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Old 08-09-2012, 11:14 AM   #1
kscarguy
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Default 400 sbc piston advice needed - at the machine shop

I took my 400 SBC to the machine shop today. I need some advice for the rebuild. It is very important to remember that this is a torque motor for my COE and will not be revved past 4000 rpm. It has a very small .427/.452 split duration cam and 76cc heads.

First - It needs to be honed to .040. (it was at .030) What piston is better, a flattop or a stock piston? The flattop will raise the compression in the 9.6 range. Will it run on regular pump gas with that compression? Will lower factory compression deliver better fuel mileage?

Second - is there any real reason to change the rods to the 5.7 length or will the stock length work ok with the low rpm design?
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Old 08-09-2012, 11:21 AM   #2
Waddayacare
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Default Re: 400 sbc piston advice needed - at the machine shop

The stock 5.565" rods are fine although the non-believers will say otherwise.

Go with Keith Black Hypereutectic flat top pistons.
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Old 08-09-2012, 11:26 AM   #3
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Default Re: 400 sbc piston advice needed - at the machine shop

A reverse dome would be best imho .But with 76cc heads you will be fine with the flat tops ,i would work on getting your quench right by decking the block that will reduce your chance of detonation by aiding in turbulance for a more complete burn.Get your pistons up at o deck there usually down .025 or so if the deck hasnt ever been cut bring em up to 0 deck and use a .038 compressed gasket.

Last edited by man-a-fre; 08-09-2012 at 11:41 AM.
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Old 08-09-2012, 11:33 AM   #4
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Default Re: 400 sbc piston advice needed - at the machine shop

If you where happy with it before then just put .040" over pistons in it and leave it at that. Flat top pistons will give you a bit more comp. a bit more HP and probably a bit more heat. The mileage may or may not be better only trying it will tell and then its to late if your wrong. I ran a stock 400 in my willys for years with stock compression and never had a problem.

Cost vs % of gain. is it worth it. Rods are just fine again Cost vs % of gain. Good luck.

Pat
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Old 08-09-2012, 11:36 AM   #5
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Default Re: 400 sbc piston advice needed - at the machine shop

You'll find a large selection of Hypereutectis pistons available is flat top or dish to dial in any compression ratio you want. The 9.6 should be ok, but I would not go any higher with the iron heads and the small cam. As far as rods - yes the stock ones will work and you'll get the stock potential out of the build based on your parts seclection. That said, the performance potential is greater with the longer torque arm of a 5.7" or 6" rod.
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Old 08-09-2012, 11:41 AM   #6
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Default Re: 400 sbc piston advice needed - at the machine shop

Flat top with 76cc head.
Lowering compression isn't going to help you get anything moving yet alone more economically.

Reversed dome if you are planning on smaller CC combustion chamber or some boost.
that would help both.


If I remembered correctly your heads are done and ready to go.
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Old 08-09-2012, 11:48 AM   #7
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Default Re: 400 sbc piston advice needed - at the machine shop

I am looking at Northern Auto Parts - It seems logical to buy their master kit for $335 with the stock pistons and then spend a little extra to get a spare performance cam (to put into yet another motor). However, for $40 extra I can also move up to the speedpro flattop pistons. I never drove this engine so I have no idea what power the old flattop pistons delivered vs stock. They might deck the block and will surface the heads so the compression could be above 9.66. The Keith Black pistons, while better, are expensive at the .040 over-bore.
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Old 08-09-2012, 12:08 PM   #8
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Default Re: 400 sbc piston advice needed - at the machine shop

I like the 5.7 rods simply because it puts less side load on the pistons. Less side load=less cylinder wear. But you go from building a pretty standard bolt together engine to an engine that is a little more complicated to assemble (mostly camshaft interference). Probably not worth the trouble of building a "specialty" engine in this case.

