I bought the Crane blueprint series cam for my 427/390 and was looking at the original GM cam specs, and I was really surprised how they reuse cams from motor to motor. For example, the 3874872 cam:
The 3874872 has a duration of 197/197 and a lift of only .398, so to say it's a mild cam is an understatement. This is what you find in the 396 2bbl towing motor. But here are a few things that surprised me:
- The 396/325 uses the same cam and heads. So the 2bbl 396 is so badly choked off by the 2bbl that it gives up 60hp!
- The cam is also used in the 240(net) and 300(gross) 396s
- The cam is also used in the LS1 427/335
That's right, the 2 barrel 396 uses the same cam as the 427/335!
My car, however, has the 390hp L36 which uses the more aggressive 3883986 cam. When I say more aggressive, it's really only in a relative sense - 214/218 and .461/.480. That's not a big cam by most people's standards but it's a long way from the 197 duration of the smaller cam!
This 3883986 cam is then used in the following motors:
Interesting stuff. I had to give some thought to selecting a cam for my 66 396/325. For me the choice was based not necessarily on returning the car to originality but what i was going to use it for. Since i was going to run an overdrive auto and the original Quadrajet, I figured I wanted something that put out good results at a pretty low rpm. Once I had those ideas, it ended up pretty much running the stock cam as that was what that cam was good for in the first place.
With it I get a good idle, a good quick pull all the way to about 80 mph, a fairly economical cruise rpm of about 2200-2300 (around 70-75 mph where I can legally get away with it) and a top end that i have yet to really explore.
I'ts a nice understressed package that seems to work well for me. At my age I'm not so interested in really winding it out anymore in any case.
Gm always liked to recycle ,when you select a cam look at what you want to use the vehicle for, im still a big fan of a 350hp cam in the 327s. some guys don't like them, old school but it works. I like a high lift , dual pattern and not to big a duration at .050, with a 112 lobe center, it works well with 10.1 to 11.1 compression, just me I guess. I use it for towing to goofing around.
If originality were not a concern I always assumed I would convert to hydraulic roller to pick up the benefits of "modern" camshaft design but these GM designs were actually very advanced for the day, even if antiquated now. They featured asymmetrical lobes and 'slow down' ramps for parking the lifter on the base circle more gently, that sort of thing. And for reasons that are above my pay grade they make a lot of vacuum.
One other random observation kind of related: If you add a four barrel to the base 250hp 350, you pick up only 5 hp. So that tells me that 2bbl is plenty big enough for 250hp. But it caps out around 265hp, which is all it can make on the 396.
Also I guess up until you hit that carb limit on the 2bbl the motor is otherwise identical to the 396/325 for in-town, towing, and torque stuff.
Back in 1980 when I went to change the cam in my 454 Parisienne (my original one, the engine is now in my 2+2) I was going to put in the high lift hydraulic cam, likely the 3883986. I bought it at the local GM dealer, but when I got it home I found it had the grooved rear main, suitable for 1965/66, but not 1970. I took a little doing, but I got them to take it back and refund my money. In went a Crane, which I think I still have in my garage somewhere.
Are you saying Zora Arkus Duntov designed the carb I grew up with on my 250hp 350cid Laurentian?
I don't know that he specifically designed that carb, but he would have spec'd the sort of requirements needed for his vision & version of any powerplant package he was developing. He was the primary Corvette engineer and as such never actually spec'd any single 2-barrel engine.
Chevrolet also used a dual pattern 280/288 degree cam in the majority of small blocks, from the 307, 350 2 barrel, 4-barrel, 4-barrel high compression engines from the 1960s through the 1970s. Engines like the base L14 307; L65, LM1 & L48 350s.
They also used a small 2-barrel for the 307 with 1.44" throttle bore, the more typical 350s used the common 1.69" throttle bore, and a similar 2-barrel was used on 400 small blocks with 1.69" throttle bore but with a 1.40" venturi rather than 1.25". The 1-year only 396 2-barrel also used a 1.69" throttle bore but with a 1.09" venturi (same as the 327 2-barrel in '69 according to published SAE specs).
I wonder what Oldsmobile used for a 2-barrel on engines like the Turnpike Cruiser package (400 2-barrel / TH400 / 2.56 axle) or the 425 & 455 2-barrel engines? Probably nothing that a Quadrajet couldn't do better or more efficiently, but then again they were built to a price point.
__________________
67 Chevelle Malibu Sport Coupe, Oshawa-built 250 PG never disturbed.
In garage, 296 cid inline six & TH350...
Cam, Toronto.
I don't judge a man by how far he's fallen, but by how far back he bounces - Patton
Looks like 3896929 was the go-to cam for most of the common smallblocks back to 1962. In fact it's the broadest part I've ever seen listed:
1963 - 69/ALL WITH 283 - 307 - 327 - 350 ENGINES
So basically all pedestrian small blocks of all displacements from about 1962 through at least 1971!
I assume that's what you mean by the 280/288, which must be "advertised" duration because the real specs @ 0.050" are 195/202 duration and .390/.410 lift.
The wildest smallblock cam seems to have been the 370hp cam (3849346) - 254/254 and .485/.485 which is bigger than all the big block cams save the L88.
That's probably 95% of the engines they made in those days. Imagine being the engineer responsible for that cam... you've just set the driving behavior, idle, fuel usage, and power characteristics for literally 50 million cars over the next decade!
I don't have catalogs going back to 1963, but it looks like it might have been the same part all the way through. Here are the catalogs it appears in, which is a lot of them. So I don't think it'd be a universal service replacement if it was also listed in all of these, but that's just a guess... it was eventually superseded/replaced by 14060651 but I think it might be the original.
The oldest book I've found it in so far is 1967.
