Theory
Overlap is the portion
of the engines rotation for which the exhaust valve is still open right at
the end of the exhaust stroke, and the intake valve is also open to give a
head start to the incoming charge. The exhaust gas is whistling out of the
exhaust port at the same time the intake charge is starting to trickle past
the intake valve. Lots of overlap works great at high rpm because more
intake charge manages to cram itself into the cylinder, but lots of overlap
will also make the engine run badly at low rpm, as exhaust gas manages to
make its way back up the intake manifold, diluting the incoming air/fuel
charge, and depositing soot on the intake runners, carburetor, etc. If you
have more than about 40-50 degrees of overlap, you're looking at something
more like a race cam than a street cam.
Lobe centre angle, on the other hand, is the number of degrees between the
point where the intake valve is at maximum lift and the point where the
exhaust valve is at maximum lift. This angle is commonly in the 112 degree
to 108 degree range you mentioned, which is why I think you're really
interested in LCA, not overlap.
The two angles are related - for a given lobe design, as the LCA gets
smaller, the overlap gets larger. 112-degree LCA's usually give you lots of
low rpm torque, but not as much high-rpm power. 108-degree LCA's give you
lots of high rpm power,but poor low-rpm torque. 110-degrees is somewhere in
the middle, usually a good compromise for a hotted-up street engine. This
is all relative, a 455 with a 108-degree LCA cam is still going to make
more torque than a 350 with a 112-degree LCA cam, of course, all else being
equal.
Advancing the cam 4 degrees always moves the torque curve down a few
hundred RPM..just as retarding it moves it up a few. Just MAKE SURE your
cam is ground to a 0 timing. Comp Cams for instance are ground with a 4
deg advance in the cam profile, so you would be advancing it 8 degree's
on the chain.
Advancing has one other added advantage. As the chain stretches, and it
will retard the cam back towards 0 instead of beyond 0, thus keeping a
more constant power band over the life of the cam/chain.
Actually in a high CR, big cam application (actually any high CR where
you want to run pump gas) you want to increase your overlap. The
overlap is what bleeds off cyl pressure to allow use of pump gas.
For example: My 72 350 was built with the TRW 10.25:1 forged pistons
and milled heads for a true 10.0:1 motor. With a JM 22-25 cam with the
lobe centers tightened up to 106 degrees it ran fine on 93, even 91 octane.
The 350 in my 68 has the same pistons and head work for almost the same
CR. The cam in it is 110 lobe centers plus a smaller lobe profile. What
this translates to is much less power since to run on pump gas the timing
has to backed off dramatically. Detonation with this motor is simply a fact of
life since there is almost no overlap, thus no chance to bleed off cyl
pressure.
[ Thanks to John Carri, Stephen Hoover
for this information. ]
Experiences
Each cam manufacture has certain charactoristics ground into them.
For example, Comp Cams is +4 degrees advanced.
Cams for Highway Gears
For a 2.14:1 geared car, don't go any higher than 218° @ 0.050"
lift on duration. I think the Performer has 214° @ 0.050, so this
would be a good choice. I had a Comp Cams 268H in a 350 once, with 2.29
gear also. Talk about smokin' some tires!
350 Cams
I had a 350 and went through the same things that you are. My
first cam was an Edelbrock Performer. Great cam for driveability and
torque, but it definitely likes a set of gears and a bit more stall than
stock.
My combo was 3.42 gear with a 2600 rpm stall. I was using an Edelbrock
performer intake with a somewhat modified quadrajet. My compression was
about 9 to 1, and that is important to factor in when choosing your cam.
Performer works good with lower compression ratios like that. Performer
also works good in heavier carsn like my 82 Delta two door. The only
drawback was that it didn't lope at idle.
Performance with that cam and those gears and that converter was EXCELLENT
for a small block in a big car. The car got off the line Really good, and
pulled hard to 5000 rpms, and didn't do to bad on gas if I drove on the
front two barrels of the carb. If you want to REALLY be able to smoke your
tires and put your friends back into their seats, than try that combo. I
should also tell you that at first I ran the cam without the 3.42
gears (2.73 is what I had at first) or the 2600 rpm stall, and even though
Edelbrock said it would be fine like that, it wasn't nearly as quick. The
Performer cam is really a midrange cam, and so it wasn't as powerfull as
stock off the line. As soon as the cam got into its powerband it easily
pulled harder than the stock 2 barrel cam. The people at Edelbrock (and
most other cam companies for that matter) don't want to admit it, but the
real powerband is from about 2400 rpm to about 5000 rpm. The cam could
hold its own off the line, but just didn't really come on until well
underway.
When I went to buy a torque converter, the guy at the high
performance shop told me stock or maybe the next step up was the max that
I'd want with that cam. Well, an Olds 350 is a short stroke/long rod
motor, and everything I read about that combo said that they shifted their
power into higher rpms as opposed to the longer stroke/short rod small
block chevy motor which, against popular belief, should actually focus more
of its power lower in the rpm band (for a given cam combination. Obviously
a stock cammed Olds isn't going to make more power at 7000 rpms than a pro
stock cammed small block chevy). To make a long story short, the 2600 rpm
stall worked GREAT with that cam. I also got a set of headers after I
already had the gears and converter, and holy cow did those things help!
