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 (production, can't afford a Stage or TA unit), I don't want to see north of 5000rpm since it will sit there for minutes at a time WOT at Bonneville

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 strokethat 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 plans 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.

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,

 

[dr_frankenstein]

why are you limiting your RPM?

 

[CTX-SLPR]

why are you limiting your RPM?

Durability concerns. Stock block at WOT for a 2-3 miles, I'm affraid of pushing the bottom end out of the motor with that sustained load and rpm. Lower rpm should reduce the stress with the same 350-380hp target (for now).

 

[dr_frankenstein]

I wouldnt worry about the block with that ratio and the long rods, but i think you will need to spin it alot higher to make the number. Oil control will be the most important and I would consider the effects of that rod stroke ratio to our 120* crank split with it being so oversquare. Most calculations and data is related to V8 90* throws.

I feel like it will make the breath of the buick too shallow at rpm under 5K.

But for the block limits, I think you will be fine. Detonation is what spits bottom ends out.

this is just my thought and opinion though.

Flame Suit ON!

 

[ijames]

This comes up fairly often at www.speedtalk.com, one example is this thread: http://speedtalk.com/forum/viewtopic.php?f=15&t=39340. It's been awhile since I read the whole thing but I know that near the middle there is some discussion on this. For the bottom end the rpm limit is set by the maximum piston speed, and increasing the bore and shortening the stroke slows this down so you can turn more rpm. If you can keep the torque the same, hp will go up since hp = torque * rpm / 5252. The bigger bore lets you use bigger valves for better breathing, and the shorter stroke wins so the friction goes down. About the only downside is that the bigger bore will have a lower detonation threshold. Of course, all of that assumes you can keep the valvetrain under control. This is why F1 engines are so oversquare and turn such crazy rpms. Whatever sets the rpm limit (rules, valvetrain, etc.) the biggest bore is best from my reading.

 

[CTX-SLPR]

I wouldnt worry about the block with that ratio and the long rods, but i think you will need to spin it alot higher to make the number. Oil control will be the most important and I would consider the effects of that rod stroke ratio to our 120* crank split with it being so oversquare. Most calculations and data is related to V8 90* throws.

I feel like it will make the breath of the buick too shallow at rpm under 5K.

But for the block limits, I think you will be fine. Detonation is what spits bottom ends out.

this is just my thought and opinion though.

Flame Suit ON!

Lucky for me then that I've got an oddfire! Crank is a Molodex 2.66in oddfire unit with UK made forged SBC rods. I'm still worried about blowing the bottom out of a production block and sustained high rpm's. Compression is ~9.1 from my estimates (not cleaned up the deck to know final height) using a set of old BMS flattop pistons. The guy I was using for machine work in CO has been 200+ on the salt and built Indy Lites motors for Loophole Racing so I'd say his concern about 350hp in a production block is founded on some knowledge. I'm still trying to keep the rpm's down but that does mean boosting it to 26+ psi at ~85% VE.

This comes up fairly often at www.speedtalk.com, one example is this thread: http://speedtalk.com/forum/viewtopic.php?f=15&t=39340. It's been awhile since I read the whole thing but I know that near the middle there is some discussion on this. For the bottom end the rpm limit is set by the maximum piston speed, and increasing the bore and shortening the stroke slows this down so you can turn more rpm. If you can keep the torque the same, hp will go up since hp = torque * rpm / 5252. The bigger bore lets you use bigger valves for better breathing, and the shorter stroke wins so the friction goes down. About the only downside is that the bigger bore will have a lower detonation threshold. Of course, all of that assumes you can keep the valvetrain under control. This is why F1 engines are so oversquare and turn such crazy rpms. Whatever sets the rpm limit (rules, valvetrain, etc.) the biggest bore is best from my reading.

Like I said to the Dr., I'm afraid of the block coming apart, not the rotating assembly failing. I'm sure I'm barely 50% of their limits save the pistons with the thinner circle track crowns. I do get to run some seriously high octane gas though at the event so that will certainly keep detonation farther away combined with an ice box L-A intercooler and maybe water injection. Alcohol is not allowed in Gas classes and really I just want it there to allow me to unseal the lid and pour in DI water from a bottle to say I'm now running "Fuel" but not mess the tune up. Come to think about it, the high rod ratio might make things harder on the block since more of the power is pressing down on the mains trying to push them out. See my thread on Stock Block Sustained High RPM for more on that discussion.

