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Poor performance after cam change !!!

 
napa219
I love my Chevy Chevette!
Posts: 7
Joined: 06/06
Posted: 07/31/06 07:02 PM

I am having some performance problems with the 396 in my 1969 Chevelle,  It was running well, but using some oil so I pulled the heads and had the guides and seals replaced.  While I had it apart I installed a new Lunati Voodoo cam part number #60205 with new springs, lifters,retainers, and locks. The engine is a 1966, 396 with 11-1 compression with oval port closed chamber heads, mild port work and 2.19 Intake and 1.72 Exhaust valves. After I installed the engine I set the initial timing at 12 degrees with a total of 34 degrees at 3200 rpm. I took it to the drag strip expecting to run quicker than I had ran before, The best time I had before was a 12.39 at 107 mph, It ran much slower, it missed and ran very poorly at any rpm over 4500 , I pulled the plugs and they all appeared to be firing properly, none appeared to be fouled or running too rich. I reset the timing to 8 degrees and this seemed to help some, but still it is missing at higher rpm, I changed the advance springs, to heavier ones so the advance would come in at higher rpm, this did not help. It almost acts like valve float at higher rpm, I did use the proper springs provided in the cam kit.   Any ideas about what could be causing the miss at higher rpm? I have changed dist. cap, rotor,plugs and wires. I am running a Crane Cams Hi-6 Ignition and coil, with an XRi points eliminator in a rebuilt points stlye distributor. Any help will be greatly appreciated,I'm out of ideas !! Thanks!!! Someone suggested 4 degrees advance on the cam? Do you think that would help?? Lunati's tech line says it already has 4 degrees advance gound in to the cam.



Edited 8/1/2006 7:18 am by napa219



Edited 8/1/2006 12:51 pm by napa219  

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 08/01/06 04:12 PM

The VooDoo is a very aggressively designed cam with a lot of "Intensity". That's to say the lift over duration ratio is high and fast. So it's likely that you're running into float or bounce at a fairly low RPM because the springs can't keep up to the rate at which things are happening.


Youre not alone a lot of people are experiencing these type difficulties with this cam, I suspect that Holley is going to either go back to the drafting board and soften the "Intensity" ratio or only sell this cam with a parts matched kit of lifters, push rods, locks, keepers, and springs.


This/these cam/s are very deceiving if you compare like durations against a Comp Extreme Energy, you will see that for identical durations the VooDoo has more lift, to do that, the rate of lift is faster and therefore, the rate of valve opening and spring compression is also faster on both opening and closing. 


The other thing this cam needs is stiffer rear gears and a looser converter.


Bogie





Edited 8/1/2006 4:23 pm by oldBogie  

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napa219
I love my Chevy Chevette!
Posts: 7
Joined: 06/06
Posted: 08/02/06 06:52 AM

I did purchase the kit with springs, retainers, lifters and locks. I am running a 3500 rpm convertor and 4.11 gears. I was think of going to 4.56 gears, but, I figured that would make the problem worse, by getting into the higher rpm range faster. I am going to try to degree the cam in with some more advance to see if that helps. any other suggestions?? Thanks!! Bill napa219, Or could you suggest a cam that would work better??



Edited 8/2/2006 7:03 am by napa219  

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oldbogie
I mow my lawn and find Chevys
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Joined: 08/03
Posted: 08/02/06 05:05 PM

This is a very intensive cam as in a lot of lift for the duration, which means the rate of lift per degree is pretty high. You said that you replaced the lifters, springs, retainers, and locks from the kit and I guess that I'm stuck with assuming Holley put the right stuff in the kit.


Still it sounds like the engine is having trouble with valve float or bounce. You didn't mention push rods, nor rockers. You can be getting flex anywhere in the system. This could be any of these items. The same goes for rocker studs which could be moving around on you. You didn't state whether the springs, retainers, locks or lifters were part of the Lunati kit, or just new stuff you picked up. If non kit parts any of these items could have the wrong mass or stiffness characteristics for the accelerations they're being subjected to.


The problem with your changes to distributor timing is that the advanace really needs hard stops. The advance boss in the distributor has a certain amount of degrees built in by vurtue of the slot length. If the weights are not throwing to the slot limit they will be unstable and hunt up and down several degrees of timing. In terms of total timing, the slot length is always the same unless you bush the posts of the counterweights or change the length of the slots. What this means is that changing the base timing with excessive advance can put in way too much total advance when that number plus the slot length combine for a bigger total than you think your getting or can result in the situation where the centrifugal never rests at the end of the slot and then you can get the hunting I previously mentioned if the springs arrest the motion of the counterweights before they rest in the max length of the slots. Reducing the base timing will reduce total timing. Excessive timing or hunting of the advance can cause the engine to run out of steam or begin bucking at a moderately high RPM like in the  range of 4500 to 5500. Insufficient timing makes the engine doggy everywhere except idle and will clip off the top end revs.


