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383/dyno update....fast68plymouth

Moparts Member
Posts: 2692
From: so burlington,vt usa
Registered: Aug 2000
posted 06-19-2002 10:20 AM
when all of the testing is done, i will do a series of posts relating to the various "stages" of testing, but i thought a brief update of last nights results might be interesting.....
i had installed the inner springs the night before, but hadnt made any pulls yet.
although some had indicated they thought an increase in power would come from this, i wasnt expecting one.
ive been doing this stuff for several years now, and have a pretty good idea of what surge and float sound like, act like...and look like on paper.
the peak power was occuring before any of the signs showed up, so i didnt think any more power would come from the extra spring pressure...and it didnt.
the weather was a bit different last night than the night before, so the dyno's correction factors would be put to the test.
with the inner springs installed, i re-baselined the motor before i made any other changes.
in a nut shell....im still looking for 25 more hp.
when all the tests are done 440/451 Jim will be helping with graphing some of the results, but here are last nights peak numbers....
stock Holley 700DP carb-442tq/423hp
Proform 750HP carb-439tq/418hp
stock 68 383 AVS carb-438tq/419hp

i played with the jetting on both the Proform and the AVS carb, and this was as good as they got.
i had tested the Proform carb against this same 700DP on a dirt modified motor last sunday and it was clearly better by about 5-6hp across the board.
just goes to show, this stuff doesnt always work on everything.
on this motor, with the stock intake anyway, the 700DP was better.

Zippy, i probably will try an open, and 4 hole spacer with the better working carbs(which ever those end up being) when all the other tests have been done.
once i know which of my own carbs works best, i'll really try to tweek the best numbers i can out of it.
some of the carbs arent mine, so they wont be getting alot of time on them.
as long as the A/F ratios are in the mid-12's to high-13's, they will be left as-is.
however...if one of them looks like it has a clear advantage over the others, i may explore the possibilites that carb has to offer.
i will tell you this though....last night felt very much like when BradH was up here testing....nothing seemed to make much difference.
well, thats not quite true....the AVS was very rich when i first put it on, and was down about 12-14hp from where it ended up.

John...i bought it as a remanufactered carb from Holley....and then i re-rebuilt it
while i had it apart i richened up the idle circuit, and swapped it over for use with the AFB 2-step metering rods...and since i expected it to be lean...i jetted it up.
i got the carb about 6 years ago, so i dont have a clue as to what it had for jets in it when i got it....but i ended up leaning it out by 2 sizes front and rear, and used the next larger set of rods in the front as well.
while i was in there, i opened up the IFR some more since the idle was still very weak, and lean...and the throttle response was...well, using the word "response" doesnt seem to fit what happened when you opened the throttle quickly.
its not too bad now....i tightened the rear door spring until the bog went away....which took about 5-6 turns.
it behaves pretty well right now, with just a slight tip-in hesitation. the idle is still a little lean, so when i cure that, im sure the tip in problem will go away.

383 dyno update #2.....

well.....a few points were made clear last night.
1st- if you have headers and a cam, an aftermarket intake manifold is going to be about the best hp/dollar you can spend.
i dont want to take away the impact of the final results, but lets just say that one part did alot more than i was hoping for.
2nd-MP stuff still just sucks!! the only MP parts on the whole motor are the valve covers, and the distributor.
the distributor came with a rotor...which of course decided to fail at about 5500rpm at 630 last night.
poof!! POP!! BANG!!....we're taking an unscheduled break while i try to hunt one down.
the only other Mopar motor at the dyno shop has the tach drive distributor in it, which uses a different rotor.
i had my 93 Dakota with me, but its different too. soooooo, back to my shop to find one. all in all....about an hour and 15mins waisted because of a crappy $2.00 part.

3rd-i had a Demon carb work pretty darn well, right out of the box!! thats a first for me. it was the Speed Demon 850DP, which i would normally have selected for use on a 500+hp motor.
it was marginaly lean on this motor, so on a 500+hp 440 it would be even more lean...but on the 383 it was "okay". i wouldnt run it this lean in the car, but a couple of jet sizes would be all it needed.
this would be the least amount of work i think ive ever done to get any Demon carb working this well.
1st time for everything i guess.

i tested all the Holley and Demon carbs last night, and all but one are pretty close in power...and the one is down considerably.
today i'll be testing the 2 Carters and the Ebrock carb...and then move onto the water pump test.
last thing will be the header swap.
i was hoping to have another day, but a paying job for the dyno came up, so i need to wrap it up today.

