LT1 1970 - 1972 Horsepower

[Erv M.]

During the three year production run of the LT1's I have read the horsepower drops each year and significantly on the 1972 model.

Is this a real horsepower drop apples to apples or did the rating change from the flywheel to the rear axle?

[Bill M.]

Re: LT1 1970 - 1972 Horsepower

During the three year production run of the LT1's I have read the horsepower drops each year and significantly on the 1972 model.

Is this a real horsepower drop apples to apples or did the rating change from the flywheel to the rear axle?

Erv:

1970 is 370 SAE gross (no accessories, no backpressure, etc.)
1971 is 330 SAE gross (compression dropped from 11:1 to 9:1)
1972 is 255 SAE net (same basic engine as 1971, rated with accessories, production exhaust. An as-installed rating.)

All above ratings are measured at the flywheel.

Drop when measured at the tires is about 15-20% lower than at the flywheel.

[Roy V.]

Re: LT1 1970 - 1972 Horsepower

I agree with Bill, GM didn't even put a HP sticker on our air cleaner lids for 1972.

[Edward C.]

Re: LT1 1970 - 1972 Horsepower

I have a question on the 71 LT-1. Did the CEC (Conbined Emission Control) With it's retarded timing and timing controled by which gear it was in have any influence on the HP rating of 330? Similar question on the 72 but I do not know what emission controls were on the 71 255 HP. Thanks Ed

[Jack H.]

Re: LT1 1970 - 1972 Horsepower

Many thought so and 'castrated' their CEC equipped cars in this belief. But, the true loss of HP was the more emissions friendly lower CR profile of the engine itself. Remember, once you're on the highway at crusing speed, the CEC solenoid is energized and passing vac to the distributor for proper advance. BUT, the spark profile changed along with the drop in CR...

[Warren F.]

Re: LT1 1970 - 1972 Horsepower

There were further timing and re-calibrations done to the '72 LT-1.

1971 LT-1 net horsepower is 275hp. 1972 LT-1 net horsepower is 255hp.

[Duke W.]

Re: LT1 1970 - 1972 Horsepower

There are at least a dozen ways to measure "horsepower", and a transition was going on in that era from "gross" to "net", and keep in mind that the gross ratings were inflated by about ten percent relative to what a real producton engine would do, but massaging the heads will achieve the gross rating.

There are "327 LT-1" configurations out there than make about 290 SAE corrected RWHP, and the inlet system is choked due to the 327 SHP inlet manifold. The next "upgrade" to get to 300 is the LT-1 manifold, but it's not original and won't fit under the C2 SB hood with the original air cleaner.

With the 70-72 LT-1 you have a properly matched inlet manifold to go with massaged heads, but the low compression verisions need the '70 pistons with an as-built CR of about 10.5:1 along with head work to make the 300+ RWHP. By comparison a decent massaged L-72 is limited to about 360 SAE corrected RWHP because the exhaust system is choked. That's why the LS7 has 3" pipes and a dual path muffler.

Also remember that "horsepower" refers to the peak at the top end of the rev scale where you spend .00001 percent of driving time. What counts is the entire power curve as expressed by "torque bandwidth" and a good high performance road engine should make 80 percent of peak torque at 2000 or it's going to feel soggy in normal driving.

The above 290 RWHP "327 LT-1" makes 80 percent at 2000, 90 percent at 2500 and easily pulls from 1000 revs in top gear while making useable power to the valvetrain limiting speed of 7200.

A real high compression 350 LT-1 should make more top end power due to its better manifold, and should make equivalent torque bandwidth with a spark advance map similar to a '65 327/365, but the OE LT-1 map had to be set up for emissions, not performance or fuel economy, and that will show, but changing the spark advance map is simple and cheap.

Duke

[Bill M.]

Re: LT1 1970 - 1972 Horsepower

One more data point:

When GM released the 300 SAE net '92 LT1, they bragged that 300 SAE net surpassed the SAE net of the '70 LT-1 (no specific number for the '70 LT-1 was given).

I've driven both, and a 6500 redline is more fun than 5700.

[Bryan M.]

Re: LT1 1970 - 1972 Horsepower

A real high compression 350 LT-1 should make more top end power due to its better manifold, and should make equivalent torque bandwidth with a spark advance map similar to a '65 327/365, but the OE LT-1 map had to be set up for emissions, not performance or fuel economy, and that will show, but changing the spark advance map is simple and cheap.

Duke

Duke,
I haven't looked in the archives, if it's there tell me and I'll look for it, but can you give me the specifics on how to set-up the spark advance map on the 72 LT-1 for maximum HP?

[Timothy B.]

Re: LT1 1970 - 1972 Horsepower

Bryan, If you purchase one of those Mr Gasket quick advance kits use the light silver springs and install the brass stop bushing. That may be all that's needed so try that with your original weights etc. and you will see a improvement.

Other than that, the distributor should be put on a machine to play with weights, springs etc. but this way you will know the advance curve.

The vacuum advance needs to be connected to full manifold vacuum and taylored to be pulled to the stop at approx 2" vacuum less than idle vacuum. To determine this, run the car with the control unhooked at idle and inital timing set at approx 28* BTDC and reset air/gas mixture screws for max vacuum. Adjust idle to spec and make a note of vacuum reading.

