[cobalt327]

ALL the 3.25" main blocks (including 421 and 455 "HO", but not SD) are "weaker" than 389 and 400 blocks (3" main). Yes, that means the cranks aren't quite as strong.

The 455 blocks aren't as strong; I believe 455 cranks are strong, albeit the large journal diameter drives up bearing speed for a given rpm. But that's a different issue than strength IMHO.

 

[cobalt327]

The 3.25" main blocks are NOT as strong! The cranks are, but that is a moot point. I had a 455 block start to split after only about 150 1/4 mile passes. I had put a lot of time, and money, into this block to keep from having this kind of failure, but to no avail.

What I said:

The 455 blocks aren't as strong; I believe 455 cranks are strong, albeit the large journal diameter drives up bearing speed for a given rpm. But that's a different issue than strength IMHO.

This was in response to:

ALL the 3.25" main blocks (including 421 and 455 "HO", but not SD) are "weaker" than 389 and 400 blocks (3" main). Yes, that means the cranks aren't quite as strong. (Emphasis mine)

A 455 crank used in a large journal block won't fail due to the crank being weak, rather it's extenuating circumstances like the block architecture flexing or lubrication breakdown from bearing speed/surface area combined w/excessive rpm or rod big end/stock bolt deflection.

A 455 crank turned down to fit a 3" main block for example, would be a strong crank. In fact, a 455 crank can be stronger than an original 3" crank, due to the ability to add generous fillets.

 

[cobalt327]

With a 389 block and a 0.030" over bore

Might make more sense to go to a standard 400 bore (+0.0575").

if I increase the stroke by 0.060" using an Eagle 4.35" crank and 6.800" rods, and if I bolt on 6X heads (1977) I found with 93.74cc chambers on top of Mr. Gasket head gaskets at 0.038", I get a total engine displacement of 457.84 or 458 and a compression ratio of 9.3:1.

? A 389 has a stroke of 3.75". A 455 is 4.21".

Did you mean a 4.25" stroke crank? Plugging a 4.35" crank into a 389 bored to 4.12 will give you a 464 cid. The SCR using your numbers is about 9.8:1

With a 400 block and a +0.030" over bore, if I increase the stroke by 0.050" using an Eagle 4.25: crank and 6.700" rods, and if I bolt on the same 6X heads and everything else, I get a total engine displacement of 460 and a similar compression ratio of 9.32:1.

A 400 stroke is 3.75" like the 389. Adding 0.050" to that nets you a stroke of 3.8". The 3.8 stroke and a +0.030 over 400 will be a 411 cid. A 4.25" crank is 0.040" more stroke than a 455.

Using a 4.25" crank in the 0.030" 400 nets you 460 cid. SCR is about 9.73:1.

NOTE: CR is calculated w/a quench of 0.040" and a flat top piston having a 'generic' valve relief volume of 5cc.

I'm ignoring piston compression height/rod lengths for the time being.

 

[cobalt327]

For your build if you want a 455-plus engine I'd go w/the correct 400 block for stroking (there are good and bad, basically avoid the casting number 500557 if you want to build a stroked 400). Then I'd use a 455 stroke crank w/3" mains and an aftermarket forged rod in the stock Pontiac length.

The 400 w/o stroking still makes a fine engine, as does a stock stroke 455, especially for a heavy street vehicle. Nothing wrong w/a 389, especially considering you'd be making custom motor mounts anyway.

 

[cobalt327]

Whatever block you decide on, just be sure it has the provision for a block mounted starter. About '63 back had bellhousing mounted starters, this can make things a lot more complicated. There's a lot more, but outside the scope of this post.

• Pontiac info/sites
• Pontiac engine info

 

[cobalt327]

If I stuck with a stock configuration for either of these blocks what SCR would you recommend?

9:1 is uber safe. No more than 9.5:1 w/an iron Pontiac head regardless of the year or casting number, when used w/a streetable camshaft. Higher SCRs requires more and more cam to offset, and the engine becomes less streetable and/or more dependent on octane.

When you get a chance read the Quench article from the Crankshaft Coalition wiki- it explains things better than I can here.

SCR:
• Static compression ratio calculator, this is the one I use, there are others. Get used to using just one- for example some calculators use a negative number for a piston dish, others use a positive number (catalog info for pistons also use both ways of expressing dish/dome volumes, so be sure you're understanding what they're trying to say). By using one calculator for all your computations you'll get used to how to correctly enter the various values. On this calculator, they ask for a head gasket bore diameter. I use a number that's 0.030" to 0.060" larger than the bore diameter of the piston if I don't have the exact measurement. The HG bore diameter isn't that critical overall so fudging a bit doesn't hurt.

A couple calculators for dynamic compression ratio:
• Kelly DCR calculator
• Wallace Racing DCR calculator
• KB/Silvolite DCR calculator

• Various Calculators from Wallace Racing.

 

[cobalt327]

Never mind. Thicker gaskets just expand the quench space beyond the desired 0.035" - 0.045" dimension. Would have been too easy.

Good catch!

