Somewhere in my stumbling and learning I seem to remember reading that a shorter stroke and bigger bore was better for a turbo engine. I would like any information that anyone can give me in support of or counter to this philosophy.
I blew up my craigslist 472, so I am starting over. I want to build an engine for the application this time, its going to take a while to get it together but my first question is displacement, I can go from 429- 5xx inches. I am not sure a stoker will be necessary for my power goals though. The ford 385 series standard bore is 4.360 and stroke ranges from 3.59 for a 429 to 4.50 for a big stroker. The application is a half-ton 4x4 truck which will always weigh at least 4500 lbs, and common wisdom says to go for the biggest cubes possible. I feel like there must be a more elegant way to make this decision. Some of these trucks at one point came with a 300 inline 6 and it moved them fine, it was a rare option to find a 460 in a half ton (2wd only), so anything in the big block family is probably ample even without a turbo.
Ultimate power goal is 550-600 HP, I think this should be easy on a turbo big block and id like to turn a meager 5000-5500rpm. I will use an s480 with the 96mm exhaust housing (The kit is already completed and functional), The truck wont see a drag strip very often, it will occasionally tow or haul about an ATV and is destined to be a fun street and desert toy. I will probably use a boost controller and keep it low PSI - low output most of the time just to save the wear.
I'm not really interested in a blanket "build the most cubes you can" answer, I would like to gain some understanding of why a certain bore/stroke ratio or one end of the spectrum works better than another.

 

More cid = more torque down low, but hp potential limited by head flow, so a 514, and a 460 with the same head will have about the same na hp potential, but the 514 will make it lower in rpm. Long strokes give more time to burn air/fuel, but increases piston speed dramatically.
It's always been my thought that a smaller bore makes head gasket retention much easier, but that doesn't help much in the case of the 385 Ford with its huge bore. The beauty of forced induction is that you don't need the large cid to make good hp/tq.

I've always liked the 429/460 platform, but it probably wouldn't be my first choice for a budget minded turbo build. The turbine on your S480 will become small with much of anything for airflow improvements.

 

Thanks Mnlx. I am somewhat regretting the 460 as a choice for this project, for many of the reasons you listed. But if I went down to Windsor size I would be in it for another trans, engine mounts and another refab on the turbo piping... none of which appeals to me. I also know that single turbo was a poor choice, but I hope that it will be capable of my modest goals, maybe this is enough of a reason to go to the 429 over the 460. I am not interested in going to twins at this point.

So back to the topic at hand, I do have an understanding of how bore/stroke influences power delivery, I have a reasonable understanding of how engines work and why certain dimensions are chosen (I am no master engine builder). I am still a bit lost on what makes a bore/stroke ratio better for a turbo application, or am I looking at it wrong? Maybe there is an ideal bore diameter where the flame propagation is at its most efficient, cylinder heads are cool and happy, there are enough head bolt locations from the factory, and maybe there is a stroke at which piston speeds aren't too great and torque is still adequate.

Should I be looking at modern OEM's and their turbo applications? For instance the flagship 3.5l Ecoboost 3.30" bore and 3.49" stroke (.9455 B/S) seems to work quite well. Or do I look at what this board favors? The gm 5.3 (I cant decrypt why this is preferred over the 6.0, I cant make sense of whether its economics or if there is a physical reason) carries 3.78 bore and 3.622" stroke (1.044 B/S). I wonder also why people choose the 302 over the 351w, is it just packaging? I know the 351w factory block is much stronger.

Maybe there isn't anything to the bore/stroke, maybe in the instance of the 460 its just too big, it wasn't engineered for what I am trying to make it do, and it was a poor choice for an engine. If this is the case now is the time for me to figure that out, before I drop in another engine and am disappointed or destroy it.

 

I have about the same amount of knowledge on the subject that you have then.... I was an automotive machinist/hp engine builder for about 10 years long ago, but i'm no engineer. Small bores, and compact combustion chambers promote good flame travel, and an efficient burn compared to old school stuff such as your 460. I believe physical size, and price dictate 302/351 or 5.3/6.0 more than anything. Most everything seems to be near square today, and makes the most sense to me for packaging (bore centers, and deck height), and all around hp/tq/rpm, but i'm sure there are better reasons someone chooses a certain bore/stroke. I big bore is good for head flow, but engines like the 6.0 really didn't take advantage of that until the rectangle ports showed well after the introduction of the 6.0, and head flow has progressed even with the smaller bores. I'll be watching for better answers.

 

As far as my logic goes shorter stroke gives less leverage to break rods, smaller bore diameter helps when the piston wants to go sideways, relieving force from the piston sleeves helping the piston not to go sideways and transfering it to the rod in a more steady way. But thats just my thoughts. I might be totally off with this.
Also smaller bores mean more meat on the block, kind of a plus on high number boosted engines. Hense why the 4.8 and 5.3 ls based engines are preffered on boost. Also that you can get iron blocks, better heads than in a stock ls1 for way cheaper price.
Shorter stroke is more likelly to rev higher for sure.

I believe david friedburgher explained how a smaller bore, shorter stroke helps on boosted and nitrous aplications

Smaller ci can move as much air as a bigger ci engine on higher rpms. So yeah bigger displacement will ocationally make the same power on lower rpms. Longer rod will help on torq production in the lower range.

