I know I am probably incorrect in this statement, but I believe that real engineering is done with pencils and paper first, dynos and testing come later.

With a 383, the stroke is lengthened and the bore, albeit minor, is widened a little (.040). But most of the cubic gain is from the stroke. As a result, the piston force is presented for a greater duration over any RPM as the piston travels a greater distance, resulting in greater torque being represented in lower RPMs..(duh). The downside, of course, is that as RPM rises, the piston's force begins to wane as the efficiencies decline and it then becomes more opportune to shift to another gear to regain the torque curve.

1) SO, should timing be adjusted to reflect the longer stroke, as in retarding the timing to allow the burn to be more effective during the powerstroke?
If so, how much?

2) If the engine is rebuilt with a -31cc dish piston (JE forged), or other combustion chamber enlargement method, then is the VE less/more than posted in the stock tables? If so, then what percentage should it be down/upgraded over what RPMs?

I know this is quite empirical, but there are some basics that follow the Bore X Stroke calculations inherent in our engines. (i.e. 18 in/hg.) and I am sure that a pure GM engine engineer would whip out his slide rule and calculate exactly what VE, what timing, at every RPM conceivable, yet when I compare an LT4 to and LT1 tune, they are slightly different, yet the only differences in design are the intake/heads and the the Compression ratio is raised a from 10.5 to 10.8. Both are 350s. In the LT4, VE is less than an LT1. Is this due to the higher velocity of the more restrictive LT1 intake/heads or just the opposite due to the better flowing LT4 intake/heads. (i.e. the garden hose constant: larger tubes = lower velocities; lower velocities = lower pressures...with same volume!)

 

I know you were being facitious, but pencil paper and slide rules? Had to laugh. Yes, design should come well before build and test. Unfortunately in the amateur aftermarket, the design phase is often skipped. Rarely is that so in OEM or successful high-end (high $) engine building.

Fairly sophisticated engine simulators are reasonably priced, and if you know what data to input (MOST important) you can simulate just about everything the engine does with any bore/stroke combination. OEM engineers have VERY sophisticated sims to work with as you might suspect.

1. How about simulating to find the starting point for timing and then fine tune it on the dyno? Give the engine what it wants.

2. Isn't Volumetric Efficiency (VE) dependant upon a number of factors working together? Factors like port flow, runner length, valve event timing as well as rpm all enter into it, don't they?

Why do you say that VE on an LT4 less than on an LT1, both in stock form? If the LT4 produces more maximum hp isn't it pumping more air at hp peak than the LT1? I'm confused. Help me out here, UD. You also seem to be talking NA, but the subject mentions SC'd. Now I'm more confused.

The main reason a 383 produces more torque than a 355 at low-mid rpm is because it's 7.9% BIGGER, not because it has a 7.8% longer lever arm. Given the same flow capabilities, hp will be about the same, BTW.

 

Agreed. That is my preference too.

Agreed. WHich just frustrates my tuning attempts even more!!!

I say that because I was comparing a stock Y-body LT4 tune (*.lt1) with my stock LT1 tune and the VE for the LT4 was (and I don't have the numbers with me at this entry) definitely lower at 80-100 KPA. However, the graph of the VE curves were a lot smoother. Seems that GM engineers did a lot more testing and tuning and testing and tuning. The Stock LT1 file is crude in comparison.

However, your logic is correct too, regarding the volume of air being pumped, but is that a direct relation to the VE? Also, I apologize for mixing SC and NA. I have an SC, but the gist of my argument is about NA.

I partially agree, but when you say HP will be the same, I think HP needs to be qualified with its modifier, RPM, to understand where the power curve is similar and where is begins to differentiate. (assuming identical cams and timing)

Also are you inferring that a 350 with longer stroke vs bore will produce identical power as a 350 with a wider bore and shorter stoke, or even a square (bore=stroke) dimension? I would be very skeptical to believe that those three designs would have identical power curves (torque/HP/RPM).

 

Longer stroke can contribute to more torque due to the longer arm(more leverage in effect) like a breaker bar compared to a ratchet. Hp for a given cubic inched regardless of bore/stroke will be very similar, if you can rev them to the same RPM, it's peak for points of HP/torque in comparison to RPM will change though. Big bore motors like to rev, and long stroke motors not as much. So no, they will not have identical power curves, they are different motors. I don't think that is what OldStroker was saying, just that for a given cubic inches X amount of power is made.

 

Yep. It pretty much depends on how much air the engine pumps. Using the same heads, manifold, etc. makes the airflow about the same thru the engine.

It's enlightening to investigate different bore/stroke combinations on a sophisticated simulator. One soon discovers that the optimum valve events for a 4.03 x 3.48 (355) are a little different from those needed by the same engine with a 4.03 x 3.75 (383) bore/stroke.

Assuming the 383 produces the same peak hp at a lower rpm than the otherwise identical 355, the 383 is making more torque at hp peak rpm than the 355. The area under the torque curve of the 383 should be greater than the area for the 355 because of the larger displacement.

As far as differences in bore/stroke to get the same displacement with the same heads & intake system, the torque/power curves may be somewhat different, but not all that much. The large bore/short stroke engine probably benefits from a little more head flow because the larger bore may cause less shrouding of the valves, and the lower piston speed for a given rpm reduces friction losses. If you optimize the three combinations, say 4.33 bore x 3.25 stroke, 4.03 bore x 3.75 stroke and 3.90 bore x 4.00 stroke, using the head flow when tested on those bore sizes, you might be surprised. Not everyone quotes head flow numbers for various bore sizes so you might actually have to flow heads to do the comparo.

If you carry it to the extreme, like you were actually going to build the engines, it gets trickier. For example, if the engine is port flow restricted (stock LT heads on a 383) you might find some help in rod length, which can make the engine think there is more port there. It's not an easy comparo to make accurately, but it is fun.

 

I used to think the same thing until i began reading David Vizard's books and the tests he performed. Given the same displacement, using different bore and stroke combinations, his engine dyno tests showed there was VERY little difference, almost nil, in the torque curves (in both shape and values along the rpm range). He mentioned that longer strokes increase piston speeds, and increased piston speeds increase frictional drag between the rings and cylinder wall... effectively canceling out the effect of the longer moment arm. He also mentioned the bore diameter effects that OldSStroker mentions (valve shrouding, time for flame propagation, etc).