Don Terrill’s Speed-Talk

Mike, in the Engine Masters Rod Ratio Test Results post you said:

Normally the higher the rod/stroke ratio, the shorter the seat duration needs to be for both the intake and exhaust profiles, and the intake needs to close earlier, and the exhaust needs to open later. Intake opening and exhaust closing don't change much, if at all.
For example, if you had a had a 12:1 358" SBC, that came off the corner at 4,500rpm, and turned 7,200rpm at the end of the straights, with a 5.7 rod, you may run a 296/300 @ Seat with a 102 ICL and a 110 ECL. If you were switching over to a 6.125" rod in that same engine and gearing, you would go to something around a 292/296 @ Seat, with a 100 ICL and 108 ECL.

In previous posts you have commented:

"I calculate seat duration from Stroke, Rod Length, and Max HP RPM"

"At a given optimum port velocity, seat duration is directly related to Stroke and Max HP RPM" - but no mention of r/s ratio

So looking at the intake lobe only:

If duration is directly related to stroke and max hp rpm (time based no matter what the r/s ratio) then how does rod/stroke ratio affect the duration?

Is the change in duration based off set rod/stroke ratio changes - like a 10% r/s ratio change likes + - 2 degrees duration change and 20% likes + - 4 degrees etc

If I am focusing on making the most power, over the usable RPM band, then the rod/stroke ratio effects seat duration.
Like in my example, I was looking at making the most power from 4,500 to 7,200rpm, and not just focusing on 7,200rpm.

If I'm only focusing on power at max HP RPM, then the seat duration wouldn't change with a rod/stroke ratio change, but the shape & area of the lobe would. That would change the duration at .050", .200", and other lift points.

CamKing wrote: ↑Fri Mar 20, 2020 10:44 am If I am focusing on making the most power, over the usable RPM band, then the rod/stroke ratio effects seat duration.
Like in my example, I was looking at making the most power from 4,500 to 7,200rpm, and not just focusing on 7,200rpm.

If I'm only focusing on power at max HP RPM, then the seat duration wouldn't change with a rod/stroke ratio change, but the shape & area of the lobe would. That would change the duration at .050", .200", and other lift points.

so Mike, speaking in broad generalities, a smaller port volume will support a given power level at a given rpm in an engine with a longer R/S ratio than an engine with a lower R/S ratio, is that correct? So can we link these factors together, and say that a longer rod ratio will make power at a given rpm with a slightly smaller intake port CSA AND slightly less duration than a shorter R/S ratio engine? I realize there are other factors involved, but speaking more in general trends than specifics, is this accurate?

So if we had a situation where an engine was intake port limited (say rules that required a spec head, but didnt specify rod length?), it will tend to be 'happier" with a longer rod?

falcongeorge wrote: ↑Fri Mar 20, 2020 4:03 pm

CamKing wrote: ↑Fri Mar 20, 2020 10:44 am If I am focusing on making the most power, over the usable RPM band, then the rod/stroke ratio effects seat duration.
Like in my example, I was looking at making the most power from 4,500 to 7,200rpm, and not just focusing on 7,200rpm.

If I'm only focusing on power at max HP RPM, then the seat duration wouldn't change with a rod/stroke ratio change, but the shape & area of the lobe would. That would change the duration at .050", .200", and other lift points.

so Mike, speaking in broad generalities, a smaller port volume will support a given power level at a given rpm in an engine with a longer R/S ratio than an engine with a lower R/S ratio, is that correct? So can we link these factors together, and say that a longer rod ratio will make power at a given rpm with a slightly smaller intake port CSA AND slightly less duration than a shorter R/S ratio engine? I realize there are other factors involved, but speaking more in general trends than specifics, is this accurate?

So if we had a situation where an engine was intake port limited (say rules that required a spec head, but didnt specify rod length?), it will tend to be 'happier" with a longer rod?

I never read him say anything like that unless it was in another thread; only that the required lobe profile will be "different".
You sure seem to be presuming an awful lot here.

