So what is the advantage of centering overlap with TDC?
Overlap Triangle
- Thread starter Danny Bellmore
- Start date
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Newbomb Turk
Well-Known Member
Power. It matters more if you have a functioning header. That’s why wide LSA’s don’t matter with manifolds. You don’t have the header helping with chamber filling.
Engines with manifolds and junk hearts are numb to LSA changes which means they are far less sensitive to cam timing than if you have a tuned header.
How bout with turbo engines?
Newbomb Turk
Well-Known Member
Boost is not the same. Neither is nitrous. Or high level competition engines.
They will open the LSA up (and lose power) and move the overlap triangle to the right which is retards the cam and probably hurts power a bit around peak torque) but they do all that because if they don’t the valve notches and the geometry of the piston top make combustion less efficient enough that it doesn’t make up for the power gain by a narrower LSA and a centered overlap triangle.
It’s managing power and efficiency.
TT5.9mag
Two atmospheres are better than one
When you have positive pressure on the back side of the intake valve you are far less reliant on inertial wave or reflective wave tuning or scavenging to fill the cylinder. The small changes that really seem to effect an NA engine won’t seem to do much in a boosted combo.
Danny Bellmore
Active Member
410, 4 inch stroke, 6.123 rod length, 13-1 compression, peak torque 4800-5000 range, weight 3250 with me in it, 5600 converter and 1 7/8 headers. solid roller
Newbomb Turk
Well-Known Member
Danny Bellmore
Active Member
Race gas
Newbomb Turk
Well-Known Member
272/288
110/107
Lobes I found that I like (what I like lol they may not suit your more sensible taste) are from Bullet:
Lobe # .020/.050/.200 LL 1.5/1.6
R302/450 302/272/196 .450 .675/.720
R316/460B 316/288/212 .460 .690/736
Or Howards:
R271/4544 299/271/194 .4544 .682/727
R288/4544 316/288/211 .4544 .682/727
I didn't look at Comp's stuff but I'm sure there are plenty of good lobes in there.
BTW, those numbers are for peak torque at 5K and Shift at 7k with a 10.5:1 effective compression ratio.
Edit: I had the ICL at 108 but it needs to be at 107.
Edit: I put my glasses on. it should have an ICL of 106 so 110 plus 4 degrees. I got it right this time.
TT5.9mag
Two atmospheres are better than one
Those numbers only put peak torque at 5k rpm? 272/288 @050 on 410 inches seems like it would make peak TQ higher than that. At least in my head.
Danny Bellmore
Active Member
Thank you very much for the information.
Newbomb Turk
Well-Known Member
I also forgot to ask if you have cylinder heads have 5/16 or 3/8 rocker shaft bolts. If you only have 5/16 bolts I would chose less aggressive lobes that also had lift between .620-.640 just to help reduce stress on the bolts.
Newbomb Turk
Well-Known Member
I ran the numbers again to make sure I didn’t end up with a green weenie but that’s the math.
So far, this math hasn’t lied to me. The math I’m using starts with finding intake duration. It’s based on peak torque rpm, stroke, compression ratio and bore size. And it’s weighted more to stroke and peak torque rpm.
Then it takes the intake duration and rpm for peak power and rod to stroke ratio to determine LSA.
Then it takes the LSA and uses whatever effective compression ratio you choose to determine ICL.
The last calculation is exhaust duration. It uses intake duration and a combination of piston speed and crank pin speed to calculate it.
One of the limits I’ve found is what I consider low rpm, low compression stuff. Say 9:1 and peak torque at 3200 and peak power at 5200.
It will almost always end up with a low 100’s or as low as a 98 degree LSA. If you then get the ICL, it will likely be 6-8 degrees advanced.
On the exhaust lobe, I can’t make the numbers make sense with exhaust manifolds. I get some pretty wild numbers that you would probably never would think of.
I suppose if you were doing a FAST build or something the numbers might make sense, but PRH or someone who is actually building or running a class like that can say if they have cams with very very tight LSA’s in their stuff.
