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Curtis
Curtis UltimaDork
10/8/19 12:33 p.m.

Lots of stuff already covered here, but some misinformation as well.  I'll just add to the mix as a 30-year hotrodder who used to build custom engines and drivelines.

The 302 Ford is a big bore, short stroke.  The 305 is a small bore, longer stroke.

Small bores not only limit valve size, but you have to remember that valve size (being a circle inside a circle) is exponential.  When you reduce bore size by 10%, you reduce the area available for valve surface by about 16%.  You also induce valve shrouding which (contrary to popular belief) is not simply the distance from the valve edge to the cylinder wall, it is the additional area consumed between the curve of the bore and the tangent.  That is a technical way of saying: even if you have the same 1.66" valve in a 4" bore, if you put that same 1.66" valve in a 3.5" bore, it will flow less because there is more bore wall close to the valve.

Therefore, a 305 may be 87% the displacement of a 350, but it doesn't have nearly 87% of the power/tq potential of a 350.  You can throw all the money you want at a 305 (and some people do), but here it is with rough numbers.

Buy a 305 for $150.  Rebuild it yourself with $300 in parts.  Throw $2000 worth of goodies at it and congrats, you have a 300-hp motor with 375 lb-ft of torque.  That torque peak will come at sky-high RPMs.

Do the same thing with the same parts on a 350 and you have a 450hp motor with 450 lb-ft of torque, better idle, lower torque peak, and something you can actually sell for money.

Throw into the mix the fact that 305s were always destined as a low-output motor, the blocks were cast relatively thin.  All of the stock heads are low-compression, poor flowing, and pretty worthless.  Trying to make big power with a 305... or even just trying to warm it up a bit... is a pretty pointless thing.  You could very easily spend $1000 hopping up a 305 only to find out that you only make as much oomph as a $250 stock 350.  IMO, the 305 is a completely disposable engine, and should be disposed of rapidly.  In the hotrodding world, most of us would sooner purchase a crate 350 than even turn the key on a 305.  Those of us who also have a foot in the GRM world could see the benefit of a running 305 being something to get us on the track.  Being on a track and going slow is better than not being on a track at all.

On the bore/stroke thing, bore and stroke have little to do with things in the realm of lower RPM motors.  If you spend enough money to make a 305 rev to 6000 rpms, the stroke will be the last of your worries.  The old wive's tale of "long stroke makes more torque" is 2000% BS.  If you equip a 455 Buick and a 455 Olds with the same head flow and cam specs, they will make the same power and torque at the same RPMs desipte the Olds being a huge stroke and tiny bore and the Buick being the opposite.  The nice thing about the Buick is that you at least have the option to make it rev with all that bore space to add flow.  With the Olds you don't.

Where you really get into piston acceleration is not with bore/stroke ratio, it is with rod/stroke ratio.  It doesn't matter what size the bore is, the piston will accelerate the same with a given stroke/rod ratio.  A short rod will have long piston dwells and high rates of acceleration and side loading.  Long piston dwells can be great for torque production, but hell on piston pins and bores/skirts.

Torque peak RPM is decided by the combination of parts, and it doesn't matter what you do to bore and stroke, the decision is made by displacement.  Long strokes don't make torque, cylinder pressure and when in happens in the angle of the stroke makes torque.  Sure, you can take a Chevy 302 with 400 hp and shift at 7000, but a 400 hp Caddy 500 will do the same 400 hp and shift at 4500 and make 500 lb-ft at 2000 rpms.  That 302 will look cute in the rear view mirror.

TL,DR: Displacement is king, but that doesn't mean that a 305 has 87% the potential of the 350, nor does it have the same potential as a Ford 302 because of the 305's tiny bores.

If you're building something that just needs to start, run, and move a car, a 305 is fine.  If you ever plan on even a tiny warm-up and more power, don't spend a dime on the 305.  Just get one of the larger bore engines.

Vigo
Vigo MegaDork
10/8/19 2:54 p.m.

Where you really get into piston acceleration is not with bore/stroke ratio, it is with rod/stroke ratio.  It doesn't matter what size the bore is, the piston will accelerate the same with a given stroke/rod ratio. 

I agree with your general point but bore size does correlate with piston weight which is basically the main factor that everything else has to be built around. Piston weight is the tail that wags the dog as far as the rotating/reciprocating assembly's  design. 

 

Curtis
Curtis UltimaDork
10/8/19 6:51 p.m.

Good point.  In the hotrodding world, piston weights are already somewhere between heavy and hippo, and there is plenty of room to shave weight off a larger-bore piston making the difference less catastrophic.

In my world, a hypereutectic or forged piston that fits in a 3" bore versus one that fits in a 4" bore is inconsequential.  They'll both handle 6500 rpms, which is more than most V8s will do, but your logic is perfectly sound.

My point about acceleration vs rod/stroke ratio was the angle of the rod.  Short rods cause much greater acceleration of the piston than long rods.  While that is good for torque, it means that (everything else the same) a short rod will have much greater forces applied to the pin/piston than a long rod.

