RPO469C calls for 270h.p. dual quads and a hi-lift cam. the spec guide says the lift on intake is .393 and exh is .399. a 230h.p base engine is .3987 intake and .3987 exh. so why is the 270 h.p. considered a hi-lift cam? does it have to do with duration? also where is the best place to find a good replica of this cam?
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Re: 1959 270 h.p. cam question
I'm glad you asked the question since I'm fixing to search for the same camshaft. I've had good luck in the past getting repro muscle car cams from Crane, but the current group of telephone techs there don't seem knowledgeable on the older stuff, and their reference material seems to have many flaws.
Recently I've been trying to buy an LS7/ZL1/Second design L88 camshaft from them, which I've bought in the past, but lost the number. For some reason the GM high perf parts catalogue now refers to L88 as 435 horse, and Crane has accepted this as gospel and has the same screw up in their book now and wants to sell the 396/375, 427/425, 427/435 for the early L88. The same tech also stood me down the other day that the LS7 & 70 model LS5 are the same camshaft and just wouldn't hear of the fact that one was mechanical and the other hydraulic............ said his info comes straight from GM and therefore my firsthand experience must be mistaken.
Bottom line in all this is............ Crane has all this stuff available, INCLUDING the early solid lifter camshaft for the '59, BUT finding somebody there who can pull it out of their archives without an incorrect substitution may prove a challenge.
Now I'll show my ignorance on early smallblock SHP camshafts............ is the 270 & 283 horse 283 camshaft the same as the 340 & 360 horse 327 camshaft? -
Re: 1959 270 h.p. cam question
I've discussed the "high-lift" anomaly before. The lifts you quote are the gross lobe lift including clearance ramps times 1.5, which yields .3937/.3997", but went you subtract out the .008/.012" clearance ramps from the .2625/.2665 gross lobe lift you get less lift at the valve than with the base cam at any rocker ratio, which is about 1.37:1 at the lash point and about 1.44 at maximum lift.
Clearly, the name "high-lift cam" is a misnomer. Maybe it was so-named to throw off the competition as to what it actually does.
The Duntov cam doesn't work very well on 461 and later big port heads because it just doesn't lift the big inlet valves far enough to achieve maximum flow coefficient, but it's okay for the small port/valve 283 heads, which is what it was designed for.
Google on 3736097 or Duntov cam or Duntov camshaft and you will find plenty of clones. Speed Pro's part number is CS113R.
DukeComment
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Re: 1959 270 h.p. cam question
I have a couple of near new 098 cams if you are willing to take a chance on one less than brand new. Will sell for a reasonable price.
Thanks,
WayneComment
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Re: 1959 270 h.p. cam question
Sorry! I meant to have emailed this and can't remember my password to delete.Comment
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Re: 1959 270 h.p. cam question
Duke, I was both shocked and amused to see you recommending my favorite cam. Now I am worried though that the LT1 cam may have been discontinued. heee. JohnComment
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Re: 1959 270 h.p. cam question
The Duntov cam works well with the small port heads on a 283. After all, that's what it was designed for, but it would work better for a road engine if the LSA was spread out to about 114 degrees, and the inlet indexing delayed about 2 degrees. This would give it the same basic ,050" timing specs as the L-79 cam.
The 461 and later big port heads have a relatively resticted exhaust port compared to the inlet port, so they need more exhaust than inlet duration.
But then if you pocket port them, the E/I ratio increases so much that you need more inlet than exhaust duration, and no OE or aftermarket cam meets this requirement. However, if you combine a 30-30 lobe on the inlet side with a Duntov lobe on the exhaust side with proper indexing and LSA, it beats the LT-1 cam.
I don't have a favorite cam. I like the LT-1 cam with big port heads because it produces the broadest torque bandwidth compared to any other off-the-shelf mechanical lifter cam, but I knew it wasn't ideal.
The key to the "magic cam" is to understand the engine's complete inlet and exhaust flow characteristics including exhaust back pressure so you can design a cam that produces the best VE across the entire rev range.