Of course, that’s getting to the point that valve notches would be so deep that you just can’t fit all that lift around TDC and have a piston worth a crap.
My kid is coming home from college for Christmas and I’m going to have him look at the math and see if we can change the math a bit to be able to do stuff like that.
And I need to point out this math makes the assumption you are optimizing the engine for the rpm that’s been calculated. And really everything else.
Newbomb Turk
Well-Known Member
Because it’s not predicting power output. That’s what head flow says.
That’s why the same head may make 1.5 HP/CFM and that same head on a different engine will make 2.0 HP/CID.
It took me awhile to get my head around that.
It also has to do with some assumptions the math makes. One of those is it won’t account for someone building a 420 inch engine (this is why you have to account for bore and stroke in the math but it is weighted more on stroke and where you want torque and power peaking) with 273 heads.
The math assumes the engine builder has a grasp of what heads the engine needs to make the torque and power where you are telling the math.
TT5.9mag
Two atmospheres are better than one
Thanks that’s very cool and if you’re verifying what it’s predicting on your dyno then the math is good. I’d like to mess around with that program. Is it something you designed or did you buy it? If so who wrote the program?
Newbomb Turk
Well-Known Member
I can type out the math in a PM and send it to you.
Or, you can find and buy a copy of the the book “The Horsepower Chain” by Don Terrell.
If you can find a used one it will be cheaper. If not, you can go to speedtalk.com and order one if any are left. I think Mike told me when they are gone they won’t do another printing. I forget what I paid for mine in 2016 but I think it was 50 bucks.
Last time I looked I think they were 80 bucks, and that’s if any are left.
Edit: I just looked on ST and Mike has not taken down the post so I think they are still available. And it says 40 bucks. So I have no idea what I paid now lol.
I remember it was more than a DV book by a bit.
Understood but..the air velocity is number one and the cams job is to control the velocity of the air through the port. The ability of a specific engine combination to use what the port is capable of is probably why some engines make more power than others with the same head. Using the port length and CC's you can calculate the average size of the port. Then using the bore and piston speed you can calculate the air velocity at each degree then in turn calculate the average port velocity that the engine is creating or demanding through a given port. If that number is too low or too high then power will be less than optimum.
Newbomb Turk
Well-Known Member
Again, the math has to have assumptions.
IMO, air flow isn’t that important when selecting a cam IF the builder has a grasp on what the heads should be for the application like I pointed out above.
You wouldn’t put fully ported B1 heads on a 408 inch big block with 9:1 compression and a peak power rpm of 6k would you? I know I wouldn’t do that.
Nor would I build a 632 big block with 915 castings (or any other garbage head like that) with 15:1 compression and peak power at 8500.
The math can’t account for poor engine building.
Newbomb Turk
Well-Known Member
That’s easy then lol. Go to page 78 for intake duration and page 130 for LSA, ICL and exhaust lobe duration.
The nice thing about having the book rather than just the formulas is you can figure out what the FLRQS is (which is piston speed and crank pin speed) and things like CRX and EPE too.
You are pooping in the tall cotton if you have the book. I mean I can do the math to calculate FLRQS but there is a chart I can look at and find it quicker that way.
Newbomb Turk
Well-Known Member
If you like math you’ll have fun. I have a love/hate relationship with math lol. When I get stuck I have my kid bail me out.
When he gets home for Christmas I’ll have him look over the math and see if he can adjust something to make it more accurate with exhaust numbers with manifolds.
Also if you have the book it gets into cylinder head math, induction math…pretty much all the math you need to build an engine.
No i wouldn't and the reason is the air velocity would be to low for the 408 and too high for the 632. I liken it to sucking on a straw where my mouth is the piston, the straw is the port and my finger covering the end of the straw is the valve. How far i move my finger from the straw, how fast i move it and how long i hold it away affects velocity. I will dig out my book and take a look, i guess i missed that section somehow.
Newbomb Turk
Well-Known Member
It gets a bit confusing because the actual formulas are so far apart in the book.
At the very back there is all the header math and stuff like that.
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