stew'86MCSS396

Hi there, First time posting...has anyone attempted to use the 1 11/16" TB on a big inch motor, like a 383? I just wanna play!!! I know everyone is going to recommend the 2" TB and 90# injectors but the engine is basically stock with cast iron manifolds. Prolly going to use a faster computer since my harness is already set up for it. I have all the stuff I need for data logging and chip burning, too. If I keep the rpms below 5500 rpms, ya think the 1 11/16" TB can handle the air delivery with the ultimate mods done? 2" stuff is practically non-existent on the rock and I suppose I could just spring for 90# injectors, which brings up the question of where is a good place to buy injectors? Door is open...fire away! stew'86MCSS396

ShiftyCapone

iTrader: (7)

I am sure you could without much of a hitch. Providing you have a small cam and restrictive heads I don't see why you could not. Now this will be a very low power 383 though. Maybe in the 250hp range, give or take some. There are a few guys who have 383 TBI's but they use the larger TBI units. I assume shipping a new Holley 670 unit from jegs or summit is out of the question? Since hawaii most likely doesn't even have junk yards your best bet is to find someone selling those injectors or go with new ones from Holley or turbo city. GMparts direct also has them but you will have to saw off one of your legs for them.

ssxmac

iTrader: (1)

IIRC someone said the stock tbi flows 470 cfm? or was it even 370? just think how big of a carb you'd throw on that 383. Just my $.02, i wish i had a 383.
-chuck

kdrolt

On an LT1, the stock 2-bore 48mm diameter horizontal throttle body can flow enough air to support 420+ fwhp as measured by engine dyno tests.

Airflow capability of any throttle body (or carb) is related to the cross sectional area of the bores. So a 2-inch-diameter (50.8 mm bores) 2-bore throttle body can flow 12% more than a 48mm unit, hence the power capability is 470+ fwhp.

The stock factory TBI is a 2-bore unit with 1 and 11/16 inch diam bores (or 42.86 mm diameter). This is 89.2% of the diameter of a 48mm unit, and therefore 79.7% of the area of a 48mm unit, so the power capability is 79.7% of the 48mm unit --- or 335 fwhp. Power capability means that it's a ceiling --- and you don't get to the ceiling unless you do everything else right (intake, heads, exhaust, fuel, tuning).

The above is only intended to show what the airflow capability of 42.8, 48 and 50.8 mm diam 2-bore throttle bodies, and hence what the power capability is for each when extrapolated by the 420+ fwhp dyno tests of HRM on a 350 LT1 with the HOT cam. And yes I know that the L99/LT1/LT4 throttle bodies are horizontal flow vs the vertical flow on a TBI engine (an irrelevant difference), AND that the L99/LT1/LT4 throttle bodies flow only air while the TBI units flow air+fuel... but the volume displaced by fuel flow is insignificant so the above math still holds. And the throttle shaft impediment wasn't included in any of the above, though it could be easily enough -- but it won't alter the basic results.

The stock GM small block TBI unit can flow enough air to support well over 300 fwhp. Fueling and tuning are the problems in getting to that level.

The airflow on all of the above are limited by the Mach Index, when the airflow speed divided by the speed of sound (Mach Index defined), is greater than 0.5. This also holds for runner sizes within an intake manifold, and for the airflow past an intake valve. It also holds for the exhaust valve flow, but the speed of sound is greater in the exhaust due to the greater gas temperature so higher exhaust flow speeds can be handled even with small(er) exh valves..

There are prior threads that discuss this. David Vizard mentions most of the above in several of his books, and his justification is found in the book (and work) of C.F. Taylor of the Sloan Automotive Lab at MIT. HTH.

ShiftyCapone

iTrader: (7)

The volume of fuel displaced IS significant. The mass of the fuel takes up space. Space that air takes up in a dry intake system. There is much more involved besides throttled air alone. TBI systems also have the injector pod in the way which drastically effects the way air is drawn in. Not every motor that makes 300hp needs the same throttle body size.

anesthes

iTrader: (12)

^^^ He's right.

