E/BGR Engine Build Thoughts

[gasblender37]

Fordboy,

Would you elaborate a little more on:

F1 type power curve.  ADVANCE the point of peak flow demand.   Longer rod/stroke ratio DELAYS this.

Thanks,
Skip

[fordboy628]

Fordboy,

Would you elaborate a little more on:

F1 type power curve.  ADVANCE the point of peak flow demand.   Longer rod/stroke ratio DELAYS this.

Thanks,
Skip

Skip,

F1 type power curve:
Current Cup engines are running rod/stroke ratios in the 1.7/1 to 1.83/1 range.   Unrestricted long track engines are producing peak power @ approx. 9500 rpm.   They would be running 10,000 rpm, or more, if they could.   Raising the rod/stroke ratio to 2.05/1 on an engine 100 cubic inches smaller, and with 4 valve heads, is going to drive the power peak to ~11,500/13,000 rpm, unless the valve motion/flow potential is severely restricted.    Is this what you want to accomplish?    Not if you are planning to turbo charge it.


ADVANCE the point of peak flow demand.
Flow demand/piston demand is a function of build geometry, and is calculated as the displacement in cubic inches (or whatever) * efficiency ratio (Volumetric efficiency) relative to crankshaft motion in degrees.

Piston motion, ie displacement, velocity and acceleration is not uniform, and it is not symmetrical to 90 ATDC.   This is due to "the cosine effect" of the geometry.    The crankshaft's rotation displaces the con-rod big end laterally during operation, effectively shortening and lengthening the con-rod, thereby altering uniform piston motion.    For V-8s of typical geometry, @ 90 ATDC the piston will have traveled more than half the stroke.  Way more, like 5 to 8% or so.   Radical geometry combinations can drastically impact piston motion.

So since the piston travel is not uniform, peak flow demand usually occurs between 70 to 80 ATDC.    The peak demand is closer to TDC with shorter rods.


Longer rod/stroke ratio DELAYS this.
A longer rod has the opposite effect, and peak demand is delayed by some amount of crankshaft degrees.   The geometric effect can vary based on the difference in length.


So what?
Depending on the flow characteristics of the inlet tract and the motion of the valve train, a savvy builder might want to alter/take advantage of the flow/piston demand.

For example:   4 valve/cyl heads are going to have superior low/mid lift flow compared to 2 valve/cyl heads.
1)   Fulfilling demand sooner will probably benefit a 4 valve combo by allowing the closing of the intake valve "sooner".
2)   Delaying demand will probably benefit a 2 valve combo, by giving the inlet tract time to "catch up".
3)   Etc.

Be aware that it is not just flow demand that is altered here.   Requisite valve events are altered as well.


BTW:  This is a very complex subject, about which not all engineers/engine builders agree.   I suggest you obtain some of the collegiate textbooks from the reading list on my tech thread:  How do I make my engine better, Racing Engines 101.     C. F. Taylor is where I would suggest you start.

Also:  Since your engine will be a "blower engine" you will receive some advice that "none of this matters".    Think very carefully about that.    I suggest that instead of thinking of your engine as a "blower engine" or "turbo engine", think of it instead as a normally aspirated engine operating in a "high air density environment".    JMO.

 
Fordboy

[Dynoroom]

Also:  Since your engine will be a "blower engine" you will receive some advice that "none of this matters".    Think very carefully about that.    I suggest that instead of thinking of your engine as a "blower engine" or "turbo engine", think of it instead as a normally aspirated engine operating in a "high air density environment".    JMO.

 
Fordboy

This is so very true. No matter what the engine type.

[RansomT]

Ding, ding, ding...we have a winner.  Fordboy, you just have filled in a mystery for me!

If you look at the evolution of OEM performance motorcycles over the past 10 years, you notice that peak HP has been moving higher in terms of rpm. The rod/stroke ratio has also been moving higher.  Furthermore looking at different bike classes, the smaller bikes (600cc) have a rod/stroke ratio in the 2.1 range with a peak power around 15K, the S1000rr has a rod/stroke ratio at a tad over 2.0 with a peak of 13.5K, while the 'Busa has a peak around 10.5K with a rod/stroke of 1.83.

