Help with 400 build (or do I do an LS swap?)

[C.I.Blazer]

Well it looks like the engine is bad on my new to me 71 Blazer. Although it came stock with a 350 and 4 speed manual, the truck currently has a 400 (511 casting) with a two barrel carb mated to a TH350. Trying to figure out what to do now. I am very new to this so please bear with me and excuse my ignorance.

My ultimate goal is to simply have a streetable, driveable vehicle. Only off pavement driving would be forest service roads and very mild trails. I do limited towing of a couple of jetskis and maybe a trailer of dirt or gravel every once in a while. I would prefer something that has some oomph to it just for fun. I want to keep the horsepower reasonable and not have to start upgrading all the other components of the truck (at least not yet). I would love it if I could keep running 87 octane but will get better gas if I need to.

Does something in the 300 HP range sound about right for this motor and this application? What is the best way to get this horsepower? I would like to start with the things that are easiest to do now with the motor out and maybe leave me some options for increases down the road that are easier to add. Not sure if that makes sense.

My plan is to start by sending everything to the machine ship to be cleaned and checked. I have read that the 400s (especially the 511 casting) are prone to cracking so they will magnaflux the block and see if I have a good foundation to start with. Assuming that is all good, how would you build this? I have read all about vortec heads, lift rods, cams, boring .030 over, etc. but have not idea how to build a complete package that works together and meets my needs. I also want to start pricing the parts out and get an idea of what this will take.

I love the idea of an LS swap and it sounds like that may be in the same price range. Especially if my block and crank are bad. I need to try and get a better handle on the cost of each. Actually, more importantly, I need to get a handle on the expertise needed for each.

Thanks for the help everyone. This is quite learning journey for me.

[mongocanfly]

a good used LS swap will be a better option than having to do full machine work rebuild on the 400
are you planning on keeping the t350? the LS will bolt right up to it but youll need the flex plate adapter and motor mount adapter plates
of course youll need wiring harness and fuel system and exhaust and ECM unlock
my opinion would be go with the LS swap...you wont regret it..

[YoungPup1977]

I would fix the frame first, install a sbc 350 crate engine under 2K. Go back with ram horn exhaust manifolds. Simple and you are on the road sooner than letting the blazer sit for a while trying to figure out the used LS engine. But again, every one has a LS in their blazer or a 600hp 383 Stroker just to drive it a few miles per year. I am trying to understand that logic but this a forum and dreams do come true for the ones with empty wallets.

[cadillac_al]

I'd be happy enough with a $1500 crate engine.

[garyd1961]

A GM crate L31 with a good 4 barrel and good flowing exhaust would get right around 300 hp and with a lot of low end torque. You could use your th350 trans and all your brackets but you will need a new intake, flexplate, and you have to run an electric fuel pump. The electric fuel pump scares some people but I have been running a cheap Holley Mighty Mite for a couple years without any problems. I do keep an extra one under the seat just in case but haven't needed it yet.

[C.I.Blazer]

Great input. Thanks all!

I really want to make this a fun driver rather than a sit in the garage and only drive it once in a while car. It will be my only vehicle although I do have a work vehicle and we generally take my wife's car if we are going anywhere on the weekends. I would estimate around 500 miles a month. Local driving with some short (20 mile) trips on the highway. Does 300 HP sound about right for what I am after?

I thought if I did an LS swap I would try and find one with a tranny as well so I could have the overdrive. Does that make it overly complicated? Our other car (my son calls it his now) is an '04 Suburban so we are learning a lot about working on the 5.3.

It seems like every time I look at Crate motors they are all in the $3000+ range. Is that just because my eyes go right to the high horsepower models? If you wanted a basic crate, which would you go with?

L31? Haven't really thought about that. Another internet rabbit hole to go down.

Doesn't sound like a whole lot of love for rebuilding the 400. Do I just scrap it?

Really appreciate all the input. Please keep it coming.

[b454rat]

Rebuild the 400. Get some 5.7" rods with ARP bolts. Keith Black or whatever pistons you want, with a slight dish to keep compression reasonable. Get crank cleaned up, get new balancer and flexplate. Get the rotating assembly balanced. Find a good set of Vortec heads, known for cracking, matching intake, and a mild cam, say Comps High Energy or Extreme, 262, or 268 for a nice lil lumpy idle. 750 3310 carb. Motor will make gobs of torque, and pull like a freight train. LS won't make the torque of this motor down low, where you need it. I love 400s, had them in many vehicles, cars and trucks, every one ran great. I have a block all ready to go, just need funds to finish it.

[geezer#99]

If you need to get a handle on the expertise of either motor, how did you determine your 400 is bad?
Did you diagnose it or get someone’s opinion?

[mongocanfly]

if you go LS and want od..your cheapest choice will be a 700r4...if you use the 4l60 that's behind a 5.3, it gets expensive to adapt it to your transfer case...more importantly though is your gearing and tire size....those need to be accounted for before you commit to a od trans...with the wrong gear ratios and tire size, a od trans may be useless

[garyd1961]

Jegs had the L31 a while back for under $2K. You might want to check out the L31-R also. It's a 4 bolt main with heavy duty valves.When I got mine it was like an extra hundred bucks for the heavy duty version.
It's really hard to beat the L31 with all you get for the price. One of the best things is it has a hydraulic roller cam.

[Captainfab]

Yeah the SB 400 doesn't get much love by the majority. Here is some reading for you.

http://www.67-72chevytrucks.com/vboa...d.php?t=612016

[C.I.Blazer]

Lots of great food for thought. Geezer#99, yes I am new to this and it has been a great learning experience. I do have some friends and people at work that have a lot more experience then I. One of the helped me do a leakdown test which showed four cylinders with 20% leakdown, 1 with 50%, and three with 80%. He recommended that it was time to do a rebuild.

b454rat & Captainfab, thanks for adding to the conversation. I have done a lot of reading of your posts on this site and because of them have a lot of interest in rebuilding the 400. I have found a local machinist with a good reputation and am trying to come up with a parts list to get an idea of the cost of the build.

