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FullTimeRacing 10-31-2008 12:42 PM

High Compression 400 sbc
what is the highst comp ratio that should be used in a 400 sbc 817 block?

I have a std bore block low mile I want to build to a 420cid with 3.875 stroke.All opt are open

this will be for drag racing only.

zipper06 10-31-2008 02:51 PM

We're running 14.5 with cosmetic gaskets in a 434" motor no issues as of yet, with close to 200 passes.


FullTimeRacing 10-31-2008 02:56 PM

I was told the cyl would bollon in time.

zipper06 10-31-2008 03:01 PM

We fill the block up to the bottom of the water pump inlet with hard block and use total seal rings. Leakdown is still within 2 to 3%. I'm sure that an after market block will seal better, but it's what works for us.


thedude 10-31-2008 03:02 PM

Hello, with a factory block, especially a GM 400, the real question is, what is the target horsepower figure you are trying to accomplish with this build? If you intend to go with a higher compression ratio and the cubic inch that you specified, I would estimate with a properly matched induction combo (roller cam/intake/heads), you will be pushing somewhere around 600+ horsepower depending on how well the combination is put together. I personnaly don't reccommend using the 4 bolt main 400's, simply because the main webbing where the outer main bolt is drilled and tapped through during factory procedure makes a great breeding ground for stress or cracking. If you must use a factory GM 400 block search for a 2 bolt main and have a reputable machinist install splayed 4 bolt caps. Please keep in mind after all this trouble and machine work/exspense, you could have probably bought a reasonably priced aftermarket cylinder block such as Dart's new entry level SHP (there are many posts about this block on the forum). Aftermarket cylinder blocks seem out of reach on a tight engine budget but machine processes to get that old iron suitable for higher horses will put you into the $1000+ range, unless you have a good engine machinist that owes you a favor. Long story short factory iron is good for between 500-550HP MAX, not to say you can't put more into these blocks, many will tell you they have with great success, as have I but with great preparation put into the block. Years and years ago I built a 670HP alcohol 377cid out of a factory GM 400 with splayed caps, I wouldn't go to that trouble again though, deck plugs, splayed caps, grout block fill, pan rail clearancing for alum. rods, etc., etc. I hope this sends you in the right direction and you are doing what any good racer does,....homework. Thanks.

FullTimeRacing 10-31-2008 03:22 PM

I have brodix track 1's no port work on a 360 cid hv intake 1050 dom and was looking to put more cube's under it.260/[email protected] 630 lift cam,c 12 gas

the block i have is a two bolt std bore. cyl look beutiful dont need to be bored.Was just going to have it honed for piston clearance and deck

I know i'm going to need a small base circle cam

us7race 10-31-2008 07:15 PM

You don't have to run a small base circle cam if you use Eagle Stroker Rods...That is what I have in my motor...

FullTimeRacing 10-31-2008 09:06 PM

love the info,13.5 to 14.0 would be the most i'd want to go.not looking to set a record just go a lil faster.

johnracer 11-01-2008 04:08 AM

You'll need to consider that, IMO, even a 406 will be DONE by 6500rpm with those Track 1's. A 420"er will be even lower. It'll make great mid range power but won't like rpm. You'll need to regear to keep it in the power band.....

FullTimeRacing 11-01-2008 05:46 AM

I was thinking a power glide would make good use of that power

right know I shitf at 6500 and go through the lights at 68 to 7300 depending on the weather.

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