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1970 402 Big Block Rebuild HELP

  
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1970 402 Big Block Rebuild HELP

 
championlab1 championlab1
New User | Posts: 3 | Joined: 10/12
Posted: 10/01/12
11:44 PM

Ok fella's need a world of advice. Ive got an all original 70 c-20 402 with 72k original miles...  Im looking to pull the motor and trans and rebuild them both.. At this point I got a guy whom will do the whole motor stock for $1495.  What I would like to do is kick this motor up to about 375 hp .. I'm hoping I can get some constructive advice about where to spend additional $$$ to get the best, longest lasting rebuild.. Im a bit of a novice, so detailed information would be greatly appreciated.. steel crank? hi perf cam? hi perm distrib. ?  would I have to, or should I upgrade the transmission I will be rebuilding..?  are there any particular ports I should have honed over the standard rebuild? Stick with the 4bbl rochester?.. basically, this motor's never been touched, so I could go in any direction, but im not looking for 500hp.. some kind of build that would have this motor last.. I'll be keeping the truck my lifetime, so..trying to do things right the first time... If you thing I should upgrade the trans, give me your input.. any schools of thought or personal experience would be greatly appreciated. Im not against putting 400hp under the hood.. just trying to keep the truck practical...If you got a complete parts list for your suggestion, please do include it in your reply...  I'll be checking in on this post daily...
Thanks
Bruce  

gtomustang gtomustang
Enthusiast | Posts: 457 | Joined: 06/09
Posted: 10/02/12
11:44 AM

Are you going to tow with this?  b/c the 396 your engine is based on, made 425hp in L78 form, but with 11:1 compression, which puts towing out unless you can access 105 octane fuel.  Factory forged cranks will do fine, but a forged 454 crank will create 440cid in your engine, and cubes can be your friend for making power below 5000rpm.

one thing to stay away from is the factory intakes, many were lo risers, an aftermarket intake was always suggested.  since your rochester is a spread bore carb (venturis are different sized), you'll need a spread bore intake or swap to a Holley carb, which is easier to tune--not better, just easier--than a Q-jet.

You could go with the L34 factory hydrailic liftercam, 325hp, or the solid lifter 350hp cam and then bolt on some better heads to make up the 50hp difference...or step up the cam, get a rough idle and have the heads done later. like you said, put your money in the bottom of the engine first, and you can buy a good set of heads later.

a factory TH400 transmission will do fine, since they came behind the 425 L78 and L89 aluminum head 396's that got the big block its reputation.  following the factory means the research was done for you already.  get an HEI distributor, and tune accordingly.  
Just because you can't do it...doesn't mean its impossible

championlab1 championlab1
New User | Posts: 3 | Joined: 10/12
Posted: 10/02/12
12:06 PM

No,  the truck wont be a tow vehicle any longer, with the exception of ramdom need to tow something...
Admittedly, I'm way over my head for this project... thus my intention to have a qualified shop in oregon or washington (suggestions appreciated) my fear is, people will sell you anything... not to critique and particular shop, but without adequate direction from sources like this, ive no context to what im being sold for this rebuild, or that the end result will be to my liking....this will basically be a daily crusier steet ride..  But ive read some 20 detailed " methods" for building a 396/402 frankly, some in complete opposite in philosophy that at times my head spins as to what directions best to follow..
I read one particular thread where it seemed that one individual honed his stock heads etc, and had a more "stock" approach to building the motor, and made some good HP with an end result of a smooth running through all ranges motor... I suppose, thats what im looking for...but I just dont have a clue specifically, what all needs to be done to achieve that result, which then leaves me at the mercy of the shop I'll have to the work.. Ive yet to pull the serial numbers from the vehicle to verify what exactly is in it.. but being stock to this 70C20 pick up, my guess is the 400 trans, and steel crank are in it.. I had a friend whom had a 69  396ss camino. I suppose what Im looking for is that particular end product... I cant speak to the exact HP it put out or if he had upgraded any components, only know the thing ran smooth, and STRONG...  My hope is to find someone here, or somewhere, whom has been down this road, and when there project was completed it was EXACTLY  what they expected... I'd much rather get the 375 hp+ from some what stock components where the engineering been done, then a hodge podge of aftermarket stuff that will have me tuning it every 3000 miles...
more thoughts or experiences greatly appreciated.  
thanks  

championlab1 championlab1
New User | Posts: 3 | Joined: 10/12
Posted: 10/02/12
02:52 PM

After further review, I guess the best way to sum this up is....how do I convert my
RPO L-47 310HP stock motor, into a RPO L-78 375 HP stock motor....  http://www.superchevy.com/features/chevellemontecarlo/sucp_0810_1970s_golden_chevy_era/photo_07.html

pictured, to a tee, and what would it cost? ... this motor is a spot on mirror image of what in my truck, Im just not sure if the L-47 would appear Identical externally.....  

apparently i cant get the photo to upload due to it being a html...but you can view the link.  

pepsi1 pepsi1
User | Posts: 81 | Joined: 10/11
Posted: 10/17/12
05:26 PM

I had a 375/396 soild lifter version. Back in the day. My Buddy bought a 1968 Corvette, with this engine. He drove a 435/427. I was given that 396. I pulled it out of moth balls after nearly 20 years. I had a 1968 Camaro I ran 11:20 @ 122 MPH. It still had the stock valves etc. All I did was clean it massage it a little, put it together.  

pepsi1 pepsi1
User | Posts: 81 | Joined: 10/11
Posted: 10/18/12
11:29 AM

If your trying to dulicate a 396/375.

4 Bolt Block some #3855961,#3902406,#3916323,#3935440,#3955272,#3969854 These all Could Be a 2or4 bolt. This #3855962 is a 4 bolt

A Cross Drilled Crank.

3/8 Dimple Rods 7/16 If you can find them

Forged pistons

Heads #330864 #330865, Oval Port with open chambers.

Heads #3856206 #3856260 #3872702 #3904390 #3909802 #3917215 #3931063 #3964290 These are Oval Port Closed Chambers. 396,402,427,454

Heads #3856208,#3873858,#3904391,These are Rectangular Port Open Chambers.

I hope this all helps,Good Luck..

Bob  

pepsi1 pepsi1
User | Posts: 81 | Joined: 10/11
Posted: 10/18/12
11:32 AM

There are many more part numbers for heads.This should get you started....Good luck

Bob  

pepsi1 pepsi1
User | Posts: 81 | Joined: 10/11
Posted: 10/23/12
10:36 PM

You know we go out of our way to help people, and they still can't tell us if we helped or not, let us know one way or the other. Smirk

Bob  

dond1965 dond1965
New User | Posts: 48 | Joined: 12/13
Posted: 12/01/13
04:02 PM

water-methanol injector allows high compression ratio with iron heads,and 87 octane fuel!keep the Q-jet for economy.use a spread bore alloy high rise 180 degree manifold,tri-y headers with cross over pipe.convert the heads to use 2.19 x 1.88 inch valves.use rhoades hydraulic flat tappets,cam gear drive [for supreme reliability] and econo roller rockers.yes use a pointless ignition distributor. a 4L80E trans is your best friend! must use a stand alone controller,has overdrive!for a cam might i suggest something like the COMP #11-306-4. 222-226 degree duration at 50. 0.525 valve lift straight across. 110 degree LSA. yeah i know im a little late with this post,maybe it could help some one!