Is this a cam problem?

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killnine

killnine

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So, slant six, 198 with a super six intake and 2bbl carter carb. The PO said he upgraded the cam using a mopar purple stick. I've never confirmed any sort of cam numbers at all. Anyhow, I've tuned the hell out of the thing to try to get timing and fuel and everything just right, and it seems to run pretty well, starts well, idles well when it's warm now. No backfires or hesitations or stumbles. However, when I get on it from a slow speed (not kicking down) or from a dead stop, it really seems like it drops about 500 lbs when it hits around 2000 - 2500 rpm, and on up, like it really kicks in. Is this because the dufus put in way too big of a cam? It's not the torque converter, because he never said anything about changing the torque converter, and it breaks away from the brakes like I would expect at around 1.5-1.8K. Besides, I know what torque converter flash normally feels like, and this takes way too long to happen. Any ideas?
 
What gears? Stock compression?

My $.02, it's not 'way too big of a cam', but just a couple steps bigger than stock. I dunno the purple stick specs off the top of my head, but sounds about the same characteristics as the Comp 264/OCG819. Mine, in a 8.9 compression 225 with 2.76 gears and stock stall. This combo would really like more gear and stall, and a bit more than stock compression. As a feller over on .org says, 'there's no such thing as too much cam, only not enough motor!'

Oh, and the 198's shorter stroke aint helpin you out much. I'd say either upgrade the motor/drivetrain a lil to match the cam, or install smaller cam ( I hear the Comp252/OCG818 come on just above idle, but that's in a 225)
 
Mine did the same from a dead stop till 3,000 then pulled like gangbusters to 6,000.Was running a 2.94 gear.Converter would flash no more than 2200.Installed 4.10's but still needed a looser converter.
 
It's possible the cam was installed just by simply lining up the dots. Very often that could mean the cam is installed/timed retarded. Specially if the timing gear isn't ground/marked right.
A 'late' cam will shift the engine's powerband to a higher rpm, which could explain your findings.

Degreeing the cam will tell the true story here.


I had 383 once in which I purposely installed a retarded timed cam as a test. The engine couldn't spin a tire on wet pavement from a stop, but once rolling and the engine got above around 2000rpm it would break the tires loose.
 
Mopar performance had several different solid lifter "Purple Shaft" cams for the slant six, and 2 hydro cams. It would help to know which one is in the engine.

P4120243..... 244 dur......,436 lift........102 center line
P3690766.......268 dur.......450 lift.......108 cl
P3690768.......286 dur........520 lift......108 cl
P4286679......268 dur........460 lift.......104 cl
P4286681......276 dur........490 lift.......102 cl
P4529342......268 dur........460 lift.......108 cl
P4529343......284 dur.........528 lift......102 cl

I have the first one (P4120243) in my 170 engine street driver, pulls good from off idle.
Have the 3rd (3690768) one in my 225 race motor, needs a good converter and gears
 
That's the thing, without knowing what's actually in the engine and how it's degreed, you are just shooting at your foot in the dark.

Initial timing and advance cure has a huge effect.
 
Exactly. Until you know what you have, we cannot know what you have, nor can we advise you about what you have.

I see it everyday at work. Customer: "Yeah, I needs me a timing belt foe a Honda"

Me: "What year and what kind of Honda?"

Customer: "I don't know. Ain't day all de same?"

If you don't know what you have, you cannot be helped.
 
BigBlockMopar said:
It's possible the cam was installed just by simply lining up the dots. Very often that could mean the cam is installed/timed retarded. Specially if the timing gear isn't ground/marked right.
A 'late' cam will shift the engine's powerband to a higher rpm, which could explain your findings.

Degreeing the cam will tell the true story here.


I had 383 once in which I purposely installed a retarded timed cam as a test. The engine couldn't spin a tire on wet pavement from a stop, but once rolling and the engine got above around 2000rpm it would break the tires loose.
Click to expand...

This is quite possible. Sounds like I'll need to degree the cam and measure the lift. Fun stuff. Any harm in running it like that, other than basically losing power?
 
