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Different bore/stroke within same engine

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RETHomer

RETHomer

Automotive
Dec 29, 2001
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Can anyone determine the advantage of running two different bore/stroke combinations within the same engine?
I just read a V8 racing engine buildup in which the author said he will use two groups of 4 cyl. displacements that will be within 0.01 ci of each other.
Didn't say how cylinders would be grouped (left vs. right bank, corners vs. inner, odd vs. even).
Did say the connecting rod length-to-stroke ratio would be similar for each grouping. Any thoughts?
 
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If you are limited to a particular manifold then different cylinders could see different VEs, so you might want to adjsut the cylinders volumes to suit.

Some classes check displacement by measuring one cylinder only. If you happened to make sure that only one plug was accessible, then you could make that cylinder smaller than the others (!)

The outer cylinders tend to run cooler than the inners, so again you might want to vary cylinder size for that.

Cheers

Greg Locock

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This is a tatic used among a minority of the top engine builders, i have only heard of it used on american V-8s.

Without commenting on it's benefit or lack of, the theory is that a combination of 4 cylinders with large bore short stroke and 4 with a longer stroke and smaller bore (with all variables maximized for each 4 cylinders ie different cams ports etc) will have a longer useable power band than if you just split the difference and made them all the same. The cylinders arent always exactly the same displacement but are close. This isnt done experimentaly because it compounds the variables, It is two halfs of previously sucessful engine combinations that are fully developed. Some who are sucessful swear by it.

 
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With a crank with 4 big end journals, as is the situation with all V8 engines I have ever seen, the cylinders which share the same journal obviously have to have the same stroke.

I wonder what effect it has on crankshaft harmonics if the strokes are different. I wonder what effect different weight rods and pistons and different piston speeds has on balance.

I know a SBC has charge robbing problems when two adjacent cylinders fire consecutively. This is exaggerated by the SBC port layout and firing order.



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This development isnt always used with different strokes and bores but the question was posed that way.

Often it is just the cam specs, port specs and piston dimensions that are different.
 
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Well I can certainly see a reason to increase the comp and inlet valve duration on #7 on a SBC, with the firing order of 18436572 and numbering the cylinders as they are from front to back on the crank pins, hence 5 and 7 are adjacent

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Pat, you did not here this from me, but...I know a guy that ran a drag racer in the 50's that had a SBC 283 that had only ONE cylinder that was stock! The one NHRA always pumped!

As to varying the cam timing, I use an APT "scatter pattern" grind on my Mini's engine. It utilizes different timing events for each cylinder to help equalize the power variation caused by the abominable port layout of the BLMC "A" engine. I have also seen different CR's used in this engine to equalize exhaust temps although I do not do this myself. I use a rather mild 13.87:1 across the board.

Rod
 
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ouch, diffent bore and stroke in a same engine. I am worried about the cylinder to cylinder variation that would contribute to slightly different torque generation during combustion. NVH will also be an issue
 
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audi i think, is working on a variable displacement engine where something crazy like the head can move increasing stroke under load. read it in a car mag about a year ago.
 
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I can see that moving the head can change compression, but I can't see how it changes the stroke.

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The new/recent Hot Rod magazine did a moderately scientific Bore/stroke test using a Chevy V-8. Constant displacement.

Their dyno results and most (but not all) of the engine builders interviewed seemed to validate the conclusion that can be drawn from the old power formula.
HP=(bmep)(stroke)(Piston area)(power cycles/min)/constant
 
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That is what I would expect under most circumstances, except for very minor differences through things like different surface area to volume ratios and different inertia in the reciprocating parts. By minor differences I mean differences that only show up when measuring to the 3rd or 4th significant figure.

There can be very minor differences in the breathing due to rod to stroke ratio differences.

Depending on the airflow in the cylinder heads, the longer stroke might be power limited before the shorter stroke as it will reach maximum piston speed at lower rpm

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Thank you all for the interesting insight.

I looked at the link mentioned by incandescent, then called a friend who builds Pro Stock engines. He said he has heard of this theory but never tried it nor has he talked with anyone who has tried it. He said the theory is to build a V4 with strong top end horsepower and a V4 with more torque. The theory is that that a Pro Stock shifts around 9600 and drops about 1000 or 1100 rpm with each shift. Instead of building a V8 as a compromise in that rpm range, mate two smaller engines specific to each different rpm.

I understand the concept of manipulating the ports, ignition and cam timing and even the compression ratio to equalize cylinder performance, especially in a single 4V application. If I remember Smokey's wisdom correctly, he suggested each cylinder should be tuned to match the others in the pursuit of 8-cylinder harmony. But I had never heard of manipulating those same dynamics to split the engine's personality as a means to run better at two different rpm. My gut instinct says the two setups will conflict instead of complement. It makes me think of running an off-road vehicle with two shocks: one for small bumps and one for large berms. I don't think it'll work.

Am I just not thinking outside the box enough?
 
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As I see it, if you tune 4 and 4 for different speed ranges, you get some cylinders optimised at a wider range, but you never get all 8 optimised at one time. I would think that all 8 optimised for a narrower range with suitable gearing to stay in that range would be the ideal. I know, some classes do not have the option to use more than one gear, or the time taken to change more than negates the advantage.

If I needed to widen a torque band, I would be more inclined to spread the cam lobe centres as the first step.

I would certainly be prepared to optimise every cylinder rather than equalise every cylinder. Equalising implies bringing all back to the lowest common denominator.

Regards

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I believe there is some merit to optimizing individual cylinder valve timing events, CR, and ignition timing for a multiple cylinder engine based on what I have read and personal experience. I don't think altering the B/S ratio is the best method to achieve that objective.(optimizing each cylinders output)------Phil
 
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Long ago, road racing V8s with IR fuel injection often had long and short induction trumpets, which supposedly were for the purpose of broadening the power band. It pretty obviously would do that, at the expense of peak power. Whether it was optimum, I don't know, but they won a lot of races.

See on a McLaren M8C:


Sorry for ridiculously long URL!

Al Seim
 
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In the 2-stroke Moto-GP days, most bikes had twin, contra-rotating crankshafts. That gave no net gyroscopic effects and also allowed each cylinder to be crankcase scavenged. Although I never saw any evidence to support it, the cylinders could easily have had their opposing banks given different crank throws. God knows they tried everything else!
 
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The McLarens used Big Block Chev heads. These heads have a long and a short inlet port. The different length trumpets were to correct for the different length ports. The widened torque band argument was a red herring to mislead the gulible.

Regards

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