Taking Big Bore Short Stroke Too Far

[CTX-SLPR]

I'm sure most of us have either been indoctrinated by magizine/forums, come to the conclusion through reading of informed sources, or maybe even for some of us (not me by the way) through experience that given a fixed displacement, big bore, short stroke makes the most power. I'm not going to dispute that but I do have a question within a certain set of parameters that are NOT the norm for this kind of discussion.

1. I'm working with a small motor, 3.0L (183cid) turbo V6
2. I'm really rpm limited due to the design of the block, I don't want to see north of 5000rpm

Looking at Bore vs. Stroke ratios and this thing is ridiculously over-square in the combination of parts I have now. 3.800in bore and 2.66in stroke for at ratio of 1.429 and with 6.3in rods, that's a rod ratio of 2.368.

The other option is a 3.5in bore LC9, horrible smog 3.2L V6 with a 3.8's 3.400in stroke that is the reverse of the later 3.0L (3.8's bore, 2.66in stroke), with either an Indy 2.900-3.070in or production 3.3L 3.160in stroke cranks to still fit in under my 183.99cid displacement limit.

I have to wonder with my rpm limitations if I've taken the big bore, short stroke thing a bit too far with the current combo or am I over thinking it? Going with an LC9 based engine would require a complete retooling of my parts inventory for everything short of the main bearings AND likely limit me to a production block but a production block that no one wants with other parts that no one wants is how I got to where I am anyway so it's not too expensive. If that buys me an engine that will make more power under my rpm target without blowing the bottom out of the block it might be worth it. Depending on cranks as well, the rods (and therefore bearings) might still work and I'd be out just the crank and pistons effectively.

Please comment on the theoretical THEN weigh in on the practical since I'd like an education as much as I'd like a specific answer.

Thanks,

[Revolutionary1]

Most bore/stroke combinations have some practicality in their making, so that must not be ignored. If you are speaking of a racing application, then many bore/stroke/rod combos are derived from some driving rule that sets the stage, such as "you must use the factory bore spacing and minimum deck height of x.xx inches".

On the theoretical side, a smaller bore is less prone to detonation as there is less surface area of the piston/chamber, and fewer places for detonation to be triggered. On a forced induction application, or a high powered pump gas deal, that makes a difference. If you look at the Engine Masters engines, the best ones are fairly close to square.

In practical terms, if you have an HP target, then you must have an intake/exhaust mass airflow (or at least CFM) target to get you there. A small bore will possibly limit the valve size to get a curtain area big enough to physically flow the air/fuel you need. You have a low rpm target so that needs to be considered when doing your airflow math.

Everything is a compromise. If you think the small bore heads will flow the air you want and make the power with a blower/turbo/nitrous, then investigate further. If you will save a bunch of money by sticking with the big bore, then do it. I highly recommend buying Engine Analyzer Pro from Performance Trends. It does a very good job of showing the TREND of what combinations work.

[SuperBuickGuy]

what is your maximum rod angle? I think that issue is more the problem than anything else because you're side-loading the skirts and bore wear

[CTX-SLPR]

what is your maximum rod angle? I think that issue is more the problem than anything else because you're side-loading the skirts and bore wear

With a rod radio as big as it is currently (~2.4) with the parts on hand (big bore, short stroke) I don't think that's a problem in the least. It's a long rod with a tiny stroke. Bore aside, that's a longer rod than normal in an SBC with 3/4 the stroke length. For example the 2004 CTS-V 5.7L race motor was an LS7 with a 4.8 crank and I assume close to the same 6.275in length rods, that's a rod ratio of ~1.9 on a factory endurance motor. I'm actually not worried about it being side loaded in the slightest.

Think about it with geometry, the angle is smaller with a longer rod and smaller with a shorter stroke.

Revolutionary1, I'll have to consider the software. The same heads were used on both the LC9 and the LC2 turbo motor though the ability to upgrade valves is a big question with a 0.3in smaller bore. I'm guessing that the valves are likely maxed out stock and the bigger units need the 3.8in bore to clear. Got a formula for unshrouded air curtain flow based on valve size?

[Barry Donovan]

5000 limit?
its a paradox.
go for the bigger stoke.

I mention the little boxer in your posts, 2.62 inch stroke, 3.65 bore.
I could not even imagine a conterbalanced weirdo dangling on main bearing bolts.

go for some stroke, slow it down.

[Revolutionary1]

Check out the PHR article http://www.popularhotrodding.com/tec...b/viewall.html for info on valve curtain area and flow coefficients. I heard the author is an ok guy ;)

[nesabo]

I met that guy. Yeah he pretty cool!

