Engine sluggish under 40mph

Engine seems a little sluggish between 0 and 40 mph. However, after 40 mph it seems to run like the wind with strong power and torque. I need to know if I’m missing something here. This is the engine setup:

1. Engine: 351 Cleveland (H Code)

2. Transmission: C6

3. Holley 600 CFM, 4V, 4160

4. Edelbrock intake

5. Petronix I

6. Engine is probably original and has approx. 160K miles on it.

Starts Well and engine doesn’t miss throughout the whole power range. The idle is good and steady too. Engine seems to stutter as it gets going but once up to 40 mph it’s gone. This maybe the way the design is for the specs that are listed. If so I can accept that until the Engine needs to be rebuilt. Just checking in on the forum for advice?

Engine seems a little sluggish between 0 and 40 mph. However, after 40 mph it seems to run like the wind with strong power and torque. I need to know if I’m missing something here. This is the engine setup:

1. Engine: 351 Cleveland (H Code)

2. Transmission: C6

3. Holley 600 CFM, 4V, 4160

4. Edelbrock intake

5. Petronix I

6. Engine is probably original and has approx. 160K miles on it.

Starts Well and engine doesn’t miss throughout the whole power range. The idle is good and steady too. Engine seems to stutter as it gets going but once up to 40 mph it’s gone. This maybe the way the design is for the specs that are listed. If so I can accept that until the Engine needs to be rebuilt. Just checking in on the forum for advice?

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What rear gear do you have?

What are your initial and total timing readings?

Have you done a compression test?

Is the rest of the engine stock?

Are you sure the transmission is starting in first gear?

Making sure the trans is starting out in first is one that I would have overlooked, but is a very real occurrence. My dad had a friend (Ed) with a T-bucket, 302/C4 combo that ran "OK" but not stellar. Ed asked Dad to "tune it up"... shifter was simply misadjusted and starting out in second.

Ed never dreamed the car would stripe asphalt like it did after the adjustment!

Axle Ratio Code

2 2.75:1 Standard Differential

8 degrees BTD at 600rpm, no Vac connected

Plugs looked nice and grey tinted with no signs of improper wear on engine. It also runs fast at the top. So I assumed the compression was good.

The rest of the engine is stock.

Engine starts in first gear I can feel the others as it goes up. I will check again on the first gear by using the selector. Great question. What would the cause be if it wasn't starting in 1st?

2:75 are dog highway gears:s

Set initial at 12 make sure total doesn't exceed 38 when you do this. Hook up the vacuum advance. I bet this is the problem.

If the trans weren't starting in first gear it could be mis-adjusted shifter as mentioned or fubar'd tranny.

ah a fairly original H code.

first clevelands tend to be dogs at lower rpms, that is what made the 302 more of a peppy motor.

sound like you have a good setup.

with 2.75:1 gears, you will not be lighting the rear wheels up. you should be able to churp the rear tires a bit when you stomp on it, but unless your on lose surface you are not going to do a burn out.

now that said you can check a few things:

Make sure the vacuum advance is hooked up and working.

make sure the kickdown for the transmission is hooked up and working. the kickdown forces the transmission to drop a gear when you stomp on it. if should kick down the trans when you floor it you should feel it.

you can also Tweak the shift points on the transmission with the transmission modulator.( get back to this in a second)

make sure your mechanical advance is working as well.

you can also mess with advancing the timing for more power you have 8 initial now you could go to 10 degs and see how it reacts.

you could raise rpms a little, 625-650

you could replace the factory heavy springs on the mechanical advance with mr gasket lighter ones so the mechanical advance comes in faster, giving you more timing and more power in the lower rpms range.

there are a ton of little things you can do without getting into replacing intakes, and exhaust manifolds.

First drive the car, have somebody time 0-60 LEGALLY on the highway or something so you have a baseline.

next make sure you feel the car go through all 3 gears as you accelerate. 1st goes quick.

now if you punch it and she shifts into 2nd and you feel like its not holding 2nd long enough or its holding 2nd way too long before it drops in 3rd, you can adjust the transmission modulator with a small flat head screw driver on the C-6, its not easy to get to since the transmission cross bar blocks it, but you pull off the rubber vaccum hose from the modulator and then turn the little screw inside in 1-2 turns to shift 2nd-3rd faster or turn it out to hold gear longer.

basically its a seat of your pants feel.

if your looking to turn a H code into a 9 second 1/4 mile 0-60 rocket that is another story.

these cars were maybe 15-17 in the 1/4 lets say 8-12 second 0-60, as a rough guess.

you want to accelerate faster then your talking rear gear swaps and intake/carb/exhaust swaps.

the H code was the basic 351 car 2.75:1 is great on the highway. i had them in my car. they don't make 2.75:1 gears anymore so when i replaced them i put in a set of 3.00:1 gears. there isn't much of a difference but on the highway i notice its a little more noisy then it used to be.