Knurled.
Knurled. MegaDork
10/8/19 7:10 p.m.

In reply to Curtis :

Short rods also work the port harder, and are a little more resistant to detonation...

 

(40 page long thread later - seriously, rod length is one of those subjects like EFI vs. carb that turns into a total geek fight)

 

The guys who build good race engines for a living say, we make the piston, we set the stroke, and the rod length is the one that conencts the two.  Usually the lightest piston practical will have a short compression height, so the rod will be fairly long anyway.

Streetwiseguy
Streetwiseguy MegaDork
10/8/19 7:23 p.m.
Curtis said:

 

 

 Short rods cause much greater acceleration of the piston than long rods.  

I'm gonna just go ahead and question your usage of the word "much" here, mmkay?

93gsxturbo
93gsxturbo SuperDork
10/9/19 1:35 p.m.

A big part of the 302 vs 305 hot rodding came that a 351W will not fit in place of a 302W, but a 350 SBC will fit in place of a 305 SBC.  Couple that with very few (no?) 351W high output motors were offered by Ford and you had a plethora of Corvette, Chevelle, Camaro 350s ripe for the pickin, and the 305 Chevy was the odd man out.  

So if you have a 5.0 Mustang, heads, cam, intake, etc were the only boltons that made sense.  Adding displacement was a major undertaking, either in the form of a stroker kit or a major engine swap that did not share many components and was physically larger.  Because of the ease of bolt-ons relative to the difficulty of adding displacement and being factory installed in Ford's pony car, the 302 came out as the big winner over the 351W.  

Now if you had a Camaro, it was a weekend project- no more parts required than a head/cam swap on a 302- to pull out the 305 and install a 350 with a few hot rod goodies.  

So blame the 351W on the 305's demise.  If the 351W was a like-for-like swap of a 302, it would have likely been the hotrod motor of choice, leaving the 302 in the dustbin like the 305.  

 

Knurled.
Knurled. MegaDork
10/9/19 1:45 p.m.

In reply to 93gsxturbo :

That's pretty much it... the only performance 351Ws were in somewhat rare versions of the Fox Mustang late in its life.  Before then, the performance Ford was the 351C, and people used to swap Cleveland heads on Windsor blocks to get the best of both worlds, before performance Windsor heads existed.

GCrites80s
GCrites80s Reader
10/9/19 1:51 p.m.
Daylan C said:

In reply to pres589 (djronnebaum) :

The Vortec TPI base is hard to get and expensive but you can get an Edlebrock Pro Flo in Vortec flavor.

Yeah people don't know that over the past 10-12 years the TPI system has turned into Ferrari stuff. Owners are completely on their own.

pres589 (djronnebaum)
pres589 (djronnebaum) PowerDork
10/9/19 2:08 p.m.

Curtis, you state that the 350 will make 50% more power than the 305 for the same mods.   You stated that's due to valve shrowding by the smaller bore of the 305 versus the more open bores of the 350 allowing for better breathings.  While I question the 50% diff (I bet more like 20% or 25% but this is all talk, not naming specifics) I generally agree.

However, you then pretty well contradict that by saying the large bores of the Buick 455 are no better than the smaller bores of the 455 Olds.  What gives?

Curtis
Curtis UltimaDork
10/9/19 2:41 p.m.
pres589 (djronnebaum) said:

Curtis, you state that the 350 will make 50% more power than the 305 for the same mods.   You stated that's due to valve shrowding by the smaller bore of the 305 versus the more open bores of the 350 allowing for better breathings.  While I question the 50% diff (I bet more like 20% or 25% but this is all talk, not naming specifics) I generally agree.

However, you then pretty well contradict that by saying the large bores of the Buick 455 are no better than the smaller bores of the 455 Olds.  What gives?

You're right, I did contradict myself a bit.  Clarification... I was pointing out the old myth of stroke = torque.  My point was that displacement = torque.  If you had a 455 Olds and a 455 Buick  AND both of them were not flow-limited, their dyno charts would look the same.  Let's say, for instance, you built them both as a 200 hp wheezer, neither would be flow limited and their dynos would be about the same.  The benefit to what I was saying about the Buick is that, if you want to turn up the oomph, you have more bore area to generate the flow you need for higher power applications.  The point was that the Olds wouldn't have more torque just because it has a longer stroke.

To the 305/350 point, I have actually built a few 305s for customers, and about fifty 350s.  Same compression ratio, same head flow, similar cams, and yes.  40-50% more power is in the ballpark.  Mind you, these were relatively hot motors.  When the 305 hits the ceiling, the 350 has a lot more room to keep going.  Your 20-25% figures would be correct if we were talking about 250-hp engines.  My point was that hopping up a 305 is pretty pointless given the small bores which limit breathing.

Daylan C
Daylan C PowerDork
10/9/19 3:29 p.m.
GCrites80s said:
Daylan C said:

In reply to pres589 (djronnebaum) :

The Vortec TPI base is hard to get and expensive but you can get an Edlebrock Pro Flo in Vortec flavor.