The L31-R looks like a great price. Looks like the advantage of the moving to the L31 is a newer design with vortec heads (which I would want to buy for my 400 anyway) and the one piece rear main seal? Only modification I would have to do would be the electric fuel pump. Add the carb and intake which I would have to do for a rebuild anyway.

Mongocanfly, for some reason I am liking the idea of an LS swap. Nothing about this truck is stock so it seems like a good candidate. Lots of questions but I will start another thread in the LS section. Thanks!

[YoungPup1977]

not sure if its about the 400 not getting any love, its what it costs to rebuild, along with how the vehicle is going to be used. most likely it would cost more than a cheaper 350 crate motor to rebuild the 400 is all. Plus the vehicle would be back to use sooner with a crate engine, instead of waiting on the machine shop. How much of the engine work can the owner do himself ? (fan clutch thread )
I think Jegs or Summit sells the 1600 dollar 350 that would work fine in the blazer.

Plus if the owner rebuilds the 400 and builds it to 300 plus HP what will it do to the 350 trans that most likely is weak as the 400 engine that was pulled....all the owners money went into the engine no money to rebuild the trans....no one thinks of the impact to the trans after building a 300 plus HP engine. oh, yea I forgot, swap it over to a used LS and a used 4L60E !! than lose 3/4 in the high mileage 4L60E. Ongoing saga.....

[C.I.Blazer]

Youngpup, appreciate the reply and thoughts. In hindsight, I think my error came in the HP rating change from 1971 to 1972 which I understand came from a difference in calculation as opposed to actual power. My goal is not to create a hot rod, but simply add a little more power. I understand the consequences of too much horsepower without the added drivetrain upgrades and do not want to go down that road. Simply trying to get a decent daily driving fun vehicle. I agree that the less expensive 350s and L31 would be easiest, cheapest, and fastest and offer plenty of oomph for what I want to do. This is probably the most sensible route.

How much of the work can I do myself? That is a question I ask myself every day. I am doing this to enjoy the journey, learn a new skill, and try something I have wanted to do my whole life. Maybe I see it through or maybe I lose interest in a year as many have. Either way, I tried. I have reached out to some local car guys that have a lot of experience and have some help regardless of which way I go.

Again to all, thanks for the advice. I imagine you sometimes get frustrated with newbies like me but I am sure we all appreciate your patience and lessons. I know I do.

[Captainfab]

The L31 in stock form is rated at 255 HP.

What side of WA state are you in, East or West?

[geezer#99]

Here’s an L31 two bolt main.

https://www.jegs.com/i/JEGS/059/0282/10002/-1

And here’s one with a Chris Straub cam that adds 100 hp.
Plus an intake carb and headers of course.

https://m.youtube.com/watch?v=yDaE0V--Dfw

[b454rat]

vortec 350s are great engines. In you case coule down falls. Need an electric fuel pump, melonized distributor gear, and Vortec specific intake. Everything else will bolt on fine. With the 400, everything off your 350 will bolt on. 400s aren't any more expensive to build than another small block. Yeah the pistons won't be as cheap as 350s, but there still not expensive, and going by what you want/need, don't need forged pistons, cast would be fine. Best thing to do to a 400 is use 350 rods. Puts less side load on the pistons as they go up n down in the bore. Plus, everybody is doing an LS. Early K-5s are cool as ****, I havne't seen one in my area in a looooooong time, they need to have a small block in em!! Just my $.02. And you'll love the torque of the 400.

[burnin oil]

Old motor I had that is cheap. 400 flat top pistons with just valve reliefs cast in. Stock rods. 300hp 327 cam and stock heads. Gobs of torque and could be lugged down to 400 RPM or spin up to 6k RPM. Ran on 87 octane but would diesel at shutdown with bad gas. 15MPG for a HS student who beat on it reguarly. Stupid simple and cheap. Now if you want a fire breather that gets alot more cash.

LS motors are great but old school SBC put the torque out better at a low RPM. This is important to us stick shift guys.

I forgot to mention this. Check for machine shop prices in your area. This is what will make or break a rebuild or crate motor decision.

[garyd1961]

The L31 is rated 255 hp with the factory fuel injection but with a good 4-barrel and headers it will be right at 300 hp and will have a lot of low rpm torque which is better than hp in these trucks.. You don't really need a cam swap.

[YoungPup1977]

Originally Posted by C.I.Blazer

Youngpup, appreciate the reply and thoughts. In hindsight, I think my error came in the HP rating change from 1971 to 1972 which I understand came from a difference in calculation as opposed to actual power. My goal is not to create a hot rod, but simply add a little more power. I understand the consequences of too much horsepower without the added drivetrain upgrades and do not want to go down that road. Simply trying to get a decent daily driving fun vehicle. I agree that the less expensive 350s and L31 would be easiest, cheapest, and fastest and offer plenty of oomph for what I want to do. This is probably the most sensible route.

How much of the work can I do myself? That is a question I ask myself every day. I am doing this to enjoy the journey, learn a new skill, and try something I have wanted to do my whole life. Maybe I see it through or maybe I lose interest in a year as many have. Either way, I tried. I have reached out to some local car guys that have a lot of experience and have some help regardless of which way I go.

Again to all, thanks for the advice. I imagine you sometimes get frustrated with newbies like me but I am sure we all appreciate your patience and lessons. I know I do.

Either way you go, its your wallet and your journey...….

Make it the best for you. Good Luck !