Punto 1.9 Stroker turbo

[Kranked]

Hi,
I am now building a 1.9 stroker Punto GT and would like to get details from the connecting rods.
I am using a modified 176A9000 Sporting engine block, Wössner stroker pistons and Stilo crankshaft. My quiestion is what is the safe line of the stock Sporting conrod or maybe the GT is stonger? The rev limit will be max 7000rpm.. Any ideas?

Thank you very much!

K.

[WhizzMan]

Do those pistons allow the use of stock parts to increase the stroke in an existing engine? What engine do the con rods come from and if that's the engine where the crank comes out of, what is the weight of the stock pistons on that compared to the new pistons? What is the rev limit on the rods in the engine they came out of?

[Kranked]

Do those pistons allow the use of stock parts to increase the stroke in an existing engine? What engine do the con rods come from and if that's the engine where the crank comes out of, what is the weight of the stock pistons on that compared to the new pistons? What is the rev limit on the rods in the engine they came out of?

Hi,
yes, those pistons made for the stoker 1.9 engine with stock con rods, and in this case I get 8:5 CR. The con rods are from Sporting 176A9000 engine and I used them with 7600 rev limit in my 1.6 Turbo. The weight of the pistons I don't know maybe someone can tell it.

[WhizzMan]

Usually, con rods and con rod bolts can handle a certain weight at a certain rpm at a certain stroke. You will need to know the data for the original setup in which they were used to be able to tell anything about the new setup. Either that, or do a lot of (destructive) tests and calculations to find out. That's why I'm asking about these. In general, if the stroke doesn't change and the pistons get lighter, the rods should be able to deal with a higher RPM. If the stroke increases, the RPM will get lower, all other things being equal. Do you still have the original pistons (with pin and rings) and the new piston so you can weigh? Ask Woessner perhaps?

[Guy Croft]

Sorry to be blunt but it's pointless trying to deduce the 'safe limit' or probable life of production rods. The chances of a race rod ever breaking in a modified production motor are pretty remote but as for prod rods you have to test the limits yourself.

Production rods can break for a variety of reasons (most of which are unpredictable) and whilst in the past I have used them way outside their design envelope, these days with the vastly increased cost of everything I would avoid them on a long stroke motor (79mm or more). You can do stress calcs on them all day long but it does not guarantee zero breakage. I know this for a certainty. Failure could easily be from some metallurgical defect.

All you can say about production rods is that if they are NEW when fitted and you race them at up to say, 7500 for 12 races - then they are safe for 12 races. They may break on the 13th event. This is how I worked out the life cycles for race engine parts in my first book - run them till they break (due to time or rpm) or state the longest period for which I ran them and they did not break and change them at that point. You can figure out for yourself which method is likely to be more costly.


GC

[WhizzMan]

For all practical purposes, Guys method probably works best. Especially if you do a lot of modifications and you're billing by the hour, or on a set budget, you don't want to spend too much effort on this for a customer. However, I think that for a hobbyist willing to spend time and only making a single modification, it may be possible to do some calculations that have merit. If you keep to factory specifications of your con rods and bolts, you should have some form of predictability on how likely they are to fail. If you know the factory spec for forces on the rods and bolts, you could try and calculate the forces in your revised setup. With only one factor changing (piston weight in this case) it might be an exercise that will give a useful outcome. There will still be math involved, but it should be manageable. This way, you should get a relatively safe margin for max RPM. You could probably run the engine faster, but simple calculations will not be able to tell you how much.

[timinator]

If you keep to factory specifications of your con rods and bolts, you should have some form of predictability on how likely they are to fail.

Hi WhizzMan, I share your need to understand what is logical or predictable when building an engine. My life experience has tempered the mechanical engineering education certainty I once had. If you add 10% more power to an engine, and drive the car once, the next time you drive the car it will be impossible the notice the difference. Most likely that 10% will be from a cam change that raised the peak rpm of the engine. The chances that the engine will now be driven at an overall higher rpm range are good. The stress the engine now sees will be more than the 10% that one would think. Now throw a missed shift at redline in just for fun. Or maybe passing another car after a long hot pull up a steep grade by dropping two gears and not noticing that you went 1000rpm over redline. If one uses this logic then putting together the best possible long block is not that hard to justify.

Keep in mind the rod or bolt does not have to break for the engine to fail. Stock rod stretch can fail a bearing before the cyclic loading breaks the beam. Have seen plenty of big ends that needed re-sizing because they were not round, or were tapered , or barreled if indeed they did not start that way.

