406 Thoughts

scoti

Master Mechanic
Supporting Member
Sep 5, 2019
499
93
Texas
So the 50k mile sbc in the 78 Malibu functions but is lacking. It's a basic D/D set-up: 305/th350/7.5"/2.73 set-up. Biggest complaint would be the leaks (back china wall, rear main, front timing @ oil pan) & fuel/catalyst vapors that seem to seep into the cabin from the factory exhaust. If I'm going to pull a motor out for clean-up & re-sealing (especially a low compression/small cubed one) I want to make the most of my time & effort. With that intro, I have acquired 2qty 2-bolt 400's over the years:

Combo #1 was "rebuilt" by an unknown & was part of a package deal w/other parts obtained years ago that's been sitting bagged on an engine stand. It's .040 over, 5.7 GM rods, has Hyper/4-valve relief Flat-tops, turned OE crank, & fairly large Lunati flat tappet cam (0412: 246/[email protected]; .515/[email protected]; 108/104 LSA/ICL; 25-6500rpm range).

Combo #2 is a recent acquisition that came from a buddy that I helped swap a 427sbc into his A-body Elky. It's well used, .030 over, cast dished pistons (w/one damaged), short-rod, OE turned crank, 64cc Dart Iron Eagle 200's, & a hyd flat tappet of unknown size/ID. I know this motor has seen it's share of laughing-gas.

In 2012, a local mass rebuilder did the rebuild on my duallys 7.4l motor & for a stock D/D it's worked just fine. My thoughts were to do a similar budget D/D build-up on one of the 400's but add/change some stuff that I have just sitting around the shop already. For my 7.4l rebuild, I had them 'upgrade' it by utilizing flat-tops vs. dished pistons & upsizing the cam to an 'RV/Towing' profile from their vendors catalog. No changes to the Peanut-Port heads other that a valve job but I added 1 3/4" full length headers & Edel-B TBI intake. It's a nice, very mild combo w/excellent throttle response.

If I used one of the 400's for the 78, the flat-tops impacts compression depending on the heads used. While the 64cc Darts my buddies motor came with could be cleaned up to work w/replacement dished pistons, I have a new/old set of AFR 190's w/74cc chambers I can use as well. The AFR's definitely out-flow the Dart IE's. My problem has been trying to put together a decent combo that doesn't start getting stupid. I like the thought of using the AFR's but selecting a mild cam to get a well thought out D/D combination similar to my Dually has been delaying decisions.

I'm not worried about using a flat-tappet cam & it would definitely work better toward the budget/rebuilder agenda. The rebuilder utilizes Melling cams & they had several decent/mild choices as part of the rebuilding. I could also install/upgrade to a mild hyd roller (less break-in concern/issues & better/broader power range). I have additional supporting parts for the larger displacement choice sitting around the shop as well: 1 5/8" primary x 3" collector full length as well as 1 5/8" x 3" mid-length headers; full 2.5" H-pipe/Flowmaster exhaust, Edelbrock RPM intake; 8.5" GN/T-type 3.42 posi rear end; & a 88-94 Serpentine accessory set-up.

My wants are simple/basic: a mild, D/D, low maint, 87ish octane combo that is fun & not a PITA. What I don't want? LS swap suggestions.

It's not involved in stop-light contests of speed.

I'm open for idea's suggestions that fall inline w/my above info.....
 
Last edited:

bracketchev1221

G-Body Guru
Jan 18, 2018
849
93
Go with the 74 cc heads and get the compression no more than 9.5-1. That will help you get to 87 gas standards with the aluminum heads. Pick a small hydraulic roller with less than 220 degrees at .050 and it will idle like a sewing machine in a 400.
 
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Texas82GP

Just-a-worm
Supporting Member
Apr 3, 2015
6,311
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Spring, Texas
I'm no engine guru, but I think given the choices I'd put a smaller cam in engine 1. Since it's relatively fresh, I think I'd spend the money on a retro-roller setup rather than rebuild the short block if it passes inspection. What would your compression ratio be with those 74cc heads you already have? I'm guessing you want around 9-9.5:1 compression ratio at most with those aluminum heads for drama free 87 octane fuel use.

Whatever you do, make sure the steam holes are drilled in the heads you go with. They are a must for street use of a 400 as far as I'm concerned.

My car had a 400 in it when I bought it. It ran hot and leaked everything. I pulled it to clean and inspect it. It was 0.030 over with dished pistons. It had iron Bowtie Phase II heads which had not been drilled for the steam holes. It had a spread bore Performer intake with an adapter and a Holley double pumper. Unknown cam. Point of telling you all this? It was a poorly matched, poorly executed combination and it ran about like it had 165 horse power. You don't want that.

