Technical Dynamic compression...will this work?

You should read this....
https://www.hotrod.com/articles/make-503hp-with-a-350-junkyard-302/

OK it is magazine sensationalism and it has Nitrous [but they made 375HP without nitrous]
So your goals of 300HP is realistic without even opening up a longblock

Spend your time on the car instead of overbuilding the engine. There are actually more benefits from a good handling car than a powerful car

 

None of the ones that you find on the web will be 100% accurate, and they are all (almost) over-estimates of flow.

If your plan is to use a carburetor that has balanced primaries and secondaries (and smaller Edelbrock's don't), you want to look at one-half of the CFM rating, versus your desired freeway cruise speed.

Find and online calculator that shows you what RPM that you will be at, at the prevailing speed of your local freeway/highway. Type THAT number into the carburetor calculator, and see what number you get.

That's your cruising CFM. That should be under 1/2 of the overall CFM of the carburetor, but not by all that much.

If your carburetor is too small, you will have the secondaries open at cruise, will be wasting fuel, and will have no power overhead to pass, or climb a hill.

If your carburetor is too large, the flow rate through the venturis will be too low, resulting in fuel that is not only poorly metered, but not properly atomized (read: dripping, not spraying).

The sweet spot is the maximum velocity through the primaries, at cruise, without endangering instantly opening the secondaries with the slightest blip of the throttle.

Let's say you have a non-locking torque converter, which we will ignore for now, and assume that top-gear is 1:1. You said that your rear gear is 3.31:1. For argument's sake, lets say you have a 27" rear tire.

At about 2750 (give-or-take transmissive losses), you will be running at 65mph. At that speed, your engine combination, with the GT40 heads, will require about 188CFM to operate. That is all that it is going to pull in to the engine.

Remember, carburetors to not put air and fuel into an engine, they allow air and fuel into an engine. The pistons draw it in, for as long as the valves are open, for as much as they are open, as restricted by the intake ports, manifold intake runners, throttle blades, venturis, choke, and air cleaner.

You can tinker around the edges, by changing cams, valve sizes, porting, manifolds, and all the rest, but you will never (save for boring and stroking, or forced induction) have an engine that will draw in any more than 289 cubic inches of air/fuel mixture, per revolution, per period of time, divided by 4, and that is if 100% V.E. (Volumetric Efficiency, were possible, in a naturally aspirated internal combustion engine, and it ain't). If it were at 100% V.E., at cruise, you would be pulling in ~225CFM. Double that, and you're looking at a 450CFM carb.

That is a fixed number. No guesses required.

A 500CFM carburetor is just fine.

 

If you are only going to tune one vehicle, probably not. I have two, because I work on multi-bank engines.

I tune everything with a wide band O2 sensor. That is how I know everyone insists that running an over-large carburetor does not understand Stoichiometry.

 

^^^THIS^^^ 1000x THIS.

Listen to Kerry!

 

I don't intend to neglect the suspension, or any other component, in favor of the engine. The difference is, I've got a plan for my suspension already, and for bodywork, wiring, transmission, drivetrain, brakes and all of the other systems, but at the time I started this thread, I didn't have a plan fully formulated for the engine. I had some options, but didn't really know which of those options was best. Hence, my reason for starting this thread in the first place, to confirm or disprove some of my assumptions in the 12% of a plan I had at the time. There's been a lot of good info in this thread that has done a lot to educate me on things I may have missed or was overthinking. I don't feel like a mild 289 is an overbuilt engine. Overthought, maybe (I may be suffering from a mild case of analysis paralysis) but I don't feel like it's overbuilt. I get what y'all are saying, this really isn't rocket surgery, but at the same time I'm still new to this process and I want to make heads or tails of all the other research I've done (especially since most of it is Mustang-based and may not apply to a larger, older car), and make sure I'm at least close to the right track. I appreciate all of your input.

 

TOTALLY AGREE!

The biggest mistake that many enthusiasts make in choosing a carburetor FOR STREET USE is determining CFM requirements for WOT under dream conditions, and then buying the next LARGER carburetor.

How often do you cruise at WOT?????

For street use, determine your cruise RPM, and then the CFM requirement, then find a carb that is close to that ON THE PRIMARY SIDE! This is exactly why all of the car companies used spread-bore carbs on most of their passenger vehicles after the spread-bore was invented. The secondary side probably is less important for a cruiser, but may still be considered.

For the secondary side - vacuum secondary if you use Holley, air valve carbs if you use Carter or Rochester.

Its like the hillbilly youth that got his first set of boots at age 17. When he got home, his uncle asked him what size he bought. The youth replied: "The salesman said 9 was my size, but 10's felt SO good, I bought 11's"

Jon.

 

Mathematics and physics are an enthusiast's best friend. Those who ignore mathematics and physics had better have a friend in an engine shop AND be independently wealthy!

Jon.

 

So, it's math time.

RPM for 3.31 gears, 27.1-inch tire diameter, and 70 mph is 2872, call it 3000 just in case. That's assuming a 1:1 high gear. Does the formula apply for overdrive as well, or does that add another consideration?