(without getting in too much detail for a forum post) The mystery for me is why when stroking a Busa motor it is progessively more difficult to keep the peak HP at the same RPM as the stroke increases. Even with higher flowing heads, bigger cams, better exhaust: The power goes up, but the peak rpm slowly drops as the stroke increases (as the rod ratio drops).  You can see it with cam timing, the "sweet spot" keeps moving lower in the rpm range.  Now of course you make more power with a bigger engine, but the peaks (for TQ and HP) creep lower as the stroke increases.

[fordboy628]

The mystery for me is why when stroking a Busa motor it is progessively more difficult to keep the peak HP at the same RPM as the stroke increases. Even with higher flowing heads, bigger cams, better exhaust: The power goes up, but the peak rpm slowly drops as the stroke increases (as the rod ratio drops).  You can see it with cam timing, the "sweet spot" keeps moving lower in the rpm range.  Now of course you make more power with a bigger engine, but the peaks (for TQ and HP) creep lower as the stroke increases.

As you stroke engines, usually what happens is: the rod becomes shorter by necessity.   That is going to have the impact you describe.   The only way to retain rod length would be to increase the height of the block and/or shorten the compression height of the piston.    Not sure how easy this might be on bike engines.


F/B

[RansomT]

We retain the same rod length by effectively increasing the block length by 1/2 the stroke increase, however that still makes the ratio smaller.

[fordboy628]

We retain the same rod length by effectively increasing the block length by 1/2 the stroke increase, however that still makes the ratio smaller.

Yes.

How feasible is it to increase the block height enough to use a longer rod that retains the original rod/stroke ratio?

F/B

[gasblender37]

Fordboy,
Thanks for all your insight.
Skip

[entropy]

Ding, ding, ding...we have a winner.  Fordboy, you just have filled in a mystery for me!

If you look at the evolution of OEM performance motorcycles over the past 10 years, you notice that peak HP has been moving higher in terms of rpm. The rod/stroke ratio has also been moving higher.  Furthermore looking at different bike classes, the smaller bikes (600cc) have a rod/stroke ratio in the 2.1 range with a peak power around 15K, the S1000rr has a rod/stroke ratio at a tad over 2.0 with a peak of 13.5K, while the 'Busa has a peak around 10.5K with a rod/stroke of 1.83.

(without getting in too much detail for a forum post) The mystery for me is why when stroking a Busa motor it is progessively more difficult to keep the peak HP at the same RPM as the stroke increases. Even with higher flowing heads, bigger cams, better exhaust: The power goes up, but the peak rpm slowly drops as the stroke increases (as the rod ratio drops).  You can see it with cam timing, the "sweet spot" keeps moving lower in the rpm range.  Now of course you make more power with a bigger engine, but the peaks (for TQ and HP) creep lower as the stroke increases.

Ransome,
increasing the CR seems to help rpm/hp  

My Carpenter kit NA 1635 Busa's had climbing hp right up to the 11,500 shiftpoint which i liked =  280-290hp (Dynojet)
But CR was in the stratosphere:  18.3/18.5:1
When i built the motor and measured CR, i didn't believe my numbers and called Bob C.  
He listened to my story and said "perfect".
Karl

[gasblender37]

Are the 6.2 chevy rods available with 1.88 rod journals?     yes

very common in cup and on Ebay


http://www.ebay.com/sch/i.html?_sacat=0&_nkw=6.200+rods&_frs=1


http://www.ebay.com/itm/Set-of-8-Carrillo-H-Beam-6-200-Rods-NASCAR-ARCA-NHRA-L-K-Engine-Builders-/151306895026?pt=Motors_Car_Truck_Parts_Accessories&hash=item233a97feb2&vxp=mtr

If one were to decide to go the used nascar rod route, what would have to be done to the rods to insure reliability?
Thanks,
Skip

[Sumner]

......But CR was in the stratosphere:  18.3/18.5:1.....

At higher rpms on any engine you usually don't fill the cylinder like you do at lower rpm so the dynamic CR probably is going down.  Also the cam overlap can really play into the dynamic CR meaning with a big cam you could run much high static CR than with a mild cam.  Even though the motor isn't as efficient each cycle at high rpm it is having more of them so can make up for it with more power due to more power strokes per minute with the right combo.