Jim, this is a brand new carb that i just took out of the box and bolted on.
i didnt remove the bowls, but i have to assume it has the 85/93's in it.
what i have found is that the Demons often make more power when they are (IMO) dangerously lean(high 14's, low-mid 15's).
if its a motor i built for a customer, and im tuning it...its going to leave here with the A/F ratio in the mid-13's, with maybe a few lines where it touches the low 14's.
i want the motors to be jetted "safe" when they leave here. what the customer does with it after they get it in the car is up to them.
the 440ci motor may provide enough more signal at the carb that it wouldnt be as lean as it is on this motor for a similar hp output. the downleg 850 with its 1.562 venturi are really too big for a 390ci motor making the kind of power this one's making.
i dont think getting this carb to leave hard in a heavy foot brake car with this motor would be that easy to do....but ya never know.

383 dyno update #3....

well, i was hoping to have been done yesterday, but i had a few people stop down to the shop for a while, which took some time away from the testing.
ive made 88 pulls so far ...and still havent changed the timing from 36deg total, or tried any carb spacers.
those things, and a few other minor tests will happen this morning, then i'll be pulling it off.
it looks like it will have a little over 100 pulls on it when im done.
what ive tested so far are, stock intake vs RPM intake, all the carbs i had planned on testing, the water pump drive test(big pulleys vs underdrive pulley vs electric drive), 1 3/4 headers with 3 and 3 1/2 collector extensions, and a set of 1 7/8 headers with 3" collector extensions.
i dont have a good way of using the 3 1/2" extensions on the new headers, so thats not going to get tested.
my impressions so far are.....its pretty damn good!!
all the aftermarket carbs tested ran fine with the stock calibration except one...the Edelbrock 750, #1407. this one ran very poorly.....it didnt really do anything well. it was just flat out lean everywhere. idle, part throttle cruise, WOT...and it didnt like to take a load at all. when getting ready to start a pull and going to full power, the motor would fall right on its face, then try to recover....finally recover, and then you could start the pull.
i found this to be pretty interesting because i also had on hand a Carter AFB 750 manual choke version which ran perfectly fine everywhere.
decent throttle response, good part throttle cruise A/F ratios, decent idle A/F ratio with the mixture screws out only 2 turns, and the WOT A/F ratio was right where it should be, and it had a pretty flat fuel curve as well.
i was running short on time, so i just continued on my testing of other things. when i was done for the night i pulled apart the Ebrock carb to see if someone had recalibrated it. this is not a new carb, but was pulled from my friends 396 Chevy yesterday morning. he's been running on the street, and didnt really indicate there was any kinds of problems with it, but there was an indication that another friend of his may have changed some jets or something.
well....that was not the case. it had all the stock jets, rods, and springs, and had no dirt or debris inside. the calibration was just that far off from ALL the other aftermarket carbs.
IMO, the Ebrock carbs are set up for motors that are much milder than what most people are building. well, most people i know anyway
anyway....back to the trenches.....

451 Jim
Moparts Member
Posts: 1440
From: Lexington Park Maryland
Registered: Dec 2000
posted 06-21-2002 09:14 AM

We are all waiting for the data.
I better watch out what I ask for, 100 pages of dyno data is a lot to analysis! I assume some are just duplicates and checks, but I wouldn't be surprised to see 30 "final" runs.

When Dwayne sends me the SuperFlow output files, I am going to put the full dyno output data into text files and graphs, including selected comparison tables and graphs. So, these can be made available on the internet.


Moparts Member
Posts: 2692
From: so burlington,vt usa
Registered: Aug 2000
posted 06-21-2002 09:20 AM

Jim, youre correct. most of the pulls are just warm up runs after a carb swap, or pulley change, etc, to get all the temps stabilized.
as of right now there are 31 "saved" runs, and even a few of those are back-up runs.


Moparts Member
Posts: 2692
From: so burlington,vt usa
Registered: Aug 2000
posted 06-21-2002 11:26 PM

Max, i used the TRW L2315's. the spec for a 68 motor is something like +.020 deck.
that piston doesnt even come close to being positive deck height....until you deck the block...a lot.
of course the heads needed to be milled about .090-.100 to get down to the legal chamber volumes....and then when you stuff your very long duration cheater cam in and check the V/P clearance you find its -.010.
thats when the project came to a complete halt.
i never felt the motor was that good of a combo anyway, and then to realize you have to run a tiny cam, or lose a bunch of compression...well, it just took the wind out of my sails for that project.
so, its been sitting for a few years now, and i decided to do something with it.
i flycut the pistons, swapped out the cam for a nice solid...and upgraded the valve springs to go with the new cam.
then slapped it back together, and now its on the dyno burning gas.
well, thats not quite true....i took it off the dyno today.
now its just a matter of organizing the data.

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