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another carb size question

3K views 38 replies 9 participants last post by  justinspot 
#1 ·
We bought a 89 chevy stepside with a 383 stroker with vortec heads and still finding out stuff about it as I go along . Here is my issue , at standing stop I can roast the tires no problem but when the tire finally catch the engine lacks a lot of power and suffers most of the way in a street race. When I first hit the gas the engine lags by either not doing anything for second for stutters a little before catching up. here are the engine details . I have a 383 stroker with vortec heads . not sure of head specs as it is one of the few pieces of paper work the original owner could not find. running a edelbrock 1407 750 cfm carb I believe stock everything inside , small tube headers , I believe 12 inch oval air cleaner ,Mallory h.e.i. , matched comp cam , lifters etc. specs for the cam are in the pic . I also live 2000ft to 2500ft elevation . 3.73 rearend . what size carb would be good? All the calculators Ive seen say 600 to 650 cfm . Is this a good number . It is mainly street driven but would like to do good at test n tune nights at the track . engine is suppose to do 420 to 430 hp although I have never had it tested . If you need more info , I will do my best to provide it.
 

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#6 ·
What pistons(so we can figure out compression ratio)??

What are your initial and total ignition timing figures??

Carb is actually a great size for the engine, may just need good tuning after we find out what ignition timing #'s you are using.

What is the actual header tube size?? and long tube, or shorty's/mid length??

What fuel pressure?? Edelbrocks are real sensitive here.
 
#7 ·
the carb size is perfect.try more static ignition,read your plugs for colour(clean burning) buy a strip kit or w/e Edelbrock calls their version of a mini carb tuning kit? Carter used to offer a kit with 3 or 4 choices of metering rods, jets and other goodies,cheap too. also play with fuel squirter stroke,usually the biggest one is what you use
 
#14 ·
http://www.edelbrock.com/automotive/misc/tech-center/install/1000/1407.pdf
http://www.edelbrock.com/automotive/mc/carburetors/pdf/carb_owners_manual.pdf
http://www.edelbrock.com/automotive...1000/Performer_Thunder_Carburetors_130815.pdf

Do this first:
Tee off at the carb inlet so you can run a small copper tube from the carb inlet, back to the firewall, up past the rear lip of the hood and up to the cowl of the truck. Mount a 0-15 psi liquid-filled analog gauge on the cowl so that you can read it through the windshield while you are driving. This is the only way you can be sure of the fuel pressure through all operating ranges of the motor. This can be a temporary mounting of the gauge, using tie wraps, duct tape or whatever you have on hand. The gauge only needs to be there long enough for you to monitor and control the fuel pressure from whatever it is now.

Many of you younger fellows have grown up with electronic fuel injection, where more fuel pressure is used to make more power. Carburetors don't work like that. The only thing more pressure will do is to push the inlet needle off its seat, allowing the pump to blow raw fuel into the intake manifold, creating a tuning nightmare for you. Carter AFB, Edelbrock and Rochester Quadrajet will work best with 4 3/4 lbs of fuel pressure at the carb inlet.

One last suggestion, if this were my truck, I'd lengthen the air filter stud and put another element on top of the one that's on there now, doubling the air filter area. Ya gotta let her breathe!!!!!
 
#15 ·
The CFM calculators are wrong and always have been, otherwise a 350 for example wouldn't pull 20-30 more horsepower with a 750 CFM carb over a 600-650 CFM. This is a consistent example you see it time after time on a dyno, no other changes just slip a 750 on and the power shows substantial gains every time.

A bog and studder is typical of either or both the mixture falling lean and or the secondary coming on too soon. The Edlebrock Carter you have is an air valve type similar to the Qjet where the secondary throttle shaft is mechanically opened. The air valve controls how much of a breath it can take. It is counterbalanced by a spring or counterweight depending upon exact model you need the Edlebrock tuning manual, these are online. This can also be an accelerating pump and metering either mains or secondaries. This just has to be worked out with tuning.

It can also be an ignition problem where there isn't enough advance, again this is a tuning problem that needs to be worked on.

The L31 heads are small port and valve size wise for a 383. They can be tricked into a 400 horse engine with a 350, but they are woking overtime to feed a 383. The intake is important also a Performer RPM of more is needed on a 383. This, however, will limit top power and RPM it will not give you the tuning problems you're seeing.

