V8_DSM_V8again

I could not disagree more..

Also terms like "seems closer to" are ho hum.. Also which method of measuring piston speed. Mean piston speed has been used alot but is an inaccurate method as compaired to methods like EAPro's piston G forces calculation...

I also opened up the files and the 88mm is a 87mm bore and the 100mm a 85.5mm bore.. I was simulating a 2.0 crank in a 2.4 block (6mm more deck) with 12mm longer rods (6mm wrist pin move) vs a 2.4 crank in a 2.0 block with oe size rods.

Again its all wrong.
Neither of those graphs were piston speed graphs..

Once again you forget accelaration rates... At the same mean or average piston speed... or even at the same peak piston speed both engines will have different peak G forces and the curves will continue to to have a distinct shape for each engine.

Both of those graphs are not effected by heads, cams etc... Yes real engines and many other graphs in the program are greatly. I just chose the two biggest changes between two engine strokes not related to VE. Head and cam packages can be matched to either. That was'nt the point. The point is pros and cons of stroke..

Piston speed is only part of the picture.. An important one but the actual stresses acting on the connecting rod are more important than some rule of thumb number in feet per second.

Also stroke has the most effect on piston gs, piston speed, piston thrust etc... Rod ratio does to a degree but stroke is what caused the huge differences in the two engines I graphed. I could have put the long rod on the stroker and the short one one other engine . In reality no block would allow this. The simulation the results would still be alot different between the engines.

While I agree that with all things equal more displacement you do not need to work an engine as hard to achieve a given power level.. But all things are not equal and I do not agree that the best way to do it is via the use of a longer stroke. Are'nt too many engines around like the 455 olds stroke or jaguars super long strokes...

With infinate parts availablilty and budget the perfect solution would be 408 cubes with the 346 stroke...

With a properly matched turbo system either displacement can make much more power than people need on the street.

I just choose to limit primarily stroke and to a degree displacement on a street going turbo engine. Not only will I make more HP per Cu/in but I will get better MPG and it will last longer..

racer7088

V8_DSM_V8again,

You just don't get it. Piston speed is not arbitrary. You don't just "decide" to run an engine at a certain piston speed. IT decides if it's gonna run there or not based on how well it breathes there!

You don't shift at a certain arbitrary piston speed. You shift when you start losing power and the car noses over. The heads and cams and intake etc. decide how much piston speed (rpm) an engine can support. The stroke tells you how high in rpm this will be unless you are cut short by valvetrain problems etc. That's just the way it works.

At the same VE and aero limited piston speeds the strokers will have LESS force on them due to reduced rpm period. Just use your formulas and see. I kept the rods all 6.125 to simplify things but if you can do the math you will see that the rod ratio does not even come close to making up for the increases in load due to rpm.

At 5000 FPM average piston speed the "G" force a piston sees on a 408 with a 4.000 stroke and a 6.125 rod is about 3440 "G"s at 6757 rpm.

At 5000 FPM average piston speed the "G" force a piston sees on a 346 with a 3.622 stroke and a 6.125 rod is about 3711 "G"s at 7462 rpm.

At the same 5000 FPM piston speed but with a 2.000 stroke and a 6.125 rod you would have 6034 "G"s at 13514 rpm. So I doubled my rpm but halved my stroke and yet even at the same piston speed with double the rod ratio but double the rpm I now have double the "G" force on my rotating assembly and double the cycles as well so about 4 time the wear and tear!

Can you run the 346 at the same rpm as the 408 and make the same power? YES but you will have to create 18% more cylinder pressure to act on the 18% LESS stroke so you will have 18% MORE sideloading than a regular 346. Now the 408 has LOWER cylinder pressure at this SAME power level and yet does not have an 18% increase in rod angle.

Now just cruising along the 346 may have a little less friction and load but when trying to put out the same power it has more. This is also seen as BSFC on dyno charts and if there really was all this extra friction and sideloading then the larger engines would tend to have or show worse BSFC numbers. Of course in reality it's the other way around with the larger engines especially the strokers always turning in higher BSFC efficiency numbers in the real world. It's not a miracle or a mystery to most in the know.

V8_DSM_V8again

I got it along time ago. Um yes you do decided. What head, cam and induction package you put on it effects the RPM range. You just said that in your next sentance and I said it like in the first post. My entire point however was that there are pros, cons and individual limitations to high rpm short stroke vs strokers. Honestly I think piston speed alone is some played out inaccurate BS.

The average piston speed in FPM is not even close to the same thing as piston g forces.. I dont even consider average piston speed in my rpm limit assesment.. I mean I'll look at it.

In reality I look at the piston g forces as this is what seperates connecting rods.. Its not the 1000's of Feet Per Minute but the sudden quick stops TDC and BDC that streses things..