Well, you've tuned the hell out of it,so what is total adv. w/o vac. and what RPM is
is it all in?You NEED to know the comp.ratio,period.If you're not in the mood to
pull the t-cover,yank the valve cover and roll the eng. to TDC#1(after you watch
the int. valve close)compression and set one or both to zero lash then measure
valve lift at max. turning slowly by hand. You may be shy a little of the cams design
lift depending on your rocker geometry,but it mite be enuff to ballpark which purple
shaft you have.Even with the geometry way off, you won't lose much more than .020
in.,so a reading of .440 leaves out most of the bigger ones.I ran the 276/.490"
piece and it definitely needed a loose converter which i never did get in it,really
kicked in around 2700ish or so.
Either way,if your A/F ratio and timing(generally 32-34deg by 2K)are good,most
likely cam is too much for either comp.ratio or converter or both.But there is always
the chance of spark energy issues at low speed(incorrect ballast etc.)and other such
"bugs".Keep us up to date and good luck!:coffee2:
 
Killer6 said:
Well, you've tuned the hell out of it,so what is total adv. w/o vac. and what RPM is
is it all in?You NEED to know the comp.ratio,period.If you're not in the mood to
pull the t-cover,yank the valve cover and roll the eng. to TDC#1(after you watch
the int. valve close)compression and set one or both to zero lash then measure
valve lift at max. turning slowly by hand. You may be shy a little of the cams design
lift depending on your rocker geometry,but it mite be enuff to ballpark which purple
shaft you have.Even with the geometry way off, you won't lose much more than .020
in.,so a reading of .440 leaves out most of the bigger ones.I ran the 276/.490"
piece and it definitely needed a loose converter which i never did get in it,really
kicked in around 2700ish or so.
Either way,if your A/F ratio and timing(generally 32-34deg by 2K)are good,most
likely cam is too much for either comp.ratio or converter or both.But there is always
the chance of spark energy issues at low speed(incorrect ballast etc.)and other such
"bugs".Keep us up to date and good luck!:coffee2:
Click to expand...

Thanks for the info. That sounds like something I can manage over the weekend.
 
Killer6 said:
reading of .440 leaves out most of the bigger ones
Click to expand...

Why would you make this assumption?

I would take this from the assumption "We have no idea" as it may NOT even be one of the purple grinds.
 
The OP stated that the PO told him it was a Mopar unit,don't know if he would
think he had something to gain by lying about it,but true it may not be a purple
shaft.That being said,my personal experience playing around w/pushrod lengths
and shimming the slanty rocker shafts showed the worst poss. deviation from the
design ratio down would cost approx. .020" lift,leaving you w/a .460 lift cam at the
max for a .440" reading.That would eliminate the larger ones(276&284) right away,
and MAYBE include the 268 cam,which would still require a bump in stall speed.
One thing I mentioned in another thread to be aware of,the valve lash specs for
the 268 and up purple shafts are.028/.032" int/exh.,WAY looser than the usual of
.010/.020" for the stock and 244 purple unit. Having the lash stock on the larger
ones will definitely kill the low end for sure.According to mopars own bracket race
recommendations,the "342" 268 dur. cam is the first cam that req. the stall bump,
leaving the rest of the smaller cams suitable w/the stock unit.
Unless the OP has some kind of class eliminator cam w/stock vicinity lift numbers
and larger duration numbers, he should be able to use the lift check as a decent
ballpark idea as to what he is running,:coffee2:
 
Even a stock slant engine acts kind of like that. You feel a surge of torque ~1500 rpm as the long runners give their "tuned port" effect. It is like the natural frequency of an organ pipe, and increases cylinder pressure. Mopar made an early 60's V-8 drag setup with super long runners for the same effect. Ditto, the Aussie slant intake. The variable intakes on newer upscale cars is similar, and even better, since they vary the runner lengths w/ rpm.
 
Actually,and fortunately for the masses,practically everything from a Hyundai Accent
to the top of the line Mercedes uses active intake technology today.Made possible
by FI of course,so the dry flow runners can be shaped as needed to accommodate
underhood packaging,w/o fuel issues.Ditto for active variable camshaft timing/phase
control,these technologies not just the realm of the upscale anymore,econoboxes to
Vipers all get to share the great benefits offered by such.
Back to the OP's issue,is the BBD a factory supersix unit,or lifted from a 318?I ran
a 318 unit,but felt there might have been better throttle response to be had on My
engine.However, I was aware I needed a stall conv.due to the cam,so i kind of put
the blame on that.There was a diff. part no. for the six carb.,and it's poss. the air
bleeds were slightly smaller increasing the signal to jets, just a thought.
Anybody have factual details on the specific differences?I believe it was mentioned
in the mopar racing manual there was a difference,but don't think it gave details as
to precisely WHAT,all was changed.
 
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