Driving feel differences between 390, 427, 428, and strokers

[scott foxwell]

What you're seeing is a result of rod/stroke ratio and piston acceleration speed.

[My427stang]

What you're seeing is a result of rod/stroke ratio and piston acceleration speed.

Yes on piston acceleration, but no on rod ratio, the part my simple spreadsheet doesn't account for in any way is the rod. It is assuming the piston and rod drop straight down. 

However, it shows everybody exactly what you are saying and I agree with you completely.  I just don't have the math skills to account for rod angularity in a spread sheet. 

[mbrunson427]

Ross, I did something similar a while back. I have a bore/stroke spreadsheet that I always use for my bench racing. I just punched in a 390/428/445 so you can see the piston velocity graphs vs each other. It's kind of interesting. Note, the rod length does not affect max piston velocity, it affects when the piston reaches that velocity. I didn't graph those differences, I can if you're interested. I guess I could actually take the same data I have in the sheet and make a similar graph for total air displaced and see what that looks like, I might try that.

The piston velocity on the 390/427 maxes out about 1 degree later than it does in the 428 and 445 combinations.

390


428


445 Stroker

[mbrunson427]

What you're seeing is a result of rod/stroke ratio and piston acceleration speed.

I just don't have the math skills to account for rod angularity in a spread sheet.

I can send you this sheet if you care for it.

[scott foxwell]

Ross, I did something similar a while back. I have a bore/stroke spreadsheet that I always use for my bench racing. I just punched in a 390/428/445 so you can see the piston velocity graphs vs each other. It's kind of interesting. Note, the rod length does not affect max piston velocity, it affects when the piston reaches that velocity. I didn't graph those differences, I can if you're interested. I guess I could actually take the same data I have in the sheet and make a similar graph for total air displaced and see what that looks like, I might try that.

The piston velocity on the 390/427 maxes out about 1 degree later than it does in the 428 and 445 combinations.

390


428


445 Stroker

Rod to stroke ratio definitely effects peak piston velocity.

[mbrunson427]

Rod to stroke ratio definitely effects peak piston velocity.
[/quote]

Not near as much as you'd think. Almost negligible, compared to the affects that stroke has on it. Rod length is more diligent at shifting the point of max velocity, changing the acceleration rate. Not saying that it doesn't affect it at all.....

Note that the longer rod slows down the piston SLIGHTLY, but pushes the max velocity point back almost half a degree, which would slow the acceleration rate.

4.25 stroke w/ 6.49" rod


4.25 stroke w/ 6.7" rod

[plovett]

I think you guys are looking at the wrong variables.  Piston speed and acceleration, bore/stroke ratios, rod/stroke ratios certainly affect things.  But the effect is inconsequential compared to other things.  The biggest effect of bore/stroke ratio is on breathing.  And that is really just about the bore size, not the ratio.  A bigger bore allows bigger valves which allows a bigger port which allows better breathing at all lifts.

I would say we're comparing grains of sand to a boulder.

JMO,

paulie

[My427stang]

I think you guys are looking at the wrong variables.  Piston speed and acceleration, bore/stroke ratios, rod/stroke ratios certainly affect things.  But the effect is inconsequential compared to other things.  The biggest effect of bore/stroke ratio is on breathing.  And that is really just about the bore size, not the ratio.  A bigger bore allows bigger valves which allows a bigger port which allows better breathing at all lifts.

I would say we're comparing grains of sand to a boulder.

JMO,

paulie

Paulie I was only looking at the rate of change in displacement as the piston goes down.  As stated it doesn't talk about the valve, just the depression caused by how fast that space grows.

However, I am not sure that I agree with the bigger valve bigger port comment.  It's true no doubt, but I want the smallest cleanest port I can for the use of the engine.  If I have enough port for the 428 on a 4.13 bore, all I am doing is losing velocity in that port if go bigger on it with the 427 (for the same end use)  of course I could spin it higher and make more power, but we'd have to discuss what we are trying to do with each engine too.  I am still on the "if I built a 390, 427, 428, or 445 for the same RPM range" what would I expect?

What you're seeing is a result of rod/stroke ratio and piston acceleration speed.

I just don't have the math skills to account for rod angularity in a spread sheet.

I can send you this sheet if you care for it.

That would be great, My427stang   at   cox  dot net

[scott foxwell]

Rod to stroke ratio definitely effects peak piston velocity.

Not near as much as you'd think. Almost negligible, compared to the affects that stroke has on it. Rod length is more diligent at shifting the point of max velocity, changing the acceleration rate. Not saying that it doesn't affect it at all.....

Note that the longer rod slows down the piston SLIGHTLY, but pushes the max velocity point back almost half a degree, which would slow the acceleration rate.