[Huskinhano]

Look at the Ford 289. 4.00 bore and 2.87 stroke with a long rod.B/S works out to 1.39 They ran pretty well!

The 289 rod was 5.155", rod ratio is 1.796

[Huskinhano]

Something else I remember. Dave Freiburger years back wondered about the large bore short stroke vs small bore long stroke. He had two BBC built.Both had the same heads and cam. One had a large of bore and short of a stroke as possible with a long rod. The other was just the opposite. Small bore,long stroke and short rod. Both engines were pretty close in displacement. He found in dyno testing both engines made almost exactly the same power at the same RPM.

[Bob Holmes]

I remember that one. As long as the bore didn't shroud the valve, it wouldn't make a difference.

The smaller bore is better for a boosted application. Far less likely to have detonation/pre-ignition problems.

Why is the engine limited to 5,000 rpm?

[milkovich]

No one can debate that a short stroke lowers the stress on the rotating assembly but why the 5000 rpm limit?

Wondering out loud though, at this RPM your piston acceleration is pretty low even on the longest stroke combination. With all the constraints you've listed, I would think a quicker piston "action" would be desirable.

[BigAL]

I'm sure most of us have either been indoctrinated by magizine/forums, come to the conclusion through reading of informed sources, or maybe even for some of us (not me by the way) through experience that given a fixed displacement, big bore, short stroke makes the most power. I'm not going to dispute that but I do have a question within a certain set of parameters that are NOT the norm for this kind of discussion.

1. I'm working with a small motor, 3.0L (183cid) turbo V6
2. I'm really rpm limited due to the design of the block, I don't want to see north of 5000rpm

Looking at Bore vs. Stroke ratios and this thing is ridiculously over-square in the combination of parts I have now. 3.800in bore and 2.66in stroke for at ratio of 1.429 and with 6.3in rods, that's a rod ratio of 2.368.

The other option is a 3.5in bore LC9, horrible smog 3.2L V6 with a 3.8's 3.400in stroke that is the reverse of the later 3.0L (3.8's bore, 2.66in stroke), with either an Indy 2.900-3.070in or production 3.3L 3.160in stroke cranks to still fit in under my 183.99cid displacement limit.

I have to wonder with my rpm limitations if I've taken the big bore, short stroke thing a bit too far with the current combo or am I over thinking it? Going with an LC9 based engine would require a complete retooling of my parts inventory for everything short of the main bearings AND likely limit me to a production block but a production block that no one wants with other parts that no one wants is how I got to where I am anyway so it's not too expensive. If that buys me an engine that will make more power under my rpm target without blowing the bottom out of the block it might be worth it. Depending on cranks as well, the rods (and therefore bearings) might still work and I'd be out just the crank and pistons effectively.

Please comment on the theoretical THEN weigh in on the practical since I'd like an education as much as I'd like a specific answer.

Thanks,

Since you are limited to 5000rpm, go with the longer stroke as longer strokes make more torque in the lower rpms. Race motors use short strokes because they are spinning the motor 9000+ rpm. With your rpm limitations you should be building for Torque, not Horsepower, or maybe I should say you need to build for increased low end Horsepower under 5000rpm. As for rod ratios you just want to run the longest rod possible, you might even need to look into offsetting the wrist pins to help reduce side load on the pistons if the block design is as bad as you say. Other than that, stop over-thinking it, just do it.

[CTX-SLPR]

5000rpm limit is out of concern of blowing the bottom out of the BLOCK. The weak areas in the Buick V6 are the main bulkheads and with a minute(s) long charge to speed in LSR, I'm worrying about breaking the block. Buick V6's in Indy where very powerful but had trouble lasting races and that was with the Stage racing blocks with cross bolted caps and such. With a production block I'm trying to keep the rpm's down to keep the stress out of the block from the high rpms. The rotating assembly itself is probably bulletproof at these power levels and the pistons are the weak link but probably not till the 600-700hp level. Again, it's all about the block on the rpm limit though some of the Buick Turbo6 community are making me question if 350hp at 5000rpm out of a 3.0L is 1. obtainable even with a turbo, and 2. enough to worry about and the block could take 6000rpm just fine.

[TheSilverBuick]

Hmm, I've never heard of a V6 or Buick 350 ever pulling the mains out. They are deep skirt blocks with fully casted mains, unlike the 455 with windowed mains.

[Aircooled]

350hp at 5000rpm out of a 3.0L is 1. obtainable even with a turbo, and 2. enough to worry about and the block could take 6000rpm just fine.

306 ft.lbs. @ 6000rpm (350 hp) will be less stress on the mains than 368 ft.lbs. @ 5000rpm (also 350 hp)