And considering the fuel output of the 90# injectors is about the same as a Factory L98, you can forget about low 1/4mi ets..

Start looking into MPFI.

-- Joe

BronYrAur

iTrader: (1)

Or get a good fuel pump and use those 90#'ers and raise the fuel pressure up to effectively raise the flow rating of them. Much more cost effective. MPFI would be good down the road, but if you have a pretty mild 383, I'd pick up a 454 TBI unit with the 2" bores to try to avoid pulling any vacuum at WOT. Then put in a Walbro 190 or 255 lph fuel pump. Boost the fuel pressure to where it's happy and get into chip tuning ASAP.

anesthes

iTrader: (12)

Whats the max pressure you can run on TBI injectors?

I know various types of MPFI injectors will go erratic, or fail at really high pressures.

-- Joe

ShiftyCapone

iTrader: (7)

IIRC the 90lb injectors can go up to 27psi. I am not sure when the stock ones go static since everyone sees different results when tuning.

BronYrAur

iTrader: (1)

I believe all the strong P+H TBI injectors can hold a LOT of fuel pressure if needed. Prevost is running 28psi on 65's. R90 ran around 23psi IIRC. I also remember reading posts from RBob a long time back when he said he's seen injectors tested up to 60psi without failing. I'd say they can handle quite a bit.

ShiftyCapone

iTrader: (7)

The GM injectors are awesome. They can go much higher than rated. You could never do that with the older style Holley's.

anesthes

iTrader: (12)

I hear that, I just have a lot of opinions, mostly based on experience. The combination of small bore, injector pod in the way, only 2 injectors just doesn't sit well with me.

If it was a large bore MONO, or quad bore TB, with injectors maybe ofset at the base as to not impead airflow, and the SOFTWARE was better than I'd have a lot more interest in it.

-- Joe

ShiftyCapone

iTrader: (7)

Yes it has it problems but you can make good power and have a lt of fun with it. We mod these cars knowing the flaws in the back of our minds. You will be okay.

kdrolt

OK, you didn't bother to sanity-check my statement with math to see if I was right or wrong. You just assumed that it was wrong. So I guess I'll post the math myself.

To do this we need the air:fuel ratio at WOT, and we need the relative densities of the fluids involved.

I'll assume an air:fuel ratio of 12:1 by mass. Specifically, if the engine needs 12 grams of air, it needs 1 gram of fuel to go with it. The AF ratio would be larger for part-throttle cruise (more air, less fuel), but there would be no need to estimate maximum airflow demands under that circumstance. And a part throttle cruise condition makes the fuel volume fraction even smaller, so I only need to deal with the richest fuel case -- at WOT.

Gasoline specific gravity is approx 0.70 g/cc (for reference water at 1.0 gm/cc, cc=cubic_centimeter). Air density, for reasonable conditions at the road, is around 1.2 kg/cubic_meter. That's also 1.2e-4 kg/cc or 0.12 gram/cc.

So comparing the densities for liquid gasoline and gaseous air: 0.70 gm/cc for gasoline and 0.12 gram/cc for air. Inverting these we have: 1.428 cc/gram for gasoline and 8.33 cc/gram for air, which shows that air displaces MUCH more volume for a given mass quantity.

Now we also know that we need 12 times the mass of air as compared to mass of fuel to reach a AF of 12:1. That means we need to multiply the inverse densities (prev paragraph) by 12 for the air, and by 1.0 for the fuel:

99.96 cc of air needed for every 1.428 cc of fuel, in order to reach a AF mass ratio of 12:1.

That means, in a punchline sense, that the intake flow needs to be 99.96 parts of air and 1.428 parts fuel by volume to reach a AF mass ratio of 12:1, and so the fuel occupies only 1.428/99.96 or 1.43% of the intake flow. The other 98.57% of the flow volume is air. So the volume fraction of the fuel is not significant, as I stated before, to the order of the assumptions made.

