Wow, I just read that thread on Exhaust valve sizing vs. Intake
sizing (which is now locked). Pretty neat stuff...although I don't
recall seeing anything about why the intake is bigger than the
exhaust valve.

I have read and understood that the intake valve is bigger because
the induction of air/fuel into the cylinder takes more effort from
low pressure than expelling gas using the high pressure of the
pistons upward movement.



Anyway...back to my topic. I just smoked my motor and decided
to take some measurements before ripping it apart.

This graph shows the piston movement of a 3.48"
crank and 5.7 connecting rod. Good information
for those deciding on camshaft IVO, IVC, etc.

I used a dial, degree wheel and depth gauge to
measure the piston in the bore. All measurements
are +/- 0.5 mm. (1 mm = 0.39370 inch )

The blue line is the piston movement, the pink line
is the delta (change in distance mm ):

http://members.rogers.com/tdese739/PistonTravel.jpg

So a question comes now. Is VE% directly realted to DCR?

For example, a dynamic (effective) compression of 10:1 can never be achieved by 100% VE if static compression = 10:1?

If the cam bleeds off cylinder pressure that much, should we be
selecting cams that improve DCR or VE%?

I'm also guessing that all of the "bleed-off" occurs during overlap? This is also the point where scavenging occurs.

I'm not really understanding why all this pressure is relieved if
the exhaust is tuned properly and intake charge is helped by the
scavenging efffect?

(BTW: I search for DCR and came up with a few good threads.
None explained my current question.)

Also noteworthy:
A Felpro steel headgasket compresses to 0.0175"
when torqued to 60 ft./lbs.

I was also able to measure my ring land height
and deck height fairly accurately to calculate
a static compression ratio of ~ 10.6:1.

Some more pics of the damage. It turns out the
outer block is cracked almost right in two!

http://members.rogers.com/tdese739/1cylindercrack.jpg
http://members.rogers.com/tdese739/pistonthrublock.jpg
http://members.rogers.com/tdese739/cracks.jpg
http://members.rogers.com/tdese739/rodend.jpg
http://members.rogers.com/tdese739/backofpiston.jpg
http://members.rogers.com/tdese739/crank.jpg

http://members.rogers.com/tdese739/rodthroughpan.jpg
http://members.rogers.com/tdese739/oilonhwy.jpg
http://members.rogers.com/tdese739/towtruck.jpg

http://members.rogers.com/tdese739/coolantplugs.jpg
http://members.rogers.com/tdese739/modified700.jpg
http://members.rogers.com/tdese739/norockers.jpg
http://members.rogers.com/tdese739/norods.jpg

Check out the coolant in the ports!
http://members.rogers.com/tdese739/c...intakeport.jpg
http://members.rogers.com/tdese739/heads.jpg
I don't know what caused this to happen, it just
skimmed the valve and cleared the carbon...not
much damage to the naked eye but I'm replacing
the valves to be safe.
http://members.rogers.com/tdese739/1exhaustvalve.jpg
http://members.rogers.com/tdese739/coolantinbore.jpg
http://members.rogers.com/tdese739/coolantonpiston.jpg

 

Did I stump the pros? Hmmmm...

Maybe my question is a little vague...or ya'll don't like me!

For those that don't want to read my long winded first post,
this is my question:

Is VE% directly realted to DCR?

With a static compression of 10:1, can a dynamic (effective) compression of 10:1 ever be achieved if the motor can produce 100% VE?

An earlier thread stated "no". If the cam bleeds off cylinder pressure that much, should we be selecting cams that improve
DCR or VE%?

I'm also guessing that all of the "bleed-off" occurs during overlap? This is also the point where scavenging occurs.

I'm not really understanding why all this pressure is relieved if
scavenging is supposed to improve cylinder pressure.

 

As Yogi may have said,"It's deja vu all over again."

It was suggested in a few threads on DCR that a search be done on DCR and the threads studied. Especially read and understand Mr. Horsepower's posts.

Here's a start:

http://web.camaross.com/forums/showt...&highlight=dcr

 

Thanks, that helps a little more.

But this "100 VE%" value still fools me.

If the swept cylinder volume can be 100% "filled" (100% VE),
DCR is still lower in relation to SCR because the head chamber
volume stilll has to be considered?

Volumetric Efficiency is a term used to measure "swept volume"?

 

Let me do you a favor. I'll give you my ham-fisted shadetree mechanic answer and then the real pros will jump in, tell me why I'm totally wrong and you'll get the real answer you wanted in the first place. Fair enough?

