how to calculate blown engines carburation?
 

how to calculate blown engines carburation?

thnx

ps: no current engine for the moment.know the formula for non-charged.what is it for charged?

Thats easy BIG. Check with Dean at www.nickersonperformance.com he is THE man when it comes to blower carbs.

thnx jdnca1;
cool site,nice products..
but i am looking for a calculation formula..if there is...
thnx

Are you looking for carburation set as jet sizes etc or fiqures on how to calculate what cfm you should run?

how to calculate what cfm I should run?

CFM or what size of carburetor is what I mean. If you are looking at how to set them up jet sizes etc there are many variables that need to be considered. If your guessing I would leave it to the experts. ;)

This is out of an old BDS Blower book I have. What carbs do I need to run and why 2 of them?
The size of carb(s) or CFM required for a given application can be calculated by the following formula:

[(CID x RPM) divided by 3456] x [(Boost divided by 14.7) + 1] = CFM required.

The amount of CFM required will determine carburetor size and quantity. If you try and use a carburetor with less CFM than required, performance and economy will be greatly reduced.

557CI
5600rpm
6psi
1335CFM?

Back4more I question everything to. This formula is out of a BDS Blower book I have. I run a 525ci with the same numbers you have. 2-850's on mine. :rolleyes:

ok i know the first part; volumetric efficiency ia taken at %100 in this formula...
according to this back4more should have 1250-1350 ...he has 1335 ..its ok.

paradigm : it looks like you are over-carburated.. :)

why i am asking these questions : reason is :
i saw a 502 cuin blown engine 8 boost with twin 1050 dominators on top of it.its a marrine engine.rpm 6000.
?!?

any comments on this?

twin 1050's is way too much carb. or they could have them jetted very small ?!Seems like a waste.

My carb/motor seems to fit the calculations pretty close.

(461 X 5200)/3456 = 693.63425
6/14.7 +1 = 1.4081632

693.63425 X 1.4081632 = 976.75022

I run a single 950 jetted with 90's.

I had used a different formula a while ago .............. don't qite remember what it was but it came up quite a bit smaller.

paradigm : it looks like you are over-carburated.. :)

Yea I know. Had this same set up on a 540ci motor in a flat bottom hull. Turned a few more rpm with that set up to. Did not want to spend the extra $$$ for new carbs. Seems to work well for me. Still has enough to rip a bravo apart. :)

These formulas DO NOT WORK, they do not take anything else into consideration such as intercooler, or not, type of cylinder heads, cam profile boat weight, drive ratios etc. You need to speak to a carburetor builder to determine what carburetors they build will work on the application. Every carburetor builder designs their product differently, and may have a different size, or caibration recommendation for the same application.

if you are going to put blower motors in you 28 skater i want to take out a life insurance policy on you. :D

:D :D :D :D

:cool: info revisited

I'm a former carburetor builder and I can say that you are right but only to certain extent. The formulas used in this thread are correct and should be used for a rough starting point. All of the factors you mentioned are considered in this formula, however for optimum performance very fine-tuning is needed. First off the normally aspirated (NA) formula given is based on 100% VE. Now no (NA) engine is going to perform at 100% VE and that is where the combined skills of a good carb and engine builder come into play. Without getting into the long boring specifics such as flow bench and BSFC numbers, perfecting carburetors is and soon to be was a science. You can spend thousands not including the price for the carb to get one just right, but let there be no mistake. Fuel injection is far superior and in short time will dominate offshore powerboats of the future. The marine FI systems at this time may still have their bugs but will greatly improve over time.

Roby

There is another way to think about this. The engine/blower is going to take in as much air as its cylinder heads, blower speed, intercooling and piping etc. will allow. This will not change to a great degree with different carb sizes. This was proven in an article about Demon carbs with interchangeable venturi sizes.

What can change a lot is how much fuel you need to add to the air, and how the carb "knows" when and how much to add. This is where you need the carb experts to help, especially on blowthrough carbs.

The important thing to know on any blown engine is the final density ratio of the air entering the cylinders vs. atmospheric. The BDS formula takes the boost ratio into account, but you also need to know the temperature ratio. Its not too difficult to measure these. It still just gives you a starting point.

now i need to find out jet sizes.

engine:520 cu in
carbs: 2 x 800 cfm
rpm: 6000
b&m 8-71 8 pounds of boost

what size of jets i should use?