This is really just for healthy discussion for someone who is not nearly as familiar with carbs as late model EFI. So, I apologize from the get-go if this is just elementary stuff for a lot of you long-time, carb tuners. In my way of thinking, if the engine is getting proper fuel (based on A/F ratio) at WOT at peak HP/TQ, then its probably large enough. If that is wrong, please help with the education. Then I read a lot of these carb-sizing charts and formulas, based on BSFC, etc. Makes a lot of sense. Then there's information that says you can "over carb" the engine... something that really doesn't happen in the fuel injected world, as even very large injectors can be controlled rather easily with programming.

I have attached my dyno graph (chassis dyno... so it is rear-wheel numbers; best guess is ~470hp/520tq on an engine stand). I didn't build the engine in the car. I do believe it idles extremely well, has instant throttle response, and makes good, if not great, power... for what it is. Combo is a 468 BB, ~9.0+:1 CR, Pro Comp alum rect port heads, 240/250 duration, .570"/.570" lift, Herbert cams solid roller cam, LS6 low-rise intake, 2 1/4" Hooker primary long tube headers, with a custom 3" under-the-car exhaust all the way out back, topped by a Holley 750 mechanical carb. It has the choke tower, but no choke plate... the choke has been disabled... not a big deal for me down here in TX as far as cold starts.

I've been told the carb is too small, but then some of these formulas say with the rpms I'm turning (limited by the intake to some degree, for sure), I'm fine. Also, the WOT A/F ratio is very good (12.8:1... all the way across the rev-range). It's not running out of fuel, from what I can tell. For those who believe the carb is too small... would a larger carb help top-end power, even with the low-rise LS6 intake?

Just looking for opinions. The combo runs really well, and I'm most likely going to leave it alone until a) I build an engine for it someday, b) decide to get an L88 hood, run a high-rise manifold at which point I will strongly consider a new carb, and/or c) the carb needs a serious rebuild.

 

Which fuel pump are you using?

 

OP
If as you say "the combo runs great" and "the WOT A/F ratio is very good (12.8:1... all the way across the rev-range)" then what's the problem?
I don't get the point of your thread. Nothing to discuss.

 

The best way to tell if the carb is too small is to check to see what the manifold vacuum is at WOT at the power peak rpm. If the carb is sized about right, the manifold vacuum at WOT and peak rpm/power should be about 0.5" Hg. If you're getting more vacuum than that, the carb is too small and is limiting volumetric efficiency by causing an inlet restriction. Carbs will not be limited in fuel flow - they are sized and limited in airflow. You can be running perfect Air/Fuel ratios on either an oversized or undersized carb - the A/F ratio is no indication of carb sizing issues. The 750 seems small for that engine size and level of performance - I'd be running an 850 on that. I run a 750 on my 407 small block, and it pulls right at 0.5" at peak power, indicating it is sized correctly.

Lars

 

My very similar 454 had 1.5" of vacuum at 5300 rpm. With a vac secondary 780cfm Holley.
There are convertors online that will help you estimate how much HP is being "lost" but It's not major. Unless you are drag racing and every couple HP is important.

OTOH a DP gives a strong pump shot at low rpm and has strong throttle response, but between that and the rich carb calibrations they are not particularly fuel efficient. You can do way better than a DP in that regard.

Personally I think that size is just perfect unless you are turning it over 6500 rpm. A bigger carb, especially a DP can get a serious "bog" at low rpms when it is too big.

 

That's a pretty steep drop-off on HP after 5200 rpm...especially when you have a solid-roller cam system. You have a fuel supply problem somewhere: either due to insufficient carb size or because of limited fuel supply somewhere prior to the carb.

 

Maybe there isn't, and this is helpful. As you can see from the graph... the power levels out from 5200-5600rpm. I recognize this may be an ignorant question, but can a carb be too small and limit my top-end power production in the upper rpms? Sorry... I cut my teeth on late model EFI stuff... where throttle bodies and such could effectively limit airflow into the engine, and hurt power, regardless of whether the injectors are sized and tuned properly. That's kind of my angle behind the question.

 

Thank you for the reply, Lars. I certainly trust your advice. My thinking (from the EFI world) was in the right area... not fuel flow issue, but airflow restriction. And therefore was wondering if I'm limiting the airflow potential of my current combination based on the smaller carb size (we all know the intake manifold is not optimum, but its a stock BB hood). My gut was that the 750 is small, so it sounds like I might have guessed correctly... I guess a vacuum gauge run to the cockpit is in order.

Thank you. I'm not too worried about fuel efficiency, or I'd be running a vac secondary. The car has a 6-speed transmission in it, so it cruises at low rpms... and idles clean too. I get 18+mpg on the highway, although it doesn't see a ton of that type of driving. It definitely could get better gas mileage, I agree... but I'm just not overly concerned at all.

I'm running an electric fuel pump. i need to check and see what it is (didn't build the combo). i may be wrong, but given a good, stable A/F ratio at WOT across the current rev-range, I felt it wasn't a fuel problem. Wouldn't it start to go lean on the top-end if I had a fuel supply issue?

 

You will also see top-end Power drop off because of that LS6 intake. Put a high-rise on it and you probably get a little bit better top-end and then need more carb. Of course you will also need another Hood.

 

This...id leave the 750 on there. Borrow an 850 depending on vacuum but I bet it wont be worthwhile for you.

 

rectangular port heads - AND - 18 mpg - with a 750 its a match made in heaven!

 

My 2.2 impreza isn't much better than that.

