396 - what makes 375 vs 325 hp? [Archive] - Chevelle Tech

: 396 - what makes 375 vs 325 hp?

Jun 27th, 00, 7:28 AM
I'm looking at a perfect 67 Chevelle with the 325HP version of the 396. Can anyone tell me what the differences are between the 375 and 325HP versions? If I do in fact purchase it, it would be great to know I could easily do the necessary upgrades to the current engine. Hopefully, the upgrades aren't too numerous - but that's an oxymoron if I've ever heard one. (= Thanks in advance.

Neal Mulcahy

Jun 27th, 00, 7:43 AM
Hi Neal and welcome to the BEST site on the net. I don't know the answer to your question, but someone will be along soon to answer it.

Leo Paugh
Maryland Chevelle Club #017
The bitterness of poor quality remains long after the sweetness of the low price is forgotten.

MarylandChevelleClub (http://www.clark.net/pub/chevelle/mcc.htm)

Jun 27th, 00, 7:50 AM
from what I remember, it would be the 325 horse has a 2 bolt main block, cast crank, oval heads, cast iron intake, quadrajet, hydraulic cam. 375 has 4 bolt block, steel crank, square heads, aluminum intake, holley carb, solid cam. I cant remember if the 375 has 7/16 rods or the 3/8 the 325 uses. The low end 325 stuff is plenty strong, so you could just update your cam, heads, intake and carb to copy the 375 version.

70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
55 chevy prostreet
69/71 camaros
91 formula
TC member #493

Big Block Dave
Jun 27th, 00, 8:14 AM
Compressin also....If I recall correctly, the 325/350 hp are 10.25 to 1, while 375 is 11:1, could be wrong though.

Go with aftermarket parts....met someone recently with a bone stock (except for drag radials)350 hp 396 in a '67, 4 speed, 4.10 gear,(same as mine)that runs 13.8's. My 325 hp runs 12.60's....why? Because 325 hp is just a f*ckn" sticker...the block is a 325 hp block, but the cam is a solid 517" 530"(I think something like 398" is stock-ewww), roller rockers, Edelbrock RPM intake, BG carb, headers. etc.

The point is, throw some aftermarket parts instead of original "High perf" parts, and you'll make more power.

[This message has been edited by Big Block Dave (edited 06-27-2000).]

Jun 27th, 00, 8:50 AM
Ditto to what drptop and BB Dave said. You can really wake up a 325 or 350hp engine with a few changes, specifically a cam change and headers. That along with a good intake and valve job will make a big difference. To answer your question, upgrading a 325 engine to 375 would be fairly expensive if you changed heads, intake etc. But do like BB Dave and you will have a 325 hp engine running like a 375. http://www.chevelles.com/forum/biggrin.gif

The hp difference between the 325 and 350 engines is pretty much 25 hp, give or take a few. But remember, the 375 hp engine was rated 375 in the Chevelle, Camaro and Nova. Seems insurance rates were not as bad with 375 horses vs 425. In the Corvette it was rated 425 hp and it had every bit of 425 hp. In the Chevelle the 375 engine had closer to 425 hp than 375. With a few modifications on it you would be pushing 500 hp. My point is the 375 hp engine was more than 25 hp from the 350 hp engine whereas the 325 vs 350 hp engines were fairly close. Hope that makes sense.

-Mark TC# 717
ACES #1641
1967 Chevelle SS396/375
1964 Chevelle 283/195
1971 Vega 350/400

Actually registered 10/98

Gene Chas
Jun 27th, 00, 10:35 AM
Neal, one more thing to add to the excellent advice given already. If it is the original 325 HP motor, don't alter it too much, or simply keep the original parts.

My 67 had a 325 HP ( not original ) with an Edel RPM manifold ( painted orange for stealth), unknown cam and small tube headers. It ran 13.78 in the 1/4. Not bad.

P.S. I would not use the rect port heads unless you are building a 7000 rpm motor. The oval port heads on the 325 will be better for everyday street use and for mid-low 6000 rpm race use.

So the differences are steel crank, 4 bolt mains, higher compression, solid lifter camshaft, hi rise intake w/ Holley, rect port heads.

Frankly, I'd add a good cam and an Edel RPM manifold ( again orange paint for stealth) and you'll have a nice running combination. If you end up reusing the QJet, ( which I would try to ), you might actually squeeze 14-15 mpg on the highway out of it to boot.

You can be more specific once you own the car!

Welcome to our site.

G. Chaas
Gold #62/ACES
67 SS396/(427L88) (http://www.chevelles.com/showroom/chaas1.jpg)

[This message has been edited by Gene Chas (edited 06-27-2000).]

Jun 27th, 00, 11:25 AM
To answer the original question,
more compression from bigger domes on the pistons, 10.25 to 11.00 to 1 c/r,
cast to forged pistons,
two bolt to four bolt main caps, block drilled for optional oil cooler,
cast iron to forged steel crank,
first to second design connecting rods, stronger rod bolt material,
hydraulic to solid lifter cam with longer duration and more lift, although some 375 hp engines had performance hydraulic cams also,
oval to rectangle port cast iron heads. larger valves, stiffer valve springs,
5/16 to 3/8 pushrods and guideplates,
cast iron oval port low profile Q-jet to cast aluminum rectangle square flange high rize design intake manifold,
Q-jet 750 cfm to Holley 780 cfm vacuum secondary square flange carb,
closed to open element design air filter,
revised curves in the distributor, vacuum advance,
Not much, just the small stuff.

[This message has been edited by IgnitionMan (edited 06-27-2000).]

Jun 27th, 00, 1:29 PM
Everyone has forgotten the Dripper Valvecovers. http://www.chevelles.com/forum/biggrin.gif And.............. Deep groove pulleys. http://www.chevelles.com/forum/biggrin.gif And........... The steel crank was crossdrilled. http://www.chevelles.com/forum/biggrin.gif
'66 Yenko
'68 SS 427

[This message has been edited by YenkoYS100 (edited 06-27-2000).]

Jun 27th, 00, 3:48 PM
also the 396/375 had a larger harmonic balancer and large dia. alternator pulley. Even the timming chain cover was different.

[This message has been edited by joe58 (edited 06-27-2000).]

Jun 27th, 00, 5:47 PM
Dont forget the decal on the air cleaner was different, most of the time I see guys switch that most of the time and that instantly gives 50 HP!

70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
55 chevy prostreet
69/71 camaros
91 formula
TC member #493

Jun 27th, 00, 5:58 PM
All early stock stamped steel big block valve covers had drippers in them, no matter what horsepower they were. Sorry, but that's a fact.

Never saw a different timing cover, short or long water pump versions, did have different timing pointer for the larger damper, though, all early forged steel big block cranks were cross-drilled, used same half-grooved main bearing design as cast irton cranks, but Delco-Morraine 400 series bearings over 300 series for lower horse engines.

Sorry, had brain fade there for a second.

Jun 27th, 00, 7:32 PM
I have about 5 different timing chain covers for a big block Chevy in my garage. The early ones had no holes for the dowel pins. The welded on timing tabs are different sizes
for different years and HP ratings.

steve stanbach
Jun 27th, 00, 7:38 PM
It also had 7/16 rods

Jun 27th, 00, 9:12 PM
steve stanbach> WRONG. They were originally 3/8" rod bolts with hi-perf. DOT rods.

p.s. the 7/16" rod bolts were correct on hi-perf. 454's.
'66 Yenko
'68 SS 427

[This message has been edited by YenkoYS100 (edited 06-27-2000).]