I've climbed around in this DCR thing for a while and what it boils down to is this: It's still just a mathematical number and doesn't really tell you a whole lot more than a simple static compression ratio calcualtion. The whole thing boils down, near as I can figure, to simply calculating your static compression ratio AFTER the intake valve closes (whever your cam makes that happen). Whatever swept volume you give away between BDC and whenever the intake valve closes- calculate using standard static compression ratio calculations from that point and you have DCR. Being a simple mathematical calulation, like static compression ratio, it does not change based on engine RPM. Big whoop. If you have a big cam with a late-closing intake, your DCR will be lower than the same engine with a small cam. Stuff that idiots like me have learned from expereince. Shove a big cam into an engine with mild compression and you end up with very little cylinder pressure, and probably a soggy bottom end.

VE is a different story. It's a measure of how much air you can actually get trapped in the cylinder from intake opening to intake closing vs. the theoretical swept volume of the cylinder(s). Varies based on a billion factors of engine design, intake, cam design, etc. but more is always better. It varies with RPM since no engine can be equally efficient at all RPMs. And VE is usually (always?) highest at peak torque. In short, a measure of how efficiently your engine breathes at any given RPM.

 

Way to go, Damon!

That's about the best and most concise explanation I've heard.

Calculating DCR in the design process helps determine how much SCR you need or can tolerate. IMO, that's it's basic use.

VE peak probably causes torque peak more than any other factor.
VE can still remain high after tq. peak, but friction losses are catching up to you and driving brake tq down.

 

Are you sure it is frictional losses that are driving the brake torque down after the torque peak - I would think it would be due to a change in VE.

Frictional losses should be fixed vs. RPM for a given engine configuration - yet I can perform a simple cam swap, move my powerband into a higher rpm range, and make more power with about the same VE (assuming optimized cam profile) - if frictional losses were really that severe then switching from a stock cam to something like a cc306 and picking up 30+ft-lbs/hp and 1500 rpm on the torque peak wouldn't happen.

I would submit that the drop normall observed after the torque peak is due to VE drops normally, in that the valves are no longer opened long enough to flow enough air at the higher rotational velocity.

Chris

 

Good point, especially in an OEM engine. It's average torque in the operating rpm range or area under the torque curve that does the job. OEM engine might need that from 1700 (stall) to 5700 (WOT shift point). We can stand more compromises on rpm range in out hot rods. 3000-6500 for example.

Here's my thinking on what I said earlier:

Assume an NA SBC engine that peaks torque about 5000 and hp about 62-6500. At low rpm (2500-3000) VE is maybe 68-72%. Cam overlap is hurting here. Some simulations graphically display intake and exhaust flow, and reverse flow back up the intake can be seen.

As rpm comes up, reverse flow is less and VE rises. Let's say it peaks about 5000 at 100%. Let's further assume it stays at 100% because intake tuning is starting to come into play. What would happen to torque?

Indicated torque, or torque produced by the combustion process would remain about the same, but since friction torque and pumping losses are rising with rpm, brake torque, or that torque which gets to the flywheel, can be measured, and actually drives the vehicle, is decreasing. True the brake torque curve wouldn't be falling off very fast, but it would be decreasing.

It's quite possible to have max torque VE near or at the hp peak with very good intake tuning. Obviously that involves a very good combination of intake shape and length and port flow and valve timing to achieve. Isn't that what most of us shoot for?

If a simple cam change moves torque peak from 3500 to 5000 by increasing VE near 5000, I suggest that it's at the expense of low rpm VE. My gut feeling is that moving the torque peak that far with no other changes would be the exception.

Anyone?

 

^ Understood.

The fact that we are not using dynamic length runners leads me to subscribe to that theory (all else being equal).

Aside from that, a cam with excessive overlap will kill the low RPM
cylinder pressure thus reducing torque because the pressure
in the exhaust manifold is still relatively high to help pull in fresh
charge.

What I'm taking from the last post is:

it may be better to build the motor to peak lower in RPM (while
building max. VE%), to outweight the differences in high RPM
friction loss and pumping loss with N/A engines.

 

OldSStroker - Got ya, I read your post as saying that the drop after the torque peak in general was due to just frictional losses (to which I would still submit that VE losses are orders of magnitude more substantial)

But I definitely agree with you - if VE is held relatively constant over an optimized range then torque will probably still decrease to to increasing frictional losses - at least that is how I read your point.





It depends what your goals are - but in the end the more RPM you have, the higher your HP potential. I don't think the frictional losses are really that much of an issue here, since we can obviously make more power at higher rpm's easily - sure, the motor may not be as "effecient", but we would still be putting more power to the ground.

Pumping loses with respect to valve timing (what I was mentioning above) will be moot as the camshaft will be optimized for the range - the question then becomes can your heads/intake/intake path, etc. support it.

I would submit that it's better to build the biggest motor you can afford that will be strong enough for your application, then determine your drivability requirements and work from there.

Chris

 

Extremely good explanation. I would just like to thank, you, mindgame, krause, and all the others (didn't mean to leave anyone out but i'd be typing all night) for helping us lurkers gain vast insight into your automotive brains I will return to the shadows again <poof>