Yeah people don't know that over the past 10-12 years the TPI system has turned into Ferrari stuff. Owners are completely on their own.

Yeah this is why I sold all my TPI parts put kept my GTA. Plus the idea of making power past 5k sounded interesting.

Streetwiseguy
Streetwiseguy MegaDork
10/9/19 4:02 p.m.

How about this opinion...A 1972 400 small block Chev will produce more low end torque than a 1972 402 big block, due to worse cylinder heads available for the small block.  Big block needs to spin.

 

Curtis
Curtis UltimaDork
10/9/19 4:23 p.m.
Streetwiseguy said:

How about this opinion...A 1972 400 small block Chev will produce more low end torque than a 1972 402 big block, due to worse cylinder heads available for the small block.  Big block needs to spin.

 

Too many factors to consider.  Given their similar displacements and identical bores, they should be the same up until you get restricted flow (from either heads or valve shrouding).   For a given displacement, cam duration, and head flow/velocity values, the BBC would make its torque a couple RPMs lower than the SBC because of the whopping 2 extra cubes.  But we both know that BBC heads are not the same as SBC heads

But quantity of flow isn't the only factor.   Velocity of flow is a huge player as well.  When looking at factory heads, your assertion is mostly correct.  Even peanut port heads don't have the velocity of some of the larger-port SBC heads.

The golden part about SBC 400 heads is that with the millions of possibilities (if you're brave enough to drill decks for the steam holes), you can get great combinations of small-ish ports that not only flow well, but have great velocity.  That pays big dividends allowing you to match short-duration cams for that lower rpm torque peak and still be able to breathe a bit higher.  Win-win.

BBC heads aren't the greatest blueprint for that combination of flow and velocity.  Seems like most of them give you one or the other.

Knurled.
Knurled. MegaDork
10/9/19 5:08 p.m.

In reply to Curtis :

The steam holes are for the blocks, not the heads.  Otherwise there would be pockets of trapped air thanks to the siamesed cylinder design.

stroker
stroker UltraDork
10/9/19 6:07 p.m.

This thread has been very enlightening to me.  I was under two false impressions, one that the old 302 Ford was the same as the 5.0 Ford and second, that the performance five liter Chevy was NOT based on the 305 block but was actually the 327/350 block.  My original question was prompted by interest in trying to conceptualize a rough parity between a Chevy and a Ford V8 with five liter capacity at the lowest possible cost.   

I need to do more research...

ProDarwin
ProDarwin UltimaDork
10/9/19 7:42 p.m.
Curtis said:

The old wive's tale of "long stroke makes more torque" is 2000% BS. 

I'm confused by this.  A longer stroke means literally that the rod journal is farther from the centerline of the crank and for a given combustion event, the moment (torque) about the centerline will be greater.  So by definition, a longer stroke makes more torque.

 

If you want a specific engine example, why not compare the 5.3 and the 4.8 which are the exact same engine except for bore (right?).  The torque curve is almost identical, but shifter higher on the 4.8.  But it spins out a little further resulting in pretty much the same power, again just shifted to the right.

 

Knurled.
Knurled. MegaDork
10/9/19 8:17 p.m.
ProDarwin said:
Curtis said:

The old wive's tale of "long stroke makes more torque" is 2000% BS. 

I'm confused by this.  A longer stroke means literally that the rod journal is farther from the centerline of the crank and for a given combustion event, the moment (torque) about the centerline will be greater.  So by definition, a longer stroke makes more torque.

 

For a given displacement, it makes zero difference.  Keeping the displacement the same, a longer stroke requires a smaller bore, which means less surface area for the cylinder pressure to push on.

 

Think of pushing 10lb on a 10 foot long bar vs. 100lb on a 1 foot long bar... either way you end up with 100 ft-lb.

 

Now it IS a little more complicated than that when you take combustion speed and thermal efficiency and optimal compression ratio into the mix... which oddly enough, "ideal" tends to be an undersquare cylinder, because it minimizes the surface to volume ratio when at the critical time +- 15 degrees from top dead center, and can give an efficient compression ratio while still keeping a good combustion space shape.  But you really need a 4v head to be able to breathe well enough to bother.

ProDarwin
ProDarwin UltimaDork
10/9/19 8:45 p.m.
Knurled. said:

 

For a given displacement, it makes zero difference. 

Oops, I missed that part.  Yes, that makes sense.  If airflow, fuel, and RPM are fixed, torque would have to remain constant as well.

pres589 (djronnebaum)
pres589 (djronnebaum) PowerDork
10/9/19 10:36 p.m.

I agree that if you're wanting more than 300hp or so out of a small block Chevy, the 350 is the way to go.  Where a 305 is interesting to me is that smaller bore where there's less places for unburned fuel to hide and be wasted along with the low cost of the engines themselves.  I would like to think that imaginative scrounging would net something that's at least "interesting" to drive along with better mpg's than the 350 if built around the idea of a lower-RPM torque/mpg engine.  Probaly not a huge advantage though.

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