 

Some recommendations I read say shoot for 1.5" drop at WOT others say 0.5". So you (and I) are in the ballpark. Not much more to be gained.
I had two experienced engine builders tell me a high rise would be worth about 20-30 HP vs the LS6 intake, but then there is a hood issue as we know.

There is one 850HP C3 running around with a LS6 intake and a BB hood, runs 9.70s, but I am almost positive he did some serious mods to it since it sports a dominator carb and a 5200 rpm stall convertor!
http://www.superchevy.com/features/1...timate-sleeper

Hardly looks like a 9.7 sec car:

 

It is also possible that there is some mechanical anomaly in the carb or with the linkage/cable that is limiting the ability of carb to fully open. That might also present a rapid 'flattening' of the performance curves on the dyno, and it would tend to maintain A/F mixture at that 'limited' position. In fact, the throttle limitation could be mechanical with the test stand, if this was an engine-only test.

 

Carb throttle plates are verified fully open. This was a chassis dyno run.

Engine dyno, my numbers would be ~460hp/520tq... which is not bad, considering the combo which is very streetable and pretty mild, IMHO. HP being flat from 5200-5600 with a good A/F ratio tells me it is an airflow issue. Cam (based on manufacturer specs) should make power to ~6200rpm, so peak HP could be obtained ~5800-5900rpm. Exhaust is not the issue. Heads are not what I would have chosen (I will swear by AFRs, and that is what will go on if/when I do a rebuild someday), but they should also not be a restriction at the rev-range where the cam operates. We all recognize the LS6 intake is a restriction. And so that brought me to the carb sizing question.

Probably not worth doing anything at all until/unless I take the plunge and install a new hood, hi-rise intake, along with a little bit larger carb.

 

Lars is 100 percent correct. Any significant amount above .05 tells you the carb to small. Any engine regardless of the amount of air it is trying to pull in or size of carb you could tune it to good air to fuel ratio. Charts are pretty bogus on trying to size a carb. To many differences in the same size engines from VE to type of intake manifold used.

 

There is a lot of chatter on speedtalk about this. Going from 1.5" at WOT to 0.5" might give you +3.5% more hp or 14HP tops.
Many race engines are set up with this 0.5" vac level. But on a 5600 rpm 454?
But there are many other factors in play, and some engines make more power at 1.5 or even 2.0" vac at WOT. Because you are switching carbs and a lot of other things too.
A larger carb also becomes increasingly more difficult to get to run smoothly at low rpms due to poor fuel distribution with the low vac drop in the venturis. Unless you step up to different venturis etc. etc.
Right now you stated it runs smoothly at cruise at 1600 rpm and gets 18mpg. You could lose all that with a bigger carb.
Is it really worth it for maybe 14 HP?
Now with a little more cam, 10.5 C.R., better AFR heads, a way bigger carb, manifold and hood, there is 80-100 HP to be had and it's still very streetable.

I know what my biggest restriction is. It's the exhaust port flow in my 50 year old L88 alum heads. It's really bad compared to new design heads..They're only a 62% E/I ratio. In flow 311/ EX 203 cfm. Just changing those to AFRs should gain 60HP. The AFR 265 intakes only go to 331 but the exhaust jump is huge to 273 cfm (at .600 lift) for an 82% E/I ratio. The change in the exhaust flow is Huge and that's what lets the power out. The intake & carb restriction is all minor compared to that. I figure your Procomp heads are a little better than mine but your CR is way lower so that about evens us out. That cam even has a little bit too much overlap for a 9:1 motor and is not helping much. With more cam you would really need more CR. Yeah if you built it you could could make it better.

 

A dual plane needs a bigger carb than a single plane. But anyway is it possible that you have a restrictive air cleaner? Do you know the CFM rating on those heads? Rec port usually have lots of flow, are they promax 317 heads with 2.25/1.88 valves? Do you know seat and open pressure? I'm wondering if spring surge might be limiting rpm with big heavy valves. The promax 317 have a max lift of .600 and you said that you have .570 lift. another thing is that net lift after lash? What is the lash?

My original 427 had a 242/248 CC extreme Solid Roller cam and I thought that it was kind of mild. But I was exceeding 7000 rpm.

I personally don't think that 750 is too small

 

Since I didn't build the engine, I'm going off what the PO documented, as well as info I have found on the 'net. Pro Comp R port heads... @ .500 flow 336in 231ex @ .600 flow 364in 251ex 2.30/1.88 valves. My personal opinion is the head is too large for this combo, but the guy was trying to emulate the '71 LS6...and for what it is, I think he did pretty well (power/torque numbers are there). Cold lash is .012".

I had not even considered the air cleaner being restrictive. I should have removed it on the dyno and made a pull... just didn't even think about that.

In a perfect world, I'd get a new hood, run a Edelbrock Performer AirGap intake, and optimize the combo from there. I think it probably has another 30-40hp (from 4500-6000rpm) in it, split between a better intake and a little larger carb. Whether that is worth the expense or not is another question altogether.

 

With your synopsis on all counts. I emulated the LS6 so well I choked it with old heads. If I did my similar engine over from scratch today. knowing what I have learned since, I would probably ditch the L88 heads and go with the AFR 325s and a lil more cam, like a 242/250. I'd leave the LS6 intake but maybe go to a bigger carb. DD says HP should climb from 485 to 570 at 6000. That was kind of what my builders and I were expecting, just didn't know the exhaust port was that bad. As I kick it around in my head during the rest of the car frame-off, it might even happen before the engine ever goes in!