Olds exhaust manifolds must be really restrictive because I didn't have any
Idea I'd be able to FEEL such a huge difference.
So here I am with this great combination, and of course I'm not satisfied.
I got looking through some magazines and see an advertisement for
Competition Cams. They made a bunch of Claims about their 270 magnum cam,
and 90 percent of them turned out to be false. I even called comp cams and
talked to their tech people, and they had nothing good to say about my
performer cam that had been working good for me for so long. They said
their 270 mag cam would work fine with my gears and compression ratio, and
they said that I could even use a stock torque converter! I should have
known better at this point, because the 270 mag specs out at 224,224
Int/Exh duration at .050 valve lift with .501,.501 Int/Exh lift.
Now, the
performer specs out at:204,214 Int/Exh duration at .050 valve lift, and
about (not positive on this part) .448,.472 Int/Exh lift. As you can see,
the performer is much more mild than the 270 magnum, and I already knew at
this point that the performer needed more than a stock stall speed to
really take full advantage of the its powerband. But comp cams told me
what I wanted to hear: 'You'll really be able to hear this cam idle'. So I
bought it. And when I fired it up for the first time, underneath my
carport with my good buddy who helped me install it, It sounded really
tuff. I loved it!
Then I drove it. I could barely smoke the tires
anymore (when in the past I could do it so easily it was almost
annoying) and I also had a terrible bog off the line. My times at the track
improved by only .06 (thats 6 HUNDRETHS) of a second, and gas and
driveablility went south! My performer could have gone faster if I would
have bought some better tires to hook it up.
Moral of the story: find someone you trust to help you decide what you
need for your combo, and don't let that BAD idle fool you. Torque is what
moves your car. Many manufacturers put nothing but Chevy or Ford grinds on an
Olds cam. Compare the specs.
455 Cams
I use Mondello's JM20-22 camshaft in my 9.75;1 455+.030 with a performer
and an automatic and it works flawlessly, it pulls HARD from idle to 5000
RPM, sounds tuff and pulls good vucuum at about 800RPM. I'll try to
remember to bring the specs in tomorrow
I use one in my engine
too. Great cam for mild street, and sounds good at the drive-in too!
Lift-.496 intake, .512 exhaust, running duration of 266/274, lobe center
of 110 degrees, duration at .050 is 226/230. Intake opens 6 degrees
before TDC, closes 40 degrees past BDC. Exhaust opens 49 degrees before
BDC, closes 1 degree past TDC. This cam isn't going to be a bracket
racer type, but will pull pretty good vacuum and idle fairly well. And
pulls like a mother!
[ Thanks to
Mike Bloomer, Mark Prince, Stephen Hoover, Dan Mann, Mike Rothe
for this information. ]
Factory Cams
The cam casting number found on the casting itself, and bears little or
no relation to the grind performed on the lobes. Cam use is identified
chiefly by the pattern of chamfers on the front OD, the location of the
notches on the front face, and occasionally they were kind enough to actually
put the last 3 digits of the cam PART # on the back of the cam.
Here's the chart produced by Karl Sarpolis that was originally printed
in the Feb/1995 issue of JWO. All credit is his. Note that the part # is not
the casting number.
Note:
These duration numbers are all measured at a mythical
0.000" lift, which is almost impossible to measure but lends impressive
duration numbers for advertising. Actual duration is a term for lift
measured 0.050" off the valve seat. This is a more to the truth measurement
of lift.
Olds Camshaft Specifications 1964-1977
Over Duration Lift
Year Part # lap Int./Exh. Int./Exh. Application
1964 387484 52 278/282 .430/.432 330 Police Pursuit
577994 36 250/264 .380/.390 330 2BC, D88 2BC
1965 387484 52 278/282 .430/.432 400, 330 4BC ST,425 ex Starfire
388992 47 278/282 .430/.432 J88 4BC, 330 4BC AT
558528 36 250/264 .387/.388 330 2BC
388828 50 286/280 .427/.461 425 Starfire
1966 588528 36 250/264 .387/.388 330 2BC
388992 47 278/282 .430/.432 330 4BC AT
387484 52 278/282 .430/.432 425 ex Starfire
388828 50 286/280 .427/.461 425 Starfire
393964 52 278/282 .430/.432 400 AT ex. OAI
393859 58 286/286 .472/.472 400 ST ex. OAI
390961 55 281/282 .472/.472 Toro
397328 82 308/308 .474/.474 400 OAI (W-30)
1967 396188 36 250/264 .387/.388 330 2BC lo-comp
396192 47 278/282 .430/.432 330 4BC AT,400 4BC AT x OAI
397744 36 250/264 .435/.435 400 2BC
393964 52 278/282 .430/.432 330 4BC ST, 425 ex. S/F, Toro
396190 50 286/280 .472/.461 425 Starfire, Pol.