I have spent countless hours grinding, polishing, and shot peening magnafluxed stock rods. Then replacing the rod bolts and finishing with a re-size before sending them out for final balance. What a waste of time and money. One can buy a far better rod for a reasonable price from many vendors who can specify a rod based on the max rpm the engine will be operated in. GC would be happy, I'm sure, to either sell or advise anyone on what he has proven to work.

WhizzMan I know that you can get a 10% gain in power without changing any parts in the engine. Doing a good valve job with valve detailing, cleaning up the bowl, good valve springs, and degreeing in the cam will show a predictable power gain.

Tim

[WhizzMan]

All I'm saying is that the forces on con rod bolts and con rods will be the same if the same weight of the piston and the same stroke will be used. You can calculate the force on the rod and bolt if you only change the weight of the piston. What forces you will get if you increase RPM is a totally different matter. You can still calculate those, but dropping 10% in piston weight will almost certainly not give you 10% more RPM with the same forces. Sure, new rods and bolts might get you more RPM than stock rods and bolts, but that wasn't the question. Regardless, without full data you can't even start calculating or even estimating where the redline will be for the rods/bolts, so the only real answer is "nobody knows".

[Guy Croft]

For those who really want to get into this, this stress calculator may of interest. Based on a very good data table originally set up by Rob Giraud and kindly shared with me I wrote this some years ago and have evaluated numerous setups with it - I think it's reasonably dependable...

GC

Postscript: this particular 'model' as you can see is based on race pistons on the Integrale bolt-only rod. FWIW a rod of that type indeed failed in an NHRA unit at 8200rpm after a couple of laps of oval on test. The failure was attributed (after metallurgical tests on the remnants) to inadequate heat treatment (carburising) and fracture occured in the region under the pin . The 'k' factor is there partly to adjust for unpredictable phenomena like that.

[WhizzMan]

Very nice of you to share this Guy.

[timinator]

What forces you will get if you increase RPM is a totally different matter.

Hi WhizzMan, as far as the rod is concerned increasing the rpm or increasing the stroke are the same thing. The piston speed will increase and the piston acceleration will increase. If you use the same length rod in both cases the angularity will increase in the stroked engine adding further stress to the rod and cylinder wall. Torque output will also increase. Added oil and water temp too. Shorter piston compression height will place the ring pack higher up in the bore which with a turbo is not that desirable.

My attempt to use added rpm to make my point was not successful it seems, sorry.

When GC said sorry for being "blunt" I knew what that meant to me. I won't assume what it meant to him since I don't seem to be good at assuming. What it meant to me was that I have covered this topic to my satisfaction and choose not to participate.

If you note the k factor for the rod bolts it is 1.5. For the stock rod it is 2.5. The bolt is made from a clean metal with an easily tested size and shape. The rod is not and therefore has a much greater chance of imperfection caused failure.

The idea of limiting rpm by electronic means presents it's own problems. There are noted engine builders who have commented on engine failure due to their use. I would consider this problem another un-considered stress. The need to maintain a safe rpm limit due to the use of a less than optimum rod seems to be making the change pointless to me.

I also apologize for being blunt.

Thankyou GC for the calculator. Your generosity has no limit.

Tim

[Kranked]

Thanks for the advice and for the share,Guy!
If I can I will measure of the weight of the new and the original pistons and I will share it!
After building I share my pictures!

Thanks a lot for any reply!

[Guy Croft]

Tim - nice explanation of k factor

but .. acceleration (that generates the reciprocating stresses) is given by the formula shown below. It varies as the square of crank angular velocity (omega) and according to the linear ratio of rod and stroke (in effect). Speed therefore is a much more significant factor.

n in the equation is L/R (rod length/crank radius) where of course crank radius is 1/2 stroke..

Me being blunt FWIW is merely my explaining my brief intervention where I personally do not wish to dwell on a lengthy explanation. Nothing personal at all. Nothing to stop others commenting though.


G

[timinator]

but .. acceleration (that generates the reciprocating stresses) is given by the formula shown below. It varies as the square of crank angular velocity (omega) and according to the linear ratio of rod and stroke (in effect). Speed therefore is a much more significant factor.

Yes GC I believe you caught me guilty of loose talk as I think you describe it.

Took your calculator for a test drive with an engine I am familiar with, a Mitsubishi 2.0L. Ran calculation at 7k rpm, then 10% higher rpm, and finally with stroked 2.3L configuration at 7k rpm. 2.0L had max piston velocity at 76 deg. while the 2.3L occurred at 74 deg. The 2.0L at 10% higher rpm produced a greater piston pin load and rod acceleration than the 2.3L stroker. When the 2.0L is 8.2% higher rpm, 7577 rpm, the 2.0L and 2.3L had the same piston pin load and rod acceleration.

Thank you again for the calculator, and more to the point, for the time you put into this forum.

Tim