-- Edited by davepl on Tuesday 20th of March 2018 11:30:17 AM
"The 3896929 hydraulic grind; a cam that has been used as standard equipment in everything from the low-horsepower 283 to the medium output 350-ci engine. Its a perfect example of the ideal street cam for production vehicles. The timing is versatile enough to work in a wide variety of engines without hurting performance in any of them. Notice that it follows the game plan by opening the intake valve early: 26 degrees before TDC to be exact. This gets things moving and we can start adding up event timing to arrive at the number of degrees (duration) that the intake valve is open. To the initial 26 degrees of crankshaft rotation before TDC, we can add 180 degrees for the entire intake stroke and 90 more degrees for the amount of time the valve remains open after BDC. The total of these figures is 296 degrees, and thats listed as duration (in crankshaft degrees) at the lash point. This actually means the number of degrees between the point at which the valve lifts off and then returns to its seat. It is not, however, the true effective duration of the intake event. At extremely low valve lifts there is little significant flow, so we have to look at the timing from a point where the charge actually begins to gather momentum. Manufacturers disagree on just where this point occurs. This is understandable, since this initial point can be affected by a variety of factors, the most obvious of which are carburetor venturi area, intake manifold design, and cylinder head efficiency. It is generally agreed that significant flow begins somewhere between 0.010- and 0.050-inch valve lift and most manufacturers use a figure in this range as their reference checking point. The most common figure is 0.050 inch, although 0.020 and 0.030 inch are sometimes used for published specs.
At 0.050-inch valve lift, the intake event on the 3896929 cam is reduced to only 195 degrees. But this is not as bad as it seems. Recalling that the theoretical intake stroke is only 180 degrees, it provides an additional 15 degrees of duration in which significant flow can continue to fill the cylinder. And all is not lost with the low-lift flow, because the valve is physically off the seat during this period and some charge movement does occur (a certain amount of lead-time inertia isestablished for each event).
The 0.050-inch checking point is an excellent means of comparing camshafts, but only if the published figures for each cam are rated at the same 0.050-inch figure. If they differ, it will be difficult to arrive at a meaningful comparison without physical testing. If we move down the chart to one of the early race profiles (PN 2927140, for example) we can observe how the timing events are realigned to work at higher engine speeds. Where the hydraulic street cam opened the intake valve at 26 degrees BTDC (before top dead center), the 2927140 cam opens it at a much earlier 44 degrees BTDC. You have to remember that as engine speed increases, there is less time for each event (cycle) to occur. Therefore, a longer period (in crankshaft degrees of duration) is used to make up for it. You may notice that while the intake opening is significantly earlier, the closing point is nearly the same. This is because the designers wished to maintain a flat torque curve since this cam is intended for use in short-track racing, where superior torque is required to accelerate heavy cars out of low-speed turns. Cams listed below the 2927140 cam have a later closing point because they are intended for longer tracks and higher speeds where it is beneficial to move the torque curve higher in the RPM range.
When you examine the chart you will find that the 0.050-inch specification for the 2927140 cam provides 48 degrees more duration that the 3896929 cam. This is a significant breathing increase for the faster running engine. All of this is, of course, general and any cam grinder will tell you that the success of the overall combination depends greatly on the compatibility of the rest of the engine components and even the gearing, tire diameter, car weight, and a hundred other factors. With the basic information offered here, you should be able to study various cam profiles and observe how each is designed for a particular application. It may take considerable study before you are able to approximate the timing required for your engine. But before you get too engrossed, there are several other factors that can affect your selection. Chief among them is the lobe displacement angle, sometimes called the lobe-center angle.
I've always been a big believer in stock cams. As long as you are driving a car more or less in the way it was designed to be used with a stock powertrain its hard to beat the factory cam for all around enjoyment.
One other random observation kind of related: If you add a four barrel to the base 250hp 350, you pick up only 5 hp. So that tells me that 2bbl is plenty big enough for 250hp.
Thinking the 4 bbl only added 5 H.P. on the short lived 1969 LM1, 255 H.P. is incorrect.
It was advertised at 255 H.P. to give non SS Camaros and Novas an edge in NHRA drag racing.
They're still competitive in stock classes today.
It was replaced with the L65 250 H.P. Engine after a few short months of production.
Only Canadian Pontiacs had the L65 Engine available from the start of 1969 production, along with the LM1 and L48.
Chevy had to wait until around Jan. 1969 to start using the L65. 250 H.P., after the LM1 was gone
In 1971 the very same Engine, with .5 less compression, was rated at 270 H.P. though it was coded L48.
I consider Chevy H.P. ratings to be more of an "Engine I.D." than its place in the "pecking order"
Similar thing with the 1970 360 HP 454, it got less compression and was re rated to 365 H.P. for 1971.
NHRA freely refactored in those days anyway, that doesn't seem like much an edge.
The number might not be -exactly- 5 but if you look at the peak output of the 350 (255hp) and the peak output of the 396 with the same carb (265hp) it'd be 10. My basic point stands though I grant the published numbers back then are anything but gospel and were often more marketing and ID than measurement.
I'm confident about the flow rate of the 2GV based on the difference between the 265hp and 325hp motors being intake-only, even if the LM1 gain is less certain.
Good point, was there any mechanical difference between the 385 and 390?
Also didn't most of these motors switch from closed to open chambers without changing rating?
And of course there's the 396, which I never fully memorized - 375 one year, 425 the next, mechanical cam then a hydraulic, and numbers all over the place it seemed.
So why the 430 and 435 charade if the Vette wasn't subject to the AMA limits anyway? Or was the 435hp motor fairly rated and then the L88 stuck just below it to stop people from blithely picking the "top" option?
Obviously the L88 made more than 430, but the did L71?
