What are the main differences between a 70 350-370 and a 71 350 270. Was 270 the max HP in 71 for this engine. Thank you JBM 30025
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Re: 70 vs 71 LT1
The CR was dropped from 11 to 9:1 to comply with the GM management edict that all '71 models operate on 91 RON unleaded gasoline.
Also the two different HP numbers you quoted are apples and oranges. The 1970 370 HP rating is SAE gross. The 270 is SAE net - as installed in the vehicle with the accessories and exhaust system. The 1971 SAE gross rating was 330, so the drop in CR cost about 10 percent. In that one year, 1971, GM quoted both gross and net ratings.
Beginning in 1972 only SAE net ratings were quoted.
As a general rule of thumb and WAG, SAE net is about 80 percent of SAE gross, but depending on the specific engine and installation configuration it can vary from the seventies to near ninety percent.
Duke -
Re: 70 vs 71 LT1
The CR was dropped from 11 to 9:1 to comply with the GM management edict that all '71 models operate on 91 RON unleaded gasoline.
Also the two different HP numbers you quoted are apples and oranges. The 1970 370 HP rating is SAE gross. The 270 is SAE net - as installed in the vehicle with the accessories and exhaust system. The 1971 SAE gross rating was 330, so the drop in CR cost about 10 percent. In that one year, 1971, GM quoted both gross and net ratings.
Beginning in 1972 only SAE net ratings were quoted.
As a general rule of thumb and WAG, SAE net is about 80 percent of SAE gross, but depending on the specific engine and installation configuration it can vary from the seventies to near ninety percent.
DukeComment
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Re: 70 vs 71 LT1
John:
As Duke stated, the compression ratio dropped from 11 to 1 to 9 to 1 in comparing the '70 LT-1 to the '71 LT-1.
You asked for differences in these engines, the '70 LT-1 uses a different head that utilizes a smaller, different shape combustion chamber, head casting # 3973414, while '71 LT-1 used head casting # 3973487 with larger, different shaped combustion chamber.
I'm not absolutely sure of this, but I believe the '70 LT-1 used a flat top piston, while '71 LT-1 used a slight dished shaped piston.
Also '70 LT-1 370hp, while '71 330hpComment
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Re: 70 vs 71 LT1
John:
As Duke stated, the compression ratio dropped from 11 to 1 to 9 to 1 in comparing the '70 LT-1 to the '71 LT-1.
You asked for differences in these engines, the '70 LT-1 uses a different head that utilizes a smaller, different shape combustion chamber, head casting # 3973414, while '71 LT-1 used head casting # 3973487 with larger, different shaped combustion chamber.
I'm not absolutely sure of this, but I believe the '70 LT-1 used a flat top piston, while '71 LT-1 used a slight dished shaped piston.
Also '70 LT-1 370hp, while '71 330hpComment
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Re: 70 vs 71 LT1
The '70 LT-1 piston is domed. The '71 version,also forged, is not domed. The drop in CR came from the combination of both larger combustion chamber and a non-domed piston.
DukeComment
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Re: 70 vs 71 LT1
The '70 LT-1 piston is domed. The '71 version,also forged, is not domed. The drop in CR came from the combination of both larger combustion chamber and a non-domed piston.
DukeComment
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Re: 70 vs 71 LT1
The difference in performance is significaant. I had a '69 350/350 that I upgraded to 350/370 specs on the July 4 weekend of 1970. It was quite quick; really got legs over 4000 revs. I later bought a '71 LT-1, but it was a dog, comparatively. The loss of CR with the high overlap cam made a real difference. Still later, I answered an advertisment for a ZR-1; I didn't even go look at it, cuz I knew that it wouldn't run strong.
Ol' GeezerComment
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Re: 70 vs 71 LT1
The difference in performance is significaant. I had a '69 350/350 that I upgraded to 350/370 specs on the July 4 weekend of 1970. It was quite quick; really got legs over 4000 revs. I later bought a '71 LT-1, but it was a dog, comparatively. The loss of CR with the high overlap cam made a real difference. Still later, I answered an advertisment for a ZR-1; I didn't even go look at it, cuz I knew that it wouldn't run strong.
Ol' GeezerComment
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Re: 70 vs 71 LT1
Tbe combination of high overlap and low compression reallys KILLS output, especially at the low end!
If I were rebuilding a low CR LT-1 I would definitely try to get the CR back up as high as possible with the OE heads, or maybe even retrofit a set of 186 small chamber heads.
The LT-1 cam will allow operation with (honest, measured) CRs up to about 10.5:1 wihtout detonation on current unleaded premiums.
DukeComment
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Re: 70 vs 71 LT1
Tbe combination of high overlap and low compression reallys KILLS output, especially at the low end!
If I were rebuilding a low CR LT-1 I would definitely try to get the CR back up as high as possible with the OE heads, or maybe even retrofit a set of 186 small chamber heads.
The LT-1 cam will allow operation with (honest, measured) CRs up to about 10.5:1 wihtout detonation on current unleaded premiums.
DukeComment
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Re: 70 vs 71 LT1
Thanks a lot guys. I was looking at a 71 LT1 to restore and drive but I don't want a dog. Sounds like changing a few internals can bring it back to life without changing the outer configuration of heads etc. Are there noticeable differences in the heads when viewed as an assembled engine (as by a judge)Comment
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Re: 70 vs 71 LT1
Thanks a lot guys. I was looking at a 71 LT1 to restore and drive but I don't want a dog. Sounds like changing a few internals can bring it back to life without changing the outer configuration of heads etc. Are there noticeable differences in the heads when viewed as an assembled engine (as by a judge)Comment
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Re: 70 vs 71 LT1
Not sure, but I don't think there are any external distinguishing features between the 186 and 487 heads - maybe someone can confirm.
The casting numbers and date codes are under the valve covers, so they are not observable during judging. Heads are judged strickly on their external identifying features such as the "double hump" casting marks on the ends and the presense or absense of accessory mounting bosses/holes on the ends of the heads.
The 461 and 462 castings don't have accessory mounting bosses/holes on the ends, but later big port "double hump" heads do.
With proper piston/gasket selection you can probably get the '71 LT-1 back up to at least an honest 10:1. Though the '70 advertised CR was 11:1 most production engines measure about a half point lower than advertised, which is typical for most GM production engines of that era.
DukeComment
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Re: 70 vs 71 LT1
Not sure, but I don't think there are any external distinguishing features between the 186 and 487 heads - maybe someone can confirm.
The casting numbers and date codes are under the valve covers, so they are not observable during judging. Heads are judged strickly on their external identifying features such as the "double hump" casting marks on the ends and the presense or absense of accessory mounting bosses/holes on the ends of the heads.
The 461 and 462 castings don't have accessory mounting bosses/holes on the ends, but later big port "double hump" heads do.
With proper piston/gasket selection you can probably get the '71 LT-1 back up to at least an honest 10:1. Though the '70 advertised CR was 11:1 most production engines measure about a half point lower than advertised, which is typical for most GM production engines of that era.
DukeComment
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