Carb Sizing Question

[F500guy]

As I now start to work thru my post build improvements and questions, next up is the Carb. I am looking forward to playing around (not debate EFI) but started looking at appropriate sizing. I have the BPE 427 that Dyno'd at 519 around 6000 RPM, but realistically that is similar power at 5700. SO, likely looking at for my use would be pretty good at 5700 RPM, not trying to squeeze 11/10ths out of it. Online calculators say I should be about 750 CFM For slightly modified engine. Welcoming any input for the questions below and a preferred manufacture. According to the charts, I would need a full on race engine @ 6200 rpm limit to need this size carb.

211908

Question #1
What would BPE thought process be to provide a 870 CFM carb for this application

Question #2
Downside to stepping down to a 770/750 CFM in this application

Any and all input about carb sizing welcome! (No EFI Debate)

[GoDadGo]

You may get better throttle response from the smaller carb; however, if the 870 came from Blue Print Engines I'd leave it alone and enjoy the ride.
For what it is worth, my pal Dave and a Blue Print 400 SBC in his F5 33 Hot Rod that also sports an 870 carb and it runs great.
As for me, I'm running an Edelbrock 800 AVS on 383 SBC and it too runs well.

[F500guy]

You may get better throttle response from the smaller carb; however, if the 870 came from Blue Print Engines I'd leave it alone and enjoy the ride.
For what it is worth, my pal Dave and a Blue Print 400 SBC in his F5 33 Hot Rod that also sports an 870 carb and it runs great.
As for me, I'm running an Edelbrock 800 AVS on 383 SBC and it too runs well.

That is the first issue, poor throttle response out of the gate and crazy rich at warm up, so debating troubleshooting that as it has been mentioned on other post or if that is a waste of time, and better to re-size.

[GoDadGo]

That is the first issue, poor throttle response out of the gate and crazy rich at warm up, so debating troubleshooting that as it has been mentioned on other post or if that is a waste of time, and better to re-size.

What is your average altitude of operation because the carb likely may just need to be tuned.
We run between 8'0" below sea level, when in New Orleans, to as high as 300'0" above depending on if we are going North to play on those back roads.

[mrmustang]

As I now start to work thru my post build improvements and questions, next up is the Carb. I am looking forward to playing around (not debate EFI) but started looking at appropriate sizing. I have the BPE 427 that Dyno'd at 519 around 6000 RPM, but realistically that is similar power at 5700. SO, likely looking at for my use would be pretty good at 5700 RPM, not trying to squeeze 11/10ths out of it. Online calculators say I should be about 750 CFM For slightly modified engine. Welcoming any input for the questions below and a preferred manufacture. According to the charts, I would need a full on race engine @ 6200 rpm limit to need this size carb.

211908

Question #1
What would BPE thought process be to provide a 870 CFM carb for this application

Question #2
Downside to stepping down to a 770/750 CFM in this application

Any and all input about carb sizing welcome! (No EFI Debate)

Having spent time on a chassis (not engine dyno) for a few solid days, and unlimited carb changes and tweaks (early 2K's as a good friend at Bruce's Speed Shop (RIP Bruce) ) learned the ins and outs of the difference between that and tuning on an engine dyno.....I highly recommend a Holley 0-4778C, 700cfm double pumper. Add a 1:1 secondary linkage, get it tuned on a chassis dyno and be done with it. Sure, you'll find others who say different, but when it comes to carbs, bigger is not better, and the better a tune you can get on a chassis dyno that uses real rear wheel tuning abilities. With an engine dyno, you are getting wide open throttle tuning best set for a drag car, not a car running on the street. I'll now let others attempt to justify their engine dyno tune, or the cause for a larger carb......Ultimately, the choice will be yours, and yours alone. Before you make it, ask your engine builder the difference between the two, if he knows the difference, great, if he does not, then you have your answer.

Bill S.

[bobl]

I've been helping a club member with that same engine package. I think that 870 they use is really not a good match on that engine. It goes very rich at light throttle. I ended up dropping the main jets 6 sizes to be able to get it to run decently. I tried a 600 Holley on it to test and it ran great. probably a bit small for it at WOT. I would go with a 700 or 750 double pumper if I were building it.