396961 55 281/282 .472/.472 Toro
393859 58 286/286 .472/.472 400 ST ex. OAI
397328 82 308/308 .474/.474 400 OAI (W-30)
230328 82 308/308 .474/.474 400 OAI w/ 16 valve springs
1968 393859 58 286/286 .472/.472 400 ST ex. OAI
402486 48 262/274 .440/.440 400 AT ex OAI, 350 w/ 269 Pol.
400117 44 258/272 .435/.435 455 Irrig, Mar, hi-comp ex Pol.,
Toro, hi-perf.
400084 36 250/264 .400/.400 350 ex. OAI
402194 82 308/308 .474/.474 350 W-31, Hurst ex CAC Casting #389410
400165 57 285/287 .472/.472 455 Pol, Toro W-34, Hurst CAC
402569 108 328/328 .475/.475 400 OAI after 1-1-68
1969 393859 58 286/286 .472/.472 400 AT, W-32 OAI
400165 57 285/287 .472/.472 455 Pol, Toro W-34, F-85, Hurst,
455 Mar 4BC hi-perf
402486 48 262/274 .440/.440 3269 AT, 350 Pol.
402194 82 308/308 .474/.474 350 W-31 OAI
400117 44 258/272 .435/.435 455 ex. Pol., Toro, hi-perf.
402569 108 328/328 .475/.475 400 OAI ex. W-32
400084 36 250/264 .400/.400 350 ex. OAI
1970 393859 58 286/286 .472/.472 350 4BC ex. AT, OAI, hi-perf
400084 36 250/264 .400/.400 350 2BC, 4BC AT
400117 44 258/272 .435/.435 455 ex. Pol., Toro
400165 57 285/287 .472/.472 4257, 67, 4400 AT, Toro W-34
402486 8 262/274 .440/.440 455 Toro ex. W-34
402569 108 328/328 .475/.475 455 4400 ST OAI (W-30)
406768 68 294/296 .472/.472 455 4400 ST ex. OAI
402194 82 308/308 .474/.474 350 W-31 OAI
1971 400084 36 250/264 .400/.400 350 2BC, 4BC AT
400117 44 258/272 .435/.435 455 Toro w/2.73 axle, 455 ex 400 or
Perf. Eng.
400165 57 285/287 .472/.472 455 4400 OAI w/AC, 4400 AT
402486 48 262/274 .440/.440 350 4BC ST,455 Toro w/3.07
409691 61 286/287 .474/.472 455 4400 ST, AT OAI ex AC
409717 32 250/264 .400/.400 350 5400
409759 68 294/296 .472/.472 455 4400 ST OAI (W-30)
1972 400084 36 250/264 .400/.400 350 L34, 350 AT L32, 33 All
400117 44 258/272 .435/.435 455 All 4BC AT, Marine LC
402486 48 262/274 .440/.440 350 F-85 4BC ST (L34), Marine
409691 61 286/287 .474/.472 455 L75 ST,L77 AT, Marine HC
409759 68 294/296 .472/.472 455 F-85, 455 ST OAI (W-30)
1973 400084 36 250/264 .400/.400 350 L32, L33, L34
400117 44 258/272 .435/.435 455 AT, Marine, 455 LC
402486 48 262/274 .440/.440 350 ST M15, M20
409691 61 286/287 .474/.472 455 4BC, L77, Marine
1974 409691 61 286/287 .474/.472 455 AG37, 455 W-30, Marine
455 (also #562302)
400117 44 258/272 .435/.435 455 ex W-30, Marine, 455 LC
(Also #562303)
400084 36 250/264 .400/.400 350 (Also #562299)
1975 550638 28 242/250 .396/.400 260
400084 36 250/264 .400/.400 350 (Also #562299)
400117 44 258/272 .435/.435 455 (Also #562303)
409691 61 286/287 .474/.472 455 Marine (Reg.)
402486 48 262/274 .440/.440 350 Marine
409759 68 294/296 .472/.472 455 Marine Hi-perf.
1976 550638 28 242/250 .396/.400 260
400084 36 250/264 .400/.400 350
402194 82 308/308 .474/.474 455 Marine Hi-perf. F.P.
409759 68 294/296 .472/.472 455 Marine Hi-perf.
409691 61 286/287 .474/.472 350 and 455 Reg. Perf. Marine
400117 44 258/272 .435/.435 455 All, Irr., ex Marine
1977 562298 52 256/277 .247/.250 260 (thru 1982)
562299 36 250/264 .400/.400 350, 403 (thru 1980)
562301 68 294/296 .474/.472 403 Hi-perf. Marine thru 79
562302 61 286/287 .472/.472 350, 403 Reg. Perf Marine, ST thru 79
Notes: Overlap is in degrees.
Olds rates duration at zero lift, while most cam manufacturers rate duration at
.050" lift.
ST = standard trans
AT = automatic trans 2BC/4BC: 2 or 4 barrel carb
OAI = outside air induction
Roller lifters and cams were in all 1986 and up 307's, and most
1985 307's. An easy ID is the heads. 5A heads have no roller lifter, 7A
heads have the roller lifters. These lifters are of 0.921" variety.
[ Thanks to Chris Witt, Greg Beaulieu, Tom Lentz for this information ]