Bore/Stroke Gen III v. Gen I/II SBC

The Gen III (LS1) is 350ci 3.90/3.62" and the "traditional" 350 is 4.00/3.48". I was under the impression that the LS1 was pretty much a "clean slate" design. AFAIK, the only thing in common is the 4.40" bore center. Anyone know why GM chose a longer stroke/smaller bore combo? It seems like there would be more hp in a bigger bore motor. What am I missing?

Rich

Smaller bores are generally cleaner burning & emissions are a big deal nowadays.

True.... a bigger bore unshrouds the valves & should help flow.

Sleeving an aluminum block from a production standpoint to achieve long bore life can also cause some compromises to be made. Keeping the bores round with consistant cooling under various conditions comes into play, as does head sealing/gasket life. The 6L blocks are iron & 4" bore.
Can't wait to get my hands on a 6.0L LS2 aluminum block.

Also the longer stroke may coincide with the intake tract design.

.... just a few guesses here.

For refernece, another similarity in the 2 motors is the rod bearings size.

I am very impressed with these motors in general & actually prefer to work on them over a traditional small block. They did their homework on this design for sure.

The Gen III (LS1) is 350ci 3.90/3.62" and the "traditional" 350 is 4.00/3.48". I was under the impression that the LS1 was pretty much a "clean slate" design. AFAIK, the only thing in common is the 4.40" bore center. Anyone know why GM chose a longer stroke/smaller bore combo? It seems like there would be more hp in a bigger bore motor. What am I missing?

Rich

The ability to increase the original displacement had to be a factor. The original SBC grew from 265 to 383 without significant block or rod length changes. If you imagine the 4.8 as the "new" 265, that ratio (383/265) gets you a 6.93 or about 7.0L, right where the race version is.

The 5.7 LS1 met the hp/torque/performance and fuel mileage goals for the C5, but still allowed for room to grow. We're already at the "327" stage in the development of the Gen III (327/265 x 4.8L = 5.9L or just about 6 L). Is that a coincidence? Maybe next we will see the "350" version or a 6.3L Z-07 or whatever it will be called. Then again they might go right to the "383" 7L size! That would be an fairly easy 500 hp based on the LS6 power/L.

My random thoughts.

Smaller bores are generally cleaner burning & emissions are a big deal nowadays.That's the ticket. Plus, more torque per cubic inch at lower RPM, typically, thus it can be geared to be more efficient. Manufacturers are taking this to the extreme, like the K24 motor in the Acura TSX. 2.4l, with a 3.43" bore, and 3.90" stroke, and they put enough beef into it, to sling it to 7100rpm from the factory.

That's the ticket. Plus, more torque per cubic inch at lower RPM, typically, thus it can be geared to be more efficient. Manufacturers are taking this to the extreme, like the K24 motor in the Acura TSX. 2.4l, with a 3.43" bore, and 3.90" stroke, and they put enough beef into it, to sling it to 7100rpm from the factory.

Yeah, Honda/Acura is reaching about 4600 ft/min average piston speed at redline. That's a LOT, considering an LS1 would need to get to 7600 to see that piston speed. Tha's a ton of piston gs and therefore rod load, but the smaller diameter and therefore lighter piston helps.

Shoot, even the CTSV LS6 race car which started out at 7900 rpm (until it won so easily the sanctioning body cut it to 7000 max) was only seeing 4300 fpm @ 7900 and 3800 fpm @ 7000! That's what a 3.27 stroke does for you. it also keeps the friction hp down which ain't a bad thing especially if you have an airflow restrictor.

FWIW, 4 valve engines like the Acura can get good low-lift and high lift flow even with a relatively small bore. Non-canted 2-valve engines do better with the larger bores. Everything is a compromise.

i agree, especially since these motors were going in a large number of vehicles. smaller bore creates a faster burning flame front if i am not mistaken.

maybe pistons are cheaper too :p

FWIW, 4 valve engines like the Acura can get good low-lift and high lift flow even with a relatively small bore. Non-canted 2-valve engines do better with the larger bores. Everything is a compromise.I understand that, but I am dead certain that you understand it better.:)

I'll go with OldSStroker's original explanation, which is that Gen III engines are all a family. The 5.7L displacement met the C5 power goals and a 6.0L was in trucks. The stroke is the same for the 5.7L & 6.0L engines and the bore diameter increased to get a 6.0L. IOW, the bore spacing was designed for a maximum displacement of 6.0L when using a cast iron block. In the end, a lot of decisions come down to manufacturing complexity and $$$.

BTW, the LS7 is a 7.0L. That is no longer a rumor.

I'll go with OldSStroker's original explanation, which is that Gen III engines are all a family. The 5.7L displacement met the C5 power goals and a 6.0L was in trucks. The stroke is the same for the 5.7L & 6.0L engines and the bore diameter increased to get a 6.0L. IOW, the bore spacing was designed for a maximum displacement of 6.0L when using a cast iron block. In the end, a lot of decisions come down to manufacturing complexity and $$$.

BTW, the LS7 is a 7.0L. That is no longer a rumor.


7.0L? Cool!

Another reason to keep the 4.40 bore spacing is that all SBC block machining equipment was built at that spacing. To retool and reuse any of the transfer line machines the 4.40 spacing is required. I think the 8100 big block also retained to ond BB bore spacing probably for the same reason.

