Chevy Small Block 400 Questions / Troubles

[merc359]

I recently acquired a Chevy Small Block 400 verified by casting numbers. I have some questions regarding this engine type and could use some help. I am not an expert mechanic, but I have built a couple small blocks in the past. Here goes...

1. I hear that small block 400s are notorious for overheating. I am not having much luck finding out WHY this is the case, or even if this is, in fact, correct. If they do overheat, can anyone let me know why and what one can do to correct the problem in a performance application

2. Are 400 heads usable only on 400s or can you stick some other, high compression, large valve heads onto the engine? For example, I have a set of 2.02 / 1.90 angle plug bow tie heads that I had on another engine I built, a 350. Can those be put on the 400 without any troubles so long as I verify valve clearance and other obvious issues. I guess this question ties into the first one as I am curious if changing heads, or not changing heads, has anything to do with the overheating 400 stories I have been told.

3. I keep reading on other threads that a longer rod is better for a 400 small block. Usually the people cite better torque, reduced lateral pressure on the pistons, and better rotating assembly integrity. However, if I go to a 6.0" rod or the usual 5.7" rod it seems to me from a physics standpoint that I am boosting my compression by changing the area of the cylinder in which the piston travels. For example, by putting a longer rod on, it seems to me the piston will end up that much higher during every stroke which should boost compression and increase the risk of valve-to-piston contact. Am I wrong in thinking this? Any engine I have ever built I have made sure of valve clearance, I will likely do so again on this engine.

4. Assuming you go with a longer rod is there any deck clearance issues that need to be assessed? Also, does a longer rod not increase the chance of breaking somewhere in the middle? For example, breaking a 6 foot broom stick is a damn sight easier than breaking the same diameter stick when it is only 2 feet long.

Anyways, I am sure some of you veterans out there are stunned by my lack of knowledge on the matter, but there it is. Any help would be greatly appreciated.

Merc359

[Diamondshoals]

1. The 400 block has "siamese" cylinder walls - no water can flow between them. Just use a good water pump and a good radiator and you'll be okay.

2. Because of #1 air can get trapped just under the deck of the block in th ecrotch of the cylinders... so 400 heads have "steam holes" drilled in them so the air can escape into the water passages in the heads - some water will flow there also, but the holes are small - 3/16". You may use those 350 heads, but drill steam holes in them. It's not rocket science... but you do need to drill them in the right place(the 400 head gasket will show you where) and at the right angle. Car Craft should have the article online or a Google search will should find it. You can do it because you're a hotrodder.

3. You're right about the longer rod reducing lateral pressure... and that's a good thing, BUT you do need to use pistons designed for those longer rods. Pistons for 6" rods are lighter(a good thing) and the piston pin is located closer to the top of the piston - it is usually up into the oil ring... which is okay because they make a special ring that'll keep everything in perspective.

4. You're not bending that 6" rod over your knee. It's either being compressed or pulled apart. The rods(and rod bolts) do take all the punishment in an engine... that's where you should spend some money. Today's rods and rod bolts are of excellent quality. Don't worry about breaking them unless you're taking the engine "upstairs" - 7000-8000 rpm

Buy a rotating assembly that's balanced. A cast crank is fine for up to 6000 rpm. A 400 makes an excellent low rpm high torque monster... 170-180 cc intake runners and a 212-218 [email protected] cam, rpm intake, a 750 4 barrel of your choice and you won't believe the power. It's awesome.

Good luck with the project.

Bob

[merc359]

Thanks for all the information Bob.

There is only one area that you didn't address and if you could it would be great. You mentioned that if I go with 6" rods I should get pistons designed for it.

This goes back to one of the questions I posted previously. If I go with a longer rod does it have any deck clearanc problems as the piston is essentially higher up, or was that what you were referring to when you indicated the pin was higher up within the piston as this would have an offsetting effect I suppose.

Is that what you meant by pistons designed for 6" rods?

Thanks again for the help.

[MagicRat]

The piston pin sits higher up in the piston, so the compression ratio stays the same.