increased stroke effect on piston dwell near TDC?

[pastry_chef]

I've seen on a number of forums and indeed in an article in a magazine people saying that increased stroke (other variables the same) will increase piston dwell near TDC.

However, a few days ago I created a speadsheet calculator for this type of info. And my calculator results are implying the the exact opposite. My calculator is telling me that an increase in stroke (same rod length) will DECREASE the time the piston spends near TDC.
I have verified my calculator with the data on a chart on isky cams website made to show con rod length changes. So I don't think my calculator is off.

Looking for opinions, on what the truth is.

[bill jones]

-I think it's pretty simple.
-You have 180 degrees or common units of measurement from top to bottom and if you had a 3 inch stroke that means the piston had to move at a rate of .0166667" average per degree and if you had a 4" stroke you have to move the piston .0222222" average per degree.

[Harbinger]

Decrease the rod/stroke ratio and the piston velocity and acceleration in relation to TDC increase.

Regards,
Chuck

[pastry_chef]

Cool. So 1.5 votes say I am right and Marlan Davis is wrong.



Not quite that simple Bill, "averages" do not apply here.

[ozrace]

Small Block Chev with std (3.480") stroke:-

Piston postion @ TDC @ 4 degrees ATDC @ 8 degrees ATDC
.000" .006" .022"

Same engine with 4.000" stroke:-

Piston postion @ TDC @ 4 degrees ATDC @ 8 degrees ATDC
.000" .007" .026"

[bill jones]

-heres a couple more examples near TDC.

#1-350 SBC 3.48" stroke, 5.700" rod,= 1.638r/l ratio, at 10 degrees = .032", at 20degrees = .134", at 30degrees= .295", at 40degrees = .514", at 50 degrees =.765".

#2-400 SBC 3.75" stroke, 5.565" rod = 1.484r/l ratio, at 10 degrees = .034", at 20degrees = .145", at 30degrees = .329", at 40degrees =.564", at 50 degrees = .858".

#3-400 SBC 3.75" stroke, 5.700" rod = 1.520r/l ratio, at 10 degrees = .036", at 20degrees = .149", at 30degrees = .328", at 40degrees= .568", at 50 degrees = .836".

#4-454 BBC 4.00" stroke, 6.135" /rod = 1.534r/l ratio, at 10 degrees = .035", at 20 degrees = 157", at 30degrees = .342", at 40 degrees= .593", at 50 degrees = .995".

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-So if you are talking about the first 5 degrees or the first 40 degrees makes a huge difference in where the piston is.
-If you want to really see a difference check the differences near BDC because that has as much or more to do with making power than all the fuss at TDC.

[onovakind67]

Are the numbers posted for examples #2 and #3 correct? I would think that the curves wouldn't cross like that.

[bill jones]

-I have just now corrected that one odd number which you found to be wrong, it was for at 50 degrees instead of 40.

[onovakind67]

Why does the 5.7" rod on the 3.75" stroke appear deeper in the hole at 10, 20 and 40 degrees but remains shallower at 30 degrees? I would think that the 5.565" rod would position the piston deeper in the hole.

[bill jones]

-I think it's all about the fact the wrist pin centerline is .135" lower with the short rod, so the angles between the pin & the rod journal throw, and between the rod journal to the main centerline are slightly different.

[onovakind67]

How would the piston pin location affect the position of the piston relative to its peak travel? If I had a 10.0" deck and a piston with another inch of compression height would it follow the same curve as a piston in a 9.0" deck?

I have a well-worn and closely guarded copy of Larry Meaux's Pipemax V3.00 and one of the features is that it calculates piston position at 17 different crank angles, one of which is 30 degrees. For the 3.75" stroke crank and a 5.7" rod it puts the piston down .3288". For the 5.565" rod the program calculates it down .3307". It shows the shorter rod puts the piston down farther at all crank angles up to 180. Is this wrong?

[bill jones]

All I can say is the numbers I posted here is what I measured using a dial indicator and a degree wheel.
-If you don't agree with them maybe you could check an engine and see if you get something other.
-One thing that the computer programs don't take into consideration is the several thousandths clearances between the rod bearing and the crank journal, nor the thou and 2/10ths at the wrist pin and the rod, nor the thou and 2/10ths at the pin to the piston bores, and these programs don't consider the piston to cylinder wall clearances etc.
-The engines I tested to get these numbers I didn't record any of that as part of the excersise as I was just looking for "trends".

[Rick360]

For the 3.75" stroke crank and a 5.7" rod it puts the piston down .3288". For the 5.565" rod the program calculates it down .3307". It shows the shorter rod puts the piston down farther at all crank angles up to 180. Is this wrong?

onovakind67, Your numbers are right on what I calculate. There could definitely be a difference between measured and calculated since the diff is at most .008" @ 90deg.

The shorter rod will be further down (calculated) everwhere except TDC & BDC. It gradually gets a bigger difference until at 90deg where the shorter rod is 0.008167" lower. Then the longer rod starts catching up and finally is even again at BDC.

Doesn't hardly seem like enough to matter.

Rick

[onovakind67]

I agree it doesn't make much difference. I just looked at the numbers posted and they didn't make sense to me. Like Bill Jones, I just looked at trends and saw some things that raised some questions, and if there are questions to be asked I ask them. I certainly appreciate the answers.


#4-454 BBC 4.00" stroke, 6.135" rod = 1.534r/l ratio, at 10 degrees = .035", at 20 degrees = 157", at 30degrees = .342", at 40 degrees= .995".

Another trend I noticed is that the 454 piston seems to drop like a rock after 30 degrees.