My dream engine has had nothing but carb related problems. The Street Demon 750 VS was a bust! I sent it back to factory for repair and tuning, came back worse. My mech has a 750 Holly VS on now and this works much better. He wants me to get a Road Demon 625. He used one a a Vette w/less engine than mine w/great results. I used the program of cid x rpm / 3456. Using this formula a 625 is perfect. My question is this, I thought it took a certian amount of carb flow to produce X ammount of power? My engine should produce over 400 hp and 400 tq. Will a 625/650 flow enough fuel/air to make the power my engine should make? My engine is the following:
360 cid, TFS heads "pocket ported", Air Gap "ported", 218-224 HR cam .531-.539 w/1.6 rrs. Forged everything, 6" rods, 10.25 cr. The plug in this engine is the stk ex man "ported" untill the next smog inspection. I know headers will "uncork" the engine and need a little more carb. How much carb will it take to meet my needs? This is a daily driver, fussy is not acceptable! TIA for any help.

 

"............. 360 cid, TFS heads "pocket ported", Air Gap "ported", 218-224 HR cam .531-.539 w/1.6 rrs. Forged everything, 6" rods, 10.25 cr.........."

I'm not sure what's legal in Cali -- but personally, I wouldn't go with anything less than a 750 DP Holley, or their 780 Vacuum Sec. model (depending on trans, gear, how you drive, etc)

 

I would go with the Holley 750 dp mechanical for your setup. It works great with my setup.

 

The formula you quoted has been proven to reccomend to small of a carb in almost all cases. I agree with the others, a 750 would be fine. I would go dp if you have a stick and vacuum with an auto.

A more accepted formula is HP X 1.6 = cfm requirements. 400 hp wants 688 cfm.

 

I run a 355 with a vacuum 750 holley and don't have any problems . I tend to jet things lean and think that most carbs are jetted too rich. Richness causes lost power, fouled plugs and short engine life. I worry about washing cylinder walls but not about burning holes in pistons. Look at the new car that run about 14.6 air fuel ratio and last forever.
I am a fan of double pumpers and if had to do it agian I would go with a double pumper.
Double pumpers run richer then vacuum carbs and need lots of help straight out of the box. I spend about one day per carb with new carbs just drilling new holes, bushing old ones, filling in holes and rejetting.

 

I guess we're all different, and that's cool :)

Re Jetting:

I witnessed a buddies 454, with a 850DP, pick up ~25 HP @6000 (at the rear wheels), when we pulled the 80s, and went with 82's. It was a 11:1 LS-6 with headers, re-curve job, etc.

I just jetted a new 850DP, for the 468 I'm assembling, with 82's F and 84's R
(I'll fine tune it later).

Funny how every engine is unique........



[Modified by 66427-450, 9:17 PM 12/29/2002]

 

Just a few things that you need to mention are the timing and if the cam is straightup, the rear gear and transmission. I'm not a fan of the 750DP on a SBC, I know lots of folks that have them on there and like them but I have always found that the smaller the carb the better the performance. Put a vacuum gauge on the manifold and see what you are getting. The trickflows have a rather large port for the cam your running and can lead to the fuel mixture plating out in the manifold, not enough velocity.
If this is to be a daily driver then I would go small carb and vacuum sec. But really need the other info.

 

Definitely 750 DP Holley. Those formulas always err to the small side. A properly sized and tuned carb will have fantastic response. Using too small of a carb to get throttle response is a "band aid" in my opinion. Always safer as far as driveability is concerned, but a big power loser.

Check out Edelbrocks website in the Tech questions area for some of their ideas on carb sizing. You'll see numbers typically 120-140% of what those formulas arrive at. Do you really want the carb to be achieving max airflow at max HP? The odds of it becomong a restriction are very good if it is sized to max out as you reach max rpm.

There's not a streetable 327-350-383 made that won't run fine with a 750. As you get more radical 850's are appropriate.

A 4779 Holley is about the most universal carb out there. It can easily be made to run great on just about anything, including mild big blocks.

And I still see that for every Demon lover out there, there are about 5 folks who hate them. They are so common in the used carb marketplace it's crazy.


