*WARNING*... EFI hot-rodder on board here. Experience with carbs is minimal. So go easy. :lol:

Combo to be (within 8-10 weeks as heads are on order):
454" Gen VI
9.5:1 CR
AFR 305 cc heads (CNC'd chambers)
Edelbrock Air-Gap intake
Hooker 2 1/8" side-mounted headers
Hyd. roller Crane cam (~232 intake duration, ~.590-.600" lift)
3.36 rear gear, Richmond close-ratio 5-speed trans

Desired operating range from 2000-6200rpm.

Which carb? Right now I have a Street Avenger 770cfm on the stock Gen VI 454 crate engine. It will be too small, I'm sure.

The guy who designs the heads at AFR (and who is helping me with the cam choice) recommends a Holley HP 4150 950cfm carb. He also estimates engine output at ~450rwhp. I'm figuring it will be closer to 420, but maybe the AFR heads are _so_ good, I'll be surprised. ;)

Either way, I think the 770cfm carb is too small for a 500+ flywheel HP motor.

This is a STREET car. I do not mind a lopey idle, but I want good low-speed driveability. It is not a daily driver, but I want it to act very close to one...with very little sacrifice.

I do not have the best understanding of how a carb functions (read some books, but I'm way down on the learning curve here), but I do not want to kill low-end throttle response with something too large. I've been told the HP series does a good job for a street car.

Any help is appreciated as always!

 

I'd say minimum of 850 w/ your combo...& that 950 would probably work fine. I'm running a Barry Grant modded 750 that flows up to 1040 cfm & it has excellent throttle response- dropped my ET by .30-35.

 

Beware........ If the Red car in your sig. is the car you are refering to... ANY carb/manifold/ air cleaner combo other than stock CORVETTE stuff will not allow you to close the hood.. The solution is an L-88 high rise hood.

 

WA 2 FST,

Im running a 830HP Holley. I would think a 830HP or a 950HP would work great for your combo.

 

Ide try the 770 first to see how it does. You can always change to a bigger carb. Its not so much the CFM rating as it is how you have the carb "setup" i know from expierence. I had a 455 olds that was biult boderline radical with a Holley 650 DP spreadbore. That carb would power that motor strong to 6000rpm and beyond. Yur 770 can be improved for different applications. It can be jetted up and down, and lighter sec springs to get sooner 4 barrel kick in, different power valves, accelerator pumps, the list goes on. Everybody judges carbs in CFM's it seems. Theres so much more to carburation than just CFM's. 770CFM is plenty for a 454 if its set up right. Thats my 2

 

The 770 will probably cost you 15-20 hp. I agree with the AFR guy. A HP 950 would be optimal. Of course, the price of a HP 950 is a pretty big chunk of an EFI setup... Why not skip ahead a few months and just get the EFI now? What's an Hp 950 run? $600? Add the Intake at another $200-$250... 1/3 the cost of a port EFI system.

 

The L88 hood is on its way. :) Should be here Tuesday, in fact.

 

John,

Being a carb newbie, I honestly believe I could spend a lot of $$ trying different combos for the 770 carb...and still not get it right. I have actually _heard of_ the parts you are referring to :lol:, but my working knowledge of them is not great. Jetting I can do. The rest of it... its greek to this guy right now.

I know there's more to it than CFMs, and in the right hands I know I could improve on what I have.

 

Markus,

This is a fair assessment. Already bought the intake...but I know I could turn around and sell it, too.

AFR guy feels my current carb will cost me 30+hp on the top-end where the cam makes peak HP (~5800-6000).

I will go check out your webpage again regarding your EFI conversion. I know you had issues with the fuel pump(s) frying. Plus that intake is an oval port and the AFR heads are rectangle. Do they make a rect. port EFI setup? What about your throttle cable issues? That looked like a PITA for someone who's not a great fabricator. You fab up stuff in your sleep, I think. :D (meant as a compliment).

 

They also make an EFI kit for rectangle port heads. Same $. The throttle cable was easy, just one weld. I'm sure you can get a universal throttle bracket from someone like Lokar if you don't want to weld.

I had one pump fry. Seems holley had issues with that one and now uses a different brand (WalBro). They sent me a walbro under warrenty and haven't had any more problems. I did have issues with fuel starvation when taking corners with less than 1/2 tank. I had a custom aluminum tank made, or you can have a sump welded to your stock tank. Or just keep the tank full. :)

 

Don't be surprised if the 770 is all that you need. Somebody above mentioned to try the 770 first, and if it falls flat, then get a bigger carb.

I am running 533 cubes, and a little more aggressive cam, intake is the factory low rise tri power, carbs, tri power, vacuum secondaries, with stock metering plates, GM 077 al square port heads, pocket ported, and hooker 2.125" side mounts. I would not chage my intake set up for all the tea in China. The sum of the individual carb cfm rating is around 1200, but the actual cfm rating is closer to 800-820. Throttle response is right there, and it pulls past 7K without skipping a beat.

I would run the 770 first, try some larger jets, maybe stagger the sizes. I think you might be pleasantly surprised at the performance. Bigger, is not always better.

 

Well, if you want simple and cost-effective I'd say get the 800 CFM Edelbrock. They're simple to work with and decent carbs.

