Advantages of a large bore?

[Guest TestZubject]

In all the information I've seen about 400-based small block buildups, the large bore and its positive effects on flow through valve unshrouding has been mentioned. Is a larger bore always going to aid flow, or is there a point where the bore ceases to have an impact on valve flow? Does a very large bore(~4.185") have any distinct advantage over a small bore(~4.00") aside from the obvious displacement increase? Thanks in advance.

[pparaska]

The valve un-shrouding can help high lift flow.

 

But the SBC is seriously undervalved. If you go wiht a big bore, get the 2.08" valves and be sure to shape the chamber wall so it comes within about .050" of the gasket bore (4.200").

[Guest TestZubject]

Cool, thanks. Would it still be advisable to "eye-brow" the block?

[pparaska]

I dunno. Gasket seal may suffer, an if you're running pistons with a high ring package, that may not be good either.

 

Help, Grumpyvette!

[grumpyvette]

ok

advantages for the larger bore include several factors

first as mentioned it tends to unshoud the valves but it also tends to allow the engine to use larger valves, now the differance does not need to be very much to be meaningful, while the change from a 1.94 valve comon to many heads to a large race type 2.05 valve is only an increase of 2.6%or 0.110 or just over one tenth inch the flow curtain of the valve at 0.500 lift went from 3.05 sq inches to 3.22 sq inches a improvement of 5.7%

then lets look at the piston surface area

a 4" bore has a flat top piston of about 12.588 sq inche, a 4.155 bore moves that up to about 13.58 sq inches, thats an increase of almost 8% in surface area, given a peak cylinder pressure of about 700psi, (very easy to get in some combos) you jump from about 367 pounds of force to about 396 pounds of force on the crank every 90 degrees(a increase of about 8%) (remember theres 720 degrees in the cycle but only about 30 of those 720 degrees have effective pressure on the piston)but theres more to it, that 8% increase in bore on a 3.48 stroke gives you not only more effective force on the piston it also increased the displacement from about 350 cid to about 377 cid, so not only have you increased the force on the crank by 8% your increasing the displacement by about an additional 8% or now in theory your working at about a 16% advantage. while its true the bores friction area on the rings increased also the loss to that is minimal due to the limited contact area of the rings.

yeah but were not thru! dont forget we increased the the weight of the piston slightly, but decreased the flow angles from port to cylinder slightly (both factors are minor) are we overlooking something, why yeas, we could have kept that same 350 displacement by decreasing the stroke and increasing the connecting rod giving us a more effective mechanical advantage if we choose to, lets see what that gives us???

figure a normal limit on reasonable aftermarket piston speed at 4500 fpm

for a 3.48 stroke 350 thats about 7758 rpm max for a built race 350 or 8307 rpm for a 352 built on a 3.25 stroke, now that 550rpm advantage does not gain you much without some very expensive parts but the lower stress on the engine at 7000rpm works wonders on a sreet engine long life and bearing wear if you limit the engine to that. but lets say you want to use those parts and both engines make peak power at 1000rpm below redline and both make 400 ft lbs of tq there, well the standard 350 makes 400ft lbs at 6758rpm or 514 hp but the short stroke large bore 352 makes its power at 7307rpm or 556 hp

lets go the other way! a 3.875" stroke is about max in a sbc without major machine work , that builds a 396 on a 350s .030 bored block, or a 421 with a 400s larger 4.155 .030 bored block. (there usually a 4% increase in hp for each additional point in cpr up to detonation range I.E. a 9:1 engine that makes 400hp should make about 408hp at 9.5:1 cpr) with no increase in displacement, add the two factors and getting an extra 10% from the larger bore and higher cpr is not that hard to do!

now lets look at compression, if you have domed pistons youll hurt the flame speeds accross the cylinders so youll need more ignition advance, a flat top 350 piston with a 76cc smog head has about 9:1 cpr, jump to a 400s 4.155 .030 over bore and the same stroke now gives about 9.5:1 cpr

the advantage allows you to use lighter non-dome pistons with faster flame front travel for less ignition advance, that potentially could give you a few extra hp!

 

ok what did you learn? short version, larger bores have few disadvantages and several measurable advantages,and LARGE BORES PLUS LONG STROKES can significantly add effective displacement! if both make 1.25 hp per cubic inch we get 495 hp from a 396 but 526hp from a 421 sbc combo given equal efficiency, yet the larger displacement will almost always have a torque advantage over most of the lower and mid rpm ranges!!

or there are few 350 engines that can compete with a similarly built 421 !

 

btw if your trying to get the most hp per cubic inch of displacement a 4.155 bore 3.48 stroke (377 combo comes close) but the increased displacement of a 406 while less efficient has the displacment to produce a better total torque curve and similar peak power

[Guest TestZubject]

Informative as always Grumpy! Thanks!