Walter R. Malik wrote: ↑Fri Mar 20, 2020 6:46 pm

falcongeorge wrote: ↑Fri Mar 20, 2020 4:03 pm

CamKing wrote: ↑Fri Mar 20, 2020 10:44 am If I am focusing on making the most power, over the usable RPM band, then the rod/stroke ratio effects seat duration.
Like in my example, I was looking at making the most power from 4,500 to 7,200rpm, and not just focusing on 7,200rpm.

If I'm only focusing on power at max HP RPM, then the seat duration wouldn't change with a rod/stroke ratio change, but the shape & area of the lobe would. That would change the duration at .050", .200", and other lift points.

so Mike, speaking in broad generalities, a smaller port volume will support a given power level at a given rpm in an engine with a longer R/S ratio than an engine with a lower R/S ratio, is that correct? So can we link these factors together, and say that a longer rod ratio will make power at a given rpm with a slightly smaller intake port CSA AND slightly less duration than a shorter R/S ratio engine? I realize there are other factors involved, but speaking more in general trends than specifics, is this accurate?

So if we had a situation where an engine was intake port limited (say rules that required a spec head, but didnt specify rod length?), it will tend to be 'happier" with a longer rod?

I never read him say anything like that unless it was in another thread; only that the required lobe profile will be "different".
You sure seem to be presuming an awful lot here.

Never DID say he said it, theres a question mark at the end. Mike, just disregard it, it seems to have upset Walter. Not worth the hassle, I have kids at home, don't need to deal with them on the internet...

Thanks Mike for your reply. I now understand how you relate r/s ratio to lobe design

falcongeorge wrote: ↑Fri Mar 20, 2020 8:43 pm

Walter R. Malik wrote: ↑Fri Mar 20, 2020 6:46 pm

falcongeorge wrote: ↑Fri Mar 20, 2020 4:03 pm

so Mike, speaking in broad generalities, a smaller port volume will support a given power level at a given rpm in an engine with a longer R/S ratio than an engine with a lower R/S ratio, is that correct? So can we link these factors together, and say that a longer rod ratio will make power at a given rpm with a slightly smaller intake port CSA AND slightly less duration than a shorter R/S ratio engine? I realize there are other factors involved, but speaking more in general trends than specifics, is this accurate?

So if we had a situation where an engine was intake port limited (say rules that required a spec head, but didnt specify rod length?), it will tend to be 'happier" with a longer rod?

I never read him say anything like that unless it was in another thread; only that the required lobe profile will be "different".
You sure seem to be presuming an awful lot here.

Never DID say he said it, theres a question mark at the end. Mike, just disregard it, it seems to have upset Walter. Not worth the hassle, I have kids at home, don't need to deal with them on the internet...

"Is that correct ?" is the question after your statement which has the appearance that you want to put a meaning where there is none.

But, you can keep thinking that you are not attempting to incite other people to think it was somehow said. it does appear that way.

Have a nice day ... sorry I challenged your statement; er question. Rest assured, I am not upset at all and really don't care.

Walter R. Malik wrote: ↑Sat Mar 21, 2020 10:44 am

falcongeorge wrote: ↑Fri Mar 20, 2020 8:43 pm

Walter R. Malik wrote: ↑Fri Mar 20, 2020 6:46 pm

I never read him say anything like that unless it was in another thread; only that the required lobe profile will be "different".
You sure seem to be presuming an awful lot here.

Never DID say he said it, theres a question mark at the end. Mike, just disregard it, it seems to have upset Walter. Not worth the hassle, I have kids at home, don't need to deal with them on the internet...

"Is that correct ?" is the question after your statement which has the appearance that you want to put a meaning where there is none.

But, you can keep thinking that you are not attempting to incite other people to think it was somehow said. it does appear that way.

Have a nice day ... sorry I challenged your statement; er question. Rest assured, I am not upset at all and really don't care.

That is just pathetic.

upsidedown wrote: ↑Fri Mar 20, 2020 11:19 pm Thanks Mike for your reply. I now understand how you relate r/s ratio to lobe design

The confusion is partially my fault.
I use the word "design" when I'm talking about designing the actual lobe profiles for an engine, but also use it when I'm talking about choosing existing profiles for a given application.