A smaller bore should also, all things being equal; be more detonation resistant. I have no idea if this was remotely an issue for the designers though.

Another reason to keep the 4.40 bore spacing is that all SBC block machining equipment was built at that spacing. To retool and reuse any of the transfer line machines the 4.40 spacing is required. I think the 8100 big block also retained to ond BB bore spacing probably for the same reason.

Now you sound like a factory engineer.

Now you sound like a factory engineer.

Well, thank you.:) It's been 36 years since I was. The economics don't change much, but the scale does. In the good ole days you coud design and tool up a new engine for maybe 50-60 mil. Today it's more like 500-800 mil. That might be low.

My guess is that given the choice, and bottomless pockets, the LSX architecture might have had slightly larger bore spacing. The 4.40 was cast in stone...make that cast iron machine bases. Any Powertrain folks around here that can verify or deny this?

Well, the price for a new engine is very related to how much your annual volume is. It's still very possible to do an offshoot of another engine for your 50-60 million budget. We just did one.

Well, the price for a new engine is very related to how much your annual volume is. It's still very possible to do an offshoot of another engine for your 50-60 million budget. We just did one.

Cool.

Speaking of offshoots, the '61-'63 Pontiac Tempert 195 cube 4 cyl was a 389 V8 with half the cylinders cut off. It had a specific block, crank and intake manifold/carburetion, but used mostly V8 parts and ran down the V8 line comingled with V8s. It was developed and tooled for something like $600K. The built about 300,000 of them for about $2 per engine for tooling cost. I could be off by a factor of 2 or 3.

At the same time the 215 cube all aluminum V8 was designed, developed and tooled for about $65 million give or take a few mil. They built a total of about 300,000 of them also. That's more like $200/engine tooling cost. Coincidently, $200 was about the cost of the aluminum V8 option in the '61-'62 Tempest. The tooling was sold to the Brits who ran it in Rovers, et al until a few years ago. They certainly got their money out of that deal!

So what was the offshoot?

BTW, any idea of the design/develop/tooling bill for the 4200/3500/2800 GM inline family? How about the Ford Modular? The new "Hemi"?

Damn, I just typed a longer reply and then this website appeared to lock up.

Anyway, the "offshoot" is a Chrysler product that is launching next year. I can't really say more than that since I doubt Chrysler would appreciate things like that getting out unless they announce it.

I don't have first hand knowledge of the full budget for the 5.7L Hemi, but I would venture to say it's around your initial estimate. Take into account about 5 years devt., vehicle integration issues with a completely unique engine, engine assy plant with 2 engine assy lines, crank mach line, block mach line, head mach line, supplier casting tooling, rod mach & forging line, etc. It's huge.

Some interesting points. I did know about the reason for retaining the bore spacing but I didn't know if it was so compeeling economically as to justify what might be a less than optimal bore diameter? Similar reason for the rod bearing bores? Seems logical.

Rich

Some interesting points. I did know about the reason for retaining the bore spacing but I didn't know if it was so compeeling economically as to justify what might be a less than optimal bore diameter? Similar reason for the rod bearing bores? Seems logical.

Rich

I think things like rod bearing and main bearing sizes are determined by intended use, rpm power, etc. rather than tooling. Bore diameters within engines or rods or pistons are adjusted with expendable tooling so anything goes. Maybe the rod journal diameter has worked so well so long that there was no big reason to change it much. ??

I find it iteresting that in a "hard metric" engines like the LSx where the nominal bore and stroke are 99 x 92 (5.7L) that the 6.0L has the old familiar 4.000 bore (101.6 mm). It could have just as easily been 102 mm (4.016) and been closer to 6.0 L. There might be some nostalgia in the designers' minds. We can hope.

I also like that they still use the "5 on 120.65mm" bolt pattern for wheels, where almost everyone else has gone hard metric.

I just wish they would have used standard bolts (did they, not as far as i know on accessories).

I hate metric bolts.

You'll find most engines are switching to metric bolts. "World" engines are becoming more commonplace and commonizing fasteners with the rest of the world is another cost savings opportunity.

You'll find most engines are switching to metric bolts. "World" engines are becoming more commonplace and commonizing fasteners with the rest of the world is another cost savings opportunity.
metric threads suck :yuck:

i guess 2nd grade math of finding the common denominator is too much for some :o

Old SStroker...

You talk about nostalgia.. What about the 550 maranello that went up to 575? aka 5.7L?
What about the carrera? 5.7L? there are some thing that flat out work right? why? nostalgia?? maybe not... mystical?? probably ;)

Anyways...

I wondered about this, although i know nothing compared to you guys, i tend to agree with a post from the first page... the longer the stroke the more piston speed you create and thus the more fps are needed on the heads. That gave the engineers probably the sucking they needed in order to have the heads work on that design?

Not sure exactly what GMs reasoning is behind the longer rod combination but:

higher rod/stroke ratios increase reliability, reduce crank/rod stress.
Reduce piston slap also as the rod angle is better. Maybe one of the reasons the LS1 engine is quieter, along with a faster flame front.
and also allow better breathing at higher rpms.

LS1 S/R ratio - 1.68
LT1 S/R ratio - 1.64

Reduce piston slap also as the rod angle is better.

Hehe, isn't the GM Gen III the engine that started the pistonslap.com website?