Does anyone have any input on how much a 400 can safely be bored. I will be doing my rebuild shortly, and have been looking for overbore pistons. I found some on ebay that are .060 over, but I dont want to go to far. I think I saw a set that were .040 too. The truck is stricktly a weekend toy. Any help would be great.
Thanks

 

it would be better to go 040.

 

Have your shop see where it's going to clean up then buy your pistons. I wouldn't bore a block any further than I had too.

 

as a good rulle, go .030 over the first time. and go with badger flat top pistons. www.tmeyerinc.com give them a call, with shipping and rings, it'll probably cost about 200 bucks.

 

I already have a set of aussie heads, the flat tops would give me too much compression. I need a set of dished pistons, I believe.

 

just got to your local autozone or carquest, or whatever the local parts store is, they can get stock type pistons in all the sizes. what would the cr be with flat tops and aussie heads? you would probably just have to run on 93 octane. as long as its less than about 11.5:1

 

Danlee said my cr with a 10 cc dish and 3 cc valve relief would be 10.04:1 with the aussie heads. And there in lies the problem; in my area, we can only get 91 octane. But the other day I was talking with a friend of mine, he has a moter that runs 10.3:1 compression, and it will run on pump gas, he has even put 87 in it and it ran fine. So I dont know what to think anymore

 

He may run 10.3 compression, but that may be way lower in practice based on the cam he's using. He may have also retarded the timing.

 

i'd say with a cr of 10.04, 91 octane gas would run fine. and, it will run fine on 87, but under extreme conditions, it will probably start knocking and could blow out the piston rings, or worse yet the pistons themselves. but imo go with the flattops, and put the highest octane gas you can get. btw dont they have octane boosters? im not sure, but i remember seeing something like that.

 

Your friends engine may have a different combustion chamber configuration. You do not say what engine it is.

The 400 with a street type cam runs best with about 9.0:1 compression but large cams can accomodate larger static compression ratios. Mileage will suffer with a large cam. Danlee may be able to run a dynamic compression ratio calculation if you have all of the proper information.

Allowable compression ratio also depends upon altitude. If you live at a high altitude you can run more compression as long as you never take your truck to lower altitudes.

While you can get an engine to "run" with a higher compression by retarding the ignition timing and/or using octane boosters it is not running efficiently. It certainly won't produce optimum performance, HP, or make the numbers shown in programs like Desktop Dyno. It will also suck gas like a pig.

 

I would only bore it to the next oversize, I am guessing it is standard bore. Better to be safe than sorry.

 

Danlee has already done those calculations for me, and he came up with a few numbers for me that he said would work. I was merely saying that I thought the cr would be too high with flat tops and the aussie heads. The cam I will be using is the Comp Cams 275deh. My friends motor is a mopar, and I dont know what his specs are, so I might be comparing apples to oranges. As far as elevation goes, I am about as low as I can go. I am in the central valley in CA. The number on the sign when you hit city limits says somewhere in the nieghborhood of 26 or 28 feet.
But now that everyone has me thinkin, what would the cr be with the flat tops and aussie heads, and would it even run on pump gas, possibly with an octane booster. Please, let me know if im shottin for the stars

 

Call 1-800-560-4814 ext.159 (United Engine and Machine Co.) Give them your information and they will tell you what you need to know. They have helped me in the past. I wouldn't go over 268 duration on the camshaft or you will lose torque.

 

Danlee was very helpful to me as well. I went with a 10CC dish .030 Aussie head combo as well. I called Edelbrock while doing the same research you are doing for my 400. A good combo is their performer plus manifold / carb / cam combo...The edelbrock tech typed my parts into his computer, including my Aussie heads (I was pleasantly surprised to find out he knew all about them, including the CC's before I told him, as there has been a proliferation of these on Ebay lately and everyone is using them). He figured we'll be in the neighborhoosd of 10.7:1 static, but the dynamic with the Edelbrock cam shold be fine, with LOTS of torque, and with premium fuel, no detonation problems. With a more agressive cam, we would have compression that is higher which could lead to problems, but some guys have pulled it off who can get higher octane fuel (like the blue stuff they sell in NE Tennessee that is $3-4. a gallon and has octane around 100+)

I went with Federal Mogul 427P pistons (factory style .030 over aluminum pistons), and all the Edelbrock Performer Plus stuff...including the Edelbrock timing set.

Should be running next week; I'll post how it works.

Back to the original question in the thread...all my books and sources say do not exceed .040 over as a general rule. Now, with that said, you could go over that but you might be sleeving some cylinders. One of mine had to be sleeved and I am at .030 over...so it just depends on your block and how good a job your machinist does.

The pistons I am running and most of my rebuild parts I bought at H+M Motorsports 800-552-9964. You can also get those pistons and most of the Edelbrock stuff at Summit racing.com Summit calls the Pistons Sterling brand, but they are still the Federal Moguls - part number STL-427P30 (That is the .030 over piston)

 

I would not recommend using a cam with less than 63 degrees IVC with 91 Octane and 10.04:1 Static Compression. A Comp Cams 275DEH has 63.5 degrees IVC. The 268H is only 60 degrees IVC. The 10.04 Static Compression was with +0.030 overbore. If you go larger then your CR will also increase.

The 275DEH is fairly agressive cam, but it will be great in a weekend toy. I ran a Crane H278-2 with 72 degree IVC and 351C-4V closed chamber (62.8cc) heads and flattop pistons in California as a daily driver on 91 Octane.

You can retard your cam 4 degrees to get a later IVC, if you run into a problem.

My current build has CHI-3V heads (64cc) and flattop pistons (10.35:1 CR) with 66.5 IVC. My heads are aluminum and I use 93 Octane here in Delaware. I am running 10 degree initial advance without any problem.