1 turn on the transmission modulator might be just what you need to get a little more go 0-40mph, or you have to start messing with ignition timing, or distributor curve for the mechanical and vacuum advance.

i would get a stop watch and an assistant and just see what your starting with. once you start screwing around then your talking about one change in tune affecting another.

back to cleveland engines they were really good at higher rpms over 1500. that is what made the 460 so popular at tracks since they would run great in the high rpm range and maintain it easily, the clevelands love gasoline they want you to just pour the stuff down the intake. the low to mid range throttle response sucked without some tuning.

another thing to consider is a stock cleveland H code, would of had a much smaller 2 bbl carb on it maybe needing 500cfm max, when you put a larger carb on a stock cammed motor you can lose throttle signal at lower rpms because the vacuum will drop off faster then the engine wants, because the vacuum signal to the carb (call for fuel) is reduced.(larger air intake area = lower engine vacuum) and it can make a car sluggish since it goes lean at mid range. you could toss the stock carb back on if your intake is original and you may actually see a low end improvement in performace, all these 600,650,700,750 carbs are really designed for higher rpm tuning. when you want the motor to go to 6500 rpms then you need that richness on the high end.

i bet you got a 3,000 rpm torque converter<~~mine did..stock from ford...ford had to put them in 4v automatic version...cause the heads are simply too much for normal street use and the car would fall on its ass at every stop light with out the converter...die and have trouble runing till it hit the right rpm too enjoy all that fuel....i took out my motor last year too freshin it up..and what do i find? little green converter instead of a big one...and thats how i come to find out..ford had to stick those in the 4v cleveland automatic versions...alot of folks dont know that about 4v converters...same with the torino or ranchero 4v cleveland automatics...they all got the little converter too....launch that baby off the start at 2500 rpm's and hold on..cause once it hits 3,000...its all power..game on...good thing about the auto versions...allmost built for drag racing with the convert..9inch and big strong c6..can run good constant times without worry of busting much...love how ford over builds

I have the Holley 600cfm 80457 and had problems with a Ford AOD transmission. Basically, there is not enough vacuum at low RPMs. I read somewhere that this Carb in not for an automatic (at least the AOD).

Statement: make sure the kickdown for the transmission is hooked up and working. the kickdown forces the transmission to drop a gear when you stomp on it. if should kick down the trans when you floor it you should feel it.

Question: The kick down is hooked up and does work. However I really have to stomp on it to kick it in. I know where the screw is for adjustment I just need to know which way to go with it. As I remember looking at the screw it seems it may have to been turned to the left to increase the height of the screw so it contacts the arm sooner and kicks sooner. Not sure if this is correct or if this thing gets senisitive on just moving it a half turn. Need advise here?


Statement: now if you punch it and she shifts into 2nd and you feel like its not holding 2nd long enough or its holding 2nd way too long before it drops in 3rd, you can adjust the transmission modulator with a small flat head screw driver on the C-6, its not easy to get to since the transmission cross bar blocks it, but you pull off the rubber vaccum hose from the modulator and then turn the little screw inside in 1-2 turns to shift 2nd-3rd faster or turn it out to hold gear longer.

basically its a seat of your pants feel.

Question: All transmission gears seem to functioning and shifting correctly. I just get the feeling as I motor down the road at a moderate speed it sure gets up in the 3rd gear in a hurry. I do have to admit it goes from 2nd to 3rd very smoothly and you can't even feel it shift. This might be a great thing overall. Therefore based on this assumption is there a need for adjustment or its just an H Code engine doing what it suppose to do?

I found where I read about the 600cfm Holley... The issue I had was at low RPM it was very sluggish, but at higher RPM ran well. Problem was that eventually, it AOD transmission was damaged.

P/N 0-80457S (electric choke)

INSTALLATION, TUNING, AND ADJUSTMENT MANUAL

199R-7948-5

WARNING: This carburetor is not designed for use with any Ford automatic overdrive transmission.

SEVERE transmission damage may result from improper application use.

I found where I read about the 600cfm Holley... The issue I had was at low RPM it was very sluggish, but at higher RPM ran well. Problem was that eventually, it AOD transmission was damaged.

P/N 0-80457S (electric choke)

INSTALLATION, TUNING, AND ADJUSTMENT MANUAL

199R-7948-5

WARNING: This carburetor is not designed for use with any Ford automatic overdrive transmission.

