from 1-5 inHg in 1 inHg increments and then from 5 to 25 inHg in 5 inHg increments.
I found that around 10 inHg there was the least ammount of disagreement between the gauge and the output from the ecm. The difference was .6%. On the extreme ends of the scale, there was a bit of non-linearity between the two sensors. At the high side (low vacuum), the difference was a max of 3.3%@1 inHg and on the low side (high vacuum) there was a max of 4.9%@20 inHg.

Going back to the data log archive of the run w/o the aircleaner, i found that the pressure difference was 7 kPa@4700 rpm, or 2.1 inHg. Using the data above the pressure in the manifold would have been 2.1 +/- .07 inHg. This pressure drop was with the stock tbi with a .25 in injector pod spacer.

What can I make of this?

1) The flow rates of the tbis that where given are wrong, since that pressure drop would correspond to a flow rate of ~570 cfm which would mean a VE efficiency of greater then 100% at that rpm.

This is obviously probably a poor deduction since the data retrieved from the static flow tests is probably accurate.

2) The static flow rates and the dynamic flow rates are quite different and directly compairing them to one another has little meaning.

This is probably the correct statement. The average pressure drop on the engine in a dynamic situation is probably substantially different then that of the pressure drop on a flow bench under static situations. The static situation is probably only good as a selection guide for tbis or carbeurators when taking the intake manifold and other aspects of the induction system into consideration. I was probably wrong in trying to draw conclusions and any coorelations between the two.

Although, the dynamic pressure drop seen through the sensors is probably more important performance wise, since it not only tells how the tbi is flowing, but how the rest of the induction system is interacting with it. This is probably the more important of the two since it tells how adiqatly the entire induction system is feeding the motor.

dimented24x7

iTrader: (2)

woah, 6k... Does it power out to there? Thats pretty impressive if it does.

RBob

iTrader: (1)

You are correct. This effect is also enhanced by running a small dual plane. The pulsing and getting the slug of A&F moving down the runner is killing the flow. There are periods of acceleration and decelleration through the throttle body. Not enough plenum to buffer the chamber filling.

When I had the 1-11/16" TBI on it would pull down 6 KPa at 4,000 RPM (Performer RPM manifold). By 4,800 RPM it was pulling nearly 10 KPa and the engine power went flat as a pancake.

When I went with the larger TBI I used an open spacer to blend the TBI bores into the manifold plenum. The reason for the single plane was to enhance plenum volume. This also helped in keeping a steady flow through the throttle body.

RBob.

RBob

iTrader: (1)

Isky 280 Mega cam and decent (not great) heads.

RBob.