im gonna be using a 350 block bored 30 over and either hypereutectic or forged pistons with eagle forged crank and rods. and dart iron eagles 200cc heads. what cam should i use. i was looking at the comp cams XE with 220/230 duration at .050 and .510 lift and 114lobe sep. and i have a 670 tbi with 65lb injectors and a weiand stealth g single plane intake and 255lph 60psi fuel pump. will this feed th motor or do i need the 454 injectors.

 

Probably going to need the bigger injectors. Even running the 65#/hr injectors at 30 psi (up from 12 psi) is good for about 350 HP @ 85% DC on the injectors. I can see where some 85#/hr injectors at 22-24 PSI would work out. That would cover 400 HP @ 85% DC. If you plan on running the engine past 6,000 RPM then more fuel pressure would be required to cover the lessor time available for injection.

You can try the 65's at 30 PSI and get an idea of how much more the engine requires. It all comes down to how many ponies and a what RPM it is.

RBob.

 

You're going to need some bigger injectors or at least 30psi of fuel pressure. A friend of mine had a 327 with the same heads and a similar cam only with less ls and he was in the 370hp range. Get yourself some big block injectors or honestly, if you don't drive it in bad weather, go carb. There is nothing special about TBI unless you keep it for a daily driver OR are hardcore into the electronics and just have to be in control of everything . Then there are some of us that are both.
Seriously, with those heads don't expect to put many miles on them. They are a race head for the street = rebuilds always in the near future.
Depending on the transmission you use, I'd go with a nice demon 750 carb with that setup. Should produce good vacuum for a vacuum secondary if you go with an auto.
Do you plan on burning chips? I think that is the determining factor here.

 

You may already know this- here's the basics:

Engine Size (CID) X Maximum RPM / 3456 X % Volumetric Efficiency = CFM needed

so, your 355 with a redline of 5500 RPM would look like:

355 X 5500 / 3456 X .85 = 480.21 CFM

Note:
Here's where the arguments come in. Theres disagreement whether the TBI flows at 1.5 or 3 hg. Tied together to this is the question of flow with a single plane intake manifold. Since the tbi draws air from both 'barrels', what is the cfm, hg, etc. related with the single plane vis a vis dual plane?

now, that formula posted above is for cfm @ 1.5" HG. Here's the conversion for 3" HG. the formula for converting flow ratings at 3" HG to 1.5" HG is:

CFM @ 3”HG X 70.7%

so, using a hypothetical 670 holley would flow 473.69 cfm @ 1.5" HG (670 X .707). or just under the requirements.


Now for fuel, here's a website that will help you out. Fuel Injector Calculator

Here's the math
(HP @Flywheel X BSFC) / (# Of Injectors X Duty Cycle)

Of course, VE and BSFC numbers are just guesses people use. A good engine builder can beat those numbers. For the TBI, that would be very, very helpful.

 

9c1Caprice, your post covers good basics. Regarding the sizing requirements for airflow there is another step. It is important to size the TB according to the desired drop across the TB (restriction to air flow).

In the example shown (I'm just grabbing pieces from the prior post) here is the engine airflow:

>>355 X 5500 / 3456 X .85 = 480.21 CFM

And then the rated airflow of the TB:

>>So, using a hypothetical 670 holley would flow 473.69 cfm @ 1.5" HG (670 X .707). or just under the requirements.

For ease of discussion lets say that the the engine airflow requirement is equal to the 670 airflow (say maybe max RPM of 5400, what ever makes it match).

At this point it would seem as though the 670 TB would be the correct choice, the airflow rating matches the engine requirement. However, this is only true if you are willing to accept a drop of 1.5" Hg across the TB. For a regular 'ole granny car this would be OK.

For a performance vehicle this is giving up too much. That 1.5" Hg is equal to almost 5 Kpa of manifold pressure. IOW at WOT @ max RPM and 101 KPa barometeric pressure the manifold pressure would be at 96 KPa.

For a mild performance vehicle Holley (from the Holley Carburetors book, HP Books, IIRC) recommends a maximum of 1" Hg drop. For a high performance vehicle the recommendation is 0.5" Hg drop.

This is why a larger TB then one at the specified airflow rating is often recommended.

RBob.

 

you know, RBob, maybe you can answer something else brough up earlier but I havent resolved it in my mind.

I forgot who started the question, but i remember a flamewar, and I really would rather not start any of that garbage again.

Ill just state the question how I structured it in my mind and hope theres no flames.