There are a few heads w/"ideal" specs for chamber volume, valve size, screw in studs, valve angle, etc. but expect to pay a premium for them when and if you find a set in good condition.

Personally I'd rather have a set of aftermarket heads than a stroker bottom end. Aluminum heads w/a set of aftermarket forged rods and a stock crank trumps iron D-ports and a stroker bottom end, IMHO.

 

[cobalt327]

Thanks, got ahead of myself there.

What is it about aftermarket heads that makes you say this? The prices I'm seeing are pretty high. I like the assurance of a new un-used unit for sure. The chamber sizes are nice and the machining seems super precise. Is it the improved airflow? I know we've come a long way over the last 35 years! Wondering if it makes sense for my street application.

Thanks again for your comments.

From my experience, the 400 or 455 engine displacement is adequate if not almost excessive in the case of the 455 from a torque-producing standpoint, for a street vehicle such as yours. Unless I miss my guess, a 455 w/proper machining and a ported set of D-port heads w/the correct CR for the cam being used will satisfy you 95% of the time.

Whether the added airflow (and yes- airflow is the root of why aftermarket heads are good) that can allow more power while using 'less' cam than might be otherwise needed if using iron D-ports would benefit you more than added displacement (displacement favors torque production all else being equal), is something we all have to weigh one against the other.

For a hot street/strip engine I could easily settle for a 400 or even a 455 w/good heads over a stroked 400 (or a 455) that was saddled w/iron D-port factory heads. Professionally ported iron heads- while being able to flow good, etc.- are not a bargain cost-wise compared to aftermarket aluminum heads either, IMO.

Be aware these are MY opinions; others may disagree w/all or part and that's fine, too.

 

[cobalt327]

Good stuff. I think I'm following you with respect to the argument you get plenty of displacement (more than I will probably need on the street) with a stock 400 without stroking to a higher displacement. And that dollar for dollar a good set of aftermarket heads - producing greater airflow - along with the correct cam translates to more (read, efficient) HP. Is this regardless of displacement?

Basically, yes. Regardless of displacement an efficient set of heads can make ANY engine better. Obviously an engine is a system, so all parts must harmonize to get the best results.

Or would you cam the larger (stroked) displacement differently regardless of whether you use aftermarket heads or cast D-ports? And, just so I know, where can I go to get the basics on cam selection. I don't know how to evaluate/talk about cams yet.

Thanks again.

You would need more cam w/less head to reach the same power as a good head. Pontiac heads mostly call for a split pattern cam. As far as where to get some knowledge of cam selection, I'd say to ask those who've BTDT- w/the combo you plan to use. I don't give specific cam advice (it's a boring story), so I'll leave that up to others and your own research.

The questions you pose (which are good ones, BTW) are things we all weigh, as I said before. In YOUR case, a 400 w/good heads would certainly be a ton o' fun. But so would a 455- even w/6X heads. I mean, you can get WAY over 500 ft/lbs. from a totally streetable 455 combo (even w/the 3-1/4" journals) that redlines at 5000 rpm- an rpm that allows a long life from the engine even when using stock bottom end castings. The engine I built on a shoestring back a few years ago used stock rods (*GASP*) w/nothing more than ARP bolts and resizing on a stock 455 crank turned 10/10. I've written about this particular engine many times here- not because it was a state-of-the-art piece, or because it could take down a BBC w/the blip of the throttle. I write about it because for the same money as a decently built SBC, I came up w/a true Pontiac engine w/a bare minimum of aftermarket part$ that propelled my '81 all-steel Camaro (>3600 lbs) w/3.31:1 rear gears to mid-12s w/ease. Hell, the car went 13-flat leaving the line from a dead idle and leaving the shifter in D and letting the tranny shift out at 4500 rpm! It needed only 34 degrees timing, never overheated and was daily driven for years w/o a whimper and I ran the hell out of it. I could have welded the hood shut on that engine, it was so reliable and understresssed.

In your case it really boils down to what you have available to you to work with (or are willing to find) as far as engine blocks, cranks, heads, etc., your budget, and your goals. I ran a 428 Pontiac way back in the '70s. It was a radically cammed, ported D-port headed animal. It was fast- but it was temperamental. I won a lot w/it, but I did a lot of maintenance to keep it sharp. It would NOT be something I'd recommend for your case- it's the wrong tool for the job. A milder 428 could be made to perform very good, even in your case- but I cannot recommend giving up any displacement (stroke particularly). To put it another way, if the build is going to be relatively mild, build it as large as practical. That could also be said even if the build was going to be radical, but I'm talking affordable streetable engines here.

Your '50 'Liner will have some weight. Most guys who build that style car aren't looking for ultimate 1/4 mile performance; they'd rather have a smooth, powerful engine that will tote the load w/o drama. So if you can "get by" w/13-flat 1/4 mile performance, a 455/D-port combo will do it. So would a 400 w/good heads, but at a higher rpm- meaning a lower rear ratio. It's all a balancing act and everything's a compromise to some extent.