I put a couple pages on my site years ago (still there.... http://purplesagetradingpost.com/sumner/techinfo/tech--dcr%20combinations-1.html ) where people sent me different static CR and dynamic CR engines they had built so one could get an idea if they could get away with a lower octane gas even with a high static CR,

Sum

[fordboy628]

......But CR was in the stratosphere:  18.3/18.5:1.....

At higher rpms on any engine you usually don't fill the cylinder like you do at lower rpm so the dynamic CR probably is going down.  Also the cam overlap can really play into the dynamic CR meaning with a big cam you could run much high static CR than with a mild cam.  Even though the motor isn't as efficient each cycle at high rpm it is having more of them so can make up for it with more power due to more power strokes per minute with the right combo.

I put a couple pages on my site years ago (still there.... http://purplesagetradingpost.com/sumner/techinfo/tech--dcr%20combinations-1.html ) where people sent me different static CR and dynamic CR engines they had built so one could get an idea if they could get away with a lower octane gas even with a high static CR,

Sum

Sumner,
Still a slow and crappy typist, hence the rehash of your post.   Sorry,   F/B

Translation of Sumner's post into "engineer speak":

At higher rpms on any engine [above the torque peak] you usually don't fill the cylinder like you do at lower rpm so the dynamic CR probably [DEFINITELY] is going down.   [The rpm of peak torque is the point of MAXIMUM engine volumetric efficiency.]

Also the cam overlap can [does] really play into the dynamic CR meaning with a big cam you could run much high[er] static CR than with a mild cam.

Even though the motor isn't as efficient each cycle at high rpm it is having more of them so can make up for it with more power due to more power strokes per minute with the right combo.  [ABSOLUTELY!!]

I put a couple pages on my site years ago (still there.... http://purplesagetradingpost.com/sumner/techinfo/tech--dcr%20combinations-1.html ) where people sent me different static CR and dynamic CR engines they had built so one could get an idea if they could get away with a lower octane gas even with a high static CR,

[Fuel suppliers "might" be willing to "suggest" the highest dynamic compression ratio a fuel might "tolerate".   If you have need to better understand compression ratios, go back and read my post "Compression Ratios, what do they mean?"]

http://www.landracing.com/forum/index.php/topic,12353.0.html

My apologies Sumner.

 
Fordboy

[tauruck]

Interesting info.

Thanks Mike.

[fordboy628]

If one were to decide to go the used nascar rod route, what would have to be done to the rods to insure reliability?
Thanks,
Skip

Skip,

Used parts of any type need to be carefully evaluated.   Dimensional checking, visual inspection, 'maganflux' or other appropriate particle inspection(s), etc.   This goes for the rod bolts as well.   Most of the time, I give rod bolts the old "Navy Inspection", ie: chuck them into the nearest deep body of water.   Reuse any which float.   Reject rod bolts for ANY flaw or dimensional difference.   If you find ONE bolt that has been "stretched" beyond the manufacturer's listed length for the part, CHUCK THEM ALL.   It's just cheap insurance.

Only consider parts that have been used for ONE race.   Otherwise you have NO IDEA of the number of stress cycles to which the part has been subjected.

High stress + Large number of cycles = 
 
Fordboy

[gasblender37]

If one were to decide to go the used nascar rod route, what would have to be done to the rods to insure reliability?
Thanks,
Skip

Skip,

Used parts of any type need to be carefully evaluated.   Dimensional checking, visual inspection, 'maganflux' or other appropriate particle inspection(s), etc.   This goes for the rod bolts as well.   Most of the time, I give rod bolts the old "Navy Inspection", ie: chuck them into the nearest deep body of water.   Reuse any which float.   Reject rod bolts for ANY flaw or dimensional difference.   If you find ONE bolt that has been "stretched" beyond the manufacturer's listed length for the part, CHUCK THEM ALL.   It's just cheap insurance.

Only consider parts that have been used for ONE race.   Otherwise you have NO IDEA of the number of stress cycles to which the part has been subjected.

High stress + Large number of cycles = 
 
Fordboy

Thanks fordboy