The real problem with building cubes with only longer strokes, stuck with using small clyinder bores and to short of deck height for the needed longer rods.


A bigger bore helps reduce one of the biggest limitations in a two-valve engine: valve shrouding. Before the air/fuel mixture can move down into the bore, it has to move out to get around the valve. If it hits the cylinder wall as soon as it moves past the valve, flow is limited. Its better to have more room between the valve seat and the nearest obstruction.
Another advantage comes from shortening the stroke. Because the piston travels a shorter distance, piston speed in a short-stroke engine is slower at the same rpm. On the power stroke, this translates into the piston spending more time in the top inch or two of the bore-where power is made. Shortening the piston travel also reduces the area of cylinder wall that must be scrubbed by the ring package. Reducing the contact between the rings and cylinder wall means less friction.
Finally, given the same deck height, shortening the stroke means you can stuff in a longer rod. A higher rod/stroke ratio reduces the maximum angle between between the center of the wristpin when the piston is centered between TDC and BDC. This reduces the tendency of the rod to try to push the piston into the cylinder wall, also reducing friction. Generally, increasing the rod/stroke ratio pushes the powerband higher in the rpm range.
If you consider the amount of work done on a piston prior to TDC (negative) and after TDC ( positive), it helps net power to reduce the amount of negative work. net work ( the difference between positive and negative ) is called Indicated Mean Effective Pressure (IMEP). You could also say this is what were calling " net work ". While maintaining piston residence in the vicinity of TDC the idea is to increase dwell time for pressure on the piston, just past TDC. The increased pressure ( from a longer rod ), which also increases pre-TDC mixture density and flame rate, suggests the the possibilty of slightly less mechanical compression ratio ( to avoid detonation ) and less initial spark timing, all of which raises the level of IMEP or net work output.

 

So do hemispherical combustion chambers.

 

Yep..that's why I like big bores AND big strokes!! Cubes are cubes....I'll take them any way I can get them.If I can get RPM AND cubes...I'm a happy camper!

The Engine Masters Challenge has provided some real interesting combos over the years where folks were maximizing power on pump gas. It became common to use small bores,long strokes and short rods. Everything we think is all wrong usually. The idea was to concentrate the pressure over a smaller area, get the piston away from TDC as quick as possible to limit detonation tendencies etc. Very interesting stuff...Kaase's combo's used pistons that were insanely"long"...but it worked!

I don't disagree with all you said....it's dead on...but there are several ways to get things done.

JIM

 

Few things mentioned by builders in engine masters on bore size make complete sense for them. Number one there limited to low rpms of 6500. As engines increase to higher rpms and the engine has less time to get a gulp of air thats when bigger bores/bigger valves unshrouding valves comes into more play, the bigger bores can allow bigger valves to be used all of this stuff not needed as much if your limited to only 6500 rpm. Number two they have to be run on pump gas the smaller bore gives them shorter flame travel across the piston top less chance of detonation with the high compression there trying to get away with on the pump gas.
NHRA rules say 500 cubes for nhra there after power no limits to rpm or fuel, 3.50 stroke with around a 4.730 bore of course in only a drag strip deal they use a short deck short rods with the really short stroke for a BBC. The 815 cu. IHRA engines barely make more power with 315 more cubes. Why because there maxed out on cylinder bore size have to do it the bad way with stroke.

This i what screws SBC new or old engine all you can do is throw stroke at one, is very easy to just buy a 4.60 BBC on the original bore center. I remember reading about the new SBC seeing they kept the old 4.40 bore center thinking how can someone screw themselfs this bad be this much of a brainless twit. Even a small change to 4.50 bore center would have made the engine only 4/10s longer also allowed them to make the main webbing thicker for even mre strength. Reminds me of fords original cammer production engine they made its huge in width and height with a 3.90 bore center limited themselfs to 3.5/3.70 bores. They at least pulled there head out there **** put in a 4.50 bore center on the new coyoty engine.

 

FWIW the Chevy R07 is 4.500 bore spacing, SBC configuration.

 

You are talking about Rod Angularity. With that being said nobody has ever said that one can not make power with a long stroke yet with a short rod. But depending on how steep the Rod Angularity is it can have a drastic effect on the life span of the engine.

As 427 Hot Rod Stated to help make up some of the problems with the Rod Angularity they used a longer piston with a longer piston skit to spread out the load against the cylinder wall during the transition of the stroke. And as mentioned a short rod will lessen the time the Piston spends at TDC and BDC thus lessening the chance of detonation due to the higher piston speeds.

So what is the question lol?

 

That's why on really exotic heads with wedge style chamber they start offsetting the valve angle not only with regards to the deck surface (hence 11-15-18° heads) but also before/after so that the valve opens towards the center and gets deshrouded even more.

