Carlo, the basic difference between a 325 and 350 hp 396 is that the 350 hp has a little bigger cam, stiffer valve springs, and a steel crank. I'm not sure which cam or crank the 240 hp 402 has, but the hp rating is much lower due to lower compression ratio for unleaded gas, more emission controls, and the net hp rating system used (with stock exhaust, accessory drives, etc. in place) vs the earlier gross hp rating system ( engine on stand with open exhaust, no accessories, etc.). The 350 hp also had a low restriction open element air cleaner vs snorkel type for the 325 and 240 hp. von
[This message has been edited by von (edited 02-09-99).]
If you let me know a fax number, I'll send you the list from the back of the How to Hot Rod Big Block Chevy's. It has all the features of the various BBs over the years (Mark IVs, anyway). The 240HP cam was the low lift hydraulic(P/N 3874872), .398intake/.430exhaust lift used in L-35(65-69 325HP 396), L-66 (69 265HP with 2 Bbl carb!) LS-1 (69 427 335HP), and LS-3 (70-71 402 330HP). The high lift hydraulic (P/N 3883986), .461/.480 lift was used in L-34 (66-69 350HP 396) L-36 (66-69 427 385HP) L-68 (67-69 Corvette with 427 and 3-2Bbl carbs) and LS-5 (454 365HP). Though now dated with the advent of the new crate motors, the How to book is great for working on the older BBs.
The LS3 is the successor to the L35 (396/325). It was rated at 330 gross in '70 with 10.25:1 c.r. For '71 the LS3 took a drop in compression to 8.5:1. It was now rated at 300 gross/ 260 net @ 4400 rpm. Torque was 400 gross lbs/ft. & 345 net lbs/ft. For '72 it dropped to 240 net @ 4400 rpm, but the torque remained 345 lbs/ft.
I am having the same dilemma, Was the Hp rating in 72 underrated, or was there a significant decrease? I have been getting mixed responses
Some say the horses were there just the rating or terminology changed....from "gross" to "net"
some say the the horses decreased due toother factors. Which is correct?
From my engine bible, "How to Hot Rod BB Chevys" they offer the following:
"1972 Power ratings were reduced because GM changed to an SAE 85 degree correction factor rather than the 60 degree factor previously used for the gross horsepower rating method." I am not sure exactly what this means. What I had heard was that the method of measuring HP had changed from at the flywheel (gross) to at the rear wheels (net). Maybe someone else has the full picture.
The major change in horse power rating occured between 71-72. Up to 1971, hp was rated as Brake Horse Power (bhp), which was at the flywheel. Starting in 1972, hp was rated as Net hp, which was measured at the rear wheel.
The actual drop in compression occured with the 1971 models. Very probably, if you compared 71 and 72 engines that were the same, the difference in hp output would probably be negligable.
OK, so the '75 350 engine in my '69 hdtp is factory rated at about 165HP, and my '69 conv. with it's original 350 is rated at 255Hp. Considering the change in HP rating method, how close (or far apart) would they actually be?
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