If it were mine and I was putting it together for a daily driver, I'd probably go with the dished pistons. That way you don't have to even think about octane numbers.
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Old 08-09-2012, 12:16 PM   #9
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Default Re: 400 sbc piston advice needed - at the machine shop

I wouldn't go over the .030 unless you have an aftermaket or a '509' block. You are putting this in a truck so it needs to be heavy-duty and the 400sbc was marginal at best when they released it - they didn't make them very long remember? The last thing you want to deal with is heat and cracks around the steam holes, you can safely take them .030 and maybe to the .040 in a nova etc but you want reliability and heavyduty.
Don't misunderstand me as i raced the 400 block motors for years and still have one of them sitting on an engine stand that makes over 800hp, i like the 400 block motor, i just wouldn't have confidance in one at 40thou.
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Old 08-09-2012, 12:20 PM   #10
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Default Re: 400 sbc piston advice needed - at the machine shop

Running a 408 (.040 over) right now, 180-190 degrees, absolutely 0 issues.
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Old 08-09-2012, 12:34 PM   #11
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Default Re: 400 sbc piston advice needed - at the machine shop

I'm already worn down .003 so heck, an extra .0035 per side can't hurt it too much more. Other than that the block is fine. It's a 511 casting with 2 bolt mains (73 Impala).

I am still conflicted on the pistons. Original dish pistons mean I could swap better heads with smaller chambers onto it later on, but for now I will have a lower compression with the 76 cc heads. I am not a motor guy, so I really do not know how much difference the pistons will make.
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Old 08-09-2012, 12:44 PM   #12
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Default Re: 400 sbc piston advice needed - at the machine shop

For your application stay with the open 76cc heads and pick a piston that gets you the CR you want with them.
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Old 08-09-2012, 12:52 PM   #13
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Default Re: 400 sbc piston advice needed - at the machine shop

Its a combination that needs to work together, like a hand and glove.
With a combination of heads and pistons you could make enough compression to blow the heads clean off the block; you could make the compression so low that you could crank it off by hand.

Tell the builder you want a low end torque motor for a heavy truck for $x
You want the flatest highest torque curve you can get for $x.

Buick 401 made 450 ish torque at 2800 rpm with 50s technology nsilhead, small valves and a tortured exhaust runner.
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Old 08-09-2012, 01:19 PM   #14
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Default Re: 400 sbc piston advice needed - at the machine shop

I've messed with 400s for years, and I really like 'em. For your application, I think that .040 over will be ok. That's the last bore you'll get out of the block. 400 blocks had an issue with core shift, and some of them couldn't even go .040, so I would highly recommend having the block sonic-checked to see if it can be bored safely.

I know that there are a lot of posts on this thread recommending keeping the compression low, but I come from a different camp. An engine with 8.5-1 compression, smog heads, small cam, running on crappy alchohol-infused gasoline is going to be a dog. If you live in a place that's over 1000ft in elevation, it'll be an even bigger slug. The fuel manufacturers are going to continue to add more more alcohol to the fuels to cut costs and to comply with ever-increasing pollution standards. Alcohol does not make near the BTU number as gasoline does, and you need compression to help you out. Get the compression to at least 9.5-1, and take a little timing out of it if you need to. I recommend a flat top for better flame travel, and if you're still not at a good compression ratio after that, have the block decked accordingly. Compression makes torque, and torque is what you need to get your COE moving. Don't fear reasonable compression. Compression is a good thing in the right quantity.
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Old 08-09-2012, 01:43 PM   #15
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Default Re: 400 sbc piston advice needed - at the machine shop

The 400 as designed was and is very good engine for torque---the RPM band and stated purpose for your motor do not justify anything more than basic rebuilder parts---cast pistons ,stock rods,etc.--keep your goal in mind and avoid more performance oriented expenses that will not cost justify---now if it is going in your race car, longer rods,better pistons,etc.---but it's not
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Old 08-09-2012, 02:00 PM   #16
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Default Re: 400 sbc piston advice needed - at the machine shop

I seem to have three reasonable choices:

1. Flattop piston, 76 cc head, (9.66 CR), small Torque "RV" cam
2. Dish Piston, 76 cc head (8.5 CR), Same cam
3. Dish Piston, Vortec head 64 cc (9.1 CR), same cam...but costs $600 extra for heads and intake
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Old 08-09-2012, 02:04 PM   #17
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Default Re: 400 sbc piston advice needed - at the machine shop

With a 4.165 bore, 3.75 stroke, 4.25 gasket at .035 thick, 76 cc chamber, -5cc valve reliefs, and .015 deck clearance you get 10.05 to 1 C.R. That is a lot for pump premium with iron heads and a mild cam. Pulling the timing back to get away from the detonation is really counterproductive. I'd be inclined to go with a dished piston. to get the compression down to the 9 to 1 range.

