Carb question. 850 vs 800
 

My motor was rebuilt late last summer. 509 with promax aluminum heads. The builder says i need a bigger carb. Its currently got a 800 (think it was from a hp500 motor). He says i need a 850. He rebuilt the 800 for me just so i could get the motor back in the boat and running. Got to take the boat out once before it got to cold and it ran good. I was taking it easy so didnt really pish it hard.

Is there a big difference between a 850 and 800? I dont know chit about carbs. Seen some quick fuel brand new ones for about $650-700. Is that a good option? Only wanna do this once. Past 2 summers have not been great boating wise so i am trying to get everything fixed befor summer.

I know a particular build that picked up right at 75 horsepower on the dyno from switching from a HP 500 800 to a Quick Fuel 1050. Back to back, no other changes and the 1050's were not tuned ideal.

Mark

I could see a difference between a 800 and a 1050. But 800 bs 850?

Should i be look for something bigger then a 850.

Run you engine with a vacuum gauge hooked up. At WOT there should be very little vacuum. Approx. 1.5. If there is much more than that then there is a restriction. Either the carb is to small or current flame arrestor is to restrictive. Once this is done you will have an idea as far as how to proceed. Just my opinion. In theory the 800 "should" be ample for a 509 up to about 5450 rpm.

GM recommends a 750 with their 500hp 502, fwiw.

The 800's will work but you will make more power with bigger carb. Don't feel bad. My builds went from 1050 doms that I already had new and now need 1250's. Will the 1050's work yes but leaving power on the table the way the intakes and heads are ported. Btw same heads as you just more CI.

You can always run with 800's and borrow an 850 for one side and see if you go in circles.

Another thing. If indeed you find high vacuum see if you can borrow a larger carb to try. This was the case if was referring to earlier. Had a 1050 lined up to try in the event the 800 was not enough. There are numerous similar builds I am aware of using 950-1050.

yes..... buy the 850..... smokin gun had 1 for sale on here

Is the guy telling you that you need the extra 50 CFM selling you the bigger carb?

My 512 has an 800CFM carb on it.
It pulled past 6k and made 675 HP.

Looks like an 870 CFM carb for the ZZ502 crate?

http://www.chevrolet.com/performance...02-deluxe.html

No he isnt offering to sell me a carb but he did say i could borrow his 850 to see if it makes a difference. When he was running it on the stand he used his quick fuel 850. He told me it would be better with a 850. Said as long as i wasnt running wide open for long stretches it would probably be ok but he didnt reccomend it. I trust him as he has treated me good.

Before i could barely get the motor to 4900 rpms. But the motor had serious issues too.

Im not sure now cause i didnt push it last time out. Only had it out once to verify everthing was ok and then it got to cold. I still have some bugs to work out and just wanna have as much as i can done before summer

Last summer sucked bumming rides.

When it comes to EFI, everyone wants a big CFM throttle body. You never hear the EFI guys, saying "A 750cfm throttle body is plenty for a 650hp engine" .

You bring up a carburetor, and you'll hear guys suggest a 650cfm on a stout big block.

Its kinda like a cylinder head. A smaller head, doesn't always net more power , if the head's design sucks. A larger head doesn't always make more power, if the heads design sucks.

A well setup dominator, can be extremely responsive, idle well, and do everything well. Stick a 1991 dominator from a swap meet on your engine, and it could very well will prob run like chit.

It comes down to booster style, idle circuitry, venturi size, and so on. An old 800cfm holley double pumper, will probably easily get outperformed, by a modern Ultra XP 850/950, everywhere from idle to wot. There is power in fuel atomization, not just the cfm rating.

From prosystems

But really cfm is not the main level of importance. Fuel shear, atomization properties and fuel curve are your main areas of concern.

To size a carb for the application, you're looking to achieve minimal restriction at the finish-line yet have enough signal at launch as to be sure that the booster is atomizing the fuel and supplying the proper air to fuel ratio.

Horsepower equals air flow (of course). Launch rpm/trap rpm equals a reference of the range of the air flow.