Rod length also effects how an engine breaths and has a bigger effect on the G forces than it does on piston speed. It smooths out the accel and decelleration which are really what limits the engine RPM.. It is entirely possible to have a 346 runing the same or a lesser piston forces than a 408 at an RPM 1,000 higher. You just need some rod length.. The average piston speed will be higher but.. not to an unsafe level nor does it matter as the actuall forces acting on the rod are the same or less.


This is again why I think average piston speed is a worthless annecdotal factoid.

Actually it has more rod angle. Piston thrust which is what I think you mean is the force of the piston skirt against the bore it is caused by alot of variables. Also things are never linear like that. An 18% cylinder pressure wont cause a perfectly 18% increase in piston thrust. 18% less stroke wont require exacly 18% more cylinder pressure etc.

Hey to each their own...

Untill we each have a combination of tuned parts sitting on a dyno the only thing left to debate is who's *** has the most splinters from bench racing..

I'm sticking to my guns as far as what I want to build...

racer7088

Those numbers were MAX instantaneous numbers in "G"s not averages! The lower the rod ratio the worse the peak loads are for sure. No matter how bad you make the rod ratio with a stroker you still won't load the pistons or pin as high as rpm will by trying to achieve the same pistons speed in the smaller stroke motor. It pushes loads up much much faster and is way more critical on rod stretch and rod bolts etc.

Big inch engines can tool along at higher pistons speeds and lower rpm such as in most endurance apps like the 24 hour C5R Le Mans stuff much more easily than the same piston speed in a higher rpm shorter stroke engine will. That's why they run the big 4 inch cranks in the 24 hour stuff. It cuts the loads way down while still making more power.

This stuff IS dynoed and run at the track all day long! Most of the fast turbo cars in the outlaw world are 4+ inch cranks in small blocks unless there are cube limits like in the NMRA etc. Otherwise they go BIG. Remember the side loading is change at HALF the stroke to rod ratio not the whole stroke. The triangle formed is only the crank pin not the whole stroke. Seriously you need to do more of this math yourself and you will see what is up!

racer7088

V8_DSM_V8again,

Don't take any of this as a flame as it is not intended that way. Go and put the numbers in your spreadsheet and SEE what happens to rod loads and pistons loads at the same piston speed when you destroke an engine. The loads on everything go up.

If you increase power at the same inches and RPM you WILL have to increase cylinder pressure at that point too and you will now have proportionately more sideloading at that point as well. The stroker only a slight amount more angle. This side loading is directly proportional to this cylinder pressure and rod angle.

If strokers ate themselves up so easily and had so much friction they would not make more power and last longer in endurance applications. They also would not have lower BSFC numbers yet they do. This could not happen if the frictional and inertial loads went way up.

A 632 Chevy is more efficient than any of the small blocks and it has a 1.38 rod to stroke ratio and will often brush the .30 BSFC range with good heads. An 800 inch NHRA Mountain Motor will sometimes do even better and is the most efficient gasoline racing engine in the world and has a rod stroke ratio in the realm of 1.33 as well!

Food for thought.

PS. I do see people using so much stroke in turbo apps though that you can NOT get a good piston in the motor and I do not believe in that so I do certainly have limits on the stuff I build. I like stroke but everything has to be strong and fit the block!

V8_DSM_V8again

I knew the G force numbers were peak g forces.

You understand it but others dont. The average piston speed in FPM is just an average and really tells you very little about the forces acting on the connecting rod.

I'm not talking about a 24 hour motor although I am sure it would have no problem lasting even longer than the 4" strokes. There is a stacking effect of intake pulse tuning and piston speed on the NA LSx 4" strokes.

I am talking about a real world pushing the limits of a street ET... With turbos small cams (as compared to NA) keeping the low end like a street NA cam, BB CHRA's and wise turbo sizing you can make great gobs of power with the 346 and sip gas like stock at cruise with great driveability with a stand alone like BS3..

Thats all well and good but considering most peoples budget and few people here will be daily driving an outlaw car I dont know how much of that applies..

For a street turbo car I would run the shorter stroke. You are thinking about a entirely different application.

I am not using some simple spreadsheet.. Its Engine Analyzer Pro.. The same program Ken D uses.

I think a 346 relying on boost and RPM can very streetable. More so with fuel economy being considered. I think you can get more hp for the dollar with the stock stroke. Yes it will last longer than the longer stroke since both will cruise at about the same RPM with a big difference in reciprocating wear between the two..

No a stroker from good parts wont go to a POS fast or anything but the same quality rings for example will wear less when they have to travel 1/2 inch less per stroke at a lower speed.

I just dont think most people building turbo cars need the displacement. If its their first turbo project they should concentrate time and money on other areas.

krazy_ws6

I have a question after reading all of this would the waste gate solve the problem of being over efficent by letting boost out at a certain psi so then the turbo on the bigger cube engine wont be chocking it?

JZ 97 SS 1500

iTrader: (2)

Yes, we have done this on small cubic inch road race cars using CO2 to control the WG. But on a big cube motor, the shear volume of exhaust gases would require a HUGE wastegate.

Jose