Something else you can try is to back off the lifter adjustment to just short of having lash. This will remove overpumping of the lifter from the equation. High leakdown lifters can go the other way and let off the plunger to soon when faced with fast lift rates and stiff springs. Then you get short timing and the power curve just flops at moderatly high revs. This can happen on one, a few, or all cylinders.


The cam drive can cause problems. The cam is slowed when the lobe lifts the valve open and is accelerated when that valve is closing and the lifter is skating down the back side of the lobe. This causes a lot of ossicallitions in the cam which can really rattle the drive resulting in an uneven cam turning rate which really shakes the oil pump and distrubutor drives. A lot of these ossicillations travelling up and down the cam depend on the drive to buffer them. There is no rule of thumb here, some setups love the factory silent link with nylon gears because is soaks these motions up, other engines like a stiff roller chain and gears. Just one of those details you sometimes have to experiment with.


You didn't mention whether the pulgs looked fried, only  that they were not running rich. Top end lean out is not unusual and will cause a lot of missfire when you hit it. The plugs will be clean, pale gray to white. You want a medium gray to light brown. A little bit of soot is a comforting thing as it tells you the mizture isn't so lean as to toast a piston or melt a valve. A hotter cam especially one with faster rates can really upset the carb. You can get stand off where reversion pressure waves have the carb fueling a cloud that stands above the inlet while drying out the cylinders. If you're running an aircleaner with filter look for wetness on the aircleaner. A carb spacer can help this by making the plenum larger and softening the reversion pulses.


I don't know if any of this will help, it's kind of new cam tuning by the shotgun process.


Another thing is the engine may not like the 4 degrees of built in advance. I don't know why the cam builders have decided to do this as advancing the cam doesn't work for every engine. I'd rather get the cam straight up and I'll advance or retard it if my tuning process indicates there some benefit. Now I spend half my time retarding the cam.


Bogie

 

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napa219
I love my Chevy Chevette!
Posts: 7
Joined: 06/06
Posted: 08/03/06 06:57 AM

I set the cam at 2 degrees retarded to acheive 110 degree intake centerline. it still seems to really miss at higher rpm. Someone suggested to change the distributor to one with an advance lockout, I just use this car for bracket racing, not street use, and to set the timing at my total advance, Is there any advantage to doing that? This cam does not seem to like much advance. I did use the Lunati kit the only thing I did not change was the push rods. I am using lunati roller rockers with a 1.73 ratio. I have an advance controller for when I run nitrous, I turned it on to retard the timing about 12 degrees,at 4800 rpm and it seemed to help smooth out the miss somewhat. I am going to try different advance weights, springs and bushings in the distributor. If that doesn't help I think I will just reinstall my old cam. It may have been slow but at least it ran well and pulled good up to 6500 rpm. Any other suggestions would be helpful!! Thanks Bill napa219    

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GibTG
I mow my lawn and find Chevys
Posts: 1985
Joined: 10/03
Posted: 08/07/06 01:16 PM

Bogie,

This talk about voodoo cams has me interested. I knew they were an awfully fast cam but how fast is too fast? We're considering a flat tappet 268 Voodoo for a fairly hot 440 BBC going into a '69 camaro here but we are wondering about the valve springs. We are going to try to use the valve springs that came on the heads, the heads are Trick Flow 280cc Oval Ports by the way, but the springs are around 140lbs at the seat (1.95) and 420lbs at .700" lift, well of course we won't have .700" lift probably more around .550-.600" but I'm still wondering if they are going to be awfully stiff. The engine is somewhat induction limited because of hood clearance with a low-rise cheap chinese intake manifold and a 780CFM Holley. The car has 4.10 gears and a Muncie. The car is going to be a hot street ride with no track time unless a lot of things get changed (sticky tires, cut hood). I'm wondering what you have to say about a cam, besides the 268Voodoo we are considering the stand-by 274 Xtreme. By the way, the engine has around 10.6:1 static compression.











 

                                                                                      ~Gibs

 
~Gibs

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tommkatt67
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Posts: 1
Joined: 08/13
Posted: 08/11/13 05:12 PM

i have a 454  11-1 thats doing the same thing at 4100rpm. changed everything same as you did . found valves going into float. valve springs are good for 680 lift. so wat did you do to correct the problem. lunati wont warrenty the cam so here im  with a cam i cant run. they say they have nerver had any complants  

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Dave632
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Posts: 2253
Joined: 07/08
Posted: 08/11/13 07:45 PM

This is a very old post at 7 years old. Both of these sound like some kind of valve float problems. I would go with some stronger springs or run double springs.
Personally I ran some very aggressive springs in my last couple of motors. 325 lbs. on the seat and 900-1000 lbs open. These were full rollers with high dollar T&D or Jesel rockers on big inch BBs. I never had any float problems and the motors pulled strong all the way up to 7500 rpm.  
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68scott385
I mow my lawn and find Chevys
Posts: 1994
Joined: 10/09
Posted: 08/12/13 09:23 AM

Probably be a good idea to start your own thread and give full details of your problem(s) and what you've done to this point to correct it/them.

After 7 years all those members are gone.  
68scott385 68scott385 68scott385

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