 

In Australia we have the XR6 T which is 4 litre engine . Roughly 4" stroke X 3 1/2" bore . Twin cam 4 valve engine . With larger turbo, exhaust, forged internals , oil pump gears . 1000 HP is easy . From a 4.0 .bassically same bore and stroke in old 4.1 Mustang 6 back in 70's . Bearings are the same size. It's the engine dynamics . Great breathing heads . Longer stroke tends to spool a larger turbo . 7 main bearing crank. 6 bolt with girdle and alloy oil pan bolted as one .

 

Barra? That sounds awesome.

 

Interesting, I was under the impression a shorter stroke was better for spooling a turbo.

I am glad to see this thread getting some attention.

 

I think spool comes down to ve, and a long stroke probably fills cylinders better at some engine speeds. A long rod will help with piston side loading, but causes more dwell at tdc, and btc, and on occasion can be a bit more detonation sensitive. I think in the end it's negligible for most of our street/race/production setups.

 

I've been thinking about this post a bit, and I am not so sure its accurate, at least not in the narrow sense of this topic, it's too general.

1: As a rule of thumb this is likely true, but I think it has more to do with maximum cylinder pressures, when they occur, and maximum points of acceleration. Piston side loading is more of a function of overall rod ratio than stroke, we need to be looking at the geometry of the reciprocating assembly as a whole to make determinations about side loading.

2: Smaller bores only mean more meat in the block if that block has different factory bore diameters, or has been machined, every engine family will have its own characteristics and should be looked at individually, even engines in the same family can have vastly different blocks. In most of the cases I have looked at a cylinder wall isn't the fail point in an engine, though it does happen on occasion.

3: I do love roadkill.

4: A smaller engine MUST spin more RPM's to move as much air with the same atmospheric pressure. Longer rod will produce more power in middle to upper RPM ranges.

Dwell at TDC seems to be an advantage, this creates longer state of compression by keeping the chamber volume small. provides superior combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. It definitely increases detonation sensitivity once you hit a threshold, the ford 400 is renowned for this with it 4x4 bore and 1.88: rod ratio. They were terrible about detonation, especially with 70's era crap ford heads.

From Victorylibrary.com refering to short rod (not short stroke):
The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
They go on to say that this is can become a problem at high RPM because the piston can actually outrun the flame front, and that it also makes a less efficient engine overall because it wont keep heat in as well.

I read up on this some more, and think your right about it messing up VE, (Cylinder fill) which would effect spool. Short rod ratio and larger numerical bore/stroke will be better at filling cylinders at lower RPM that seems to help answer the question about stroke length and turbo spool. Still doesn't answer my durability questions though.

 

The underlined section is what makes cam recommendations so difficult IMO - it makes me into a cam guesser lol.

p.s. glad to see that I'm not the only one that ever references the Victory Library - I've spent days reading there

 

If only I could interpret or understand it without rereading it over and over again!

 

I have two people that I trust when it comes to turbo cams that I know personally. Both have lead me to make a choice that has worked better than OK. I'm not indicating that there aren't others more knowledgeable, but both of these guys describe a turbo cam's events, mostly exhaust opening, as having a purpose of creating the optimum cylinder pressure to both spool the the turbo and not choke the turbo. The event has to consider many other items such as back pressure, cam overlap, rpm range, and of course turbo specs along with head efficiency and exhaust piping. Very similar discussions to what I've read in the Victory Library pertaining to cylinder pressure.

I hope that when (if) I come back to this world in my next life that I can have the time and resources to play with cams, engines, turbos as full time priority -

 

All of this is a very complex issue. It not only depends on rod length, it depends on the original engine design and RPM range and what size and response turbo system is needed. RPM directly affects dwell time, for example. Different systems have different limits in cylinder pressure or rotating stresses. I don't think there are that many "one size fits all" rules.

 

The 400m basically had Cleveland heads . Yes the exhaust was a little proud of chamber which didn't help detonation . The main thing that caused detonation was the pistons were waydown the bore from deck to lower compression . The fix was in install taller pistons increasing comp yet STILL prevented detonation . No room for flame it all gets pumped out on exhaust stroke .. we like our 4 doors down under . Unfortunately these are NOT made any more !! These are derived from the 250 CI 6's from the 70's just Aust re engineering over the years .

 

The 4v closed chambered cleveland heads had a nice chamber, but all the 2v, and some of the 4v's were open chambers, and had no quench to speak of.

 

You are correct, but there is still a lot to learn from it. Let's not dismiss a topic because it's difficult

 

So the Hemi head is no good either ? No or little quench on pentroof 4 valve heads .. Tighter deck did help the M motors ..

 

I'm not sure quench is that important in turbo apps.

https://www.theturboforums.com/threads/how-important-is-quench-in-a-turbo-engine.323776/

I didn't have detonation issues in my turbo 400 with fitech. It was just a worn out old engine

 

Like posted, quench is less important with boost, and most new generation hemi, or pent roof heads have quench pads, and are more of a hybrid than a true hemi. The original hemi head worked, as do the open chambered heads, but without the benefit of quench, and are old tech compared to current chamber designs. I'll agree tighter deck will help the open chamber, especially in na form. There were pistons available for the the Mopars (maybe the Fords as well) that were essentially a small dome to maximize the quench.