289 * 3000 / 3456 = 250, so I'd need a 500 cfm carb by your calculation? Or do the primary and secondary flow at different rates for a vacuum-secondary carb?

 

The equation you used is for an engine with a volumetric efficiency (VE) of 100 percent.

You might find this article interesting: http://www.thecarburetorshop.com/Carburetorsizeselection.htm

Estimating your engine at a VE of 85 percent, than 250 times .85 is equal to 212.

I do NOT KNOW primary/secondary CFM on other than Carter, but if you were considering genuine Carter carburetors:

Carter AFB 9400/9410 - 200 primary/200 secondary 400 total
Carter AFB 9500/9510 - 225 primary/275 secondary 500 total
Carter TQ 9800/9810 - 200 primary/600 secondary 800 total

For your second question, flow rates are based primarily on venturii area. With Holley, the vacuum secondary versus mechanical secondary comment referred to the timing of the opening of the secondary. With the small (289) engine on the street, a mechanical secondary is likely to have hesitation when the secondary opens UNLESS you (are a friend) is an excellent tuner.

Jon.

 

This is turning into a great thread, I love the knowledge here.


Sent from my iPhone using H.A.M.B.

 

I definitely intend to use vacuum secondaries, that much is certain. So that's the reason for the various formulas, is the VE assumptions. I can look up primary/secondaries for the other carbs I'm considering, thanks for the info on Carters.

That's the great thing about the HAMB - people with all this knowledge willing to help out and pass it along.

 

OK, so we have some carb experts on here, the advice offered is sound. Surprised I have not seen a comment on the stealth intake. I have only run them on SBC's but for a dual plane, they are big, I only use them on 400ci street motors. I have one on the shelf now, I would use it on my mild cammed 350, but I know it's too big for that motor, I am buying a new performer for it.
If it were me, a little performer or performer RPM would be the way to go, for small intakes, they really make power in a usable range.

 

And yet I have seen lots of engines make the most power with a carb that is "too big". A 302 should never need a 750 according to the formula, but they often run the quickest and fastest at the strip with 750's. Personal friend had a Ford Racing 302 crate engine with GT40 iron heads and a B303 cam with a Stealth intake. He ran both a 650 and a 750 on that truck (87 or 88 Ranger, don't recall the year) and it was faster and quicker with the "too big" 750.

By the formula, my brothers 418W should not need more than 700cfm. But the completely worked over 750 that probably flow over 800cfm at this point cannot be jetted to function right at all throttle points. It needs at least an 850cfm. Probably a 950.

So while the dyno might say the 600 is enough for a 302 or a 289, the drag strip in the real world showed the engine could use the 750 to greater effect.

Also don't act smug and dismissive of someone else because you have an "advanced degree" and design for the "5th largest global automaker". The guy that has to lay out a laundry list of "look what I can do" to try to be dismissive is petty. It's unbecoming. Congrats on what you do. Probably a very interesting job. But also....I'm not impressed by your list. A-whoopy-do!!

 

LMAO. When ratio's don't make sense!

 

Just a note that 65-66 Mustangs were still 3000-3300 lbs or so; not as different as you might think than a '51. I6 Mustangs were a bit lighter at 2800 or so, but overall similar.

Lots to think about as it's your first engine build, but definitely worth it. 289s are still pretty cheap, and I'm sure you can find one on craigslist or something for ~$200-300 too, so don't get hung up on "messing up." Hell, if you could find one that is complete and spins free, I'd bet you could put it in and get it driving and you'd be surprised how peppy they can be. Did that myself as my first engine.

 

I don't want to get in the middle of this, but I will say it's apples to oranges, street vs strip. Big carbs usually are better on the strip because there is no part throttle. Street cars have lots of compromises going though.

 

Not sure if their SBC intake is different, but the general consensus seems to be that the Stealth is the best street intake for a carbureted small-block Ford, even on engines that are otherwise stock.

Good point on the weight comparison, I would have thought they'd be lighter. The short block is $50 and I've seen complete engines locally for around the $200 range, so I'm not so much worried about messing anything up, it's more a "I don't want to waste time and money on things that don't work so I can get this thing running ASAP" mindset.

 

@carbking one question on the Carter numbers you posted - I know Edelbrock carbs are very similar to AFB's, would those numbers apply as well? I can't seem to find the primary/secondary split numbers on Edelbrock's website or elsewhere, and the guy I talked to on the tech line didn't seem to be able to find it either.

 

Get the roller motor and put an Ecam in it from ford racing and then up your gears to 3.55. Make sure to change the valve springs cause they are junk stock. Throw a good 600-670 carb on it and smile every time you hit the pedal.

 

Answer to your question - I don't know.

Comments:
"I know Edelbrock carbs are very similar to AFB's" - true, both are mostly made from aluminum
Ford T's are similar to Rolls-Royce - true, both have 4 wheels

I have access to the remaining Carter records, including the assembly drawings.