Mine had significant ridges at the tops of the cylinders so my guess was it would have to go to 0.060 over to rebuild it so I moved on.

Good luck with the build. A nicely executed mild 400 with 3.42 gears should be quite snappy.
 

scoti

Master Mechanic
Supporting Member
Thread starter
Sep 5, 2019
499
93
Texas
I'm no engine guru, but I think given the choices I'd put a smaller cam in engine 1. Since it's relatively fresh, I think I'd spend the money on a retro-roller setup rather than rebuild the short block if it passes inspection. What would your compression ratio be with those 74cc heads you already have? I'm guessing you want around 9-9.5:1 compression ratio at most with those aluminum heads for drama free 87 octane fuel use.

Whatever you do, make sure the steam holes are drilled in the heads you go with. They are a must for street use of a 400 as far as I'm concerned.

My car had a 400 in it when I bought it. It ran hot and leaked everything. I pulled it to clean and inspect it. It was 0.030 over with dished pistons. It had iron Bowtie Phase II heads which had not been drilled for the steam holes. It had a spread bore Performer intake with an adapter and a Holley double pumper. Unknown cam. Point of telling you all this? It was a poorly matched, poorly executed combination and it ran about like it had 165 horse power. You don't want that.

Mine had significant ridges at the tops of the cylinders so my guess was it would have to go to 0.060 over to rebuild it so I moved on.

Good luck with the build. A nicely executed mild 400 with 3.42 gears should be quite snappy.
That's where it gets tricky. Going off of the mathematics strictly, mild cams start increasing the cylinder pressure. So I played around around increasing/changing cam size profiles to find a compromise of static & dynamic compression. I don't have the notes in front of me @ the moment but I recall just shy of 9.8:1 static & 8.5:1 dynamic during the calcs. But... That's just math & doesn't mean reality.

With limited exposure to these larger sbc's, I rely on research which mainly resulted in magazine/internet data which we all know can be cantered to favor the discussions 'pitch'. My friend that I got the 2nd 406 from was to be my educator (he had built multiple street 406's) but sadly passed before we were able to do the build. I will say the common thing I did read while researching was the 400 likes more cam vs. a 350 based build & what is more radical in a 350 is milder w/the extra cubes. 230° @ .050 duration seemed to be a common number but the combos were usually trying to yield "more power" where I'm wanting 'stout yet pump-gas friendly'. The 2nd most common thing was something I was well aware of.... The need for steam holes for a driver type combo.

I have zero desire to be the fastest old guy on the block but love being pushed back in the seat when getting on the highway. I'll look into the cam profile (Comp Cams 12-423-8 retro fit cam).
 
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ssn696

Living in the Past
Supporting Member
Jul 19, 2009
4,752
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Mid-life crisis
Thanks for the update. Interested to see how this pans out. I have a 406 in my coupe, but the project is parked until life lets me catch up. It's on its third life now, but I agree with your last sentence.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
2,236
113
Upstate NY
My recommendation is to eliminate some of your options meaning pick a block and a set of heads. Persoanlly, I'd haul all of it off to the machine shop and tell them 'go' and to stop when they come across the 1st good block and set of heads (I'd pick the AFR's 1sr ;) ) You'r blocks that have had NO2 ran through them and that are .030" over have a lot of potential to be problematic, but might be fine. I'd do my darndest to keep rom going more than .030 over.

Once you have that nailed down, then pick a piston and cam to keep you build pump gas friendly. The best part of a SBC is that parts are cheap and available.
 
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scoti

Master Mechanic
Supporting Member
Thread starter
Sep 5, 2019
499
93
Texas
My recommendation is to eliminate some of your options meaning pick a block and a set of heads. Persoanlly, I'd haul all of it off to the machine shop and tell them 'go' and to stop when they come across the 1st good block and set of heads (I'd pick the AFR's 1sr ;) ) You'r blocks that have had NO2 ran through them and that are .030" over have a lot of potential to be problematic, but might be fine. I'd do my darndest to keep rom going more than .030 over.

Once you have that nailed down, then pick a piston and cam to keep you build pump gas friendly. The best part of a SBC is that parts are cheap and available.
I only have 2 blocks. The first is the machined years ago, assembled short block that's been sitting on an engine stand for 10+yrs as described (flat top .040 Hyper pistons/5.7 rod 509 block). The 2nd one is the more recent acquisition from my friend that passed & the one that's the stock short-rod/dished piston/well used 817 block that saw the NO2.

The well used Dart Iron Eagle 200/64cc heads were part of the stock bottom end 2nd motor.
The AFR's are new/old (mock-up only then sitting on a shelf 10+yrs) 190/74cc combustion chambers.
 
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