Thoughts on compression/build

[tpepmeie]

In my opinion, valve size becomes the limiting factor (and with valve size goes port size, per my previous post). Using a stock stroke length, with the biggest valves known to fit in the LT5 chamber, the mean velocity at the valve head approaches 80 m/s at 8000 rpm. That is pretty inefficient, as even F1, etc. are typically 73-75 m/s.

To achieve the real big rpms, you would need a more oversquare engine (shorter stroke:bigger bore) and intake valves around 34% of bore area. And ports to match, of course. That would allow more reasonable air velocity as revs increase.

Then, finally, I am told that the ladder-frame sump allows the crank to "walk" in its journals at higher RPMs. Perhaps that is solved with studding the bottom end, though.

Todd

[A26B]

Todd,

I'm going to school here & havve a couple of questions.

Originally Posted by tpepmeie

In my opinion, valve size becomes the limiting factor (and with valve size goes port size, per my previous post). Using a stock stroke length, with the biggest valves known to fit in the LT5 chamber, the mean velocity at the valve head approaches 80 m/s at 8000 rpm. That is pretty inefficient, as even F1, etc. are typically 73-75 m/s.

If the LT5 has higher port velocity than the typical F1, then should cylinder fill be more efficient from the inertial ram effect.....or....is the velocity higher but total flow volume lower because of port restriction?

To achieve the real big rpms, you would need a more oversquare engine (shorter stroke:bigger bore) and intake valves around 34% of bore area. And ports to match, of course. That would allow more reasonable air velocity as revs increase.

I've read a number of articles regarding port size and it's a generally respected opinion that larger port size is not necessarily always a good thing. More Hp can be realized with higher port velocity in many cases, as a result of more efficient cylinder fill and a/f mixture. I'm assuming your comments are directed to the full-out, purpose built race engine which will operate nearly continuously in the top 20% of the rpm range.

Then, finally, I am told that the ladder-frame sump allows the crank to "walk" in its journals at higher RPMs. Perhaps that is solved with studding the bottom end, though.

Wasn't this an issue with the 375 Hp engine design that precipitated the 405 Hp bottom end design?

Jerry

[tpepmeie]

Jerry, heck I don't have a PhD in engine design that's for sure. Just a self-educated enthusiast, who has absorbed a lot of the theory from various sources.

Originally Posted by A26B

If the LT5 has higher port velocity than the typical F1, then should cylinder fill be more efficient from the inertial ram effect.....or....is the velocity higher but total flow volume lower because of port restriction?

Beyond a certain point, too much velocity can kill power big style. Once the flow is "choked", then inlet density is reduced despite higher velocity. Additionally, so-called pumping losses skyrocket, as the effort to draw in the mixture at super high velocity is too great. It is not a good thing to have higher inlet velocity than F1 or other race engines. You will find that most of the optimized unrestricted engines operate at around the same factor (called Lovell factor, to be precise).

Originally Posted by A26B

I've read a number of articles regarding port size and it's a generally respected opinion that larger port size is not necessarily always a good thing. More Hp can be realized with higher port velocity in many cases, as a result of more efficient cylinder fill and a/f mixture. I'm assuming your comments are directed to the full-out, purpose built race engine which will operate nearly continuously in the top 20% of the rpm range.

Oh, for sure. Too much port volume can be detrimental absolutely. Again, you are right that higher port velocity, & smaller ports are a good thing *up to a point*. The key here is that there are nearly universal laws around something called the Mean Inlet Mach Index (refer to textbooks by Charles Taylor for additional info). Above about .55 Mach average inlet velocity, power suffers.

You are right, for good low-rpm driveability you want smaller rather than larger ports, provided you can still get the needed airflow at peak power speed.

It's a lot of textbook, and theory. But I have researched several many successful 4-valve race engines, and the theory does hold in practice.

Originally Posted by A26B

Wasn't this an issue with the 375 Hp engine design that precipitated the 405 Hp bottom end design?

Probably right on that account, perhaps the 4-bolt mains solved the problems.

Todd

[A26B]

Thanks Todd, always a pleasure to discuss such things with you!