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under powered

7K views 26 replies 14 participants last post by  DJOHAGIN 
#1 ·
i picked up a 77 460 from a f100 truck and put it into my 78 4x4 when i had it out i drilled an taped the ports in the exhaust ports and ground them smooth then i put in a comp cams high energy cam Hydraulic Flat Tappet, Advertised Duration 268/268, Lift .494/.494, part number K34-227-4 put on a weiand dual plane intake and holley 4160 750 cfm carb and msd dis and 6al box and it has no power i changed the springs in the dis to the light silver and blue and that helped then changed the button to silver to give me 25 advance and its set at 12 inital i also changed the sec spring from silver to long yellow because the sec only just moved a bit the odd time it will back fire but not every time iv tried lugging it down to under 1000 rpm to see if it will do it all the time but it doesn't do it every time other than that the engine runs smooth but just doesnt go like i figured it should any help would be great
 
#4 ·
Didn't degree it and used the timing set sent with the cam kit from comp I asked about using earlier timing set but this cam has advance built into it is what they told me and I figured it would at let set me back in the seat a bit right now the old 351m would almost keep up to it
 
#5 ·
I was disappointed in my build when I first started driving it. '71 429, added Crane 272 cam, Edelbrock performer and Edelbrock 1407 750 carb, in a 1972 Mustang. It had less acceleration then the original 2bbl carb and stock cam. I had to set the project on the side for several years. When I got back into it and the engine was in a car again (76 Torino Elite), I started tuning it, trial and error. My carb was very lean, timing was retarded, and advance was wrong. I eventually ditched the ebrock 750 for a Qjet, but that only gained me some throttle response and (a lot of) fuel mileage. It is quicker now in my 4500 lb. Elite, than it was in a car that weighed 1200 lbs. less.

The cam is going to move the torque peak up, so off the line is going to suffer. If all else isn't dialed in, it won't feel faster as rpm's rise. You have a heavy truck, possibly geared wrong, with a low compression ratio. The cam should probably be dual-patterned, and heads should have been ported a bit (see reincarnation-automotive.com for porting guide).
 
#8 ·
It was really bad before I changed the dis springs wandering what the max advance I can do and how soon it should come in I think my sec still are not opening soon enough or opening far enough I might try one size up on my accel pump nozzle and see what that does
 
#10 ·
The intake manifold and cam shaft are COMPLETELY inappropriate for a 7.7 to 1 c/r oem 460. Too much duration (218) and lack of additional exhaust timing. You likely have a non straight up timing set as well.

No amount of tuning is going to wake this combo up.



If you want to wake it up:


Remove and deck heads .035"
Install performer intake
Install straight up timing set.
Custom short duration cam with 12 to 14 degrees additional exhaust duration. I would grind a 207 / 219 voodoo on a 108 lobe sep in at 104 ICL.

Properly curve distributor with black bushing for 18 degrees centrifugal and 16 to 18 initial.


Early small chamber castings would get static c/r in the low nines if decked .030"



S
 
#12 ·
Straight up for what year?
What is the part number?

Comp cams tech and sales leave a bunch to be desired when it comes to the BBF.


If your set does have straight up cam timing this is just one piece of the puzzle.

The information I presented will fix the problem. There is no easy work around here.



S
 
#13 ·
Listen to and heed what The Mad Porter recommends.

I followed his build advice and had him grind me a cam. I told him I wanted a torque monster.

Right now I am in the process of repairing and replacing my bent and broken rear spring perches! Oh and a twisted trans yoke.

He gave me a torque monster.>:)

He knows of which he speaks!
 
#14 ·
timing set is comp 3222 which is for pre 71 and this cam cant make this truck less power than the stock one and whats wrong with the manifold its made for idle to 5500 i knew this thing wouldnt be 600 hp but i figured it should push 300 or close to 400 its a 7.5 engine our old gmc grain truck with its stock 350 has more punch than this ford
 
#18 ·
The Dual plane Weiand intakes are good from 1500-6800 according to the Holley site. According to the comp cams website the stock springs are inappropriate for this cam.

To recap your have a dog which needs more compression and a more suited bottom end intake. In fact you would probably be better of with the factory 4 barrel intake than the one you have.

Trust me listen to The madporter, my 435HP 466 in my 4000lb f100 lays rubber when I punch it at 50km/hr. Your wasting your time thinking its your tune.

D
 
#23 ·
You're right it was the cam operating range that starts at 1500 and goes to 5500rpm.

D
 
#20 ·
If it were me, I'd be checking the spring pressures.....ALL of them, too light of a seat pressure will cause back fires. Another reason for a back fire is either crossing over in the distributor cap or an accelerator pump that is either out of time or not functioning up to par.......or a power valve not opening under a low manifold vacuum situation.
Rob
 
#21 · (Edited)
Way back when I did my first bolt on I used the same intake and cam as you with appropriate springs. It was a dog after that, I totally agree with the Mad Porter. When I did mine I used a 670cfm Holley Truck Avenger carb. The throttle plates would not open all the way at WOT. There is a screw on the main shaft that was rubbing on the manifold. I added a 2" spacer and it helped just due to the fact that the throttle plates opened all the way.

Might want to make sure the butterflies are opening fully

In the end I ended up with the heaviest(black) spring for the secondary and I think it was a 6.5"Hg power valve. Check engine vacuum while cruising.
Before the swap I had a intake off a 429 with the 2" secondaries and 1"ish primaries and it was way better on fuel and had more power off the line.
 
#22 ·
Ya mine did that too I dunno how holley can make a manifold that doesn't work with there carbs I had a spacer but it was too high so I just ground a bit off the side of the intake so it is opening iv got yellow tall spring in it came with the silver one and I'm sure they were not opening what should I get for idle vacuum and cruzin
 
#24 ·
I'm definitely no expert on tuning carbs. I put a vacuum guage on intake manifold and tuned for best idle vacuum(I got between 16 and 17"Hg). I plumbed the guage into the cab and went for a drive on the highway, cruised on flat ground at 8-8.5"Hg so I went the next size down.
As for spring selection I put the heavy black one in because you might think the secondary's aren't opening but they may be actually opening to fast. I think the heavier vehicles benefit from the slower opening.

Best thing you can do is go buy yourself one of those SA books on rebuilding and tuning Holley Carbs-I think they are about 20 dollars.

Just want you to know that even after all of this was done I was still disappointed with the mods. It didn't wake up until I had the short block mildly upgraded and some minor self port work done to the heads.
 
#25 ·
just to get you guys thinking say i live with it for now and when i get engough money i rebuild it what should i build id like to keep my cam and intake if possible and id probably spring from new heads but i have to keep the stock exhaust ports for my headers to work in the truck any ideas are welcome it must run on cheap pump gas and start and drive in the cold
 
#27 ·
As pointed out Cummins, you can run any combo of parts you want to run, but those parts need to work together.

While the Weiand will indeed run from 1500-6800 rpm, the port volume on it is quite large, which kills the lower end of the power band. Ideally, you want a power band from 1000-4000 rpm for your rig. The Weiand intake starts to get happy around 3000-3500.

The cam you selected needs more compression. Compression and cam timing go hand in hand. With low compression and the intake valve closing later than a stock cam, you are building very low cylinder pressures, and that results in low torque, and that is what you are feeling.

The Mad Porter gave you the best advice. You can choose to listen, or not.

Dave
 
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