1250 dominator too much for my motor?

Found an add for a used 1250 cfm dominator carb here in sweden.
I wonder if it will be way too much for my r5p7 motor.
Thanks

yes

Ok thanks

I`d go w/a 2-circuit 1050 Dominator properly set up.......

For e85?

Sure why not......

Ok im interested. Lets talk via pm or facebook?

way to big of carb

Quick rule of thumb is HP x 1.4 = CFM. So you need to be making about 900 hp before you need 1250 cfm.

Never been a believer in "formulas" per say, because few of them EVER suggest a Dominator for a small block, yet 9 of 10 run faster with a Dominator............However, in this case the 1250 is WAY too big. Too much throttle blade size, will result in very low air speed. I agree that it needs a 1050

Find out if that carb has straight boosters in it or if it has skirted boosters before buying it and converting it. If it has skirted booster then it is probally to big. I bought a Holley 1150 CFM Dominator and had it straight down boosters, I had it converted to E85 and it came back with skirted boosters so it flowed less( how much less I don't know ) after the conversion I tested that carb. on E85 on the same motor with a Holley 1050 Dominator on pump gas and the motor made 687 HP on pump gas with less torque than HP, the same motor 20 minutes later made 676 HP and 686 Ft. lbs of torque on E85 with that carb. My new bracket car has never ran any other fuel than E85

Thumperdart i read your post too fast. I read that you have a carb instead of that you suggest.

Cant see enough of boosters.

Pay attention to what Monte said because it is very telling.

Monte didn't say 1250 CFM was too much (really because it's a piss poor way to rate a carb) but that the butterflies (throttle blades) are too big.

Chances are, the 1050 would flow more than that (1050) anyway.

I also agree that those formula are a joke. One of the smartest manifold engineers I ever had the pleasure of talking to way back in 1984 told me to ignore those things. He had a chart in his catalog that gave RPM/airflow/CFM but he said it was a very loose guide and generally was extremely conservative.

I suggest a 1050 Dominator w/a good base tune instead of MOST of the stuff that`s shipped out w/too much emulsion and wrong set up for starters.......

Totally agree about these formulas in general cos EVERY motor I put a bigger carb on runs better AND goes faster.......

I agree those 1250 listed Holly carbs have way to big a throttle blade for most applications for folks on this board.

This is where it's a maybe, maybe not. How big is the engine? How heavy is the car? I have a 461 ci SB2 head engine, I've successfully made more power and quicker track times with a 1450, 2.25 throttle blade 2.050 venturi carb. Those heads are a spinoff of an SB2 head and with enough cubic inches, at least 400, the 1250 will be a good choice. Below that you better be spinning 9000 +.

Only 358cui.
Ill wait and save up some money and order a custom built carb for it.
Not sure the fuel pump can deliver enough on e85 though...

Just how true is this? If it is I see ALOT of guys running way to big of a carb. 530 HP builds with 950 carbs. By that math figure it should be running a 750. If you used a 750 instead would you still have 530 HP?

It's not about HP, it's about induction velocity in the RPM range it runs in. Some heads and intakes are better than others, they maintain a relatively even velocity throughout the port. The 358 would work with a GOOD 1250, one with boosters that atomize the fuel well. Using fuel that vaporizes well will also help, like Q16 or C25. Being in Sweden I would lean toward a 1050, cooler temps there will make it harder to vaporize fuels.

The formula is just simple math. An engine burns fuel to make power and the fuel needs air to burn. It takes 1/2 lb of gas per hour to make 1 hp and it takes 12.5 lbs of air for each lb of gas. So with a little bit of math you can figure out how many cubic feet of air is required to produce one horsepower.

The size of the carb is a slightly different question than how much air the engine needs. How much air is a math question. The carb size depends on a bunch of other things such as throttle response and ability to vaporize the fuel.

The best motors I've built (perHP per C.I.)and dyno tested where a lot more effecient on the brake specific fuel consumption than the standard 440 motors, they where under .42 lbs per hour down to .35 lbs per hour

Yup .5 BSFC and I'd call it a pig with a capital P-I-G.

That is fine, you can adjust the formula for a lower BSFC if you want. Use HP x 1.2 = CFM for a more efficient engine. One reason the NASCAR guys get by on small carbs is because they have developed the engines to have very low BSFC. Burn less fuel and you don't need as much air.

Drag racers typically don't pay much attention to BSFC. You can go to the track and ask 10 bracket racers what their engine BSFC is and maybe only one will even know what you're talking about. So an estimate of 0.50 isn't far off for the average bracket race type engine.

As I said before those 1250 Holley's just have to much throttle blade for MOST ON HERE.

We run a splayed valve SB Chebbie with a 2.35 throttle blade. So engine size means nothing to be brutally honest.

That is true, engine size doesn't really matter when calculating how much air the engine needs.

However, the classic formula is Displacement x RPM x VE so people have that wired into their heads. Problem with this approach is that most guys don't know what their VE is.

The other way is to just cut to the chase and convert from HP to CFM. In this formula you need to have a rough idea of the BSFC as well as air density and fuel density. But it is easy to shorthand the formula if you know a little bit about typical operating conditions. When you use this formula then you really don't care about engine size or RPM. HP requires air and fuel regardless of engine size or engine speed.

Once you know how much air the engine is going to use then you can pick a carb. The carb is usually somewhat close to the max air used but not always. Street cars go smaller, drag race cars sometimes go bigger or even much bigger. There is a bit of art involved in the carb selection. The airflow requirement is basic science.

While that IS true in most cases.........for most basic calculations a .5 brake is still a decent number to use. It gets you in the ball park

Yeah I agree. I have a ton of dyno sheets that show typical hot rod or bracket type engines with BSFC right around 0.5. I'd say it is a very typical number for anything with a "normal" cylinder head.

Your right Monte...if I was calculating fuel system requirements I would use .5 BSFC just to do the quick math.

I was suggesting that on the dyno .5 BSFC would be junk and I still stand by that. I have had my stuff go under .42 most of the time and highly prepped stuff go under .40 and I am by no means cutting edge.

I'm trying to find a dyno closer to me, but I will dyno my junk here in the next month or so and I will post it up. If the BSFC is near .5 I will stop and fix it. I can't for the life of me figure .5 being the "rule" for even bracket crap after about 1995 or so (maybe earlier).