Topic: How hard to get 400 hp out of a 400 pontiac engine

[RainMan]

Hey Guy's ,I no nothing about Pontiac engines ,and was wondering if anyone seen articles on how to  take a factory  non w72 pontiac 400 ,and get around 400 hp out of it . I like the 1 hp per cubic inch ratio .
I think I read the 400 has 200 hp,and the w72 has 20 more . How did they get that little gain there ? I had a w72 in my 77 years ago ,and I did notice she was a little more powerful imo.
I read on Ebay that the a cam change can give real big improvements in the pontiac 400. I see people are changing heads to get more compresion I think .
In the old days I don't think there was alot available for pontiac ,but someone mentioned it's better now

[cuog]

I'd get a good free flowing exhaust, agressive cam, and mill the heads for a touch more compression or get older higher flow heads with higher compression, and pistons to match, then get the carb tuned to handle the new flow, and you'll probably be close.

[Rick]

How much do you want to spend?  That's the only question.

If you only want to spend a lilttle, you can swap the exhaust manifolds for a set of tube headers or (for street use) get a set of RARE cast exhaust manifolds.  Get a decent exhaust system and open the shaker up, rejet the Qjet and recurve the distributor to match and you'll get a healthy bump.  Beyond that, you can explore getting a set of higher compression heads (don't go too high on pump gas, though) or a set of aluminum aftermarket heads.  Pile on a cam change and bigger rocker arms if you feel like screwing around with a rear gear change and you can get pretty wicked.

Just don't get too hung up on HP numbers, though.  TORQUE is what moves the cars around and makes it feel mean on the street.  Big HP numbers are great on the strip but you won't use them much on the street, because the big HP comes at the high RPM end of the engine operation.  You just won't run the engine at 5500 RPM on the street all that much, or if you do, for all that long.

[pstein]

You could just build it to whatever specs the Ram Air III or IV had.  They were under-rated at 340 and 375 hp respectively.  Comp Cams makes cams with these specs.  Then you just get some 72cc heads and you're good to go.  Edelbrock has a spec sheet of how they made 422 hp/441ft-lbs out of a Pontiac 400 using only their products.  Not too shabby.  The dyno chart they provide starts at 2500 rpm's though.  But listen to Rick.  That guy knows what he's talking about.

[ta78w72]

In 1978 the base 400 had 180hp and the W72 engine had 220.

[y88rick]

i was told you can gain 40 hp off of opening that exhaust up and around 10-15 by opening the shaker so say you get all of 45 out of doing this and then depending on your rear end gears (mine is a 2.58 so if i did such and then put in 3.23's i'd be impressed with the difference that made all by its self. changing my rear end gears on my car would definately improve the take off feeling by a long shot and with a 3.23 i could still go down the highway respectively. i can do 78 at 2500 rpms right now and around 90 at 3500 which i do not need to really go any faster than 78 anyhow thats plenty fast down the highway. i think i could acheive that still 78mph at a guess of 3300 rpms? but bye bye gas mileage too. i do think with them 3 mods you would feel like you had a 300 hp car even though it really wasnt. agreed your torque makes you the man on the streets i'd rather have 240 hp and 285 lbs of torque rather than 500 hp and 203 lbs of torque

[ta78w72]

I doubt you get that much by cutting the shaker.  Opening the exhaust is huge though.  Another 180 hp isn't out of the range of possibility though.

Gee Rainman, that's a beautiful car!

[72blackbird]

Here's the basic recipe for a 400-hp 400:
1. early (67-71) D-port heads- the good ones are nos. 670, 12, 13, 16, 48
2. forged pistons w/ moly rings
3. rotating assembly dynamically balanced/ oil clearances blueprinted
4. cam w/ under 230 dur. @.050, adv. dur. 280 deg. or larger/ around .500" lift
5. HEI or aftermarket ignition w/ optical trigger and 50kv coil
6. 750 cfm or larger carb/ factory or e-performer intake
7. 1 5/8" primary/ 3" collector header/ 2 1/2" dual exhaust

There are of course a few variables to consider like timing, final combustion chamber volume, dynamic compression ratio, and overall build quality, but a proper buildup with stock unported heads will make at least 350-375 hp.

My 406 with ported no.48s, forged TRWs, a Crane 284H cam installed straight up, Holley 750 vac sec on an e-performer, 1.5 roller rockers, Hooker comps and 2 1/2" duals, w/ a stock TH-350 w/ a 2000 rpm converter and a 3.08 10-bolt posi let my 'Bird run a 13.04@104 in street trim (wide BFGs all round, no slicks, big sway bars on, etc.).