The upshift drops RPMs where it has to start over again to gain back into the power range. This is a difficult situation for both carb and ignition to track with it can be a lot of work to tease the best solution out, especially with only a three speed automatic because the RPM gaps are very big.

Bogie
 
#16 ·
Your timing specs are in the workable window, so that should be fine.. I just wanted to be sure you weren't getting way too much total advance.

Looks like it is time to dive into tuning the carb, as has been suggested. Heed the other posters previously given advice on fuel pressure - its very important.

Air cleaner could definitely be bigger, what you have there is scarcely big enough for a mild 305.

Header tubes could be bigger too, but that isn't what is causing your bog problem, they are just restricting power some.
 
#18 · (Edited)
. The guys have pretty well zero'd in on your problem... likely carb. secondaries opening while spinning tires at higher RPMs, but then when it hooks up, RPMs drop down too low for all 4 bbl.s to be open yet... tightening up the spring slightly on the secondaries may cure that... also, when racing, avoid spinning the tires/engine RPMs so high that the secondaries open too soon... your carb. size is about perfect, as the other guys said... those calculators are for stock engines with higher vacuum, for a mild performance engine we go about 10-15% bigger in air flow CFM...

. Also, at the same time, your torque converter stall RPMs may be too low for that getting biggish 230/236 cam (way down at 196/206 durations is about basic stock)... when the tires bite, they may be pulling the engine down out of it's powerband... creating some 'falling off the cam' bog... prolly need at least 2500 RPM stall converter for that cam size... you can powerbrake up and get an idea of your converter's stall RPM range if you have a tach... your rear end ratio and rear tire size also figures into this... you can put the rear axle up onto safety stands and rotate the driveshaft to find the rear end ratio... measure the tire diameter or give the size numbers off the side... also, vehicle weight is a factor...

. This engine sounds like it was built identical to the famous Hot Rod 6 part HT383 build up... they got 460+ HP and 500+ lb.-ft. torque with your combo... (and a slightly 'happy' dyno?)

GM HT383 Crate Small-Block - Engine Build - Hot Rod Magazine

. We call it a "mild" build because it isn't an all out finicky irritating race combo, or near a stock build, either, but a good, strong, still fairly tractable, fun budget street/strip build... (as opposed to strip/street build which is biased more toward the strip)... nobody meant to insult you or your vehicle...

. As also mentioned, since this is a truck and you prolly have room up top, double up the air filters stack... (also looks impressive, LOL!)

. You need tire size and rear axle ratio close to right to let the engine rev into it's powerband in all 3 gears by the end of the 1/4 mile for best performance... but with those Vortec heads, RPMs won't be real high with 383" to feed... also, you need to be shifting at RPMs above the HP peak for peak performance, say, at least 6,000+ RPMs... do you have a tach? This calculator provides a great spread sheet of theoretical RPMs in gears vs MPH...

http://www.F-Body.org/gears (hit 'clear right' button before doing anything)

. I've seen complaints before about this particular combo, especially concerning low RPMs torque with that big of a cam, Hot Rod posted some surprisingly impressive torque numbers for 2500 RPMs, but they used an expensive annular boosters carb. with better than average low RPMs metering to get those numbers that strong that low... and a lot of precise dyno tuning... and so the typical owner of this combo prolly isn't getting as much low RPM torque as they expected from the article...:

No power 383- built similar to HOTROD HT383 build Page1 - General Chevy Technical Discussion Forums at Chevy High Performance Magazine

.
 
#19 ·
at standing stop I can roast the tires no problem but when the tire finally catch the engine lacks a lot of power and suffers most of the way in a street race. When I first hit the gas the engine lags by either not doing anything for second for stutters a little before catching up.
Carb issues could be part of it, especially if you're going too rich or too lean when the secondaries open, but I'm betting on a miss-match between the cam and the torque converter
If the RPMs drop much below 2500, especially with a heavy vehicle, you're going to fall off the front end of the velocity curve and fall flat on your face until it catches up
You either need a smaller cam, or a looser converter
 
#20 ·
the stall is 2000 rpm . looking into a bigger tube header setup and bigger air cleaner . as far as fuel pressure I will have to get back to you cause its soggy and wet out side . I do have a setup that has a gauge between the tank and carb . What size air cleaner would you recommend ? Also is there a better head I can swap on or do I need to tear down and find out piston type etc.
 
