So...

I just swapped in a new (smaller) cam into my 400.

It had a (generic) 224/234 cam, and I just installed a Comp 260XFI 216/223 and lifter kit of course.

Now, I'm pretty darn sure I did everything right, but I have a NASTY (relatively speaking), slapping sound. It's not really the "tick" of a loose rocker, it's a tad bit harsher sounding than that. (at least, to me, who knows...)

I have GM 487X iron heads (original LT1 performance Vett heads), and some kind of aftermarket alum roller rockers. (I think the pushrods are aftermarket too, but I don't actually know.

I thought, oh, no big deal, just a loose rocker, so I adjusted them again, no dice.

I PROBABLY adjusted them a little TOO tight at this point (the valves aren't staying open or anything, but it's probably past "recommended" lash setting.)


The thing is, it doesn't really sound like it's coming from that "close" it sounds like it's in the back, and deeper in the engine, but I'm not 100%.

My next thought, was that I had some kind of weird clearance issue with the springs/valves/whatever, but why would it do that with a "smaller" cam? This one has a more aggressive profile, but it doesn't have more lift, and it has less duration...


Any Ideas????


OH! I forgot! It ONLY kicks in above about... 1600-2000RPM, and doesn't seem to start until the engine has been running for a second (not, only when it's warm, but after it's been STARTED. It can be fully warmed up, and it still seems to take a second or two to start making the noise)

I might be wrong about the delay, but the RPM thing is absolute.

 

Im not an engine expert by any means, but you might want to check to see if a valve is contacting a piston. Its crazy that would happen after installing a smaller cam, but if the tolerances were really tight it might be possible because of a different pattern. Not saying this is definately the problem. Just one thing to take a look at. try rotating the engine by hand with the spark plugs taken out and see if anything binds. Hope you figure it out!!

 

Maybe the springs are too strong for the smaller cam?
It's not hard to damage a flat tappet cam during break in, especially with a stiff set of springs (this is why you must take the inner springs out of dual spring heads when breaking in an older style cam).

 

@zraffz If he ruined a cam lobe you should be able to tell be watching the rocker arms right? If one or a couple don't move the valves as much as the others then the cam is toast right? Ive never broken a cam but that would be a quick way of checking I'd think.

 

I can't really can't see the piston thing... My pistons are slightly dished, with valve reliefs.

The springs aren't anything special, they are "double springs" but nothng special.

Not to mention, there is VERY little difference between the two cams, just a little smaller duration, and slightly wider lobe separation...

As far as damage, it runs GREAT, and all the rockers move the same amount I think...

If I had wiped out a lobe, wouldn't it show?

 

Yes you are right but I have seen one cam actually round the lobes evenly. The motor was just seriously down on power and we couldn't figure it out. It didn't make any horrible noises though. We took it apart when it spun a bearing months later.

Another thing (although I haven't done it first hand so IDK if it would make a noise) is the possibility of damaging cam bearings when removing/installing the cam?


Edit:
Their is a difference between ruining a lobe due to improper break in and ruining lobes due to spring pressure. High spring pressure won't necessarily damage just one lobe. As far as I recall, any dual spring setup is supposed to have the inner spring removed during break in.

 

Re-run the rocker adjustments maybe?

 

My problem with te "messed up cam" theory, is his: why would that make a sound?

This is a VERY "mechanical" sounding noise

Something is HITTING something else...

 

Oh, added detail:

Te cam says it runs 1.6 rockers, and I'm only running 1.5 (aftermarket, so they're more like 1.52)

 

IIRC,
With 1.5 the " lift " would be lower due geometry compared to1.6 rockers
The dual spring means
{ if I'm right }
The inner spring is a " damper " to help with harmonics & spring
recoil harshnes & binding.

Less lift & more duration means the valves don't " lift " { open up } as much as previous cam.
Duration means they are open for " x " degrees longer.

Valve train knocking // tapping could be anything--
Loose rockers, valve to piston contact, bad springs, loose // pulled rockers studs, bad // stripped rocker studs in the head, bad // collapsed liters, lack of oil to liters....

 

When you installed the lifters did you soak them in oil before you put them in? First thing i would check if i was you is rotating by hand with spark plugs out. See if it spins free with nothing stopping it. If you can't rotate it by hand then its binding somewhere and you have to find that point. Im just trying to make suggestions. I would agree that its probably not a valve contacting a piston, but at this point id suggest checking everything.

 

Ok, so, I looked at the rockers again this morning, and I'm wondering...

My rockers are often "crooked." As in, they are not centered on the valve stem. In some cases the roller is actually at the very edge of the valve stem.

Also, this means that several of the pushrods are riding up against the head.

It has always been like this (old cam and new), and I'm wondering, could that be causing the problem, and it's only showing up with the new cam for some reason?

I don't have any kind of "guides" or anything, the rockers seem to be "self-aligning..."

This is an area I know NOTHING about, so any help would be great...

 

There are 3 ways of aligning the rockers: narrow pushrod hole in the head, guide plates, or self-align rockers. You must employ one and only one of them.

