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I have a 70 Lt1 coupe, and I have several questions I need help with. My exhaust manifolds were cracked (both sides). The L/H number was 38727#5 LH1 (the # is a 5,6 or 8 I think) and the R/H is 3832461 RH5. Th GM replacement R/H has the A.I.R. holes, but is marked 14081238. The L/H does not have the A.I.R. holes and is marked 3846559A LH1 SCC (and a 6/3/99 date- ouch!). I have not been able to find correct replacements. I have tried the popular Vette part magazines and several corvette junkyards. Any ideas where else to look? And about the A.I.R. holes. Now I cannot find the plugs. Any ideas on that? And since we are on the subject, why would a 1970 special high performance Corvette have a smog pump (that uses the holes). I do not have one, but the LT1 convertible in the Corvette Fever (of the 88 left controversy) magazine did have one. Am I supposed to? I will save my other 100's of questions for later. Thanks in advance. Jeph (NCRS# hopefully in the mail)
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Re: 70 LT1 Ramblings
in the july issue of chevy high performance on page 92 there is a article on repairing cracked exhaust manifolds. if you are interested i can scan the article to you. -
Re: 70 LT1 Ramblings
Jeph,
In 1970, the LT-1 was originally equipped with an AIR system. AIR systems were not installed for performance; they were required to meet government emissions regulations. Higher performance engines were MORE likely to have emissions problems.
The correct exhaust manifold part numbers for '70 LT-1 are 3872765 (LH) and 3932461 (RH).
A good place to find used parts is the big meets, such as Corvettes at Carlise (Aug 25-27). Also check the NCRS "Driveline" and Hemmings Motor News. Paragon Reproductions (800-882-4688) has some new reproduction manifolds (not sure which applications available), and they also sell NOS/used parts. Bair's Corvettes (8000-421-9644) is another possible source of used parts. Use the reproduction only if you can't find good used with reasonable date codes.
Chuck SangerhausenComment
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Re: 70 LT1 Ramblings
I would appreciate that, thanks.Comment
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Re: 70 LT1 Ramblings
OK. I knew they started de-tuning in 71 to begin getting ready to meet the requirements, but was not aware there were already some requirements in place. Carlise is a long haul, and I have checked with Paragon. I'll try the other routes and check on the possibility of repairing the R/H. Thanks, JephComment
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Re: 70 LT1 Ramblings
Jeph, try Paragon Vintage, They usually have a number of original parts for most cars. Good luck.
regards
BillComment
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Re: 70 LT1 Ramblings
Jeph----
ALL 1970-72 LT-1 engines used the Air Injection Reactor System (AIR). This system was used since its effect on performance was LESS than the other system, first used for Corvettes in 1970, which was the Controlled Combustion System (CCS) The latter system used carburetor and ignition mods to achieve exhaust emissions reductions and was VERY performance "unfriendly". So, GM only used this system on lower performance engines. In fact, even during 1968 and 1969, Chevrolet used the CCS system on most passenger car engines with automatic transmissions. The only exception was for Corvettes which ALL used the AIR system for 68-69. This included ALL automatics and even ALL L-88s and ZL-1s. The AIR system is MUCH MORE performance "friendly" than other emissions control options. Many folks don't think so, but IT IS.
As far as your exhaust manifolds go, others have advised you correctly. You require left side manifold GM #3872765 and right side manifold GM #3932461. The 3932461 manifold was used on many 1969-1980 Corvettes, but not all are drilled and tapped for AIR fittings. This particular manifold casting was manufactured both ways. Nevertheless, this manifold is fairly plentiful in both versions and you should not have a difficult time locating one. It has been GM-discontinued, though, for over 20 years so an NOS version will be hard to come by.
The left side manifold, GM #3872765, was a special manifold that was used only during the 1968-71 model years. It has an alternator bracket on one end. This particular manifold was only produced in an AIR drilled and tapped version. Actually, it was available from GM until about 6 or 7 years, or so, when it was discontinued. It is a lot harder to find than the 3932461, though.
