Dyno results, carb too small?

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On 2006-04-21 16:19, mstngjoe wrote:
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On 2006-04-21 08:59, StangNinjak wrote:
@mstngjoe: ......I ask you, on a 351 engine engine, with TFS heads and cam, rpm Intake or stealth, what would be better ? A 670 carb, Holley SA or a 770 Holley SA ? Which would work better ?
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I ran a Holley 750HP w/vac secondary (#0-82750) on a 331, AFR 185, Stealth, C4, 3.70 gear.

The thing ran like a scalded cat.

That was a street car that saw no track time at all.

I wouldn't put anything less than a 750 on the application in question.

But that's just me. [img]/forums/images/smiles/icon_wink.gif[/img]

A smaller carb is adequate......not optimum for power though. [img]/forums/images/smiles/icon_tup.gif[/img]

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Note the heads you are using. AFR 185 WILL flow that size of carb. Now would you use that same carb on a set of oem steel heads, or in this case a preformer head setup ? That head is basically just a step above stock...As the entire preformer series is.

I have a good article from Mustang and Fors that talk about head flowing and sizing with carbs. I wish my scanner was working so I could put it up here.

As in the app, yes a 750 could run it, But actually a 670 would pull better for a Street application. How do I know ? Its mines. I ran both in my 65 coupe, and my tq, accerleration improved with the 670. Though I lost some total HP up top, I gained more tq.

TQ WINS RACES. Hp is what you lie to your friends about.

This is a great therad as I used a web based carb calculator last night that said a 10:1 347 should use a 602 cfm carb@100% volumetric eff. One of the high performance Ford books i have warns against big carbs.If this is true would a 600 cfm EDELBROCK be good enough for daily driver with some "STOP LIGHT GRAN PRIX" first and secong gear fun stuff or would it be a dog-my favorite is imports on the interstate from 40-45mph roll!

Well guys the combo is changing,

for now it will stay a 357, but in the fall i'll goto 408-410ci, but for now i'm going with victor jr heads with upgraded springs, solid roller cam, I would like to pull to around 7000rpm what cam and carb would you guys suggest, and matching springs as the heads I have my eye on don't come equipped

I have been advised if i got with a solid roller then keep the cam duration in the 250 (@ .050) range with lift at .650 on both sides.
what do you guys think about that?
[addsig]

All this stuff about a 750 being too big for your motor is crap.

I had a 725 VS Demon on my 302, and while I could never get the A/F ratio quite right, that thing had more impressive throttle response than my 650 Holley does. I think it had down-leg boosters, which makes a big difference.

There is no such thing as "too big" a carb, just one that doesn't meter effectively. If you're drawing 2" of vaccuum at WOT, you're leaving some big power on the table. I would suggest, not that you go buy a brand new carb, but that you try and find or borrow a cheap 700ish cfm vaccum secondary, and just bring it to the dyno next time, and stick it on. You might be surprised.

There is no way any of the armchair mechanics can tell you what will work best on your combo. Including myself. You just have to try things out, and tweak things here and there. I just have a history of and know others who have "overcarbed" engines that have outperformed equally tuned carbs that "rule of thumb" said should be the right size.

I could run a Dominator on my 302 IF I used the proper venturi's. It's all in the combination.

I'm going with a comp 35-772-8 cam (solid roller 248-254 .614-.621) and a mighty demon 825 [img]/forums/images/smiles/icon_wink.gif[/img]

should make for some nice peak power at 7000 [img]/forums/images/smiles/icon_wink.gif[/img]
[addsig]

Any VS carb, be it a 550 or 850, will ALWAYS pull some vacuum at WOT and leave power on the table vs. a mechanical secondary carb. On a smaller/milder engine, I'd put money on a 650DP making more power than a 750VS, because the vacuum secondaries don't open 'till you have some manifold vacuum to pull them open. However that being said, I'd bet that a 750DP on the same mild/small engine will make more power than the 650DP if tuned correctly, because it will have even less vacuum at WOT. The only exception would be if the engine was basically stock, and thus at WOT on the 750 it might not draw enough air through all four venturi to meter fuel correctly.