When you bore a cylinder to .040" oversize, you are removing .020" of material.
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Old 08-09-2012, 02:19 PM   #18
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Default Re: 400 sbc piston advice needed - at the machine shop

My actual deck clearance is yet unknown. The 9.6 static CR is supplied by the piston mfr. According to Crane Cams, 9:5 is max for my cam, and 9:1 is about right. I bought the cam before I knew it was previously bored and had flattop pistons.

While my gut really wants new flattop pistons, I am inclined to run the dish piston and 76cc heads for now and then swap to the Vortec heads later...

decisions, decisions...Now I'm thinking I could use the 400 crank in my 350 4 bolt main block and build a 383....Ahhhhhhh!!!!
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Old 08-09-2012, 02:41 PM   #19
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Default Re: 400 sbc piston advice needed - at the machine shop

Quote:
Originally Posted by kscarguy View Post
I am looking at Northern Auto Parts - It seems logical to buy their master kit for $335 with the stock pistons and then spend a little extra to get a spare performance cam (to put into yet another motor). However, for $40 extra I can also move up to the speedpro flattop pistons. I never drove this engine so I have no idea what power the old flattop pistons delivered vs stock. They might deck the block and will surface the heads so the compression could be above 9.66. The Keith Black pistons, while better, are expensive at the .040 over-bore.

I built a 406 (400 +.030) for a custom 1 ton dually 1988 suburban using this exact kit with the speedpro pistins, final compression ratio was 9.8-1 with the heads having a mild flatteing/mill job and minor black deck cleanup the cam was a 268h comp cam with 1.6 ratio rockers, 1 3/4 primary headers, gm cast iron bowtie intake with a custom adapter to put a 1988 454 tbi unit on it that was stock from the suburban. Ir runs perfectly fine on any pump gas from the cheapest grade regular to premium without any knock problems.

I used all the controls from the 454 on the 406 and we have a performance chip in the ecm. The suburban gets 18-19 mpg on the highway empty and gets 13mpg pulling a 32 enclosed car trailer with a fullsized 1970 suburban in it! I drove the truck over a set of certified truck scales at our local dairy and the total weight was 18,626 pounds! They have put over 20,000 miles on it since the build and have yet to have any problems with it, It has the stock 4 core hd radiator, a/c, etc and never over heats. This summer when it was a 100+ degerees here he had brought the burb up with a 56 chevy in the back and when he pulled into my drive and was setting there it was just over 200 degerees but on the road never touches 200.

I have a 1988 3/4 ton 454 suburban with 48,000 original miles, a/c, and just a chip in the ecm and I can't touch him loaded or empty. I get 13mpg empty and 10mpg loaded. I had a open trailer with a 1969 suburban on, total weight was 16,796 pounds and he had his enclosed trailer and suburban inside and I could not pass his ass on the open road....so I'm here to say you should have no issues with the plans of the master kit upgrading to the speedpro flat tops and just put her together and enjoy.
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Old 08-09-2012, 02:43 PM   #20
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Default Re: 400 sbc piston advice needed - at the machine shop

Quote:
Originally Posted by kscarguy View Post
My actual deck clearance is yet unknown. The 9.6 static CR is supplied by the piston mfr. According to Crane Cams, 9:5 is max for my cam, and 9:1 is about right. I bought the cam before I knew it was previously bored and had flattop pistons.

While my gut really wants new flattop pistons, I am inclined to run the dish piston and 76cc heads for now and then swap to the Vortec heads later...

decisions, decisions...Now I'm thinking I could use the 400 crank in my 350 4 bolt main block and build a 383....Ahhhhhhh!!!!
Don't do it...your 400 will outdo that 383....follow your gut and go for it. I'm here to tell you it will work great....been there done that!!
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