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Engine sluggish under 40mph
#1
bulb 
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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#2
sgtjd;26040 Wrote: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?

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?

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.
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#3
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!

Pete - MotoArts Decals and Signs
'71 Sportsroof 351C-4V/4-speed - FINALLY under construction - no, wait, on hold again...
'90 Mustang 7-Up 5.0 ragtop, rolling beater
'66 Sunbeam Tiger Mk.IA, survivor
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#4
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?
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#5
2:75 are dog highway gearsConfused
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#6
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.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.
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#7
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.
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#8
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 buildsSmile

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#9
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).

1972 Mustang Grandé
2001 Toyota Tundra
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#10
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?
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