Question For Mike Jones - Duration Vs Rod/Stroke Ratio

[Stan Weiss]

David,
What I used was different engine families which could have the sane stroke, but they have about an inch difference in deck height.

Yes I have looked at wrist pin offset, but there is a very small limit that you can do there. Where it really becomes interesting is in inline engines where there has been some large offset of the crank center line. I need to see if I can find some numbers I had where the engine had a 10mm crank center line offset.

Here are a couple of graphs I did sometime ago for a 350 SBC 0.060" offset in both directs and the pin centered.

Stan

ab-ra-350-wpo.gif

ab-sf-350-wpo.gif

[Stan Weiss]

In a conventional four-stroke engine, each of the strokes (intake, compression, power, exhaust) involves a nominal rotation of 180 degrees, totaling 720 degrees for the complete four-stroke combustion cycle. An offset engine adds to the duration of the two downward strokes (intake and power) and subtracts the same amount from the two upward strokes (compression and exhaust), with the total remaining 720 degrees. A typical offset engine will have strokes of 185 degrees, 175 degrees, 185 degrees and 175 degrees, with the degrees as a percentage of offset distance to stroke length.

https://motocrossactionmag.com/ask-the- ... nk-offset/

First let me say if you have 185 175 185 175 or 175 185 175 185 depends on which direction the center line is offset.

That article talks about a YZ450F having a 12mm offset. I know the bore and stroke but had to guess at the rod length.

Note: With an offset the piston and the crank journal will not be ATDC at the same time

Bore_=_97.0___Stroke_=_60.8___Rod_Length_=_97.25___RPM_=_6500
Wrist_Pin_Offset_=_12.0
Rotation_Time_of_crank_per_degree_in_Milliseconds____0.0256410
Rotation_Time_of_crank_per_rev_in_Milliseconds_______9.2307692
Crankshaft_rev's_per_Second__________________________108.3333333
Actual_Piston_Stroke_61.32056303791
Cylinder_Volume___27.652750_CI______453.147392_cc
Engine_Size_______27.652750_CI______453.147392_cc
Crank_Angle_Piston_TDC_-5.39417223358__Piston_BDC_169.65899214486

=================

Bore_=_97.0___Stroke_=_60.8___Rod_Length_=_97.25___RPM_=_6500
Wrist_Pin_Offset_=_-12.0
Rotation_Time_of_crank_per_degree_in_Milliseconds____0.0256410
Rotation_Time_of_crank_per_rev_in_Milliseconds_______9.2307692
Crankshaft_rev's_per_Second__________________________108.3333333
Actual_Piston_Stroke_61.32056303791
Cylinder_Volume___27.652750_CI______453.147392_cc
Engine_Size_______27.652750_CI______453.147392_cc
Crank_Angle_Piston_TDC_5.39417223358__Piston_BDC_190.34100785514


Stan

[pastry_chef]

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.

Thanks Mike. Any general trend between R/S ratio and LSA?

[Walter R. Malik]

I remember a teacher in high school some 45 years ago told us in the class that if the inertia of the intake air/fuel mixture ever catches the piston before BDC ... you are in real trouble as horsepower diminishes rapidly.

I never rally placed a good meaning on that until now. I always thought it was just a matter of using to big of a camshaft but,
actual lift/degree with the same cam timing events could make a difference.

[upsidedown]

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.

Mike in the example above where you are looking for the most power from 4500 to 7200 rpm, how do you choose the rpm point to pick a lobe for duration, lift & lobe centres to cover that range. As you said it isn't at Max HP rpm so where is it?

Of course, the lobe centerlines would change, with a change in R/S ratio

Thanks Mike. Any general trend between R/S ratio and LSA?

Yes good question?

[CamKing]

Thanks Mike. Any general trend between R/S ratio and LSA?

Like mentioned in the first post:
"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."
If the intake opens in the same place, and closes earlier, then the ICL is closer to TDC. If the exhaust opens later, ands closes in the same place, the ECL is closer to TDC. Those two moves would give you a tighter LSA

[CamKing]

Mike in the example above where you are looking for the most power from 4500 to 7200 rpm, how do you choose the rpm point to pick a lobe for duration, lift & lobe centres to cover that range. As you said it isn't at Max HP rpm so where is it?