If the carb is too big or signal/curve is too poor at the launch rpms created airflow, the fuel does not properly atomize and plates out (turns back to raw fuel) on the intake. Losses of 10-12 percent of available torque at launch can easily be recognized without a lean cutout or backfire. Then as rpms increase, the plated fuel is picked up and alters the air to fuel ratio down-track as it is cleaned out of the intake. More loss of power. So you jet it down to compensate for the plated fuel being picked up and the launch gets even worse. See the dilemma.

The wider the range of rpm you're going to subject the design to, the more you need to look at the range of airflow and available options.

I'm sure you remember this old formula:

CID x RPM x V.E. / 3456 = CFM

Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings.

20.4 = 1.5 hg.

You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads.

Because, most of you have specific application designs, a custom shop/unit is typically the plan.

In the future, use this calculation as a general rule on a modified carburetor:

CID x RPM x V.E. / 2820 = CFM
350 x 6600 x .9 / 2820 = 737 CFM

Now you'll be a little closer.

A .9 Volumetric Efficiency (V.E.) number represents a pretty good combination and a 1.1 V.E. number represents an all out assault on the engine blocks stress handling capabilities.

Remember, if we have a heavy vehicle and a two speed we will require a slightly smaller carburetor, than a light vehicle and a stick. Also, if we have a booster/emulsion/air bleed configuration designed to operate and shear fuel at lower rpms we can increase the cfm. An increase in cfm is usually a guaranteed increase in power, but it takes a design that'll still pull and shear fuel at launch to pull that off. That's when the builder starts altering the entry and exit angles of the booster, the emulsion layout, air bleed configuration/well diameter, etc. All in an effort to fan the fuel cone to increase impact, supply the proper air to fuel ratio throughout the rpm band and emulsify the mixture prior to decrease plating for the air speed being encountered. All those mods cost money and they're not easy to do.

But return on investment is the deal when purchasing a carburetor. Oftentimes a customer is thinking of purchasing a this or a that, when the same money spent customizing his current model will yield more performance.

Remember, as we talked about earlier, the loss of torque we record at launch and the subsequent rate of acceleration you lose at the start of the race will be carried throughout the rest of the shifts. So a good leave (excellent fuel shear and proper air to fuel ratio at launch) is getting the reciprocating mass to carry this rate of acceleration to reduce E.T.'s. But if you have too much restriction at the finish-line, the mass will be slowed as a result and E.T.s will increase and none of us want that.

Thanks for the E's.

Have a great season everyone...see you at the races.

Using the above formula

502 at 5200rpm=833 cfm
502 at 5500rpm =881cfm
502 at 6000rpm =961cfm

Each engine is different, so , the vacuum test is a good idea.

I always thought a guy could get away with a larger throttle body on an EFI because the fuel was introduced in the intake runner and the TBody didn't have to draw fuel through a booster and atomize the mixture.

That really makes the 1050 a great choice for the build of which I speak!

Yes, that is true.

My point though, was that carbs have come a long way at how well they atomize the fuel. i woudn't go sticking a 1150 dominator on a 454 that turns 5200rpm and makes 450hp, but, if we are talking a 600+hp big block, they usually do well with a dominator , or really good flowing 4150. At least in a boat. Not many boats are running around with the throttle blades open 8% at 3000rpm, like a car cruising down the freeway. Usually, in a boat, at 3000rpm, there is plenty of velocity , as the engine is pretty heavily loaded at that point, and consuming some air to maintain 3000rpm due to the heavier load.

Anybody have any experience with


http://blp.com/cart/index.php?main_page=index&cPath=72

I like the 2 cicuit doms.

The formula MT posted agrees with the carb Pro Systems sold me, so they do seem to actually utilize the formula. If buying an off the shelf carb I'd be concerned with what it really flows verses what the advertised flow is. If you determine you need an 850 and buy one advertised as 850 is it really flowing 850 or...?

I have seen alot of them used on open dirt late model engines 20,000-40,000$ engines.

Would prolly be a great carb for big cubic inch marine engine if set up properly.
Very expensive though....cost could be a deciding factor though....
just saying.

I thought Holley "wet" flowed their carbs.

I thought carter / Edelbrock "dry" flowed theirs.

Anyone know about the quick fuels?

I was looking at a 1250 under 1k. But ya I also noticed they have some high end or at least high dollar carbs as well. Had to look twice at the price.