After this, you can determine the correct control to buy but don't forget to reset idle (inital) timing back to spec 8-10* BTDC. When you hook the new vacuum control connected to manifold vacuum, the idle timing will advance another 15*, so 15+10-12 (inital) will give you approx 27* idle timing and the control will stay at the advanced stop at idle but will retard the moment vacuum drops and avoid ping.

[Duke W.]

Re: LT1 1970 - 1972 Horsepower

Duke,
I haven't looked in the archives, if it's there tell me and I'll look for it, but can you give me the specifics on how to set-up the spark advance map on the 72 LT-1 for maximum HP?

It's basically the same as '64-'65 SHP/FI which is 24 deg. @ 2350 max centrifugal and 16 deg. @ 8" with a manifold vacuum signal.

Swap the VAC as required and find a full time vacuum source. Then get the centrifugal in as quickly as possible without detonation, and set the initial to achieve 36-38 degs. total WOT advance.

I'm sure you'll find more details/explanations in the archieves.

Duke

[Bryan M.]

Re: LT1 1970 - 1972 Horsepower

It's basically the same as '64-'65 SHP/FI which is 24 deg. @ 2350 max centrifugal and 16 deg. @ 8" with a manifold vacuum signal.

Swap the VAC as required and find a full time vacuum source. Then get the centrifugal in as quickly as possible without detonation, and set the initial to achieve 36-38 degs. total WOT advance.

I'm sure you'll find more details/explanations in the archieves.

Duke

Bryan, If you purchase one of those Mr Gasket quick advance kits use the light silver springs and install the brass stop bushing. That may be all that's needed so try that with your original weights etc. and you will see a improvement.

Other than that, the distributor should be put on a machine to play with weights, springs etc. but this way you will know the advance curve.

The vacuum advance needs to be connected to full manifold vacuum and taylored to be pulled to the stop at approx 2" vacuum less than idle vacuum. To determine this, run the car with the control unhooked at idle and inital timing set at approx 28* BTDC and reset air/gas mixture screws for max vacuum. Adjust idle to spec and make a note of vacuum reading.

After this, you can determine the correct control to buy but don't forget to reset idle (inital) timing back to spec 8-10* BTDC. When you hook the new vacuum control connected to manifold vacuum, the idle timing will advance another 15*, so 15+10-12 (inital) will give you approx 27* idle timing and the control will stay at the advanced stop at idle but will retard the moment vacuum drops and avoid ping.

Thanks guys! This will definitely be on the "to do list".

[Doug K.]

Re: LT1 1970 - 1972 Horsepower

Thanks guys! This will definitely be on the "to do list".

Quote:
Originally Posted by Duke Williams (22045)
It's basically the same as '64-'65 SHP/FI which is 24 deg. @ 2350 max centrifugal and 16 deg. @ 8" with a manifold vacuum signal.

Swap the VAC as required and find a full time vacuum source. Then get the centrifugal in as quickly as possible without detonation, and set the initial to achieve 36-38 degs. total WOT advance.

I'm sure you'll find more details/explanations in the archieves.

Duke
Quote:
Originally Posted by Timothy Barbieri (6542)
Bryan, If you purchase one of those Mr Gasket quick advance kits use the light silver springs and install the brass stop bushing. That may be all that's needed so try that with your original weights etc. and you will see a improvement.

Other than that, the distributor should be put on a machine to play with weights, springs etc. but this way you will know the advance curve.

The vacuum advance needs to be connected to full manifold vacuum and taylored to be pulled to the stop at approx 2" vacuum less than idle vacuum. To determine this, run the car with the control unhooked at idle and inital timing set at approx 28* BTDC and reset air/gas mixture screws for max vacuum. Adjust idle to spec and make a note of vacuum reading.

After this, you can determine the correct control to buy but don't forget to reset idle (inital) timing back to spec 8-10* BTDC. When you hook the new vacuum control connected to manifold vacuum, the idle timing will advance another 15*, so 15+10-12 (inital) will give you approx 27* idle timing and the control will stay at the advanced stop at idle but will retard the moment vacuum drops and avoid ping.






I'm not getting this... so with this set-up, the vacuum is at full advance at idle. The mech. advance advances the spark as the vacuum advance retards it due to the lower vacuum with the throttle open?

I have very little experience with the old fuel/ignition systems, but I'm trying to learn. (20 years exp. working on computer controlled vehicles)

[John H.]

Re: LT1 1970 - 1972 Horsepower

I'm not getting this... so with this set-up, the vacuum is at full advance at idle. The mech. advance advances the spark as the vacuum advance retards it due to the lower vacuum with the throttle open?

I have very little experience with the old fuel/ignition systems, but I'm trying to learn. (20 years exp. working on computer controlled vehicles)

Doug -

The link below will help you understand the basics of ignition timing, centrifugal and vacuum advance, and how they work independently and how they complement each other in conventional point distributors.

http://www.lbfun.com/warehouse/tech_...101Article.pdf

[Doug K.]

Re: LT1 1970 - 1972 Horsepower

NOW I get it!

Thanks to John, Timothy and Duke I get the whole vacuum/centrifugal advance conundrum. I didn't have it right in my brain until now.

I think my LT-1 will benefit from this information!