The Cal-look Lounge

I was thinking does bore and stroke differences affect optimum venturi for a given rpm?

So if you have an 90.5 x 84 and a 94 x 78, they both equal ~2160cc or so. So would they both give best specific output using same venturi? Most of us know how to figure out venturi diameter, given rpm and single cylinder disp (cc), but what happens when you change HOW you get to displacment.... does long stroke allow a larger venturi? 94 x 69 vs 90.5 x 74 vs 88 x 78.... all equal about 1900cc but would all three make best HP and/or give best driveability using the same venturi? Seems like the longer stroke motors would be more "forgiving" on big vents than equal cc motor running a shorter stroke- harder signal on fuel circuits. Or maybe vents would be same, but emulsion tubes would differ. (Guys, if you haven't and are able to- try F4's in your IDAs ;))
Thoughts?

Hmmm... you're moving the same amount of air but I could see where bore and stroke might change how the air is drawn.

Interesting question

Are you assuming the same rod ratio for each engine?

John -- You need to explain to me why Rod ratio would make any difference, other than the fact that the longer the rod the better -- --

I've always liked a big bore/short stroke combo -- I've found I could always can get more RPM and therefore more HP at of the same size engine, But I could always get more torque from a Big stroke/small bore combo -- so it comes down to what are you buiding the engine for and that would effect the venturi size (High RPM = bigger Vent/Lower RPM = smaller vent ) -- my .02 -- LOL

Rod ratio effects air speed at different RPM depending on the ratio. Jim is asking for optimums at different RPMs for a given venturi. Different rod ratios will give you different air speeds through a given venturi at different RPMs.

I guess the exact opposite. I think the carburettor would think the engine was much bigger than it is, since the same volume of air would pass the venturi on a shorter time.
And according to the late great SOB a shorter rod (as opposed to a longer rod) would generate more "snap" in the turning of the piston, and that should also help in fuel delivery... as I recall it.

Assuming same diameter intake valve on both engines, you have less shrouding with 94mm bore. Therefore more flow potential, higher piston cfm demand and the likelihood you'll make more peak power (at higher rpm) than same capacity with a smaller bore. As a result, optimum venturi size may be bigger than that for the small bore motor.

You'd probably opt for the smaller bore, long stroke option if improved driveability in the low and mid-range was top of your list, in which case a smaller intake and venturi will give best results.

Assuming each engine has identical BMEP (peak torque on both engines the same), max torque will occur at higher rpm on the bigger bore, resulting in more peak power:
(bhp = torque x rpm / 5252).

Re rod length: IMO unless you're straying well outside the norm (say 1.6 to 2.0 rod ratio), rod length has minimal influence over where in the rpm range peak flow demand occurs. Cam choice, induction length, valve diameter and port size are vastly more influential.

Optimum venturi size is the one that delivers best average power and torque across the rpm range the engine will most often see. For the two engine combinations you're discussing that's likely to be different. It's a matter of tweaking the variables to make the engine suit the application. If you want lots of power, the larger bore has more potential, plus the shorter stroke reduces piston speed to better suit the higher rpm range. Look at just about any high end n/a race engine. Rules permitting, short stroke/big bore is the way to go for max power.

If it's a broad spread of low and mid-range torque you're after (eg daily driver bus motor), the smaller bore/long stroke approach has its benefits. Longer stroke generates more piston speed, filling the cylinder more efficiently at lower rpm than short stroke/big bore... that's assuming cam, heads, carbs etc are dialled in to suit.
Horses for courses.

Alternatively, fit EFI and quit worrying about venturis forever (and if the plural of venturi is venturii)  ;)

As I understand the question, I don't think this wouldn't be true. You are moving the same volume of air over the same amount of time.  :o ???

Yes, but a short stroke  is moving the same amount of air over a shorter distance.

Jim, can you tell me more about the F4 tubes?

Getting back to the venturi question, the short stroke, short rod ratio motor even though your moving the same
amount of air the velocity speed would be higher, therefore max power the larger vents would gain more power the in a
street car the tune ablity becomes a challenge.
 any comments or am I going it the wrong direction.

I'm not saying you're wrong but I'm trying to understand here. Assuming that the RPM, the valves, the displacement and the venturi are the same, how would a shorter stroke change velocity?

John, as always: THANK YOU!

I think you have a point Rick... The piston will be at tdc and bdc at the exact same time, at a given rpm regardless of the stroke.

Whew... I thought I was gonna look like an idiot there for a moment!  ;D

axam , im not sure if this would help explain,  or i may just be adding more confusion


the bigger stroke will have higher piston acceleration  and deceleration

the smaller stroke will have slightly longer dwell 

if they use the same rod length =  the rod itself doesnt change in length but the stroke does so r/s ratio gets affected ,


ofcourse but as John Maher explains, unless we're at both ends of the extremes, this may not show any diff in result or power


if it changes the tune? as mr jim ratto asks us.. or  quizes  us?   i dont know but would love to learn  :)

I was thinking strength of signal to mains/emulsion/airs, that the longer stroke motor would send a stronger signal, earlier, bringing usable HP on earlier, the larger vent alllowing high end rpm as well, giving more overall power. I'm not talking race only, sorry I didn't mention that.
I've tried using overly large venturies in carburetors on engines that didn't "justify" them, and they were complete pigs to drive, nightmares to jet, etc. I'm talking 42mm on hot 1914, 40mm in 1776, etc.

I guess we'd have to think of situations of when there is either 1atm+ in cylinder/ports/runners and intake valve is open. In reality, I know very few of us would elect to use same exact cam timing for 90.5 x 84 and 94 x 78 motor, given the choice, but if all variables were identical, opening and closing events, valve lift, port diameter and length, valve diam, etc... what I asking, would the increased stroke, despite cylinder displacement being within a few cc, allow for better power, using same venturi diamter than the short stroke motor? And during period where cyl is above 1atm, is big vent an advantage or hinderance.

Thanks

Thanks for feedback.
 It just that in a race application (oval track)
 the short stroke, short rod and big bore motor felt stronger and more responsive as
the long stroke, longer rod, and big bore definately had a smoother and better torque
curve, but didn't feel as snappy and alive as the other combo.

also Jim, tell more about these F4's

hopefully our experts here can divulge more info regarding your question

which i do find very interesting, namely the mean piston speed of a 84 stroke would be alot higher than the 78 stroke
now how that affects things, if it does? id love to know..

Looking at this some more.... John Rayburn's statement above is correct.
But how much effect does conrod length have on air flow and velocity?
Ran some numbers  based on the two proposed engines already discussed: 78 x 94 (2165cc) & 84 x 90.5 (2161cc).
Simulations below are based on an engine speed of 6000rpm.

First, the 78mm stroke x 94mm bore engine, with 5.400" rod (1.76 rod ratio).....

[attachment=1]

Next, 84mm stroke x 90.5mm bore but with rod length increased to 5.800" to provide similar rod ratio (1.75) to the smaller stroke engine shown above.
By maintaining the same rod ratio, max and average piston speeds are identical, as are flow demand cfm and port velocity....

[attachment=2]

Signal at the venturi for both engines is the same, assuming the heads perform identically on the different bore sizes.

How about running a shorter rod on the long stroke engine? 84mm stroke with 5.400" rod drops rod ratio from 1.75 to 1.63.
This variance of .400" is probably as far apart as anyone building a street motor is likely to go so might be expected to have a noticable impact on air speed etc.

[attachment=3]

Result: max and average piston speed increases. Values in the cfm and velocity columns have also changed.
But is it enough to have a significant effect on performance?

Draw your own conclusions  ;)