Should I trade carbs??
- Thread starter y0ukantride
- Start date
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Short answer: that 750 is much too large
Long answer:
heres a pretty simple VE calculator.
Carburetor Size Calculator
always irks me to see people throwing 750s on 302s "because its a v8" its all about volumetric efficiency
rule of thumb being, most stock engines are running at 75-80% VE (its a guess at best), modified engines 85-90% and obviously the more air/fuel mods, tuning etc will increase the VE of the motor,boosted cars/full blown race cars tend to be at or above 100% VE, when it comes to carbs, bigger isnt always better.
figuring your exact VE requires a bit of math but the basic equation is VE= 3456xCFM/CIDxRPM
but you can probably just guestimate yours at 85% (that might be on the high side)
this may be more info than you were looking for, but its late and i felt like rambling.
if you're feeling pretty wild, give this one a shot
:
Volumetric Efficiency:* Calculating your cars volumetric efficiency
Long answer:
heres a pretty simple VE calculator.
Carburetor Size Calculator
always irks me to see people throwing 750s on 302s "because its a v8" its all about volumetric efficiency
rule of thumb being, most stock engines are running at 75-80% VE (its a guess at best), modified engines 85-90% and obviously the more air/fuel mods, tuning etc will increase the VE of the motor,boosted cars/full blown race cars tend to be at or above 100% VE, when it comes to carbs, bigger isnt always better.
figuring your exact VE requires a bit of math but the basic equation is VE= 3456xCFM/CIDxRPM
but you can probably just guestimate yours at 85% (that might be on the high side)
this may be more info than you were looking for, but its late and i felt like rambling.
if you're feeling pretty wild, give this one a shot
:Volumetric Efficiency:* Calculating your cars volumetric efficiency
heres a pretty simple VE calculator.
Carburetor Size Calculator
always irks me to see people throwing 750s on 302s "because its a v8" its all about volumetric efficiency
rule of thumb being, most stock engines are running at 75-80% VE, modified engines 85-90% and obviously the more air/fuel mods, tuning etc will increase the VE of the motor,boosted cars/full blown race cars tend to be at or above 100% VE, when it comes to carbs, bigger isnt always better.
figuring your exact VE requires a bit of math but the basic equation is VE= 3456xCFM/CIDxRPM
but you can probably just guestimate yours at 85% give or take.
this may be more info than you were looking for, but its late and i felt like rambling
if you're feeling pretty wild, give this one a shot
Volumetric Efficiency:* Calculating your cars volumetric efficiency
Was just looking for this. Have a page like this bookmarked on my junk yard computer but not here at home.
joel5.0
New Member
The symptom you are having with that carburetor is not due to its size. And no, a 750 CFM is not too large for a 302 CI engine, nor is it for a 79/159/238 CI (choose your way to determine CID) wankel.
Nor is a 1000HP Holley based professionally designed custom carburetor too large for a street 297 CI setup (289 .060" over).
IOW... CFM carburetor ratings based on engine size and static VE, is not the correct way to determine carburetor size all the time. Or did '65-'67 GT-350 289's have a 100% VE @7000 RPM stock? They were fitted with a 715 CFM Holley by Mr. Carrol Shelby.
joel5.0
New Member
I guess the classic diagnostic/ID cause/correct/tune/calibrate problem solving procedure is one that should be avoided like the plague then....
....BTW, is the symptom really carburetor related? What about the spark advance curve? When does full spark advance occur? Is a distributor with a vacuum advance canister used?.... 100% centrifugal?...... has the locked spark advance alternative been tested/checked?
I beleive its a carb problem ... car seems to run good bogs a lil on low end seems like it loads up .. really it just needs to be tinkered with a need to adjust my timing i need to get a timing pointer lol so its pretty low i just did it by ear and running it ...same way on the carb... i just see alot of people with 600s with basically the same setup... buddy also said it seemed riched from the black exhaust when i take off hard.. ill see if i can make a lil cam of a drive around... car seems to rev good and everything in park but under a load seems a lil sluggish until around 3 grand
joel5.0
New Member
More proof it really isn't a "too big of a carb" situation/problem. There are some basic calibration/tuning chores you need to do in order to fix it...... chores that are needed regardless of the carburetor size used. Is the vacuum advance canister working? What is the initial spark advance? What is the total spark advance and at what RPM is it reached?
Was carburetor idle mixture calibrated reading manifold vacuum? Main jets tuning/replacement reading spark plugs? Was engine vacuum checked under WOT conditions?.. etc........ chores required regardless of the carburetor size you have installed.
If a 700 CFM Holley DP is "too big" for a 302 setup, why did I have to increase the jet sizes in my junk from 69/78 OEM to 72/82 so the spark plugs could get a little tan? It idles at 750 RPM, 11"-12" Hg. of vacuum and it clears/revs/runs fine = Hear it here
lol ok ill go get some carb vacuum gauges and see what i can do.. What kind of numbers should i see from the canister on the distrb? How do i find out spark advance curve? plugs look like they are running lean very light tan almost white ill get a pic of it.... i havent had a chance to do a lil ride along bc of shyt weather.
