4.625X4.250 combo

[lucky]

Hello,
I personally never get tired of BBC combinations, mild to wild. I think it is amazing that no matter the combo, there is always someone disputing the output, others will recognize what it took to be king. Now and then, a particular combination shows to be out of the ordinary, based on parts used. Are these extraordinary build specs ordinary? Or are ordinary parts made extraordinary, and claimed as ordinary?

[FC-Pilot]

I don’t think I understand your post. I have built two of those. They were that size because that is what was left in the bores of the blocks. Both performed to my expectations.

Paul

[pdq67]

I guess at this size I would use a 4.625" b x 4.375" s crank to make a 588" engine.

What everybody's thoughts on this??

pdq67

[mag2555]

We all know that top shelf parts and what seems to be ideal Bore and stroke ratio's is no sure thing in regards to above average output for a given motor, so I don't understand the op's post!

[Frankshaft]

I guess at this size I would use a 4.625" b x 4.375" s crank to make a 588" engine.

What everybody's thoughts on this??

pdq67

It'll run. That's a good combo. I would start with a 4.6 bore though. Dart or Merlin blocks are spendy to bore to the limit to begin with. If you have an existing block that has been used for years, and you are at the point it needs to go 4.625, then you do what you gotta do.

[FC-Pilot]

We all know that top shelf parts and what seems to be ideal Bore and stroke ratio's is no sure thing in regards to above average output for a given motor, so I don't understand the op's post!

Yea, I did not understand what the point was either. I was kinda scratching my head.

Paul

[Dave Koehler]

Philosophy major?

[Newold1]

OK lets think about this carefully as pdq67 tends to bang around in the closet to much! Kinda like an "OLD WILD CHILD!"

If you are going any bigger in bore size than a 4.500" then you are done with standard GM blocks and you are using either a GM Bowtie block or an aftermarket block period.

Since BBC regular aftermarket blocks and Bowtie blocks tend to max out at a realistic 4.625" bore, WHY START THERE!? You got nowhere to go if you got to freshen or repair a damaged cylinder.

So my thought is start somewhere between 4.530" and 4.560" and give your self about .060" for that bore damage that seems to many times happen in the life of a minimum $2500 block! Think down the road and into future so you don't have to buy another block and start over. Shit Happens!

If you are filthy rich and spoiled rotten! Stop reading here!

As for stroke, I still say depending on the rpm range required for your build you can run any stroke in one of these blocks from 4.00" to 4.75" without having to clearance the block, so go big or go home! There ain't no replacement for displacement in most BBC builds! Use the 4.560" bore size. use a nice center counter weight forged 4.75" stoke forged crankshaft a nice set of 6.70" H-beam rods and a great set of the new lightweight forged coated pistons with a great ring package and gas ports if needed.

Spend good money on a great set of heads, cam and intake system and an easy 850-1000hp is easily within reach without power adders, nitrous or boost. Just a nice sweet NA BBC. Need more than that then by all means pile on the adders and be prepared to for shorter hours and more maintenance and repairs. This ain't rocket science!

[FC-Pilot]

I agree with everything you just said. I don't understand some of the short minded thinking. The two I built were short deck blocks. The first one was 4.6 and had damage that scared three cylinders. It was just going to be a basic boat engine so we bored it 4.620 and had a set of pistons made and ran it in a boat. I also had a 4.25 crank with a nick in a journal so we had it ground and threw it in. We had a set of heavily ported factory heads and a set of used rods and used cam and induction to complete the package. We had less than 4k in the whole engine.

The other one was an old DRCE block that was 4.625 when we got it with the heads we purchased. We put a crank in it and had some fun with it. Had it not been for the fact that the blocks were both either at that bore already, or in need of going that far over I would never have gone that far over.

I have another Dart short deck block in the shop sitting at 4.625 right now that I picked up for $200. Nothing wrong accept was bored too far over for the previous owner to run with the blower that he was going to put on it.

Paul

[pdq67]

OK lets think about this carefully as pdq67 tends to bang around in the closet to much! Kinda like an "OLD WILD CHILD!"

If you are going any bigger in bore size than a 4.500" then you are done with standard GM blocks and you are using either a GM Bowtie block or an aftermarket block period.

Since BBC regular aftermarket blocks and Bowtie blocks tend to max out at a realistic 4.625" bore, WHY START THERE!? You got nowhere to go if you got to freshen or repair a damaged cylinder.

So my thought is start somewhere between 4.530" and 4.560" and give your self about .060" for that bore damage that seems to many times happen in the life of a minimum $2500 block! Think down the road and into future so you don't have to buy another block and start over. Shit Happens!

If you are filthy rich and spoiled rotten! Stop reading here!

As for stroke, I still say depending on the rpm range required for your build you can run any stroke in one of these blocks from 4.00" to 4.75" without having to clearance the block, so go big or go home! There ain't no replacement for displacement in most BBC builds! Use the 4.560" bore size. use a nice center counter weight forged 4.75" stoke forged crankshaft a nice set of 6.70" H-beam rods and a great set of the new lightweight forged coated pistons with a great ring package and gas ports if needed.

Spend good money on a great set of heads, cam and intake system and an easy 850-1000hp is easily within reach without power adders, nitrous or boost. Just a nice sweet NA BBC. Need more than that then by all means pile on the adders and be prepared to for shorter hours and more maintenance and repairs. This ain't rocket science!

Newbold1,

Thanks for the kind works and I admit to being in the closet most times..

Can you install 6.70" rods in a stock deck height block?

As for max boring an aftermarket BBC block to create max displacement for the stroke used, after it's life is used up, then sleeve it and roll-on.

And imho, it's a dammed shame that GM didn't use the Packard bore spacing when they 1st designed the BBC engine!!

Can you say, "a 4.75' piston?"...

pdq67

[Newold1]

Hey Wild Child, you're welcome.

I would not build a 4.750" stroke 6.70 rod combo in a 9.80" block but tall deck 10.20" blocks do it well with a good piston. Since GM Bowtie and aftermarket BBC blocks (Dart, World,Blueprint, Brodix) all sell so close in price for the 10.2" deck versus 9.80" why bother with a short deck.
GO BIG !!
Please don't give me the headers don't fit argument, almost every header maker makes both versions for many vehicles. Anyway on the 45 degree bias at the header mounting is not even +.250" higher on a stock head and almost every good aftermarket aluminum head uses a +.400" raised exhaust ports. So get headers that fit, simple.

If you are building an engine to win, don't limit your result by only using what's easy and so cheap! JMO