DukeComment
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Re: 1959 270 h.p. cam question
Duke,
"The Duntov cam doesn't work very well on 461 and later big port heads because it just doesn't lift the big inlet valves far enough to achieve maximum flow coefficient, but it's okay for the small port/valve 283 heads, which is what it was designed for".
By stating, "big inlet valves" do include both configurations of 2.02 and 1.94 inlet valves?
The 1961 283/315 engine had the 461 heads, 1.94 intake valves along with the 097 cam. If GM continued to use the 283 for the 62 corvette what combination of components would you imagine would be used in terms of heads/ valve size cam and exhaust, induction?
John, Duke,
As the 283 continued in other chevy models what components did the hi po 283 engines include?
Thanks,
JoeComment
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Re: 1959 270 h.p. cam question
Yes, my comments include both inlet valve sizes. There is only a minor difference in flow between the 1.94" and 2.02" inlet valves. The biggest improvement on the the big port heads with the larger valve sizes is the larger exhaust valve, which helps with a single pattern cam, but a dual pattern with more exhaust duration in the form of an earlier opening exhaust is necessary for best broad range performance.
But, when you massage the heads everything changes - less exhaust duration with a relatively late opening exhaust valve is best because of the dramatic increase in exhaust flow relative to inlet flow, which improves the E/I flow ratio from about 0.65 to nearly 0.80, but long exhaust duration with an early opening exhaust valve (like the LT-1 cam) doesn't do too much harm. The biggest "problem" with the OE SHP cams is that they have too much overlap because they were developed and tested on lab dynos with zero exhaust backpressure. When you add even modest exhaust backpressure, high overlap decreases performance, particularly at the low end.
The LT-1 cam will work okay on a 283/315 engine, but the bottom end needs to be qualified because the peak power range will be 5500-7200 or better.
After '61 all the 283s were low to medium performance "economy engines".
DukeComment
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the stock corvette cams were designed
for use in racing because back then the engine was to be "stock" and the racers used open exhaust. all SHP chevy engines really pick up performance when you went to open exhaust because of the cam designComment
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Re: the stock corvette cams were designed
That's a good point. Before circa 1964 SCCA production engines had to be "stock" and no material removal from the head was allowed. The OE exhaust manifolds were also required, but you could run open exhaust in any configuration as long as the exit was behind the driver. Duntov designed his cam with one objective in mind - WIN RACES - and it won a boatload of SCCA Regional and National Championships. Back in that era it WAS a "racing cam".
But I also think that Chevrolet paid little or no attention to "as installed" power with accessories and exhaust backpressure in that era. It wasn't until the more realistic "SAE net" power came into being in the early seventies that the OEs started paying attention to the effects of overlap on as installed SAE net power with the vehicle exhaust system versus SAE gross on a lab dyno with no exhaust backpressure.
So the SHP cams have too much overlap for any production exhaust system, but they work well with open exhaust and even better with headers and open exhaust, but the backpressure of the OE exhaust system mostly negates the wave dynamics generated by headers, so headers are only marginally beneficial with the OE exhaust system and essentially worthless on low overlap cams like the base engine cams with or without exhaust backpressure.
Modern engines have much less overlap. Part of this is emissions related, but it also helps improve torque bandwidth. A case in point is comparing the current LS7 to the vintage L-79. Even though the long stroke LS7 can tolerate more overlap, the EFFECTIVE overlap of the LS7 cam is about 2.1 sq-in-deg versus about 4.3 for the L-79 cam and a whopping 5.5 for the 30-30 cam, which is why it is so soggy down low.
You can get a hint of this be comparing lobe separation angles - about 120 degrees for the LS7 cam and 114 for the L-79 and 30-30 cams.
The LS7 cam is also VERY well tuned to the LS7's E/I ratio which is quite low in order to allow as much inlet flow as possible even to the detriment of exhaust flow. Consider that the LS7 has an inlet valve about the same size as a SHP 427, but and exhaust valve about the same size as a SHP 327. To help compensate for the relatively restrictive exhaust valve and port, exhaust duration is nearly 20 degrees more than the inlet duration and the exhaust event is phased very early to open the exhaust valve early. Conversely the inlet event is phased late, which minimizes overlap and enhances power at very high piston speed because it closes the inlet valve rather late.