JIM

 

Jim,

I saw the stuff on Edelbrock's web site you are referring to. I thought the fact they reccomend a bigger carb with a dual plane manifold over a single plane (with all other motor componants equal) interesting.

My 920 cfm DP worked great on my motor when the cfm x rpm x ci forumal called for 750 or so.

 

Agree w/ the others a 750 DP is probably about right for your setup. The carb sizing formula is notoriously conservative.

As to why Edelbrock recommends a larger carb for a dual plane is due to the stronger signal provided by the dual plane -- you get less "penalty" (ie loss of throttle response) by running a larger carb compared to a single plane.

Carb sizing is in many ways a compromise as in so many things related to street performance. In general you can tolerate a larger carb w/ vacuum secondaries compared to a similarly sized carb w/ mechanical secondaries. They are easier to tune and more forgiving. However, a vacuum secondary carb will never quite match the performance of a DP if you have the ability (and patience) to dial in the DP.

Transmission type and gear ratios also play a role in proper carb selection. Tight converter, tall highway gears suggest a smaller vacuum secondary carb will work better. A manual transmission or a loose converter w/ higher numerical gears will be best exploited w/ a larger DP carb. Again you can get virtually any carb to work (large, small, vacuum, DP) but as you stray too far from either side of "optimum", it will take more work to dial it in for all driving conditions.

Good luck,

Mark

 

Mark,

I should have known that, makes since. Makes the fact I am going from dual to single plane with bigger cam and head work promising. Maybe I will get lucky and it (920 DP) will be perfect for the new motor combination.

 

I glad I'm not seeing any "too much carb " "it'll bog" talk here. Was reading a book on selecting high performance parts for buildups and almost bought it until I read the part where the author said " .....mechanical secondary carbs are for racing and not for the street and will not be discussed here " :lol:

If everthing is working correctly then a 750 Holley would be my choice for your engine and like it says above, mechanical DP for a 4 speed and vaccuum secondaries for a low stall automatic. :D

 

Thanks for all the replies. The 1.6 CFM per hp sounds much more reasonable. According to that 400 x 1.6 = 640. That sounds about right. A 650 has done well for many a 350 and a 750 can't be far off. My engine is set up for max hp @ 5500. I think that I will pull about 425 at best. I have heard good things about the Avenger 670 being a STREET carb. Either that or a 700 DP. My 74 has a M21 and 3:55s. I am really trying to get this dead on not just close! I forgot to mention my valves have been backcut for extra flow. As you can see, I'm really trying to make a killer street engine. Any further advice?

 

Put a vacuum sec carb on it. Very few street cars work better with a DP especially SBs

 

Vaccum secondaries are "safe". Meaning they work 99% of the time on just about anything. Please don't take it wrong, but they are made to be "idiot proof".

But we build our cars to have maximum performance. That means we like to play with them to make sure they are faster than the "other guy" with similar setups.

The double pumper Holley is about the most tuneable thing out there. The power is always controlled with your right foot. With a 4 speed, there is no way I would want a vacuum secondary carb. When traction is minimal, the control of mechanical secondaries is vital to getting moving well. The ability to come out hard or to "bog" it when traction is minimal is a great thing. With Vacuum carbs, about the time they get open well, it's time to shift. And that is AFTER all the work to change the stock super stout spring in there, install adjustable dipraghms, remove check ***** etc to make it open quicker. The engine could have easily handled the airflow, but it wasn't there. When traction is poor and you want to feather it. here comes that secondary opening when you don't need it. Like driving with a rubber band throttle!

I've made 750 DP's work on everything from a '82 302 Mustang GT on up to 454's, 440's etc. You can tune a DP to leave the line cleanly with 3.08's and an auto if you work at it a little. There are infinite cams, shooters etc to tune it with.

If you just want to bolt on something and forget it, get a vacuum carb. But don't be surprised when the other guy has done a little homework and pulls 2 lengths on you out of the hole and keeps pulling away from you!


JIM

 

I have nothing more to add other than support of the Holley 750DP.
I have had one on for over 30 years now. The same carb.
Still works very well. A DP does need some tuning to your specific engine.

I get great low end and high end response.
As stated, they are the best for a modified 350.