The big throttle bodies, do they suffer similar problems as carbs with too big of venturies. or, in other words, do these large tbi's create problems with too much air flow in low rpm apps. This question is in the same light as why holley decided to go with a progressive linkage 4bbl tbi linkage for a more controlled fuel/air mixture - simiilar to 4bbl carbs. i understand that venturies/ carbs design points used to meter fuel that tbi's dont have, but there are people having trouble tuning their non progressive 4bbl tbi's and holley rectified this situation by making one with progressive linkages.

Its a little convoluted of a question, but, like I said, Im putting it in different words to keep this civil.

 

No problem, I have the nomex suit in storage

TBI units don't have low airflow problems such as carbs have. A TBI bore is even straight walled, no venturi at all. The fuel flow is entirely controlled by the ECM. Unlike a carb where it is the flow of air through a venturi. As for the driveabilty with large TBI's there are a couple of items in effect.

One is that opening the blades (pushing on the go-pedal) exposes a lot of bore quickly. This can make it difficult to operate the engine in a smooth manner. Two methods of helping with this is to make the gas pedal throw longer, the other is to use progressive linkage. Progressive in this case is like what GM did with the LT1 TB's. A snail shaped cam on the TB shaft that slows the opening rate at low blade angles.

Another problem with large TBI systems is the injector size. With such large injectors it can be difficult to get the PW down small enough and consistent enough for a good idle. With four injectors the problem is multiplied.

Holley killed two birds with one stone by going to progressive linkage on their 4bbl TBI units. A slower bore opening speed, and the ability to shut off two of the four injectors.

Progressive linkage in this case is where the rear bores do not open until the front bores are opened past a certain amount. Then the PW if halved at the same time the rear injectors are enabled.

There is another item that is more tuning related then anything. With the ability to open a large bore quickly manifold filling is very rapid. A lot of fuel is required to go with the sudden surge of air. This can also cause the fuel already in the manifold to drop out of the mix and wet the manifold walls. This creates a sudden lean condition.

The fix for this is to either don't wack the throttle wide open at low engine speeds (driver learning), or add a bunch more AE. Both work.

RBob.

 

WOW that makes me feel a lot better. i have been struggling with a stumble on hard accelleration that i improved with AE but was hesitiant to give too much AE all at once. I feel now that was the correct thing to do. each 10% more helped but eventually with cold weather 40-50 deg cold engine it was a losing battle. i quit at 200% increase in values. the fact of those butterflies moving so quickly(never though of that), large runners in intake, relatively large runners in heads(2.02), 90lb injectors, the XRAM plenum design, all leads to what you are describing. next spring in the 454 TB on my single plane and off comes the plenum. do you agree in my conclusion?

 

IIRC the XRam is a plenum on top of a single plane with the two 1bbl TBI units. Yes, in cold weather best bet is to add some heat to the plenum floor to control wetting.

Keep us updated on how changing out the plenum works out. Even with a single plane & a 2bbl TBI on top I like to add a little heat to the plenum floor. Much more consistent.

One advantage the '747 has over the '8746 is more control over the AE. Most of this added control is with the delta TPS AE.

RBob.

 

no exhaust heat crossover in performer rpm so no can do. only thing is i saw was a heated holley adapter for 454 TB but they discontinued. anyway the plenum is elevated and NO WAY to get heat to it other than a 10 mile ride and even then it cools quickly. reverse in summer most likely gives you a cooling effect which is good however inj nozzle tip is about 2 inches from plenum floor and offset to sides of plenum from top of single plane by a couple inches. not the optimal design. A/F mix has to go sideways first at right angle from nozzle then hang a right angle in to intake plenum and then inro the intake runner. some where it hits a cold wall/floor and condenses and forms the GM River...

 

I take it the new manifold will be the air-gap? Fit a heat exchanger to the bottom of the plenum. Then plumb coolant through it. Can pick up a coolant feed from the front water crossover of the manifold, return it either to the large heater hose (heater core return) or to a tapped fitting in the water pump. Don't need a lot 3/8" fuel hose can be used.

RBob.

 

Edit: erase lots of words
After Edit:
Ok, Great, something I can mull over.

 

This seems counter intuitive at first glance.

Lets assume .5bsfc and 80% injector DC.

If you have 320HP you need 200lbs/hr of injector(s)
200 = 320 * .5 / .8

2 - 100 lbs/hr units or
4 - 50lbs/hr units or
8 - 25lbs/hr units

Won't those combinations require roughly the same idle PW? What am I missing or have I just misread your post altogether?

 

You are correct in the example you presented. I was thinking along the lines of going from two 90#/hr to four 75#/hr in the quest for more HP. Most of the time the need to go to a 4bbl TBI from a 2bbl TBI is to get more total injector along with more airflow.

RBob.