For a circle track application, like the example. You put peak power below the max engine RPM(7,200) at the end of the straights. How much below, depends on haw far max torque is from the RPM(4,500) off the corner. You want peak torque closer to the corner RPM, then peak hp from the max RPM(unless they can't hook up off the corner). In the example above, if they can hook it up, I would design the cam to make peak torque in the 4,600-4,700rpm range, and peak HP in the 6,600-6,800rpm range.

[Walter R. Malik]

Mike in the example above where you are looking for the most power from 4500 to 7200 rpm, how do you choose the rpm point to pick a lobe for duration, lift & lobe centres to cover that range. As you said it isn't at Max HP rpm so where is it?

For a circle track application, like the example. You put peak power below the max engine RPM(7,200) at the end of the straights. How much below, depends on haw far max torque is from the RPM(4,500) off the corner. You want peak torque closer to the corner RPM, then peak hp from the max RPM(unless they can't hook up off the corner). In the example above, if they can hook it up, I would design the cam to make peak torque in the 4,600-4,700rpm range, and peak HP in the 6,600-6,800rpm range.

The biggest issue is that there is a premium on good throttle drivers. Most act like the throttle is a light switch.

[CamKing]

The biggest issue is that there is a premium on good throttle drivers. Most act like the throttle is a light switch.

That's where my 35 years of experience comes in. If the engine builder tells me it's for a new driver, or one that thinks his right foot is an on-off switch, I will reduce the duration a few degrees, widen the LSA a few degrees, and retard the ICL a few degrees. This makes for a very flat torque curve. It reduces peak torque, and lowers peak torque rpm, but it's much less likely to spin the tires, coming off the corner. I do the same thing for limited tire classes.

Other companies will recommend a larger duration cam, because that reduces max torque. The issue with that is, the torque curve becomes even more peaky, and it's the difference in torque over a few hundred RPM, that causes the issues, coming off the corner.

[tjs44]

Mike,what do you think for a engine for a 100% street car that has a OD trans and will operate from off idle to about 2500 with occasional bursts to 3500-4000?I know we always regress to race engines but a lot of HiPo street engines are being built these day.s with ODs.Tom

[CamKing]

Mike,what do you think for a engine for a 100% street car that has a OD trans and will operate from off idle to about 2500 with occasional bursts to 3500-4000?I know we always regress to race engines but a lot of HiPo street engines are being built these day.s with ODs.Tom

Well, first you would want to design the cam to make the most power between stall converter RPM, and 4,000rpm, but that can be tricky. Sometimes when you're working with these short duration cams, you can over-cam it, and actually make more power, thru-out the usable power curve.
Say you have an 1,800 stall, and a max RPM of 4,500. A cam designed to make power to 5,000rpm, may make more power from 1.800-4,500, then a cam designed for 4,500rpm. Even though the larger cam moves the peak torque rpm up, it can still make more torque, at the RPM the smaller cam made peak torque.

[Stan Weiss]

Mike,what do you think for a engine for a 100% street car that has a OD trans and will operate from off idle to about 2500 with occasional bursts to 3500-4000?I know we always regress to race engines but a lot of HiPo street engines are being built these day.s with ODs.Tom

Tom
Are you talking about your Pontiac with the manual TKO trans?

Stan

[hysteric]

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 too would like Mike to answer this question concerning a smaller port volume and longer rod ratio. Here's a quote I saved from a previous discussion on the long rod ratio theme from memory. Mike stated:

We've tested the heck out of different rod lengths in limited class circle track engines. There are a ton of circle track classes that limit you to 358ci, stock stroke, a compression limit, and unported production heads.
In these classes, Rod length is one of the few things that can be changed, so it's a great place to learn about the effects of rod length. One thing we've proven, as the rod/stroke ratio goes up, the engine wants an earlier intake valve closing point, for the same rpm power band. If you run a cam designed for the shorter rod/stroke ratio, with the longer rod, the intake closing is too late, and it'll hurt the power.
When we cam the engines correctly for the different rod lengths, we normally see the power between peak torque and peak hp to be very close, with the shorter rod making more power below peak torque, and the longer rod, carrying the power farther past peak hp.

Hope this helps.

[tjs44]

Mike and Stan,yes TKO 5 speeds with .64 OD.Tom

[CamKing]

Mike and Stan,yes TKO 5 speeds with .64 OD.Tom

Not sure what you want to know ?

If you want to know what cam it would need, that's what my cam recommendation form is for: http://jonescams.com/street-performance/