The first two are visually obvious, the sa rocker look like this with little guide nubs that sit either side of the valve stem:

http://www.summitracing.com/parts/CC...8-16/?rtype=10

If the rockers are nearly falling off the valve tips, you aren't using any alignment. This needs fixing.

 

Ok, so, I just looked at it again, and they are NOT SA rockers.

However, the heads have very narrow slots, that keep the rockers from actually coming off the valve stem.

I guess it's just more play than I'd LIKE.

Also, comp cams says in the camshaft specs that I NEED 1.6 rockers, could that be the problem?

 

Sounds like your rockers are aligned properly using the pushrod slot method.

Assume this is the cam you came from:

http://www.summitracing.com/parts/SUM-1105/

and this is the new one:

http://www.compcams.com/Company/CC/c...?csid=120&sb=2

With the Comp one, the 1.6 rockers are only required to achieve the 499/493 lift specified. Back calculating this for 1.5 rockers, you get 467/462, which is similar to / less than the 465/488 spec'd for the old cam. So your noise doesn't sound like an interference problem caused by too much lift.

Edit: Can you post a vid of the noise?

 

Actually, the old one is a "generic" one, from "EnginePro"

It's this one: MC5892
At this url:
http://enginepro.com/chevyV8.html

It looks like it's virtually identical to the summit one you posted though.

And yeah, I can't imagine that .002 increased intake lift would be causing interference.

 

I just use the Summit line of cams for general reference, when referring to the old 'generic' cams. Like the 'rv cam', the 'mild cam' etc. Summit, Crane, GM, etc etc they all seem to be the same series of dinosaur ft cams.

Didn't change your dipstick did you? Last cam breakin I did, a new crappy chrome dipstick went in too which ended up was too wide, and getting smacked by a crank counterweight. That took me while to figger out.

 

Unfortunately no. No new dipstick

The thing that confuses me, is how completely RPM dependent it is.

It's ONLY above roughly 1800RPM or so... So how could it be a clearance issue????

 

New timing chain set-- chain & gears?
If so, did the gears line up evenly-- not slightly askew from each other vertically?

If its the old set, how many miles on it?


I'm wondering if there is some timing chain " slap " against the cover..?..

 

No, it's not new, but it was new with the old cam, only about... 6-10K miles ago? I've only had the car 3-4 years, and I didn't do the engine rebuild/swap right away.

Yes, everything seemed to line up great. No problems.


Thanks.

 

What kind of gas are you running? On my vortec motor i had the ignition cranked up and was running 87. Above 1800 I had a knocking noise like what you said and I thought it was mechanical, but turned out to be pre-ignition. I switched to 91 and the noise went away. Since this is a mystery noise im just throwing ideas out there. Not meaning to confuse you, just going off personal experiences.

 

Is the rockers hanging over the valve stem too far or not enough could the rocker be hitting the spring retainer or maybe the rockers are hitting the oil baffle in the valve cover (if they arent running staight on the valve stem)

 

Is the noise cyclic like a lifter tick, or more sporadic? Is it faster like a lifter noise, or slower like a knock - ie can you isolate it to cam or crank speed, and position in the engine?

Was anything else removed or replaced besides dist, intake, front accessories, valvetrain?

 

No, nothing else was moved. Just what was necessary to replace the cam.

I can't really tell for sure how fast the noise is. However, I can tell you that it is 100% rythmic. So, it's either 1:1 with cam speed, or crank speed.

I use a mechanical fuel pump, has anybody heard of that making noise after a cam swap?

 

That's a double head scratcher.

 

So, just for giggles, I un-bolted my fuel pump, to make sure IT wasn't making the sound...

No dice

 

To recap:

If it's only been doing it since the cam swap, and it's exactly rhythmic, something interference and cam swap related is implied. It only happens over about 1800 rpm, so it's an interference that's non-existent or only very marginal at idle. The noise sounds like it's in the rear of the engine.

More ideas:

Assume you have no ecm and a mech advance dist. Maybe the weights in the dizzy are flying out with rpm as they should, and contacting something inside the cap somehow. Is the cap or rotor on a little crooked, has a spring come off, bent shaft, etc.

Defective lifter collapsing a little under increased rpm loading. Can you isolate it to a single valve or cylinder?

Problem with the cam break-in as mentioned above, though I think you'd have noticed the resultant loose rocker by now. Or ditto rocker stud pulling out.

 

Ok, update.

I finished putting the accessories, etc... back on. I tried to isolate the sound with a mechanics stethoscope, no luck. It seems to be coming from "everywhere."

I'm at the point now, that since it's a relatively minor noise, of just focusing on other things, and essentially "driving it until something breaks."

Horrifying as that sounds, I don't really have any other options at this point, I've investigated everything I can, and I literally don't know what else to check.

I appreciate the help, and if anybody has any new ideas, please, let me know...

 

so did anything break yet?

 

The 400 sb was known for a condiition called cam walk, where the cam actually moves front to back while running, that makes a knocking sound. My experience with them, always install a cam button. Does the noise sound like valvetrain or lower? I had a problem with pushrod length, the rockers were contacting the spring retainers.