Cam question 4 Duke W.
Duke, If I were to take my 340 SHP out for a good run with it's present 079 cam and then were able to jump in an identical car with an LT-1 cam what would I notice as the visceral differences in the driving
experience?
experience?
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Jim Cear -
Re: Cam question 4 Duke W.
I assume you mean the ...097 Duntov cam.
It depends on other variables such as head flow. actual compression ratio, and spark advance map, but if you were to take a typical Flint L-76 with a Duntov cam and did nothing but install the LT-1 cam with the original lazy spark advance map, they would probably be about the same at the low end - maybe with a slight edge to the Duntov - about the same in the mid range, but the LT-1 cam would be stronger up top and would not fall off as fast past the peak.
For sure the 5000-7000 range would be the biggest difference.
Duke -
Re: Cam question 4 Duke W.
Let's go one step further; My L-76 is stock w/44k miles on the clock, but has been massaged with an aggressive advance curve, rejetted 3721SB (air bleeds, fuel and off-idle mods for today's gas), Pertronix II electronic ignition (for consistent runups to redline), valves set per Duke' method @ .008" in. & .016" ex., and factor in a 3.36 final drive ratio. Same story??
Stu FoxComment
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Re: Cam question 4 Duke W.
...more low end torque, but that would be due to the aggressive centrifugal curve, not the cam.
Getting the centrifugal advance in as quickly as possible is a CRITICAL to achieving maximum low end torque.
If both cams had the same centrifugal curve, then it's back to maybe the Duntov having a slight low end advantage. A chassis dyno test of a "327 LT-1" configuration made 80 percent of peak torque at 2000 and 90 percent at 2500.
Get a Dynojet test and see what your engine does.
DukeComment
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Re: Cam question 4 Duke W.
Duke;
I'm almost afraid to run it up now after the recent thread cautioning against risking an original numbers matching engine, what with the potential weak rod condition you fellows noted. Then again, maybe you've reinforced that approach noting that I'm wasting my time pushing to redline anyway. I'll probably limit it to say 5500 from here on an enjoy it longer as it is.
Stu FoxComment
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Re: Cam question 4 Duke W.
Given your low mileage, but depending on how hard the engine has been run in the past, you're probably okay to 6000. RWHP probably peaks at 5500 and is fairly flat beyond that to over 6000.
My engine had certainly been run hard at times, including several days of race track hot lapping shifting at the redline.
At the 115K mile teardown the use showed up as a very badly cracked #7 rod right at the bolt seat, which is where the first design 327 rods are weak. I was probably just a few trips to the redline from blowing the engine.
I was lucky!
DukeComment
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Re: Cam question 4 Duke W.
My engine, compared to yours, has done more sittin than runnin (which is probably not good either). I've tried to put on a 1000 miles a year, so I'm about 3000 short of target. Guess I need a good trip out to your country with it. I can say this; each time I pull the rocker covers, it looks just like a new engine on the top deck and that makes me feel I didn't do too awfully bad in caring for it these past 47 years. It survived a couple of late oil/filter changes, two major periods of storage, and a number of sets of fouled plugs.
Stu FoxComment
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Re: Cam question 4 Duke W.
Your question is not clear, but I consider the early (pre-'66) rods to be boat anchors unless you are doing just a "stock rebuild" on a 250 or 300 HP engine that will never see more than 5000 RPM.
The second design rods are probably okay to 6500, but they should be Magnaflux inspected, which requires removal of the bolts so the bolt seats can be checked during Magnaflux, and then you have to install new bolts which requires resizing the rods.
By the time you've done the above, you've probably spent the price of a set of Eagle SIR rods - about $250 - which are probably as good as the Crower Sportsmans, which are good to 8000, so the only thing than can kill the bottom end is oil starvation.
My current philosophy on the second design rods is "all or nothing". If you are doing a "stock rebuild" and don't plan on massaging the heads, which means a L-79 will not make useable power above about 6000, just use the rods "as is" other than dimensional checks. If the heads are massaged a hydraulic lifter engine, depending on the camshaft, will make useable power to lifter pump-up speed which is 6500-6800.
Mechanical lifter engines will make useable power to about 6500 without head work and valve float speed - about 7200 - with head work.
In the long run, spending the $250 on a set of Eagle SIR rods for any engine configuration is a cheap insurance policy.
DukeComment
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Re: Cam question 4 Duke W.
Duke,
sorry for not being completely clear. I did mean magnafluxing as well as installing quality bolts. When I had my engine rebuilt to stock specs I was told, and thought I did some research to confirm, that the stock rods were acceptable if using the stock redline. This is on a '65 fi car.
GeorgeComment
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Re: Cam question 4 Duke W.
Your '65 has the early rods. If they pass Magnaflux, they have more life left, but it's still limited by fatigue considerations.
When I rebuilt my engine in the late seventies, I replaced the one cracked (at a bolt seat) rod. At the time I didn't know about the design change. I should have replaced all with the second design (at the time I think my price was less than 15 bucks), which has the additional material adjacent to the bolt seat. I just didn't see it at the time.
In order to improve durability, I did the typical "race prep" of the era - ground down the die flash along the beam flanks and polished them - both the seven original first design rods and the new second design rod - then had them shot peened along with installing higher strength bolts, which required resizing the big end.
Fatigue failures usually start from a surface defect - like a small crack that can be detected with a Magnaflux inspection. But a rod can pass Mag and then develop a crack in further service. Polishing the surface can inhibit the development of cracks, and shot peening work hardens the surface, which further reduces the tendency to crack.
The surface of highly stressed steel has a sort of "memory", which is why fatigue life is finite. However, reworking the surface as above typically "erases" this memory, and the fatigue clock goes back to zero.
Today, I would just throw the OE rods away and buy new rods because of the availability of inexpensive higher durability rods. Back then about the only choice was Carillo, which is what serious racers used. But in relative terms they were very expensive (IIRC about 500 for a set at the time), and since my intended application was road use observing the OE redline, I decided that reworking the OE rods as above would provide sufficient durabililty.
DukeComment
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Re: Cam question 4 Duke W.
Hi, Duke
As long as we started to talk about rods
What is your opinion of GM Powdered Metal Connecting Rods versus Eagle Connecting Rods
Thanks, ScottComment
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Re: Cam question 4 Duke W.
My understanding is that they are very good, but I don't think they're available in the small journal size for 283s and 327s.
DukeComment
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Re: Cam question 4 Duke W.
Per Jerry MacNeish, who runs constantly in NHRA Stock Eliminator in his '68 Z/28 (and won the E/Stock Wally at the U.S. Nationals at Indy three years ago), the GM PM rods are junk under race conditions. The racers in that class launch at 7000 and shift at 8400, and run mid-10's at 125 mph in 3400# cars with 5.57 gears (Jerry still holds the National Record at 10.54/126). The few Stock Eliminator guys that tried them lost engines and won't even think about using them again, but that's well beyond what we'd call "severe duty".Comment
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