I used the same technique to design the Special 300 HP cam, but the end result is different because the head flow E/I ratio is very high instead of very low. In fact, not withstanding the difference in lifts, if you take the LS7 cam, swap the lobes, knock about 8 degrees duration off each and advance it 8 degrees you've got the Special 300 HP cam.
Understanding all the flow relationships and sweating the details pays dividends!
DukeComment
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Re: 1959 270 h.p. cam question
Duke:
Just wondering if you have done any experimenting with your computer program on valves with reduced diameter stems like the Manley Race-Flow. Do they improve flow or are they hype?Comment
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Re: 1959 270 h.p. cam question
The simulation programs provide best results when actual head flow data is fed into them, and since the difference between the 1.94" and 2.02" inlet valves is minor, I would suspect that the difference between conventional and "race flow" valves would be negligible, but the only why to know for sure would be to flow test the same head back to back with both types of valves to determine if there is any significant difference in flow.
What I've found with pocket port heads and the SHP cams and Special 300 HP cam is that the inlet system is effectively choked in the 6500-7200 range. This is expressed in an output called "Mach number" (it's actually called Mach index as originally defined by C. F. Taylor), which is a dimensionless parameter than takes into account port flow, valve size, manifold runner flow, and valve timing. Once it achieves a level in the range of 0.55 "more cam" only provides marginal improvements in top end power while killing large amounts of low end torque.
This is somewhat "good" because we are getting the most out of the heads while maintaining good port velocity, which is critical to good low speed driveability and torque and decent fuel economy on a carbureted engine because fuel mixing and vaporization is highly dependent on port velocity. (You should not run a head with inlet runners larger than 180 cc on a carbureted SB road engine if you want something other than a gas guzzling, torque shy cantanerous beast with poor low speed driveability.) By comparison, the LS7 peak Mach index is only about .42, but since it's fuel injected, high port velocity is not as important.
It also appears that on vintage small blocks the actual limiting factor with pocket ported heads are the OE inlet manifolds. Rather than "more cam" (or some fancy valve geometry) a higher flowing inlet manifold such as the LT-1/Z-28 manifold is what's required to increase peak power and extend the useable power rev range with little or no loss of low end torque.
It's also noteworthy that Rockester FI produces little power improvement with OE machined heads, but once the head flow is improved, the FI system provides a substantial improvement in top end power due to it's single plane manifold architecture and large, nearly straight runners, but it takes a pretty good hit on low end torque. Given the above observation, it suprises me that so few guys in vintage racing are running FI.
I recommend OE equivalent replacement valves with maybe an upgrade on the exhaust side to a 21-2N or 21-4N stainless steel to ensure longevity. "Trick parts" like high ratio rocker arms and "race flow valves" can eat up your budget pretty quick, but only provide marginal improvements on the top end. They might make a difference on a racing engine in a very competitive type of racing where small differences in peak power can provide a competitive edge, but this is not typical vintage Corvette service.
On the other hand you cannot spend too much money on upgraded connecting rods to replace the spindly originals on ANY small bearing small block, so my recommendation is to upgrade the rods and everything else OE or OE equivalent and leave the trick "race parts" to racers who might actually benefit from them.
DukeComment
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Re: 1959 270 h.p. cam question
the best way to increase low lift flow is the machine 20/25 degreea back angle on the valve head starting at the edge of the valve seat narrowed to the width of the seat in the headComment
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Re: 1959 270 h.p. cam question
Yes, that is very important and often overlooked. The best "multiangle valve seating" explanation I've seen is the sketch in "How to Hot Rod Small Block Chevys". If you want maximum top end power for your configuration, follow these instructions precisely!
This cut on the topside of the valves is important to low lift flow. What you need for good low lift flow is an "annular venturi". Without this top cut there is a severe discontinuity in the flow path at low lift, particularly on the inlet side, but three angles on the seat and this top cut on the valve achieves geometry close to the ideal venturi shape.
DukeComment
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