As far as I know, neither of these manifolds are currently available in reproduction.In Appreciation of John HinckleyComment
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Re: De-tuning vs. Emissions
Jeph,
I suppose the AIR system could be thought of "de-tuning" in a way. But not really in the way that you mention for the 1971 and later engines. That de-tuning was a purposeful reduction in engine performance with lesser power producing "grade" of components. TBarr #24014Comment
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Re: De-tuning vs. Emissions
The '70 LT-1 is a true Special High Performance small block, just like the good ol' days, but they did have to fiddle with the ignition curve, fuel curve, and add the AIR system to meet emission standards. The real detuning began in 1971 when the CR was reduced to 9.0:1 from 11.0:1 to comply with the corporate edict that all engines operate on unleaded fuel, which in those days was only available in one "regular grade" octane rating.
The ignition was retarded to increase exhaust gas temperature, which also reduced part throttle fuel efficiency. The hot exhaust and injected air resulted in oxidation (combustion) in the manifolds which made them run hotter than pre-emission days, so eventually they crack. Of course, if someone richened the carburetor over the years without reworking the ignition curve or disabling/removing the AIR system it would just make the situation worse.
My advice to Jeph would be to set the distributor up to '65 SHP/FI specs. This should eliminate cracked manifolds and really light up the low and mid range torque curve. The carburetor could probably use some recalibration, but not being intimately familiar with the LT-1 Holley setup I can't offer any specific recommendations. The AIR system should also be removed or disabled. You can always reinstall it for judging.
DukeComment
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Re: De-tuning vs. Emissions
I don't have any part of the A.I.R. system except the manifolds. And since I got the car as one of the ultimate examples of a pure muscle car, that will be one of the last pieces of the restoration I would do. I do not have the transistorized ignition (I do still have the pulse module in a box), so I am running the 1112021 high performance (350/350) distributor modified into a breakerless ignition to reduce some of the inefficiencies at the high RPM's that this car seems to love. I have it set at 8 deg before instead of the 14 deg before requirement for the TI setup. I have the carburator tuned normally, set 1/8 turn rich at idle which seems to work great. If you give the the 65 SHP specs, I will try that to see how it works. The location of the cracks in the manifold point me to an exhaust system that was not properly supported during maintenance, excerbated by the age, thermal cycles and oxidation problems you mentioned. Thanks for the help! JephComment
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Re: De-tuning vs. Emissions *TL*
Jeph,
I doubt the 14 degrees timing is dictated by the TI. I think it more likely is driven by emissions, carburetor/camshaft and available fuel considerations. The vacuum advance, centrifugal advance are also a factor, but those curves could be created in your points type distributor easily. Now days the fuel may be the major controller of timing settings.
The other guys will advise you further as we are at the limits of my knowledge. Duke's idea for using the pre-emissions SHP specs is intriguing and I will be interested in seeing what those are and how they compare to the 1970 SHP specs. What intrigues me most about his suggestion is that those changes could be made with little or no adverse impact on judging.
Terry
TerryComment
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Re: De-tuning vs. Emissions
The basic theory on ignition timing is to get it to the maximum as quickly as possible, but detonation is the limiting factor. The small chamber SB heads generally make peak torque (at most engine speeds) with 38 degrees total advance, so the trick is to get this in as soon as possible without excess detonation.
There was a running change on '64 SHP and FI distributors. They had a new centrifugal and vacuum setup. Previously the centrifugal advance was not all in until 4600 RPM. The new setup brought it all in at 2350 (1175 distributor RPM). The max distributor advance was 24 degrees at the crank, so best torque and power was usually obtained with initial timing in the range or 12 to 16 degress. Back then we had 100 plus octane leaded fuel available and there were no tailpipe emission standards. The vacuum advance can specs are 0 deg. at 4" HG. and 16 deg. (crank) at 8" Hg. The prior vacuum can didn't max out until about 15" Hg., which is below the idle vacuum of these engines. This exascerbated the inherent idle instability of these engines, because the vacuum advance would wander around.
I doubt if the centrifugal weights and springs are still available from GM. The parts manual says 1116236 for the vacuum can. Since this probably has a visible stamped number, it is likely detectable by judges, but the centrifugal advance parts are not unless they remove the distributor cap.
The '70 LT-1 ignition curve was modified to meet emissions. Don't know the specs, but they should be listed in your service manual. Basically what you should do to maximize engine output without detonation is to experiment with aftermarket advance kits. You probably won't be able to get the centrifugal advance in a quick as in '65 because of current fuel quality, but the closer to '65 specs, the better.
DukeComment
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