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Old 03-18-2009, 01:27 PM   #1
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Default 350 vs. 400 FAQ's and suggestions

Alright I was on another forum and someone was asking questions about swapping from a 350 to a 400sbc and I answered a lot of questions for him. I figured I would do a write up comparing the two motors.

Typical questions...

1. I've got LT headers for my 350 (i'm assuming any SBC), will they fit the 400?
Yes they will fit, a 400 is diffrent internally from a 350 or 305 etc. but on the outside it is basically the same. I have Hooker 2210's bolted up to my 406 right now.

2. Will the motors mounts from my 350 bolt right up and will this motor drop right in?
The answer is yes since it's still on the old sbc platform.

3. From what I understand the 400's are balanced differently then all other SBC's. Will I need a different flywheel then? Both motors are 2 piece rear main seal motors.
This is where it gets tricky, a 400 is externally balanced and the 350's are internally for the most part. Most 400's use a staggered bolt pattern which uses a 168 tooth flywheel. You can have the shop drill and tap the hole for a inline pattern and use a 153 externally balanced flywheel or a internal one with a balance plate mounted between the crank and flywheel. If you need more info feel free to PM me.

4. Will my starter work with this motor?
Depends which starter you have and such but I would venture to say no right now.

Now I will compare the motors. All externals like heads, cam, intake, headers and bolt on stuff are completely transferable if you desire.

350= 4.00
400= 4.125
350= 3.48
400= 3.75
# of Main bolts:
350= 2 and 4 Bolts
400= 2 and 4 Bolts
350= 5.7
400= 5.565

400's came in 3 diffrent casting numbers 330817, 3951509, 3951511. Those are the only 3 numbers put out by GM. Most 511 blocks are 4 bolt mains but there were some 2 bolts. The 817 and 509 blocks are the more desireable since just about all of them were 2 bolt mains and have a higher nickel count from the factory. The interesting thing about the 2 bolt mains on the 400's is that by design the 2 bolts are stronger because of the added webbing in the main areas causing less cracks and issues inside the motor. The reason for the shorter rods with the 400 is to compensate for the added length of stroke to keep a modest Rod/Stroke ratio.

Now building a 350 over a 400 is a toss up. 350's can rev higher by their shorter stroke but the 400 will make more Torque where its needed. 400+ci sbc motors are torque monster motors that make a lot of it very quickly but they are usually limited in RPM's. Most mild motors dont go past 6000-6500rpm's. The actual HP levels will be pretty similar if you built two identical motors but the torque curve on the 400 will be much higher and much wider.

Popular combos for 400's:
406 = .030 overbore
408 = .040 overbore
415 = stock bore, 3.875" stroke
421 = .030 over, 3.875" stroke
422 = .040 over, 3.875"
427 = stock bore, 4.00" stroke
434 = .030 over, 4.00" stroke

Theres also another combo comming up a lot, it's a Destroked 400.
377= stock bore, 3.48" stroke

Popular combos for 350's:
355 = .030 over
357 = .040 over
383 = .030 over, 3.75" stroke
396 = .030 over, 4" stroke

Theres plany more info and I will keep adding to this. Picking out chich to build just depends on the direction you want to go. Do you want a high revving HP motor or a crazy torque monster?

Building a 400sbc:
To build a good 400 motor the first things you have to remember is a engine is a air pump. The more air in and out effectively will make more power. Because of the increased bore sizing the motor needs more air than a 350 would.

A good flowing manfiold such as the Edelbrock Performer RPM series or a Victor Jr. are a good choice. You need the runners to have a good port size so the motor can breathe. A manifold like the Performer just doesnt let enough air into the motor. A good dual plane or a single plane are ideal.

This is where you make or break the motor. Good heads are very important. Most 400's cam stock with a set of 333882 casting heads, theyre not the worst in the world but if you want to put any real power to the ground you need a nice set of heads. Because a 400 is bigger than a 350 you need more port volume. The idea with good heads is to find the heads that flow the best with the smallest port volume. A typical street/strip 400 needs 190-220cc intake port in order to make good power as well as a good flowing/sized exhaust port. All that air cant go in and go out a tiny hole. Port velocity needs to be kept up so when the valve opens the exhaust gets "vaccumed" out. Anyting smaller than 2.02/1.6 valves are too small, even for a street engine.

Picking out a cam isnt that hard it's just finding on with suitable lift and duration for your purpose. A good cam for a 400 will have over .500 lift on either side of the cam as well as around 230* duration @ .050. All 400's were flat tappet cams from the factory so you can run a hyd. or mechanical flat tappet cam easily. To run a mechanical roller(solid) cam you just need the proper lifters and valvetrain. Same goes for a Hyd. roller, the biggest diffrence is the cam needs to be a Retro-fit cam with special lifters for the non roller block.

This one is plain and simple, any performance 400 needs a minimum of 1 3/4" primaries with a 3" collector. Anything less will choke down the motor. 1 5/8" isnt too bad but it will lose the power and the motor will suffer slightly.

Again this depends on the goal of the motor. A simple cruising motor can use a 650cfm carb or even a 750cfm. Once you get into doing heads, cam, intake the CFM jumps quite a bit. To run a good 406 a MINIMUM of 850cfm is needed. I plan to run a 850DP or a 950HP(Really flows 830cfm) series for a street/strip car.
Really just a cammed 305..
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Old 03-18-2009, 01:58 PM   #2
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Old 03-18-2009, 02:02 PM   #3
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They're not externally balanced I know that. From my understanding all 350's are internal/neutral balanced.
Really just a cammed 305..
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Old 03-20-2009, 11:20 PM   #4
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I'd like to add to this. He missed the one most important thing. When swapping heads from a 350 to a 400 they have to have steam holes drilled in them.
This can be done by laying the 400 gaskets on the heads and making them and just simple drill the holes.
Other wise you will forever be having overheating problems.
If this is a sticky on numerous other forums I'd suggest you go fix them all so nobody cooks there motors.
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Old 03-21-2009, 01:23 AM   #5
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^^^ good info

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Old 03-21-2009, 08:49 AM   #6
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Well I knew I would forget something. The steam holes hafta be drilled at a 45* and a 30* angle. Also steam holes are "needed" on race motors since they dont have the time to sit and warm up in traffic like a street motor. But for anything on the street I would suggest it. We've built a few that didnt get drilled and have done fine but I dont like the idea of it.
Really just a cammed 305..
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Old 03-29-2010, 11:31 PM   #7
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I just love my 350 zz 4 oh crate motor
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