Carbs can act strange. One of the last dyno sessions I was involved with was very interesting. Was on a revved up 502 but nothing spectacular and we tried every carb under the sun it seemed. From 750's to 950's. Holleys, including hp's, mighty and speed demons, AED along with a couple others. Can't even remember all of them but the 830 speed demon with choke horn was flat out the best hands down. Really blew my mind. Was not what I expected. If I recall it was up by 10 hp over the others. Overall not a huge difference btwn all of them but was a fun test. The kicker is though on the water the holley hp950 responded the best, idled great and best top end speed. Just sayin... interesting how things can work out. What and how??? All I can say like it's been said before is going from the dyno to the water are two different gigs. May not always apply but definitely can. The guys tuning in the bilge I have respect for especially when their looking for that little extra power. It's there just gotta have patients.

True story on a 24 challenger with a so called baker engineering 500 hp engine I picked up 2 mph on same day with carb tuning only. Carb was an 850 holley dp. That was before all the afr widebands were so readily available and aware of.

looks like bajachicken got it wrong.

My crate engine 502 with Edelbrock oval port alum heads and moderate cam would turn 5900 rpm with the stock merc Weber/Edelbrock style 750 carb. Properly jetted of course, but it idled great and had great out-of-hole and mid-range too. Personally not a huge believer in extra big carbs. Throttle bodies are different, they don't have to have a vacuum signal to meter fuel.

Pretty much identical to my set up: ZZ502 shortblock with Eddy RPM ovals but I put a QF850 on it. The supplier brochure said 750.

Not the first time nor the last. ;)

i am glad you have a good sense of humor.did you ever think about changing you,r screen name bajachicken?it has a nice ring to it.

Nah... it's actually already the "family/internet safe" version of a nickname, so there's nothing you can do to it that's worse than the original, lol. Baja actually has nothing to do with my mediocre entry level boat, but I spend months down in Baja on a motorcycle by myself.

Now back to our regularly scheduled programming, and the fact that I fear that my equipment may be too small. I can spin my 502 to 5600 with a QF850. Maybe my prop is to small. Something is too small somewhere, dammit.

It'll be awhile before i fire the boat up but where do i hook the vacum gauge up to when i do? I assume i need to do this in the water and its frozen still here in Iowa. Just rev it up in neutral, or does it need a load on it? Sorry for the dumb questions but i dont pretend to know enough about carbs. Im going to tey to get my builder to come with me this spring but hes pretty busy between his work, 2 kids sports, and trying to drag race on the side.

On the base plate of the carb there should or may be some vacuum ports. Usually on the front. Need to use the one that full vacuum and not ported. If there aren't any you will have to plumb a connection in somewhere on the intake.

It's an Eagle but pretty much a GM clone recommending a 750. I'm not saying that it's a good recommendation, but it's their recommendation.

http://i622.photobucket.com/albums/t...CBF26EBF01.png
http://i622.photobucket.com/albums/t...085A19B72A.png
http://i622.photobucket.com/albums/t...6EDBB6EBC2.png

On my QF850M the vacuum ports are plugged off (no nipples) except for the big one on the back for the pcv valve. What's the work around on this for testing vacuum?

Use the port on the baseplate that the hose for the PCV is on. Temporarily unplug the PCV hose and put your vacuum gauge on that carb bung. You want to read manifold vacuum just under the throttle plates.

So what is the carb of choice? Quick fuel? My last motor had a prosystems holley and you would have sworn it was fuel injected the way it ran/ideled. But i know it had been sent back and had some $ invested in it.

In the little searching ive done the quick fuel marine version is the all black one. Any opinnons on this carb?

In the application I am referencing the Quick Fuel 1050 was set up correct out of the box from what I have been told. They were run the boat with a wideband monitor.

Bumping this thread.


Ordered the 850. QF. Recieved over the weekend. Looks like a nice piece. Installed and about ready to go.

I run an older holley mechanical fuel pump. Where does the vent hose from the pump suppossed to go?

For the life of me i dont recall where it was hooked up to if anything before the engine build.

Some setups go to the flame arrestor, some right into tue carb if the carb has the appropriate fitting

There is a nipple on the tall merc flame arrestors.