 

3.766 stroke doesn't like any shelf pistons for longer than stock rods, they'll be custom. I finally ended up with 6.7" rods, pin height should be around 1.200. Waiting on block work before I order them. 4.630 x 3.766 = 507 CI.
Now if I can just sell my dang house I can buy her a decent C5 and build my '81 the way I want it rather than compromising.

 

Yes but its a very expensive nascar engine no one will own.

 

No question lol. Just don't see a good reason to build a big SBC with all its problems. The little bit of weight difference of BBC no big deal and it opens the door to much bigger engines with a much stronger foundation ( block ) and standard off the shelf heads that flow air.

 

I'm glad this conversation has overflowed to another thread. Sorry I have not been on much, with Scott Foxwell getting here and trying to get the head shop in place I have not been online.

 

In addition to bore/stroke ratio, given rod angularity does matter, rod/stroke ratio is also relevant to the main topic. And, despite what many a nay sayer has said about it, there is something to be gained in high-perf engines by improving R/S ratio. Did my first long-rod build some 30 years ago, so I'm not surmising about something I haven't done here...

FWIW, since I'm going to be into custom slugs for my own 427 BB redux anyway, I've decided to use 6.385" rods (+ 0.250" longer than stock), which I had left over from an aborted 496 BB build, resulting in ~1.7:1 R/S ratio. This is very close to Smokey Yunick's recommended rod length for high-rev 427 BBCs and, while not the primary reason I'm doing it this time around, according to EAPro modeling is worth a few HPs on top end, at least for the particular type of build.

As for B/S ratio, the 427 BBC comes in at 1.13:1, which is another reason it's not commonly an apples-to-apples comparison between SBC's to BBC's of equal displacement.

Yes, I'm always looking for new BB converts.

 

so, is there much difference in the efficency between a 427 and the 454?

 

Well, the stock 427 BB uses the same rod length as does the stock 454 BB, so it does have a better R/S ratio (1.63:1 vs 1.53:1), and lower max rod angularity (17.84* vs 19.03*). My long-rod build is at 1.7:1 and 17.12*, respectively. These seemingly minor differences are more substantial than they may appear. Until you put your foot down a similarly built 427 theoretically shouldn't use quite as much gas, but when you do they really like to rev (up until valvetrain limitations). And, it's internally balanced. YMMV, but that's what works for me.



.

 

Same reason why I went to a 4.125X3.250X6.250 combo with a 1.92 rod/stroke ratio.

 

& a 15.07* max rod angle (from vertical), btw.

 

Little Mouse I am sure my reply was a bit long lol.

As for weight difference like everything else it all depends not only the makeup of the Engine but also what kind of vehicle and for what purpose.

But I have to agree that the Big Block as for heads has so much more going for it than a Small Block.

But in my humble opinion for a street driven car it is hard to beat a simple 468 CI or a 496 CI. Not only for the Cost of the build but the power that can be produced VS the Dollar Spent. I mean really with a Big Block Chevy that will produce in the Neighborhood of 550 to 600 HP with Tons of Torque is a great Street Motor. I also feel that this type of Engine in a Corvette is probably pretty darn close if not the top of Power that should be installed in the Corvette unless one is willing to spend a great deal of funds on the rest of the drive train to be able to handle more Horse Power and the most important of all Torque.

And to try and keep away from Sticky Tires lol.

 

Well just consider for some reason in this day an age you only wanted to build a 477 BBC. A simple 4.50 block, You still have several overbores left to you, only have to use a standard 3.76 stroke crank all kinds of room left for longer rods. Take one look at the size of the main webs and caps up beside your SBC. I think a person should keep one bare BBC block and a SBC sitting side by side standing up on the bell housing flange as a reminder of why not to build the SBC lol. I just think at a certain point say a 406 SBC that if you want to go bigger just go into a BBC for durability. I still kick myself every once in a while for selling my perfectly good 4 bolt BBC.

 

Little Mouse the Beauty of the Big Block is that if you keep in the realm of sanity lol. A 2 Bolt Main is a very strong bottom end. Just as the Factory 3/8 th's Rod Bolt Rods with a good set of Aftermarket Rod Bolts and some special attention.

I try and attempt to tell people that they can build a darn strong Big Block for pretty cheap using mainly factory parts. But it seems as if people just have money jumping out of there pockets to buy this or that lol.

But then again Little Mouse you have been here awhile and I am sure you might remember this discussion on the hows with 427 Hot Rod and a few others. For us this is just Old Stuff but to be honest it is pretty darn relevant even today.

 

Correct. I went to a too specific design requiring special parts. I do not regret it as this was my own choice, but if the only thing you aspire are streetable lbs/ft then a stock big block is all you're going to need. (provided cost of fuel is not a factor)