I did not post concerning the intake, as intakes (other than multi-carb intakes for Pontiac) fall outside my area of expertise.

I do not know if it will fit the Stealth intake, but Holley produced a carburetor for Ford (Holley refers to it as the "Ford design") marketed as Autolite or Motorcraft. It is known as a model 4100. Most of these are vacuum secondary, and unless one is looking for a specific number, readily available and inexpensive. Would be an excellent alternative to a genuine Carter.

Jon.

 

I thought they shared a lot of the AFB design, I hadn't looked into it much though. Just seen it mentioned in a couple of places.

 

In regards to the manufactures CFM rating...…..not all use the same criteria. Ratings can be dry
or wet. The latter can subtract 10-15% off the rating. If I remember, Edelbrock for one rates theirs
as dry CFM. This can significantly reduce the actual flow especially if trying to tune for ethanol blends.

 

And you are still trying to justify being wrong, with specious comparisons.

Thanks for playing, but your game's over.

 

Sure. Whatever you say. Not justifying anything. Just saying what I've seen in the wild. From my observations the formulas are conservative. Often excessively.

As I said several posts back, do whatever you want.

 

Anyway, back to relavent, and correct information.

Here are the venturi and throttle bore sizes for Edelbrock carburetors.

Primary, versus secondary flow rates can be computed as percentages based on those numbers, keeping in mind that the existence of the choke assembly (butterfly and cross-shaft) makes a slight restriction on the primary side. It should not be more than 1%.



For the OP, ignore any and all drag race based advice, unless you are building a drag car.

0-60 times do not matter, either.

You need a setup that allows you to comfortably drive the car on only the primaries (at the maximum possible velocity), using the secondaries only when absolutely required.

Look a picture of the underside of a Rochester Quadrajet, and you will get the idea.

The selection of these throttle blades, and their attendant venturis was no garage-accident.

Small primaries are what you want on the street.

(Only the people who don't know how to tune a Quadrajet bad-mouth them. Ford even put them on top of the 429 Cobra Jet.)

 

The formula applies to overall ratio. so a 3.31:1 as above would be correct for a manual with 1:1 top gear.
if it had a 0.78 od you multiply them together 3.31 x 0.78 = 2.58 overall ratio [2240RPM @ 70mph]

Also non lock up converters have approx. 10% slippage so the ratio for a C4 would be 3.31 x 1.1 =3.64
That is why most manufacturers have a higher ratio for Auto trans vehicles.

Luckily most OD trans [AOD] have a lock-up converter which is a nice feature.
---------------------------------------------------------------------------------------------------------------
While on the Carb discussion. All the flow numbers are calculated at wide open [both primary and secondaries] 99% of the time the butterflies are partially closed so the only way to get these numbers is to measure vacuum and use a flow bench.
An Engine will make more power when it is over-carbed but only on the dyno or dragstrip.
But in Road Racing [and applicable in street driving] the lap times are usually worse with too much carb. Why because most of the power is needed near the bottom rpm range of each gear change.
Again these theories can get proven wrong by somebody that thinks [or drives] differently.

Generally the engine will need 1.67 CFM per horsepower.
So if you are targeting 300HP you will need 501 CFM [at wide open throttle]

A 500 CFM carb will allow a 302 engine to turn 5721 RPM at 100% volumetric efficiency [which is possible]
To get 300HP at 5721RPM the engine needs to make 275Ft/Lbs of torque at the same RPM which is quite achievable
But in street driving, even if you are driving aggressively the engine needs to get up to those RPMs, which is why the engine needs to be responsive down low.

A modern car only needs about 40-50HP to cruise at a constant 70MPH [usually on the idle circuits of a good carb, and ticking over in O/D]

 

I'm definitely wanting to use an automatic (it's a factory auto car, and I'd hate to change that) but I'm on the fence about whether to use an AOD or C4. I hadn't thought about the converter slippage with a C4 though. If I go with an AOD (0.67:1 overdrive) with the 3.31 rear gears, that will put me at 1900 RPM at 70 MPH, is that too low?

 

Not with the right cam!! And if the TV cable is correctly adjusted.
If you apply a bit of power the converter unlocks and kicks down ,so the rest of the trans behaves just like a C4.
At 1900 RPM the Engine would only need about 138 FT/LBs torque to cruise at a constant 70MPH

The AOD is a good choice even though it is off topic here.
You may notice by now, I am favourable to Hydraulic Roller Cams! This is because you can have short duration @.050 yet still get a good cylinder fill because the cam "ramps-up" faster.

You can throw together a cheap engine that can "light up the tyres" when prompted, and you can still stack your car with your buddies and cruise on the open freeways

By the way my C7 is only doing 1350RPM at 100KMH [63MPH] which is our speed limit.

 

Not with the right cam, as has been mentioned, but not without the right carburetor.

You need one that's really good at metering fuel at 1900RPM, and that ain't gonna be a big one.

 

I definitely understand the benefits of a roller cam - but if I wanted a Fox-body Mustang engine, I'd just drive a Fox-body.

So how big would be too big?