Bob

[Jim1855]

I ran a 4150 780 DP, no choke from ProSystems on my 427. The 780 drives well once above ~ 2,000 RPM and runs to 7,000.

I have run the 670 & 770 Street Avenger vac & choke carbs. As well as a ProSystem 940 DP, no choke.

The 670 provided the best drivability but the 770 had more power. The 940 was too big until about 6,000 and even then probably too big. The 780 is the best across the RPM range.

I'm running an Edelbrock Vic Jr, AFR 225 heads and 1-7/8 headers. All of this was a previous car, 2,650#, 3.27 gears. 440 rwhp on a Mustang dyno.

A 750 is a good/safe bet. I would certainly run a double pumper and don't see a need for a choke or horn on a summer car.

Jim

[F500guy]

What is your average altitude of operation because the carb likely may just need to be tuned.
We run between 8'0" below sea level, when in New Orleans, to as high as 300'0" above depending on if we are going North to play on those back roads.

Most will be 500 feet or less, but I may opt to drive up to a ski resort or over a pass 4500-6000. According to my engine supplier, they are tuned for sea level. I have a low degree of tech support confidence in my supplier (tested it once), it is a rock'en engine but will go to other experts for specific information.

[F500guy]

Great input, this mirrors my research and thanks Rebostar planting the bug to look into it. I Think I would still want a choke, damn cold in the mornings sometimes in the PNW, especially fall and spring driving. I have plenty of time to work it out, it actually runs OK now, starts right up, good fast idle to warm up, nice lopy (if that is a word) sound at 800 idle, just a little smokey.

[Grubester]

For my Mk 4 roadster with BP 347, Holley 600 CFM (4160 Street Warrior), I was going to avail myself of the chassis dyno & tuning services of FTW Tuning in Springfield, Oregon.
I got a response from him that setting up my carb is exactly what they do. (and a rear-wheel HP output... why not?)

[cv2065]

On my last 427 from BP, I stepped down to a QF 750 Brawler w/mechanical secondaries. Once tuned right it felt just like fuel injection. Throttle response was excellent. I've got the same carb for this Roush 427. I'm also using an electric choke.

According to the CFM calculator, even a 750 is a little big on a street car. At 6K RPM max and 427 cu/in, the 'optimal' size is a 630 CFM carb @ 85% efficiency. I'm good with it though.

[F500guy]

Thanks for all the info everyone, I decided to go with a Holley 670 street avenger.

I want to maximize my "Smile Miles" this summer, this should be a easy plug in and I will just change the vacuum spring to get running, then I can play with the pump shooter as needed later, but hopefully better than the 870 out of the gate. Long term down the road after I have a painted and mostly completed car I will probably move up to maximize output. I have a nice data system that I can hook up a Lambda sensor, throttle position and RPM along with speed and Acceleration, So I can play with tuning on my own. I ordered a tuning book as well for some light reading...:)

[PNWTim]

You might reach out to Aldridge Motorsports for feedback as well. Those guys have been racing and building Fords for 60 years. I believe Denny retired and they have scaled back their operations significantly but might be worth a shot.

[johnnybgoode]

Thanks for all the info everyone, I decided to go with a Holley 670 street avenger.

I want to maximize my "Smile Miles" this summer, this should be a easy plug in and I will just change the vacuum spring to get running, then I can play with the pump shooter as needed later, but hopefully better than the 870 out of the gate. Long term down the road after I have a painted and mostly completed car I will probably move up to maximize output. I have a nice data system that I can hook up a Lambda sensor, throttle position and RPM along with speed and Acceleration, So I can play with tuning on my own. I ordered a tuning book as well for some light reading...:)

I agree with your sizing selection but would suggest a DP (with a manual choke) vs the VS Street Avenger. These cars work wonderfully well with a DP (light + 5 speed) I'm running a QF SS650 on a 500/520ish stroked 351W and it's a perfect match in my opinion. Lots of adjustability and runs like EFI once dialed in. https://www.youtube.com/watch?v=Jao9vziGZl0. Scott