 

[rag231]

Think about the large bore, short stroke, short deck Indy motors of old. They ran sustained rpms in the 10000 range whole races. I know that it was with stage blocks and heads but with the technology that the TSM guys are bringing to the table these days, I believe anything is possible. Are you able to run 4 billet caps and girdles in that class. If done right I sure it can handle your rpm limitations and hp that you need to be competitive. If it is not top secret what heads and valve train are you planning to run? I would be more concerned with oiling and my valve train durability. I would be scared to pump too much oil to the top of the motor and starving my bottom end. Is it going to be possible to develop a cam that can make the hp and turn low rpms with a short stroke crank. With most of the V6 engine formulas the crank stroke moves the rpm up or down to reach an "x" hp level. What is the optimal rear end gear to run with your proposed combo?

 

[ijames]

I'm sorry, I thought you were also asking about the theoretical trends as you go from under to over square and where the optimum is, perhaps versus maximum rpm. Never mind, then. Contact Bill Anderson (EightSecV6), he ran a TSM car for a while with a short stroke and turned at least 1000 more rpm than others at the time so he probably has a good idea of the bottom end durability. Yes, it's drag racing not endurance racing but I never heard of any block or bearing life problems. His shop is now in Stevensville, just across the Chesapeake Bay bridge, so he's only 2 hrs away from you now. Every couple of years he hosts the MAGNA club meeting, but unfortunately for you the most recent one was just a few months ago so it will be awhile before the next time.

 

[CTX-SLPR]

I'm sorry, I thought you were also asking about the theoretical trends as you go from under to over square and where the optimum is, perhaps versus maximum rpm. Never mind, then. Contact Bill Anderson (EightSecV6), he ran a TSM car for a while with a short stroke and turned at least 1000 more rpm than others at the time so he probably has a good idea of the bottom end durability. Yes, it's drag racing not endurance racing but I never heard of any block or bearing life problems. His shop is now in Stevensville, just across the Chesapeake Bay bridge, so he's only 2 hrs away from you now. Every couple of years he hosts the MAGNA club meeting, but unfortunately for you the most recent one was just a few months ago so it will be awhile before the next time.

You caught me there! I did say the theoretical before the practical. I guess I'm mainly concerned about being already off of the deep end vs. moving around the 1:1 ratio for that discussion as well. The big areas along those lines are theoretical rpm range and detonation resistance as you dwell the piston at tdc longer with long rods.
I'll have to see if I can get plugged in with the Turbo6 community here.

Think about the large bore, short stroke, short deck Indy motors of old. They ran sustained rpms in the 10000 range whole races. I know that it was with stage blocks and heads but with the technology that the TSM guys are bringing to the table these days, I believe anything is possible. Are you able to run 4 billet caps and girdles in that class. If done right I sure it can handle your rpm limitations and hp that you need to be competitive. If it is not top secret what heads and valve train are you planning to run? I would be more concerned with oiling and my valve train durability. I would be scared to pump too much oil to the top of the motor and starving my bottom end. Is it going to be possible to develop a cam that can make the hp and turn low rpms with a short stroke crank. With most of the V6 engine formulas the crank stroke moves the rpm up or down to reach an "x" hp level. What is the optimal rear end gear to run with your proposed combo?

True but they were 3.9x2.9 motors that results in a lower bore to stroke ratio. Is the statement "don't destroke it too much, that is where your torque is" true? I know those Indy motors had little need for low end power with the sheer number of gears they had and how much speed they could carry through the corners. Is there a point where you start killing off your bottom end from a practical perspective vs. just trading some down low for more up top? The extreme RPM's of F1 sure seem like they have done that. Additionally the methanol fuel certainly helps with detonation resistance.

My Stuff, allowed by the rules for Production:
Stock block (billet caps, girdle, and even stage/TA blocks are allowed as long as it's the same deck height, bore spacing, bell housing, cam height, and number of head bolts used) and stock heads (can be aftermarket but same port order, valve actuation method, and number of head bolts used). Valvetrain is a set of KB/T&D roller rockers connected to a solid flat tappet cam via big pushrods. Planning on adding several extra oil returns back to the pan from the heads to help keep the oil in the sump. With a TH200-4R, the run to speed would be 3000-5000rpm with a 25in tire and 3.42 rear gears. Upping the rev range to 6000 and dropping the OD would require a 2.93 rear gear but the pull to speed still starts ~3000rpm. Actually trying to keep the rear gear high to keep from lugging the motor, several salt veterans have warned against running too low a rear and not being able to get the car up to speed for a charge as a common noob mistake.

 

[CTX-SLPR]

Distilling out questions:

Does ramping the bore vs. stroke ratio up too high effectively make the bottom end of the power curve unusable with the cam choices it drives or can a cam bring the power back down?

If you keep the deck height the same, do longer rod ratios needed with shorter strokes have a similiar but lower order effect on the power curve?

Related Question:
Does rod ratio affect stress on the main bearing area of the block by taking the side forces off of the walls. Basically does it up the forces trying to push the main caps out of the block in a meaningful manner?

Thanks,