Reconditioned they're less than $300 from Summit
http://store.summitracing.com/defaul...9962&x=16&y=10


[Modified by Black Snowman, 2:25 AM 5/18/2003]

 

I have to agree with a couple others in that trying the 770cfm first, you have nothing to lose. The 950 will make the most TOTAL hp/tq but at peak rpms. I would reccomend a 850cfm carb since you are wanting a car that will act like a driver. The 850 will have much better throttle responce and torque at a lower rpm vs the 950. If this was a drag car, I'd say go with the larger carb, but for the street you want: throttle responce, dependability, and decent mileage which all can be had with the 850.

I'm partialt to Demon carbs, but that's me. :D

 

Wes,

Your mechanic is right on. Please look over the article I posted. The 950 (same exact carb as mine) was not only the largest, but made the most power throughout the rev ranges tested. As I recall, there was only one exception where a smaller carb made slightly more power at one particular rpm band.

I am thrilled with mine!

Chuck

p.s. The picture/article did not expand for me like it is supposed to when I clicked on it. Try right clicking and save it as a desk top item. Much larger and easy to read. Delete when bored or memorized. Article was using a small block Chevy of unknown displacement.


[Modified by Chuck Harmon, 2:08 PM 5/18/2003]

 

George,

There is a conversion for 2 barrel to 4 barrel ratings, 3"Hg vs. 1.5"Hg. As I recall, the approximately 1300cfm 2 barrel rating was equal to about 1000cfm using the 4 barrel measurement at 1.5"Hg.

Your tri power, like some others on this forum, moves alot more hot air than is often admitted! :jester

Chuck


[Modified by Chuck Harmon, 3:14 PM 5/18/2003]

 

Here is an equation to determine cfm requirements:

cfm = [ (RPM x Displacement) / 3456 ] * VE%

Assuming you are running close to 100% VE;

cfm = (6500 x 454) / 3456 = 853 cfm

Another factor, I believe a paper air filter flows about 5 cfm per sq. in., therefore, for your 950 carb will require a filter 4.3 inches tall. A K&N filter flows about 6 cfm/sq. in, it will have to be 3.6 inches tall. You may need a "Extreme" top from K&N just to get the air flow through the filter.

If AFR is suggesting 950 cfm, I am sure they have dyno'd enough engines to know. You can also ask the cam mfg. and if they give the same ballpark, then it will be a good starting point.

If you are going to be around 950 cfm, why not got for an Edelbrock Pro-Flo fuel injection with 1,000 cfm?

 

If you are only going to turn it 6200 than the 770 will be plenty. I don't believe at 6200 you will see any HP gain with a bigger carb but you will definately see a loss of low speed torque.

 

I would go with an 850 DP or a 950 HP. I have a HP 1000 on mine and it works very well. If you follow the advise below I would go with the 950 or 1000 HP

I think anything less than an 850 would cost you some power. Do not believe a big carb can not be streetable, it just takes a little tuning.

I would also go with a bigger cam (at least 240 at .050) and try to get a little more compression ( 10 to 1 or more). If you do not go with a bigger cam now, you will later :yesnod:

A little more compression and a bigger cam will get you approx. 475 streetable rwhp.


The article Chuck posted is a good referance, bigger carbs (up to a point) are usually a better choice.

 

Guys...thanks for all the excellent feedback. It's all appreciated and taken into account. Seems about half of you say to stick with my 770 and the other half say go with something bigger.

FWIW, the AFR tech feels the 770 will cost me 30+hp compared to the HP 950 carb.

Chuck's article is very interesting in that even the carb that supposedly would be too large for a given application actually performs the best over the entire rev-range...losing neither low-end TQ or top-end HP.

69 NOX Ratt, this is probably for a different thread, but 1) I am going to be running about as much compression as I can. The engine is so new that I am not going to yank it apart to run a dome piston, so I am having the heads angle-milled to 108cc to bump CR to 9.5:1. I could have had them angle-milled a bit more, but wanted to save some meat for cleanup at a future date, if necessary. 2) I am going with the 232 cam b/c I want the car to be really streetable. I have over-cammed an engine before and it just ticked me off...cam stayed in there all of 40 miles. I've found more luck in _my experience_ with smaller cams. Then again, those were all blown motors, so the blower may have made up for the smallish camshaft as far as overall power delivery. ??? The head tech at AFR gave me two choices... 232/232 (single profile cam due to the head design/flow characteristics), .580" lift, 110LSA or a 244/244, .610" lift, 110 LSA.

I'm concerned with too big of a cam also due to the relatively low compression.

The smaller cam should put my peak HP rpm right around 5800-5900rpm. Where do you think I'd be making peak power with the larger cam? Is it going to ruin streetability? I want this thing to behave at engine speeds ~1800+rpm. I do not have an OD, and have a 3.36 rear gear, so it is at 3000rpm at 70mph. Also, I want longevity. That's one reason I'm sticking with the hyd. roller. Easy (read: NO) maintenance and a very long life.

Maybe I'm chickening out with the smaller cam, but I believe I'll still be in the 420-430rwhp range with it, and I may spray it later if I feel the need for more power.

Thanks!

 

I understand where you are coming from. I went conservative with my cam (solid 248/248 @ .050) so it will behave on the street. To compensate I will have at least a 200hp shot os N20 for when I want to "play". :reddevil