Geno

[SoonerTA]

What exactly is recurving the distributor? 

I have been planning on getting the H.E.I tune up kit from Pertronix which includes a new cap, rotor, 4-pin performance module, Flame-thrower coil, dust cover, adjustable vacuum advance, high performance advance kit, wiring harness and capacitor.   

Would this cover "recurving" the distrubutor?

I am wanting to bump up the HP on my '78 TA with the 400ci and auto trans.  I have just purchased the oversized RARE exhaust manifolds and 2 1/2" exhaust.  I am in the process of trying to find someone to rebuild and rejet my q-jet.  Am I on the right path with helping out my ignition next?  I am wanting to get the Ram Air IV cam but will come at a later date when I will probably rebuild the motor.  I am just wanting a nice increase without the rebuild.     

[Rick]

Recurving the distributor means tailoring the centrifugal advance to better match what your engine/cam combination needs to maximize the power out of the fuel it is consuming.  Generally during that process you get specific recommendations as to what your vacuum advance should be doing also.

The first criteria, of course, for an engine to make power is to optimize the air flow into/out of the engine.  In the end, it's just an air pump, so maximizing your volumetric efficiency (% of air that your engine could *theoreticially* pump) is probably the most important thing to making more power.  You're on a good track already as you have a good intake manifold (stock iron pieces do really flow well, although they are a bit heavy) and you are reworking the exhaust with good pieces.  The 6X aren't bad, although some of the older 1.77" exhaust valve heads flow a little better.

Your next step is to optimize the air/fuel ratios and you are addressing that by having the Qjet massaged and rebuilt.

The final step is to get your ignition timing set up so it lights off the air/fuel ratio at the best possible time throughout the range of RPM that you are using.  So I'd say you have the right approach here.  Incidentally, centrifugal advance is what makes the difference when the engine is under load, whereas vacuum advance is the mechanism that maximizes your fuel economy while cruising at a steady speed down the road.  You have totally awesome acceleration without any vacuum advance, but your fuel economy will suck -- so don't neglect your vacuum advance.

One word of caution -- I'd be careful about throwing a RAIV cam in a 400/auto, especially for street/highway use.  It's entirely possible to screw up a great car by putting in too much cam.  Your rear gear may be pretty numerically low (maybe as low as 2.41:1 or 2.56:1) and that doesn't play well with a big cam.  You don't want your car turning into a log wagon and getting beat coming off every stoplight by a VW Beetle.

[RainMan]

^^ That's some good info ,thanks.

You know if I ever get an SE clone ,then maybe the direction a Guy should go is with pontiac's 455 . Maybe  400 hp  out of the 400 was an unrealistic idea?
I think a Guy might be better off building a torque monster instead.
From what I have seen ,the 455 is basically the same block as the 400 ,but longer stroke crank  . I have to say I don't like a lumpy cam shaking the car all over ,well within reason anyways.
Like they say there's no substitution for cubic inches  (at least they used to)
Someone mentioned that by using aluminum heads you can run a higher compression without detonation ? That is news to me ,but great if it works that way .
I'm not talking 10.5 to 1 compression here ,but it sure would be nice to get at least in the  9's. 
I bet pontiacs 455 should be capable of under 400 hp  with tons of torque without getting really carried away on the build .
Hopefully someday I find out lol

I recall going for a ride  in my friends 75 oldsmobile years ago ,and although it was the olds 455 , the way it pulled you into the seat was just incredible .
 

[Rick]

Aluminum heads dissipate heat faster than cast iron ones do, which means they are less subject to detonation problems.  "Conventional" wisdom is that they are good for 1 point more compression ratio than cast iron heads for that very reason.  However, I've run across one engine builder who claims that you can only get 0.5 - 0.7 point more CR using them.  I've also seen some analysis that indicates that in order to equal the output of a cast iron head you MUST run the higher compression ratio.  I have not been able to confirm either of those last 2 points with any concrete data (such as dyno runs or time slip results) as of this point -- still looking, but haven't seen it yet.

Keep in mind that there are some complexities to determining just how high of a CR you can run on pump gas.  While just about everyone is familiar with the STATIC compression ratio ((swept volume + end volume)/end volume), there is also the DYNAMIC compression ratio that comes into play also.  Basically the engine does not begin compressing the air/fuel mixture until the intake valve closes, and this is (for reasons of flow and chamber charging) always after BDC on the compression stroke.  That means that the cylinder isn't compressing the mixture through the entire stoke, and so the dynamic compression ratio is lower than the static CR.  Most pump gas will burn adequately at an 8:1 ratio and frequently (depending on the static CR and the cam you are running) the dynamic CR is much lower than that.

