396/425HP?????

[Rick68]

Just got my new issue of Muscle Car Review and there is a tech article about a stock 396/375 HP rebuild putting out 425HP.The only deviation from stock is the use of roller tip rockers and pointless distributor.That was also using the stock manifolds.With headers and rejetting the carb(primary side only #68 to #72) it went up to 457HP.

[X33RS]

I haven't seen the build but it's doable.   Without details it's impossible to speculate, but there are so many tricks to make power with these things.  Just a good blueprinting will bring up the HP.  For instance, these engines are supposed to be 11:1 compression.  Well technically they weren't when they came from the factory.   All of the stock engines I've disassembled, including my DZ, fell short of advertised compression.  Pistons are usually in the hole .020 to .025, decks are uneven from front to back, heads usually don't CC what the factory says they are, and even cranks shafts come up short on stroke.  All this costs HP.
   With all that said, My DZ that had never been apart came in at 10.6:1 on some cylinders and and 10.4:1 on others front to back.  Deck was uneven, pistons were in the hole, and stroke was short just a hair.
   By blueprinting we go through and correct all those things.  Crank shaft stroke is corrected during machining,  pistons are brought up to or near zero deck, and heads are checked for correct CC.   So now it's still stock advertised compression, but now it's actually AS advertised, and we have increased power output as a result.  It's still stock technically speaking when looking at advertised numbers.
  Going through the entire engine with this kind of detail is what brings those power numbers up like that.   Of course then there are happy dyno's too, but that's another subject.

[Mike S]

 Did the roller tip rockers also include roller rockers too or was it just roller rocker arms.
I read that a flat tappet cam setup can consume up to 10% or an engines HP compared to a roller setup (lifters and rockers).
Outside from point bounce I can't see a pointless distributor contributing much to HP output.
Also, wasn't the 375 HP rating on the conservative side from the factory? I thought that was practice back then due to the auto insurance industry starting to take notice.
  Then again, I thought motor HP ratings were done measured at the flywheel with no factory style exhaust manifolds installed which would inflate the actual ratings compared to what a street motor would see with a factory exhaust.

Mike

Mike

[Z28Project]

I believe that all of the Chevy special high performance solid lifter engines were rated on the conservative side back then.

At least from everything that I've read.

[firstgenaddict]

Vortec pro builds a oval port motor which puts out 600+ HP... it's all doable, cam shaft matched to the flow numbers on the heads is the largest factor to making big usable power.

[Rick68]

The cam is a flat tappet cam like original and the rockers are roller tip only.The block was not decked because they not want to loose the original block stampings on the pad.The engine is from a 1969 L78 Chevelle.Remember that in 1965,had the Corvette and full size Chevrolets had the L78 rated at 425HP.

[Rick68]

There was also no porting done to the heads

[maroman]

Remember that in 1965,had the Corvette and full size Chevrolets had the L78 rated at 425HP.

I think it was lowered to make the insurance companies happier. In early '66 the 427 Corvettes were rated at 460, then lowered to 425.

[MO]

The block was not decked because they not want to loose the original block stampings on the pad.

Blocks can be decked without disturbing the pad.

Every engine that came off the assembly line was different. L78's were probably anywhere from 350 to 450 hp. Add headers, rocker change, distributor work, carb jetting, blueprint, dyno tune, and 457 hp should be easily achievable.  And who knows what else was done that the article didn't report.

[Kelley W King]

I read the article mentioned. I too believe it is possible if not likely. I also believe from the factory all were not equal. I have owned several L78,s and hipo smallblocks and some we just called ( a strong engine). I presently have a 69 CE L78 with factory intake ,carb, cam, and headers and it is one of the better ones I have had and really do not know why. I still have a 365HP 327 that always ran ok, but after bending a rod years ago I had a pro do the longblock. We dropped the comp to 10.0 to 1 for gas and he installed a different cam. I could not believe the seat of the pants loss even though he balanced it. Afraid the compression change did it, I on a Sunday afternoon put the old cam back in. It felt as if it gained 50+ hp. I am a believer that the 365 distributer, 2828 Holley, cam, and 202 heads were a team that worked together. I still drive it today.

[X33RS]

Yep, some did run better than others.  A lot of that had to do with the explanation I gave above as just one example.  Nearly every virgin engine I've done has the deck out of whack.  Almost always higher on one end.  That drastically affects compression ratios from front to back.  Not just GM but a lot of the Fords I do have been the same way.  I have also run into factory cranks, never turned, that come up short on stroke.   This mass produced stuff was never really perfect.   You just get some virgin engines that are better than others.

327's are my favorite sbc's.  I ran an L79 in my nomad for years, drove it daily.  Essentially like your L76 but instead of the 30-30 solid cam, it has the 151 hydraulic.  That motor was virgin (still is) but it was such a strong runner.  It started showing signs of getting tired (using oil) so I pulled it.  Years of 6500 rpm abuse at the hands of a youngster      Would love to tear that one down and start checking things.  I'm betting I'll find things much closer to advertised specs than most others.  My 69 302 was horrible.  Deck heights were off, stroke was short, which had compression ratios all over the place when comparing front and rear banks.  Wasn't all that close to the advertised numbers.
   However really no need to knock the compression out of these things.  I ran pump gas in that L79 for years with 38 degrees of timing and it never hinted at detonation.  Not much seat timing even with the milder 151 cam allowed that 11:1 (advertised) motor to live fine.  So when I built the DZ, we made sure we had exactly 11:1, and with the 30-30 cam that is much more aggressive than the 151, plus in a smaller motor,  we figured it would bleed off plenty of cylinder pressure.  I have 800 miles on the engine with 91 pump gas, currently 36 degrees timing, and it's running perfect.

I read the L78 article, it really didn't go into all that much detail so it's impossible to speculate.  I did notice a few clues that they were definitely tweaking things.  For instance it does mention they added some seat pressure on the valve springs.  This is a key point.  Also something we did when building my DZ.  Just 10-15 lbs. of seat pressure is huge in valve train control and will contribute to some HP gains in the upper rpm range where valvetrain control is crucial.  Especially when that valve slams shut on the seat.   All kinds of little things to do in these engines to find power without affecting drivability or originality/value.
   Any good machinist/builder worth their weight, if the owner is willing to pay,  will go over it with a fine tooth comb and correct what is needed, make things better than factory, along with a few tricks.  That's the different between a good running engine and a great running engine. 

[X33RS]

Just another tidbit I noticed on that build.  That engine, to me,  seemed to peak a bit early considering that factory solid cam and knowing how that same cam performs in the 427's.  They showed peak at 5800 with HP falling off after that.
   That tells me that there may be a couple things going on.  They may have advanced that cam a bit when installing it trying to bring the power in sooner, maybe looking for a little more torque early in the rpm range.
  I've also noticed these "supposed" blue print factory cams aren't always exactly like the OEM stuff.  Comp for instance tends to grind a couple degrees advance in, and the lobe profiles on some of them seem to be a bit more aggressive.   They didn't give very much on the specs of the cam used, other than showing OEM type duration and lift from what I read.  That doesn't reveal much.   All speculation though, since they really didn't have much else to say about the engine.