I'm not much for spoon-feeding (not that you've asked to be spoon fed). It's one of those "teach a guy to fish" deals. But I think you can read between the lines enough to know what I would lean towards. Because of how/where I was raised I never spend frivolously. To say "I have X stroke aftermarket crank, w/Y-brand BBC rods!" means VERY little to me, unless these parts are actually needed to reach a particular goal. This holds true regardless of the make on the valve cover.

 

[cobalt327]

Nothing wrong w/a 400 block and the now-affordable aftermarket cast 4.25" crank and decent rods, as has been mentioned already. That's if a rebuildable 455 core will set you back more than the sum of the parts to build a 400 into a stroker.

I wouldn't recommend a stock Muncie M20 behind the big Pontiac and 4700 lbs- along w/a relatively high rear ratio. That's just asking an awful lot from that transmission. The M22 "Rockcrusher" is somewhat stronger but it has close ratio gears (not needed or even wanted w/an engine having a wide powerband like most Pontiacs). Now there are aftermarket solutions to the strength issues of the Muncie. AutoGear has posted about the availability of different (stronger) Muncie gearsets and cases, etc. Then there are stronger manual transmissions (TKO, etc.), but these are costly. Pluses are OD and/or low first gear ratios.

The main reason behind using a TH400 is strength (it has plenty) vs. expense (not that much, even w/good soft parts and 34 element sprag, etc.). And setting up a vehicle for an AT is uber easy- no hydraulic TOB or Z-bars, no clutch pedal assemblies, etc. An AT can be easier on drivelines, too. Not as much fun, granted.

 

[cobalt327]

If you plan to use tube headers (which you should), avoid heads that haven't got the end holes to mount the headers or manifolds. One example is the 7K3 head. There are brackets that allow the use of headers, but the brackets are expensive and I don't know how they hold up. More here.

Avoid "shorty" or block hugger-type headers. Avoid 3-tube Pontiac headers. There have been guys saying that they make no real difference in performance- to this I say BS! The ONLY thing going for them is they're marginally easier to install in some apps, and they're generally cheaper. You can use cast iron RA-type manifolds, but for the cost difference- unless the fit was enough better to warrant them- I'd just as soon use headers. YMMV. Not that I need to bring in outside sources that have done back-to-back testing (like I did in the early 90s), but here is some testimony from someone else who saw a large drop in performance when using the 3-tube headers vs. 4-tube. Jim Hand is recognized as a legit source for Pontiac info that can be trusted. BTW the closer to stock the less dramatic the difference will be. More testing comparing Pontiac RA/HO manifolds, shorty headers, and long tube headers: Headers vs. Manifolds - High Performance Pontiac Magazine

 

[cobalt327]

The newer TH400 sprag assembly are not as strong as the older TH400 tranny. The newer trannys started about 1971. The older trans (~pre-'71) had a smooth race on the hub and a true sprag whereas the newer TH400 got a "roller clutch"-type affair and a hub not compatible w/the 34 element sprag (see comparison below). Some info on the sprag can be seen here.

I would expect any aftermarket tranny to feature the 34 element sprag but it would be in your best interest to double check.

Undesirable clutch hub and dog clutch, left. Stock 16 element sprag, center. 34 element sprag and smooth hub, right.

The grooves cut into the dog clutch hub makes using a 34 element sprag impractical. The smooth type hub can use either the stock sprag or the stronger 34 element sprag- which is recommended for a high torque engine like yours.

Guys are now using parts from the 4L80 to upgrade the TH400 sprag assembly. You can research this if the need arises; a thread w/some info: http://www.hotrodders.com/forum/direct-drum-175248-2.html#post1248354.

Hopefully AutoGear will chime in on aftermarket solutions to the strength of the 4-speed Muncie boxes, as well as other options available to you for a manual box.

 

[cobalt327]

The 4L80E would be found where a TH400 would have been previously. Heavy duty 1500 to 3500 series trucks and like that. One downside is no provision for a mechanical speedo, and another is they ordinarily need a control box to operate w/o the original vehicle computer, although full manual valve bodies are available that need no computer.

And the 4L80 is BIG! You can look into a 700R4, it's not as heavy duty stock but can be built to be durable. Another AT option is a TH400 w/a Gear Vendors OD attached to it.

I agree a manual would be choice in your car, though.

 

[cobalt327]

Thanks for simplifying the AT option. I think a TH400 will stay in the running. I'll let a few more issues shake out before making a final decision about transmissions. Just so I'm clear, you still believe a TH400 with the 34 element sprag can handle the tonnage, right? Or would you lean toward the 4L80 given the current 4,600-lb weight of the Streamliner? I can always try to shave more weight somewhere inconspicuous on the car. After all it's gotta "look" slow (I mean, original) to be a sleeper, right?
Thanks again.

The TH400 is equal to the 4L80, at least 75% of the parts interchange or so I'm told. Either can handle the load of your vehicle and engine combo. They both are/were used in the same applications- trucks w/large load capacity and curb weight. So there's no real strength advantage one over the other. That the 4L80 has OD is its biggest asset IMHO.