SEVERE transmission damage may result from improper application use.

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That is a warning specific to use with an AOD transmission because it has no provision for hooking up the TV cable. As you apparently found out, failure to hook up the TV cable or operating the vehicle with an improperly adjusted TV cable will result in transmission damage. For the OP application the carb should be fine.

yes just note the AOD is the (Automatic OverDrive)

if your running an original 71-73 transmission there was no overdrive gear, the original C6,fmx,c4 were 3 speed with no overdrive gear.

The kick down is important on the 3 speeds but as far as i know not having one hooked up will not ruin a 3 speed transmission but it will make the car very sluggish to accelerate and for passing.

try a 2500 rpm 1st gear launch....could be carb..but like im saying..my car acted the same way..mine is a 1973 4v cleveland with c6...shifts fine......and doesnt really start to rpm out till after 40mph normaly...like 50 or 60..it started to really pull...after i figured out it had a high rpm converter..well yes...of cource it felt sluggish at low rpm..cause it wont let it have all its torque and power will 3,000 rpm's..you have to keep that motor rapped out when racing...it was designed for rpms..big heads..small crank..4 bolt main.

Many 1972 -1974 351C-4V CJ engines were mated to Ford's C6 automatic transmission. Ford used a special high-stall (3000 RPM) torque converter (D20Z-7902-A) and flywheel (D1ZZ-6375-C). <~~from ebay motors http://reviews.ebay.com/Ford-351-Cleveland-Engines-Key-Facts-amp-Information_W0QQugidZ10000000001410755

Question: The kick down is hooked up and does work. However I really have to stomp on it to kick it in. I know where the screw is for adjustment I just need to know which way to go with it. As I remember looking at the screw it seems it may have to been turned to the left to increase the height of the screw so it contacts the arm sooner and kicks sooner. Not sure if this is correct or if this thing gets senisitive on just moving it a half turn. Need advise here?

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You can easily adjust when you want the kick down to come on. if you back the screw out (counter clockwise) it will engage the kick down before pushing the pedal to Wide open throttle. if you move the throttle by hand you can see the plate come back and make contact with the screw and at the point at which the kickdown starts to engage, you can try backing off the kickdown screw and have it come on early so as you punch it the kick down comes on at say 97% throttle and you stomp the extra 3% into the pedal.

its all tested by your butt sitting in the car and seeing how it reacts.



Question: All transmission gears seem to functioning and shifting correctly. I just get the feeling as I motor down the road at a moderate speed it sure gets up in the 3rd gear in a hurry. I do have to admit it goes from 2nd to 3rd very smoothly and you can't even feel it shift. This might be a great thing overall. Therefore based on this assumption is there a need for adjustment or its just an H Code engine doing what it suppose to do?

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you may want it to hang in 2nd a little longer, if you have a tach see if you can tell at what RPM 3rd is coming on, if its shifting Way early in the 2000rpm range then its in 3rd too fast and has to work to get back in the power band.

you could try increasing the modulator adjustment screw 1 turn and then drive and test again.

took me a little while to adjust mine mine, i had the adjustment set way too far counterclockwise and when i stomped on it it hit 1,2,3 before 3000rpms,

i came in about 10 turns before i got 1,2,3 to hang out too long, then i backed off 2 turns to get the feel just right.

1st will shift fast, 2nd you want to hang just a little and then into 3rd, you will feel it and hear it in the engine, if your sitting in the car and you stomp on it and go man 2nd went way to fast i'm in 3rd then you could tighten turn the modulator 1 turn clockwise and test again, eventually 2nd will hang in there longer.

Shift kits just make the trans Snap into gear with no lag, the transmission will love that because its not allowed to slip and wear the clutches, but it will feel very abrupt to you as it SNAPS from 1, to 2 to 3, it a sporty shift with no lag some kits come with heavy 2nd gear spring that tries to hold 2nd longer, but you can accomplish almost the same with the vacuum modulator.

at least with the modulator adjustment it won't effect half throttle shifting and smoothness. the modulator is driven by the engine vacuum falling off as you stomp on it, and as your speed builds back up so does your vacuum amount that drives the modulator for the next shift point.

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can't really compare my car to another H code since i rebuilt it as a 4V which made 300+ on the dyno, but if you figure 50-60% drivetrain loss when dealing with a C6 transmission, i could be looking at maybe 200hp to the rear wheels. my 0-60 is like 5-6 seconds which is about the same as my daily driver sedan today. my ford feels faster then my sedan but not by much. my sedan is also rated 300 hp at the back of the motor, at the rear wheels with 4wd is most likely about the same 180-200hp at the wheels