Just how all that plays together is something that I'm exploring right now.  I've a series of threads going down in the TA Tech section of this forum entitled "CAM TECH" where I am examining all of the facets of cam design and selection, one issue at a time.  There are a lot of issues, and there are a lot of opinions with a lot of good information and some bad information out there.  I'm attempting to wade through it all with the hopes of gaining a better understanding of the subject in general.

The 455 is perhaps the best Pontiac engine at developing torque, and with 455 cubic inches you get power, but it's not the engine to wind up tight into the RPM band.  The long stroke, which makes such great torque at lower RPMs, makes for a lot of mass moving a long distance compared to engines with a more "square" bore-to-stroke ratio.  But it's great for the street and IMHO perhaps the best match for the TAs and the way they are driven today.

Just my take...

[SoonerTA]

One word of caution -- I'd be careful about throwing a RAIV cam in a 400/auto, especially for street/highway use.  It's entirely possible to screw up a great car by putting in too much cam.  Your rear gear may be pretty numerically low (maybe as low as 2.41:1 or 2.56:1) and that doesn't play well with a big cam.  You don't want your car turning into a log wagon and getting beat coming off every stoplight by a VW Beetle.

I guess I received some bad information about the RamAir IV cam.  I had no idea it was that aggressive.  I am wanting a mild cam for my street car with little highway use.  What would be the best cam for my application?  Should I call up Crane Cams and tell them what I have and see what they suggest?  If I do that, I would wonder if they are giving me good advice.  I have spoken with "tech help" before and it seems like they just know there product and not when to use them.     

[RainMan]

http://www.highperformancepontiac.com/tech/hppp_0811_1967_pontiac_gto_400_engine_build/index.html

^^ look at the specs on this pontiac engine build ,WOW

[Rick]

Cam selection depends heavily on what your application will be.  That sounds like a weasel answer but it's true.  The choices are quite different between street/highway use, drag use, and race use.

Street/highway means you get it out of your garage and drive it around public streets, making stops and (mostly) observing sane speed limits.  Drag use means you trailer the car to the track and use it for 1/8 and 1/4 mile runs.  Race use means you run it in circle track races on closed courses as quickly as you can make laps.  Those are my definitions, others may offer different interpretations.

For street/highway use, you want your engine's peak torque to set right at your normal highway cruising speed.  Peak torque happens to also be the point where your engine has its highest volumetric efficiency, which means you will get your best fuel economy at that point in real-life driving.  It will also have it's best throttle response at that point, and you'll find that the car moves away from stops much quicker than it would if the peak torque was higher iin the speed range.  You'll still retain some decent top end, but it won't be a Bonneville Salt Flats racer.

For drag use, you want your engine to make its maximum possible horsepower and the transmission/rear gear/tire size to all work so you don't run out of RPMs at the end of the run.  There is considerable art to those selections, and I am NOT a drag racer, so I'll leave the description of all of that to others.

For race use, you need your HP to fall in the speed range you'll be making to be competitive.  Obviously, that varies with tracks and courses, so those guys do a lot of cam changes and rear gear changes.  Again, I'm not a racer so I'll leave description of those details to the guys who are.

You sound like you want a street/highway car, so my advice would be to decide what rear gear/tire size you're going to use and compute how many RPM that requires to drive it at your normal ihighway cruise speed.  Practically speaking, that could be anywhere from 55 - 75 MPH.  Then I'd find a cam that develops peak torque as near to that RPM as possible, and use it.  If there is more than 1 cam make/model that falls out of that selection process then I'd look at the width of the power band and pick the one that's the widest.  But that's just my twisted sense of the process -- YMMV.

EDIT -- with due respect to RainMan, if you put the cam in that article in your 400 it won't run on the street very well.  It has its torque peak at 4,000 RPM, which works out to be 91 MPH using a P255/60/R15 tire for your peak torque point with a 3.42:1 rear gear ratio.  Conversely, to drop that to 70 MPH you'd need a 4.46:1 rear gear.  Additionally, that cam provides for lift in the 0.550"+ range and your heads won't flow enough to need more than anything over 0.475" (or thereabouts).  That's just too much cam for a streetable Pontiac 400, IMHO.  Just my $0.02.