High elevation/high horsepower carb question

[crashandburn]

This one's for my old Buick, but I bet some people here have some answers. I've got a '72 Skylark with a 471 big block Buick in it. It made 611 hp on the dyno. At the end of the dyno run, the brake specific fuel consumption spiked, showing that the carb ran out of air, and it really needs a bigger one. They dyno guys said that if I swapped the 950 for a 1050, it should be good for 630-650. I didn't bother with the carb swap for fear that it would make the driveability problems that it already has worse due to having less vacuum signal to the metering blocks. I have since converted from the auto to a 6 speed standard and most of those problems are now gone.

I have recently moved from CT (500 feet above sea level) to Colorado Springs (6000 feet) and lost some power to thinner air. I wonder if going up in carb size would get back some of that power, or would it just cause more driveability problems because now, aside from the vacuum in a bigger carb, there is less air to start with.

Also, my 950 is a 4150 series carb (traditional Holley). 1050 is the biggest 4150 they make, and I wonder if at that point you are really pushing the limits of a 4150 and should just go to a 4500 (Dominator) which really isn't made for street use.
And if I'm going that route, maybe I should go to an 1150? Would the bigger bores make up for the lack of air density? Or just exaggerate the issue of not having very good vacuum signal?

The bottom line is that the car does not scare me any more. That is what makes the car fun. It is scary. If it doesn't scare me then I have to figure out something else. I'm even considering selling my engine and building an 800 hp twin turbo 6.0 LS.

 

[diesel4me]

Dual quads perhaps?...maybe a blower or turbo?..

Sounds like a scary ride,with that size engine and a 6 speed!..

 

[Hogback Fabrication]

The CFM rating is a measure of flow at a certain restriction (1.5" hg if I remember right for 4 barrel carburetors).

In CT, there was more air trying to force it's way through the carb. Here in CO, there is less. Really, your 950 CFM carb in CO probably drives worse on the street than the 1050 would have in CT. And you can practically guarantee that you're making less power regardless.

CFM matters less at altitude because the air pressure is lower, so the pressure differential across the restriction (carburetor) is less.

Hate to say it, but if you need more, it's time for more air

 

[crashandburn]

The CFM rating is a measure of flow at a certain restriction (1.5" hg if I remember right for 4 barrel carburetors).

In CT, there was more air trying to force it's way through the carb. Here in CO, there is less. Really, your 950 CFM carb in CO probably drives worse on the street than the 1050 would have in CT. And you can practically guarantee that you're making less power regardless.

CFM matters less at altitude because the air pressure is lower, so the pressure differential across the restriction (carburetor) is less.

Hate to say it, but if you need more, it's time for more air

So it's not a matter of more surface area, a bigger funnel to get the air in, but more a matter of the lower air density causes less restriction in the carb? That sucks but does make sense.

 

[crashandburn]

Dual quads perhaps?...maybe a blower or turbo?..

Sounds like a scary ride,with that size engine and a 6 speed!..

It's got 11.5: compression, so forced induction isn't really an option, although I have considered knocking out the pistons for some deep dished ones and change out my ridiculously huge cam for turbo grind and giving it some boost. A twin turbo big block buick would be pretty bada$$.

 

[dyeager535]

And just FYI, while I don't really dabble in carbs anymore, the reading I've done on the topic indicates that booster design can and should be used to make large carbs work on motors that are "too small".

That whole overcarbing attitude that was (and still is probably) prevalent seems to be waning...even Summit at least now lists two different CFM figures for carb calculations, street and race, indicating that there is beginning to be at least some recognition that how an engine performs can't be calculated via two variables.

As expensive as carbs are though, dumping the coin for a "good" carb and all the parts certainly makes me pause. For an "inferior" and "antiquated" technology, you sure wouldn't know it based on selling price.

 

[63chevyll]

Over carbing happens all.the time here.
750-800 is good.for most.big blocks here.

Ive even put 600s on 454s to make them streetable. Then put a 850 for the strip

 

[dyeager535]

Obviously altitude would be a factor. I've never had to deal with the elevations you guys do!