I still say that the best way to choose a carb size has nothing to do with engine size or RPM, and everything to do with manifold type (dual/single plane), HP output, and intended usage. Most engines are roughly the same efficiency, and thus need X amount of air/fuel to make Y amount of horsepower.

So a 750 carb is equally at home on a 350hp big block OR a 350hp small block, as they are both going to need to ingest roughly the same amount of air to make that power level. With the same size carb a dual plane will be more responsive and economical than a single plane because each cylinder only sees a 2bbl carb with half the CFM rating. And for a given amount of WOT performance (lack of vacuum), a dual-plane intake will need a carburetor significantly larger than a single plane. And if you intend to use the car as a daily driver, and are willing to sacrifice some performance for economy, a smaller carburetor wil make less WOT power, but will atomize the fuel more completely at lower airflow levels.

This is all assuming that you are comparing apples to apples, same carb type and booster design, with the only variable being venturi size. When you throw into the mix all the different brands of carbs, venturi designs, and the fact that their rated CFM is really only a ballpark figure and thus a 750 edelbrock does NOT flow the same as a 750DP or a 750VS or a Qjet 750, then you see the kind of varied responses that we are seeing here and from everyone else in the performance world.

_________________
'86 Bronco fullsize, 460, E4OD
'85 Ranger, 340hp 289, T5


<font size=-1>[ This Message was edited by: Motorhead on 8/5/06 7:48am ]</font>

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On 2006-04-22 01:23, Spali32 wrote:
A smaller carb is adequate and more street ability. A larger carb is great if you have your foot floored all the time, but it's a dog under 4,000 rpms.
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The only reason this is "advertised" is because it is a simple way to have good off idle power for someone who doesn't know how to really make a carb work.

Adequate is up to the owner. A 408 with 200 hp and 250 torque may be adequate to someone.

Wrong on the more streetable part.

Wrong on foot to the floor and a dog under 4k rpm as well.

My (I feel like a broken record at this point) custom built Holley double pumper sports an 850 main body with the choke horn milled, a 950 baseplate, annular boosters, and some other secrets the builder performed. He said it should flow about 900 cfm or more.

Sits on top of my mild 289 with a Victor Jr. that has runner extensions welded in (no porting) and a 1" clover shaped spacer. Idle quality, idle strength, throttle response, bottom end, top end, and part throttle driving are substantially better than with the previous tuned 625 cfm Carter AFB and dual plane Stealth intake (the big carb and the stealth run about the same on my combo at the track as the Vic Jr. in case you were wondering). Nothing else changed on the combo.

The car also picked up over 5mph in the quarter mile. Traction limited by the street radials I ran at the time it only dropped about 4-7 tenths. Not bad for a carb that is waaaay too big.

My combo is only a 9:1 comp 289 with a small crappy solid flat tappet cam (repro of the old hipo cam) and mildly ported 289 heads with stock size valves and a three angle valve job. Shift at only 6,000 rpm.


<font size=-1>[ This Message was edited by: 289nate on 8/10/06 4:57am ]</font>

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On 2006-04-21 08:59, StangNinjak wrote:
@mstngjoe: This is true as well. I read that article before, and another one on the same topic. there are ways to make a big carb work on smaller CI engines true. But with the current combo he has, specificlally the heads, getting to big of a carb would be bad. I ask you, on a 351 engine engine, with TFS heads and cam, rpm Intake or stealth, what would be better ? A 670 carb, Holley SA or a 770 Holley SA ? Which would work better ?
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The question is what is too big of a carb? Not that too big of a carb will hurt power. Everyone knows that.

For your example it would depend on who tunes and modifies the carbs.

Work better for what? The strip? The street? For someone that just wants to bolt on a carb and go without even changing the jets?

I would rather go with an HP Holley instead of the SA.

Run a 750HP on my 10.8:1 Vic Jr. 347 and it is definately not a dog anywhere??? Guess I better go smaller. [img]/forums/images/smiles/icon_lol.gif[/img]


<font size=-1>[ This Message was edited by: gork1rogues on 8/11/06 3:35am ]</font>