good carb book

[lostforawhile]

I picked up a Haynes Techbook on clearance the other day,it's the Rochester carburetor manual book number 10230
even if you don't want to do a Rochester conversion,there is a huge amount of useful carb info and theory in this book. I'm probably going to be modifying my extra intake to fit one of these carbs,it seems to be the closest match i can find to the factory carb,without all the crap attached to it. heres a really useful formula for determining the cfm of your engine
I am using ~ for divide since my keyboard does not have a divide key
VE stands for volumetric efficiency which I would say is about 85 percent due to this engine having two intake valves



cubic inches~2) x maxium rpm~1728)xVE%=cfm

cubic inches =119~2=59.5
maxium rpm =6500 rpm ~1728=3.761
59.5x3.761=223.78x85%=190.21 cfm

[lostforawhile]

has anyone figured out the actual cfm of our factory carb yet?
trying to figure out something wierd the GM iron duke four cyl engines came with a rochester varajet but the smallest varajet is 375 cfm,yet those motors were only 2.5 litres. something doesn't add up,I know the formula for our engines cfm is correct have checked it several times, but it seems like 375 cfm is awfully big for a 2.5 litre. they made a mono jet single barrel thats really close to our cars,but single barrels are really outdated. i wanted to go with the varajet because it functions really close to the factory accord carb without all the electronic crap. does anyone know the cfm of the webber carbs that work on our cars?

[A20A1]

Domestic makers also may inflate the CFM values some to help sell the carbs.

I think ours is a bit lower then the 32/36 weber aprox. 325 cfm.

We have a bigger throttle bore on the secondary then the 32/36 and smaller or near match on the primary in the stock carb compared to the 32/36. However we have a much tighter area in the venturies then the 32/36, so I'd say in the 250 - 280 cfm range.

I think its more important what the primay flows... after that the secondaries can offer much more cfm and jetted right should give the proper air/fuel mix as long as there is enough vaccum created in the venturies to draw out fuel.

I just saw someone on here that said they used a 38 weber... and that isn't even progressive. Not sure how they liked it but it must be working. I wrote something down somewhere about swapping booster venturis from one series of carb into the 38 or a DCNF... I'll need to look for it.


the 38 weber is aprox. 375 cfm

Also plenum design can offer a boost in the vaccum signal... divided plenum is about 50 extra cfm


EDIT:

the 32/36 and 38 may not be that far apart in CFM, some site even list them as the same: 300 CFM


I wrote in another thread about tiny throttle ports. They seem to hurt low end either in response or tq. In any case that makes sense.

Add to that what I recently discoverd about the 38 and the 32/36 being pretty close in CFM but the difference is how they respond in the low end.

Now the 38 kind of takes the theory for a spin cause it's a synchronous rather than a progressive. But what you're looking at is how much air flow you allow at what RPM, and that tiny primary in any carb not only a 32/36 is there for one reason and that is for economy... Funny thing is they tell you the carbs that have massive secondaries give good milage as long as you don't open the secondaries. :D

I still stand with the idea of engine load and having a vaccum secondary helping maintain vaccum to keep the carb responsive and only opening when the demand is there.

But with the updated knowledge I would say stick with carbs where the barrels are more the same size, but progressive and vaccum secondary.

[lostforawhile]

I'm going to try to do the rochester conversion simply because it's easy to get parts for,pls there are lots of parts for modifying it out there. the varajet is set up like our carbs with the small primary and big secondary. it was developed to work on gms small front wheel drive four cylinder motors. probably the closest thing out there to a stock carb thats not electronic, sounds like a close match. the webber is too hard to get parts for should you break down,rochester parts are available at most autoparts stores.