The above also applies to the fuel volume fraction in the airstream at the entry point (under the injector), because I didn't take into account the partial vaporization of fuel into a gaseous state, nor did I include the manifold heat needed to accomplish this, nor did I include any gas law that relates pressure, volume and temperature. But those things happen only partially (vaporization) and progressively during travel in the intake manifold and the intake ports in the head --- so it is sufficient to deal only with the flow condition at the throttle (which was the original point).

I will lastly add that the above applies to a TBI engine, a carbed engine, or even to any port FI engine (like TPI), because the math doesn't care about whether the combine mixed-flow of air and micro fuel droplets is in a TBI bore, or a carb venturi, or in the runner of an intake. All that matters is that the flow is in some form of equilibrium and that the AF is in the neighborhood of 12:1.

QED.

kdrolt

Wrong on both points.

Do a search here on TBI and find the posts where I provided the links to the TBI airflow bench measurements. The links point to testing done by a Crossfire owner (who visits TGO btw), and he tested the TBI both with the injector pod/tower, and without. The links point to threads at the CFI Vault website. Removing the injector/tower did improve airflow, but by a negligible amount. IIRC the testing also included spacers to raise the injectors, and they also showed increased airflow, but negligibly so.

And as far as comparing motors that each make 300 hp --- because they make the same power at the flywheel, then the airflow requirements will be essentially the same for each engine. The only differences will be the VE (volumetric efficiency) and the BSFC (brake specific fuel consumption). An engine that has a poor VE (from poor airflow pumping) and a poor BSFC (from a poor combustion chamber) might need a larger TB, but not substantially larger than for those on the other engines.

To first order, if you know what the target hp is (say 300 fwhp), then you can size the throttle body needed based on the cross sectional area of the bore (monoblade) or combined area of the bores (2-bore for TBI, TPI/L99/LT1/LT4, or 4-bore for a 4v carb). This only addresses the airflow needs for the throttle, and it doesn't consider the need for pressure differential (vacuum signal) in a carb.

ShiftyCapone

iTrader: (7)

1.43% seems rather significant to me. 45lb/hr injectors are putting almost 15 gallons of fuel in the TBI unit per hour. Not to mention that we are assuming a fixed AF ratio along with air density. Those are just the small injectors. I will look up the flow bench results, but you yourself still stated that the pod is still a retsriction. Your view of negligable just must be different than mine. The air flow requirements are not the same for every 300hp motor. Period. Most SBC are close because of the deisgn similarites. Engine design, cam profiles, intake style, and chamber style will affect that the most. Your way is accurate to help you get choose the right one though. So I get your general point there.

anesthes

iTrader: (12)

Well duh, obviously without some sort of airfoil or redesign it won't matter much. We're talking about the basic design of the unit, which is poor.

Which is part of the reason I went with a monoblade TB on my mpfi setup, vs a twin or quad. I want the blower blowing all it can without having plates and such interfearing, or 9 miles of TPI runner.


GM didn't build TBI for performance. They built it for economy.

GM didn't build TPI for performance, they built it cuz it looks mean
and sold cars.


-- Joe

kdrolt

1.43% volume is for fuel means that you'd lose 7 cfm on a 500 cfm airflow throttle. Does 7 cfm still seem significant relative to 500 cfm? And if you had a flow bench, would you be able to reliably measure a difference of 7 cfm on a 500 cfm TB (without a large sample size of data, and probably a lot of averaging)? Probably not, so it is not significant either from a measurement viewpoint or from an engineering viewpoint.

As far as 15 gph of fuel consumption is concerned, the number means nothing because it's not in relation to anything else. 45 lb/hr of fuel also means 540 lb/hr of air consumed --- which just restates the AF ratio in terms of lb/hr for each.

As far as the fixed AF ratio, and fixed air density --- c'mon. What kind of change in either do you expect over the short time period you plan to be at WOT ? Those picky details are not important when sizing the TB is concerned.

And the injector size isn't relevant either. The GM injectors that fit into a Rochester TBI unit are all externally the same -- so they all fit the sbc or bbc TBI units. What makes them different (mechanically) is the size of the orifice that opens/closes to allow/prevent fuel flow. The smaller injectors use a smaller orifice; the larger injectors use a larger orifice.

