BigWhiteGTP

I'm not ragging, I just want to know why it's done. I see all these guys with 355's and 307's and such and I am asking why do you guys do it?

Is it cheap to do? Does it make a substantial HP/TQ increase? Is it worth it for the price? Or is it just to get as much power from the block as possible? Where do you find replacement pistons, cuz they need to be wider now, right?

How much can a 305 be bored out, what is the end C.I.? Same question for the 350? And how do you get the final number if an engine was bored over .060 or whatever?

Thanks guys.

dr1

generally for cleaning up the bores when rebuilding
and afterwords its why you end up with such numbers as 355 because of the size increase

DuronClocker

iTrader: (1)

Yeah, sometimes you can get away with just honing during a rebuild, but sometimes the cylinders have a ridge at the top or scoring in the bores. Sometimes they can even be out of perfect alignment. Boring them a bit wider gives you a nice smooth straight cylinder wall again. Its actually very easy to find pistons in common sizes like .030" over. Some sizes are harder to find, like .040" over, especially for a 305. I was going to buy a re-machined 305 block that was .040" over (which would make it a 312), but I couldn't find any pistons I wanted to use for a .040" over block.

The guys who bore their motors straight to .060" over are for max performance, but I couldn't justify it. For the extra very minimal power you'll gain, you are eliminating the possibility of re-boring the block in the future, which means you'd have to go and buy another block again. Its not terrible I guess, but it seems to be a waste of 160lbs of iron during rebuilds. I suppose it might help get air into the cylinders with a bit more space between the valves and cylinder walls though I suppose.

ede

what you're seeing is people who don't know how to correctly idintify their engine. there is no such thing as a 355, it's a 350 with some maintance work done to it, ie 350+.030 or 30 over 350. going over size with the bore wil lresult in next to no preformance gain and if fact all else being equall will lower the compression ratio. the only reason you'd want to overbore and engine is to make the cylinders round and straight again.

crazy3rdgen

iTrader: (1)

I always heard that with a base 350 ci block, after awhile you bottom out on a certain amount of torque numbers possible, (saying its n/a) Compared to a 350 block thats bored 60 over and the displacement is greater. You could probably reach more tq then the base 350 ci.

Correct me if i'm wrong guys? But thats what i've been told. And also that just boring a block wont give you a lot of hp gain.

rx7speed

how does it cause a loss of compression?
and I would say there should still be some performance gain being your allowing for more air into the motor....

though I would agree 5cid on a 350cid motor

ede

same piston shape or design, greater volumn to compress= less compression, slight but still less

AlkyIROC

My old 469 is really a 454 + .070.

My new 540 is just a 4.250 crank with a 4.500 bore. No overbore yet. 0.030 gives me a 548. 0.060" over gives a very common 555. Next jump up from that is 0.100" over for a 565. Taking it to the max with a 0.125" over gives a 571/572 depending what you want to call it but it's not the same bore/stroke as the 572 crate engine.

Because of the different bore/stroke combinations and the different sizes of overbores with the SBC, many different sizes of engines are available.

Everyone knows the 383 uses the 400 crank but it also requires a 0.030" overbore. Keeping the stock 4" bore but using the stroke from a 400 only gives a 377. A stock 400 block with a 350's crank gives a 372 but boring the 400 block out 0.030 now makes another 377 using a different bore/stroke than the other 377.

An old, high mileage engine should get an overbore to clean up the cylinder walls. You normally do an overbore if there are any deep scratches also. 0.030" means you're taking 0.015" off the wall. That should take out most of the bad scratches. You don't normally buy a new block and bore it out to an oversize until it needs it. Theoretically, each overbore is a major repair process to extend the life of the block.

Once a block has been bored to max, there's not much you can do if it needs another boring. If the block is rare or expensive to replace, all the cylinders can be resleeved and you'll start out with a fresh unbored block again.

DuronClocker

iTrader: (1)

Actually, over-boring an engine RAISES compression. Do the math on any compression calculator. The combustion chamber is the same size, but the total swept area of the cylinder is a bit larger volume. Larger volume uncompressed, but the volume difference is made up when the piston is at the top, so the compressed volume is very similar to the compressed volume before boring.

Assuming all else is equal, you will gain about a tenth of a point going from a 350 to a .030" over 350.

The only reason I call my 350 a 355 is to distinguish that it has been rebuilt with a different rotating assembly than was stock. Though, technically, it is a 355ci motor.

Dialed_In

iTrader: (3)

You beat me to it

ME Leigh

Ed's getting old.

rx7speed

grater volume to compress should mean more compression
take a 100ci motor and a 10ci head (we are talking single cylinde here)
that's 10:1 compression right/
now bore the motor to 110ci you still have the same 10ci head so now the compression goes up to 11:1
or am I missing something?


dang it I was beat

Dialed_In

iTrader: (3)

I think that's what DuronClocker said. Larger swept volume of a cylinder = more air to compress into the same final area (combustion chamber, quench area)

rx7speed

that's what I said to as well as I was beat
noticed it right before I was ready to post but figured ah what the hell I already spent those few hours trying to type of up an explanation that our wonderfull ede can understand since he is getting a little old
you know we have to take care of our seniors when they start losing that mind of theirs

sorry ede
blame it on ME leigh he is the one telling us your old