CamKing wrote: ↑Fri Mar 20, 2020 10:44 am If I am focusing on making the most power, over the usable RPM band, then the rod/stroke ratio effects seat duration.

Mike, would you please explain how r/s would affect seat duration? Or any other valve train activity?

And, how does r/s ratio affect other aspects of engine design, such as lift, duration, rpm, combustion?

If useful, I could calculate almost any engine parameter (position, velocity, acceleration,,,), for a specific engine for you.

David Redszus wrote: ↑Sat Mar 21, 2020 1:14 pm

CamKing wrote: ↑Fri Mar 20, 2020 10:44 am If I am focusing on making the most power, over the usable RPM band, then the rod/stroke ratio effects seat duration.

Mike, would you please explain how r/s would affect seat duration? Or any other valve train activity?

And, how does r/s ratio affect other aspects of engine design, such as lift, duration, rpm, combustion?

If useful, I could calculate almost any engine parameter (position, velocity, acceleration,,,), for a specific engine for you.

Simply put.
If you change the R/S ratio, you change the piston motion curve.
If the piston motion curve changes, you change volume requirement curve.
If the volume requirement curve changes, the valve lift requirement curve changes.

I was farting around with an earlier version of Controlled Induction yesterday and a change in rod from 5.7 to 6.125 left the seat duration alone but changed the duration further up between a degree and a degree-and-a-half depending on what lift number you looked at. It didn't change the lobe centers and I don't think it changed total valve lift but apparently I threw my little note that had the numbers away.

Edit: Found it. The shorter rod called for a hair more duration with no change in total valve lift nor lobe centers. A half-a-degree at .050 up to 1.5 degrees at .400 lifter rise.

For me the change in cylinder volume is what helps generate depression.

4.06" bore, 3.75" stroke

5.7" rod green line (SBC)

6.625" rod red line (Pontiac)

Stan

RevTheory wrote: ↑Sat Mar 21, 2020 3:09 pm I was farting around with an earlier version of Controlled Induction yesterday and a change in rod from 5.7 to 6.125 left the seat duration alone but changed the duration further up between a degree and a degree-and-a-half depending on what lift number you looked at. It didn't change the lobe centers and I don't think it changed total valve lift but apparently I threw my little note that had the numbers away.

Edit: Found it. The shorter rod called for a hair more duration with no change in total valve lift nor lobe centers. A half-a-degree at .050 up to 1.5 degrees at .400 lifter rise.

That's why I don't use C.I.
Of course, the lobe centerlines would change, with a change in R/S ratio.
Look at Stan's graph, and tell me how it couldn't.

Stan Weiss wrote: ↑Sat Mar 21, 2020 3:54 pm For me the change in cylinder volume is what helps generate depression.

4.06" bore, 3.75" stroke

5.7" rod green line (SBC)

6.625" rod red line (Pontiac)

Stan

ab-cvc-389-57-6625.gif

Stan
That is a very large difference in rod length and ratios; 1.52 vs 1.77.

As your graph clearly shows, the shorter rod moves the piston faster prior to 90 deg ATC, but slower after 90 deg ATC.
What the shorter rod gains before 90 deg ATC, it gives back after 90 ATC. How does this reflect upon any possible difference in air flow?

Since the volume is determined by the piston area (which is fixed) and the piston position, we can simply look at
the differences in vertical piston and calculate the respective areas under the curve.

The shorter rod has a larger area under the curve (deg * position) than does the longer rod; but the difference is only 1%.

The maximum difference in piston position between the two rods occurs at 90 and 270 deg and amounts to 0.045".
At 20 deg +- TDC, the difference in position is only 0.005".

The most interesting of all are the piston acceleration curves near TDC and BDC. What is the airflow effect of a
double humped acceleration curve at BDC?

Have you looked at the effects of pin offset on piston motion with regard to stroke, rod, etc?