The choice of injectors, for use on a given engine, depends on providing smooth idle control (usually near the smallest fuel flow delivery possible), on providing fuel to satisfy the designed (or intended) power output at WOT, and providing tuneability for all rpms and loads. Making all three happen gets very tough when the engine power goes above 300 fwhp and you have only 2 injectors (as in a GM v8 running TBI). This thread was discussing the issue of airflow only.

As far as the pod restriction, IIRC it was several cfm change by removing the pod. That means several (under 10 cfm) as compared to the base flow of 500-600 cfm (the number depends on the bore size of the TBI unit). A change of 10 cfm is insignificant in terms of sizing the power/airflow of a TBI unit when the unit is capable of 500+ cfm. It can also be said that the injector pods are not really as much of a flow restriction as many here would think.

As far as the --- you're still wrong. We're still talking about SI (spark ignition) internal combustion 4-stroke engines, and therefore power output is directly related to airflow moving through the engine. It doesn't matter whether you are comparing a 300 fwhp inline 4-cylinder engine to a 300 fwhp 12-cylinder boxer engine. The fact that they both have a 300 hp peak output means that they both have the same airflow requirement assuming they're both tuned for the same AF ratio (i.e. in the 12:1 neighborhood).

kdrolt

TBI flow measurements here:

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

Dewey316

i will also add, that the fuel being introduced early, adds a cooling effect to the air, raising the density. so it makes that 1% have even less of an effect.

there is no argument there about the airflow, BUT math is great, we can debate this all we want, we can talk about VE, potential energy of fuel vs HP vs airflow needed, etc. until we are blue in the face. the point is, how many large cube TBI motors, really make that power you say it should? in the end, toss the math out the window. how may big cube TBI engine pull vacume at higher RPMs with teh smaller TB. end of story.

ShiftyCapone

iTrader: (7)

This is the point I am trying to make. By air density I was refering to the HUGE variance of ambient conditons that an engine goes through. If it doesn't make that much of a difference guys wouldn't have different tunes for every month of the year. This is becoming more and more common for todays EFI tuners. I am not talking about any temp or pressure change other that what is outside of the car at this point. I am not going to argue anymore. I really didn't mean to get you all fired up. We are really butting heads about the same thing but I am being stubborn about it. You can size a TB just like you mention as I stated earlier but it is not exact. That is the point I was trying to make. I have spent just as much time in the classroom devoting a significant part of my eductation to IC engines and vehicle performance as anyone on this board. It boils down to how literal you want to be along with personal experience with high powered TBI set-ups that should "make the power you have figured out on paper" but in reality don't. We all step back and scratch our head when our TBI calculations leave us stummped. Most of this is based on my frustration with TBI right now. I am helping Chuck! get his 454 TBI, vortec set-up to run and it seems like no matter what we do the car runs like death. With a carb it could make 300hp. You deffinately know your stuff so I do not think there is any point in beating this to death anymore. I hope I didnt make you to mad

stew'86MCSS396

Okay...with all that said, I might be crazy enough to just try it and post results for all you R&D types. Oh and the half truth, I wanted to make sure I get pure unbiased opinions which I got by leading you all to believe this was going on a mildly built 383. The screen name says it all but I can assure you that it's mild. The cam is slightly bigger than the 325 hp stuff of yester year. Don't want to advertise how pathetic my track times were but let's just say 2.8 60 ft and trapped over 90 mph with a holley 600.

The only one question out of all this...some of you made mention of cranking the fuel pressure way up. Can this be done by modifying the oem FPR into an adjustable one or do I need to get an aftermarket one? Also, how much more fuel flow can I expect to attain by doing that?

I'm going to do an island wide j-yard search for a 2" TB one of these weekends...you never know what you may find and alot of times the yards don't even know what they have. thanx again for all the feedback and if you'd like to add more, by all means do! stew'86MCSS396