From what I see here, many are still stuck on the old "350 needs 600CFM max" when in fact GM was not doing that 30 years ago. Even with the limits/stops the Q-jets had, GM was running 50CFM more on the primary side on 305's and 307's than on the 350's and maybe even the 454's, so done right, there is certainly validity to running "more" than basic calculations have indicated for a long time.

I don't stick my fingers too much into my buddies circle track car, but I know they don't do any actual testing to see if the car would perform better with more or less carb (or even to see if there is restriction to airflow in general). Some of that has to do with cost, but some of it I think is just not thinking there is anything to be had there. Personally I would think even a simple vacuum gauge would give you an idea of this when you are really pushing the engine. Definitely a lot of factors, but once you step into headers, exhaust CFM (at least theoretically) isn't the limitation...even 1 5/8" is "big" for a 350 that sees at best 4000RPM.

Even if you can use a larger carb, the cost/benefit may not be worth it. Certainly not just to experiment.

 

[454Sub]

What about a vacuum secondary carb instead of the mechanical one?

That may help with the lower vacuum signal and only provide the air the engine can actually handle. Then allow you to fine tune the secondaries with the springs.

 

[crashandburn]

What about a vacuum secondary carb instead of the mechanical one?

That may help with the lower vacuum signal and only provide the air the engine can actually handle. Then allow you to fine tune the secondaries with the springs.

They don't make a vacuum secondary carb that big. Also, as far as part throttle driveablity, the main concern is the primaries. So unless I went with a spread bore like a Q-Jet (NO!) I would still have huge primary bores and crappy vacuum signal.
The two main factors in dealing with carb size selection are really displacement and max RPM. Most of the big blocks that we deal with in reference to trucks will rarley if ever see more than 4500 rpm. In my case we are talking about a 471 CID engine that turns 7000 with ease, and does it often. The formula is simple. Cubic inch times max rpm divided by 3456 gets you in the ballpark. For some reason, the old big block Buick motors love CFM, even after changing everything about the engine as I have. So that formula puts me at 953 CFM, which according to the end all, tell all, the dyno, was not enough and ran out of air at about 6200. Not terribly undersized, but undersized nonetheless.

 

[ktmoutfront]

I have a friend who squirts medical oxygen into the air cleaner. Swears it helps. The jurry is still out in my mind.

On a side note or high jack, when are all of us Springs guys going to do a meet n greet?

 

[carmantx]

The buick intake and head design are different than other engines, and they need and use more cfm than a Chevy. I run a 950 on my 464 Buick and dyno about 640hp and spray it a 100 shot at the track. Altitude change will make you need to jet differently to compensate. I would go to a 1050 though if possible. There are many variables though. On my 500 hp 462 Buick, I run an 890 cfm quadrajet that I built, because I like quadrajets.

 

[carmantx]

How is your 950 set up now?

 

[63chevyll]

Need a smaller.carb...time to come to realization you have 25% less power.thats Colorado for ya.

just jetting it helps. I thought that also. But finally saw the results I wanted by going to smaller cfm carbs

 

[big pappa b]

Welcome to the land of less HP for the $$$. 20-25% less is a given. I haven't finished my BBC Malibu which made 600hp at near sea level. Not sure if I'm ready for the disappointment

 

[crashandburn]

Ok, a couple to answer...

Brad and Big Pappa: Yeah, I know.... my math says I thoretically lost 17%, which in my case is nearly 100 hp (ugh). The car is running ok I think, other than some quirks I that I have to tune out such as acc pump and floats. I had it apart a few times and now the jetting is good according to my wideband AFR gauge, but the rest of it is pretty out of whack and it goes dead lean and stumbles. Once that clears out it is snappy and responsive.
Carmantx: I think I have 72's in the front and 76's in the rear. I forget, I had it apart a few times. I have TA Stage 2 Track Eliminator heads on it and an SP2 intake. It's funny, the heads are totally redesigned with raised intake runners, huge cam, power comes on at 3500, and yet it still acts like a Buick and likes big carbs. Like I said, the 950 wasn't enough at sea level.
ktmoutfront: The Pikes Peak guys have onboard oxygen too, so your friend probably has the right idea. You're introducing pure oxygen in an oxygen deprived environment.

We should definitely do a meet and greet, guys. I was thinking about going into the pass next week and burning through some ammo if that interests anyone...