Actually I mis-worded that. I should have said it effects piston acceleration, not velocity. The acceleration rate is what effects the induction.

[scott foxwell]

I think you guys are looking at the wrong variables.  Piston speed and acceleration, bore/stroke ratios, rod/stroke ratios certainly affect things.  But the effect is inconsequential compared to other things.  The biggest effect of bore/stroke ratio is on breathing.  And that is really just about the bore size, not the ratio.  A bigger bore allows bigger valves which allows a bigger port which allows better breathing at all lifts.

I would say we're comparing grains of sand to a boulder.

JMO,

paulie

Bore size is arbitrary. If you're dealing with a two valve head; one valve, one port, then yes. Bore size will be the limitation. There are some very small bore engines with some very deep breathing multi valve heads on them that make very good power at some very high rpm's. It's all about piston speed. Piston speed dictates the demand. Demand dictates the induction. It's all math.

[Barry_R]

I will point out a single set of pretty clear observations.

These are directional - not absolutes - but are pretty dang well proven.

First is that displacement always wins.
Its "air in - air out" no matter what.
More air in means more fuel can be consumed and more fuel equals more potential power.
Only way for more air in a defined architecture, in a defined time period is a bigger engine or more RPM

In a two valve engine you are always valve limited.
We are only talking two valves because that is what we are building.
If - IF - we "fix" the displacement, a bigger bore allows a bigger valve, and will be the winner.
I do not think its all about piston speed.  I think its about the displacement delta.  The head does not see the piston - it sees the change in volume.
In every single racing category where displacement is limited, they eventually end up at the maximum possible bore, the largest valve that will be functional (not the largest that will mechanically fit - but the largest that will work properly), and then go for the highest possible RPM.

Same path has been followed in NASCAR as in Pro Stock - or F1 for that matter.  Only way that changes is when durability, rules, or checkbooks interfere.  You cannot really use current OE development as a gauge because they are wrestling with packaging, weight, fuel economy and emissions as "rules".  A large bore provides greater opportunity for emissions issues due to crevice volume and the amount of working surface that gets wetted in the cylinder among other things.

Take all this stuff and translate it into street FE engines and you bump up against limitations that have nothing to do with the theoretical.  Here we simply end up with an arbitrary RPM range (say 6000ish), existing head castings, pump gas, rationally priced parts, and available blocks in an aged architecture.  When you go to that place - displacement wins.  If you put a working mans budget into the equation displacement wins easily. 

[chris401]

Considering all the above information I am interested how this would scenario play out. Slight over bore 427 vs 428. Pick any vehicle, cam, gearing, intake, head, compression ratio, valve, ect., OR dyno OR calculator. Identical build with only the bore and stroke changing.

Compare the power and rpm ranges of:


"428" 4.13                   × 3.98 = 426.5442180 CI

"427" 4.2378510454 × 3.78 = 426.5442180 CI

[chris401]

Considering all the above information I am interested how this would scenario play out. Slight over bore 427 vs 428. Pick any vehicle, cam, gearing, intake, head, compression ratio, valve, ect., OR dyno OR calculator. Identical build with only the bore and stroke changing.

Compare the power and rpm ranges of:


"428" 4.13                   × 3.98 = 426.5442180 CI

"427" 4.2378510454 × 3.78 = 426.5442180 CI

Probably best to use a flat top piston with no valve reliefs.

[cammerfe]

I can assure you that, 'way back when', I swapped the 390 engine in my '67 Cougar XR7 GT for a 427, (it would have been in the latter half of '68), the difference in the 'feel' of the car, and the fun factor, were all out of proportion to the additional 37 cubic inches.

KS

[427Fastback]

Just some thought.My friend who recently passed had/has a 69 Mach I SCJ auto that I looked after for over 25 years.I long ago pulled the 3.91's and put in 3.50's.Engine has a duraspark ign,blue thunder intake and a 268H cam.Both have well tuned Holley carbs

My car (1968) with the 427.Same gears,same ign,same exhaust with a 270S comp cam.C7-F intake.Very similar set up but my car is a 4spd.Most important..same engine tuner (Me)

Very similar power but the 427 would easily out rev it.The 428 would at no point out torque me.Nor was it capable of beating my car as I simply would out rev it..That being said I certainly was not able to simply out run him.

Now my car is a 4spd and his is a 3spd auto.As it is a SCJ its internal rotating mass is much more than mine as the SCJ has Lemans rods.A standard CJ would probably spin up faster.If both cars had the same trans I doubt anyone would notice the difference other lifter noise..

In a year or two when its finished I would be interested in driving my friends 68 fastback CJ 4spd car and see how it feels compared to mine..

JMO.....Cory