#22 ·
Also is there a better head I can swap on or do I need to tear down and find out piston type etc.
I wouldn't worry too much about swapping any major parts yet ...
Focus on the simple stuff first: Fuel pressure, float level (I don't know about the Edelbrock carbs, but Holleys are very sensitive to changes in float level), tuning, and stall speed (I suspect you need something closer to 2500)
Until you get those right, none of the other stuff is going to do you much good; and once it's running right, you might decide it's good enough as is
 
#21 · (Edited)
. Stall RPM sounds too low for that cam... is "2000" the converter rating... or did you determine that by testing?

. As mentioned above, you can just stack a 2nd element on for doubled air cleaner airflow... also, if the aircleaner lid is currently too close to the top of the carb., a 2nd element would cure that, as well...

. Bigger primary tube headers would hurt low RPM torque even more... are you currently at 1 1/2", 1 5/8", or 1 3/4" ? ... 1 1/2" could get restrictive at higher RPMs on a 383"... 1 5/8" I wouldn't bother changing for bigger...

. Might be able to see piston top through sparkplug hole... maybe a shop in your area has the video capability to do that...

. Before doing anything else, I'd focus on carb. tuning and the torque converter... obviously your engine combo can already make serious power... did you check out the links above?

. Rear end ratio? Type vehicle/weight(I assume it's standard cab, short bed, one of the lighter full size pickups)? Tire size? Shift RPMs?
 
#25 ·
. 2000 RPM stall rating may be for a 350" engine, a milder 383" engine may be able to slip it up higher RPMs... however, with your bigger 230/236 cam, your 383" engine may not have any more torque than a milder 350 way down at 2000 RPMs... although a 2500 stall would help, if going to the trouble of changing the converter, might consider a 3000 stall... since you have a usable 3.73:1 rear ratio for a 3000 stall... and your 230/236 cam doesn't really pull really hard till above 3000 RPMs...

. What is the outside tire diameter?

. When you get it running right, have to decide if the 1 1/2" headers are holding you back too much... since trucks are heavier than cars, smaller tubes for more low RPM torque to get the truck moving are often preferred, but depends on what it's mostly used for... also, 1 1/2" may be a good match for the exhaust port size/area of stock Vortec heads, I don't know...
 
#27 · (Edited)
. Plugging 285-50R20 into the middle link I gave you above gives 31 1/4" tall tires... that reduces the effectiveness of your 3.73 rear by quite a bit as far as acceleration goes... might want to slip some 22" - 24" tall rear tires on for dragging... that would reduce your current top speeds in gears down from current 64, 112, 162 MPH to more like 45-49, 79-86, 114-124... give more acceleration in each gear and allow the engine to wind all the way out in 3rd gear by the end of a 1/4 mile... right now your engine is prolly lugging pretty good trying to accelerate in 3rd gear... well, actually, you're not even going to reach 3rd gear in the 1/4 with 31" tires... just racing in 1st and 2nd gears... you'd need a 5.13:1 rear end gear to run good with those 31" tall tires...

. Maybe a spare pair of wheels with 275-50R15 (25 1/2") tires or 345-35R15 (24 1/2") tires... or even some shorter ones...

. Of course, if shorter tires put the rear of the truck much lower than the front, you may encounter a problem of air trapping under the truck much above 75 MPH and the truck wanting to dangerously fly up into the air...
 
#31 ·
. Plugging 285-50R20 into the middle link I gave you above gives 31 1/4" tall tires... that reduces the effectiveness of your 3.73 rear by quite a bit as far as acceleration goes
You'll get a little higher flash speed off the converter with the lower gears and more torque multiplication in second gear (not to mention better traction), so you're really not losing much more than a couple tenths peaking in second gear.
If we're talking high-12s at 112, the 3.73s sound about right; and you still have another gear for cruising
 
#30 ·
First,short tires are not good for drag racing. If you try short tires and you like the feel of the gearing,then choose the correct gears for the cars application.
On a low horse power engine,small tube headers are fine,
The 2200 RPM stall is not a race converter and will not launch like a race converter, but its good for a street car. Not really worth changing out for a slightly looser converter...
 
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