Negative Overlap

[Ardy]

I found this posted somewhere and thought I would toss it out here for comment.


"I was reading an interesting article about TC96 cams in a Thunder Press that was stuffed in with my Zanotti purchases.
According to the author, a major problem with the TC96, and the explanation why traditional Stage 1 modifications seems to be less effective than on Evo and TC88, is that the 96" have (he says) negative valve overlap.
Now, I'm no specialist on cams, valves or much else, but it seems that a normal tractable street bike would have several degrees of overlap, maybe 10° or so, so that the exhaust is still open when the inlet valve opens. Of course, this makes the motor less efficient, as some of the inlet charge will find its way out of the exhaust, and it's the elimination of this, for emissions regulations, that has resulted in H-D's choice of cam. With the negative overlap, the exhaust valve closes early, and the inlet opens late, which means the motor can't breathe, and it won't matter as much what air filter or pipes you put on, it still won't breathe.
This may be in contrast to Evos and TC88s, where the breathing was restricted more by the pipes and filter than by the cams....?
Apparently, the stock TC88 has a low overlap of about 2°, from what I remember.
Another interesting point he made was that, by closing the exhaust earlier, the engine acts as a compressor towards the end of the exhaust stroke, trying to force gases out through a closing valve. What hadn't occurred to me (but did to the author) was that the compression of the gases leads to heat, and he puts that down as one of the key reasons for the high operating temperatures of the TC96. I'd always figured that the overheating was caused by weakness in the mixture (less fuel = less cooling) but if he's right, then only a change in cam, to give a positive overlap, would not only free up the power in the motor, but also eliminate the 'overheating'. Compressing gas will sap power as well of course.
If what he says is right, then only way to do an effective tune on the TC96 would have to involve a mild cam change, as well as the usual stuff."

What do those who understand these things think?

[mayor]

here's some data from BigBoyz web site (http://www.bigboyzheadporting.com/tccams.htm) that backs up the cam specs mentioned in the article:



I don't exactly support the articles conclusion that a stage one kit doesn't reacte quite the same in a 96" compared to 88", at least when caparing apples to apples (EFI).  I do think that stage 1 carb bikes probably saw better hp or tq per cubic inch ratio results.  My guess is the typical disparity between the stage 1 results of an 88" and a 96" is related to the stage 1 EFI downloads. I think that if a stock EFI 96" had an equally good tune as a stock EFI 88" the C.I. ratio results would be somewhat similar.

[Ardy]

I have read other places questions about heat on the 96 and cam choice and never really understood the line of questions. What is the reasoning behind neg overlap and does it cause more heat like the poster implied? EPA? No other cams but stock ones are built that way. Does the intake opening -12 BTDC mean the intake opens before or after actual TDC?

[Rininhoj]

Yikes!
I never thought to check the specs on my stock cam, '06 injected. The negative overlap explains a lot. For one I didn't realize it was different than the earlier FI cams. I knew that they (HD) had to meet tougher emissions #'s in '06. But I thought they did that all with ignition timing & leaner AFR @ idle & cruise. This explains why the addition of a Tmax-AT & SupperTrapp SM still don't have my temps down where I think they should be.



Oh well, I already have the Andrews TW21, new lifters & the SE Hybrid tensioner plate/oil pump on the way. This gives me comfort knowing that besides just power there will be other benefits.  

Kinda strange, I can see where this could reduce emissions somewhat. From a "what's dumping out the tail pipe right now" stand point. But to me the bigger picture is that I must now open the throttle more to get the acceleration I would have gotten with a positive overlap cam. Also that -5° brakes down into 1.5° of compression & 3.5° of vacuum where there should be some swirl chasing out the burned gases & pulling in fresh mixture. Granted there is very little piston movement within 3.5° of TDC but it cuts cylinder fill & efficiency. There for more fuel burned in the long run to get from point a to b.

Ardy,
I take the negative (-) to mean that the scale is based on Before Top Dead Center, but that in this case this value goes the other way. Hence this cam is actually opening the intake After TDC.
Rininhoj

[Admiral Akbar]

Leaving the exhaust open linger definately helps.. In fact, I noticed a reduction in vibration going from a smog cam to a cam with overlap (wasn't looking for it BTW).. If the valve closes early and doesn't let all the hot gases out, wont the motor run hotter?  Max

[Eleft36]

I found this posted somewhere and thought I would toss it out here for comment.


" I'd always figured that the overheating was caused by weakness in the mixture (less fuel = less cooling) but if he's right, then only a change in cam, to give a positive overlap, would not only free up the power in the motor, but also eliminate the 'overheating'. 

The "over heating" reference needs to be related to a factual set of data.
"Over heating" as to what?

Combustion alone is the source of heat that creates power, usually 450* to 580*.
The "air cooled" engine is designed to cool with external "air" flow.

Heat naturally goes to cool, even at 100* ambient, the cooler air is 350* to 480* cooler than the combustion a 1/4" to 3/8" inside a vessel designed to radiate the heat into the ambient air surrounding it.

So where is the "over heating" ?

Al

[FLTRI]

Leaving the exhaust open linger definitely helps.. In fact, I noticed a reduction in vibration going from a smog cam to a cam with overlap (wasn't looking for it BTW).. If the valve closes early and doesn't let all the hot gases out, wont the motor run hotter?  Max

More Power = More Heat. If the engine makes more power @ say 1000-2500 rpm it will produce more heat, right? So since high overlap cams are inefficient @ lower rpms they make less power which also means less "hit" (vibration?) than a higher cylinder pressure (less overlap) engine.
Just my $.02,
Bob

[Steve Cole]

If you hold some exhaust in the motor it lowers cylinder temperature. This is commonly called internal EGR. It used by most all OEM's to help with emissions. What happens is some exhaust is retained in the cylinder which gives less fill on the intake stroke as well as less oxygen. When properly tuned you will have less combustion occurring therefor producing lower cylinder pressure and heat. This lower the NOx output and is why all OEM's are doing it.

[Admiral Akbar]

Leaving the exhaust open linger definitely helps.. In fact, I noticed a reduction in vibration going from a smog cam to a cam with overlap (wasn't looking for it BTW).. If the valve closes early and doesn't let all the hot gases out, wont the motor run hotter?  Max

More Power = More Heat. If the engine makes more power @ say 1000-2500 rpm it will produce more heat, right? So since high overlap cams are inefficient @ lower rpms they make less power which also means less "hit" (vibration?) than a higher cylinder pressure (less overlap) engine.
Just my $.02,
Bob

Not sure I can still find the sheets but both made the same torque up until about 3000. The cam with overlap rolled up to about 8-10 more fp tq.. gave the more about 8 more HP.. 520 was flat at 80 then rolled off to 75 hp. This was an 88ci SnS evo motor in softail.. The 2 cams were a SnS 520 and a Crane H290 adv 4.. May take a while to find the first sheet, I found the crane runs.. Stock SnS 88 ci motor made 85 hp 92 tq, with 9.4 to 1 cr.. 2 inch intake, 1.6 ex. 1.8 intake ports.. Don't buy the velocity thing.

Max