Billiumss

iTrader: (11)

Since most cams have an advertised duration measured at .050", does this formula determine the proper overlap of a cam:

To calculate overlap:

- Add the intake and exhaust durations
- Divide the result by 4
- Subtract the LSA
- Multiply the results by 2

Here are a few examples:


TR 224 224 + 224 = 448
448 / 4 = 112
112 – 114 = -2
-2 * 2 = -4

TR 230 230 + 224 = 454
454 / 4 = 113.5
113.5 – 111 = 2.5
2.5 * 2 = +5

TR Custom 230 + 230 = 460
460 / 4 = 115
115 – 114 = 1
1 * 2 = +2

Stock 01 Cam 196 + 207 = 403
403 / 4 = 100.75
100.75 – 116 = -15.25
-15.25 * 2 = -30.5

MTI Stealth II 224 +220 = 444
444 / 4 = 111
111 – 116 = -5
-5 * 2 = -10

MTI X1 230 + 227 = 457
457 / 4 = 114.25
114.25 – 112 = 2.25
2.25 * 2 = +4.5

TSP 231/237 231 + 237 = 468
468 / 4 = 117
117 – 112 = 5
5 * 2 = +10

TSP 225 225 + 225 = 450
450 / 4 = 112.5
112.5 – 112 = .5
.5 * 2 = +1


TR 224 = -4
TR 230 = +5
TR Custom = +2
Stock 01 Cam =-30.5
MTI Stealth II =-10
MTI X1 = +4.5
TSP 231 237 = +10
TSP 225 = +1


If the above formula/numbers are correct, then everything I’ve read on LS1tech.com, the Internet, books and magazines, the more overlap you have, it will give you:

- More HP on the second half of the RPM scale
- You will loose some low end torque
- Rougher idle
- Have more exhaust smell at idle due to reversion
- Lower vacuum at idle
- More tuning headaches


So what are your thoughts guys? I just want to make sure I understand how overlap affects things. If I am wrong, please correct me.

Bill

PREDATOR-Z

Well easier is {(DUR int+DUR exh)/2} - 2xlsa= degrees of overlap at .050

You basically got it right also:

1- the more negative overlap the better chance to pass smog (sniff tests)
2- the more negative overlap the better for N2O shooting, hense high lsa cams fair better under juice.
3- For every 1* positive overlap you'll make roughly +2rwhp more at peak
4- positive overlap is only a headache with MAF tuning> speed density tuning is not affected by that.
5- positive overlap on split pattern cams are more of a pain to tune than an equal pattern cam with same positive overlap.

This is just what past through my head, i'll think of more.
Good post BTW.

ROJO99

I pick up some helpful info. I am researching a cam for the spring and there alot of low milage cams in the sale section so this helps. One questions on LSA, whats the (+) mean? I.E. 112+2...

PREDATOR-Z

It means there is 2* advance ground in the cam. Rough expl:

224/224, .581/.581 114+0 lsa (straight up) &
224/224, .581/.581 114+4 lsa (+4* advance)

will mean that the advanced cam will peak a little earlier about 200 rpm earlier than the other. and will bring the power curve on lower in the rpms.

Basically it changes the VE's.

Ed Curtis

That's a bit too general...

There are a lot more specs to a camshaft lobe than the .050" duration figures that you have to consider.

For example, you can have one camshaft with 230* @ .050" on both intake and exhaust but have two totally different lobes on the intake and exhaust side. What dictates whether or not this cam will deliver as promised? The design, not just these random specs. I'm sure with enough data, one could create enough 230*/230*/112* type camshafts and I'd bet that each one would act completely different from each other, even with the same .050" duration specs.

Also, having less than zero overlap is not a mandate for passing the sniffer. Many a positive overlap at .050" lift camshaft designs have done just this. It's all in the "design" and not just one parameter.

Ed

Scalpel

iTrader: (4)

Another example of why most people that want to get their $'s worth should let pros, like Ed and others design cams for us.

Sport Side

iTrader: (6)

One of the things I've noticed that degreeing a camshaft a few degrees advanced, is that it really throws off the valve events. This technique really turns the camshaft exhaust bias'd.

Seems almost every camshaft off the show room floor has a good bit of timing advancement too. Is there certain valve events that are being shot for, or what? What's the reasoning behind this...


Not to off topic is it....

Ragtop 99

iTrader: (7)

Advance is ground in for one of 2 reasons:

1) assumption that the timing chain will stretch and over time retard the cam to where you originally wanted it.
2) advancing a cam increases low end power

I'm not sure how valid #1 is with an aftermarket chain and #2 frequently represents sub-optimal cam design.

DAPSUPRSLO

iTrader: (14)

You can also find out your overlap at .006" which is advertised duration and .200" which is a good midlift number. You just need to used advertised and at .200" numbers in the above formulas. Like EDC said, there can be tremendous differences in the lobe design leading to camshafts that act very different for the same duration number at .050".

jrp

iTrader: (16)

after putting my FTI cam on a cam doctored you can really see the whole spectrum and get better insight to the design and thought that went into it.

FWIW my cam checked in at 15.1* positive overlap @ .050

Billiumss

iTrader: (11)

JRP, what cam are you running now with +15.1 degrees of overlap?

I know ramp rates can change overlap some but say you have a Comp Cams 224 cam and you have a TR 224 cam on the same LSA, how much of a overlap difference?

Are there pics of the different lobes somewhere on the net? (side view) Has anyone every done a overlay of different lobe designs?

Will shops tell you all the specs of a cam if you ask?

Does anyone have the 'full specs' on the MTI Stealth II cam? This might be my next cam.

PREDATOR-Z

Gross vave lift int (.581) exh (.581)
Duration @ .006 Tappet lift, int (273) exh (269)
Valve timing @.050:
int -4 BTDC, 48 ABDC
exh 46 BBDC, -6 ATDC
Specs of cam installed @ 116.0 ICL
DUR @ .050 int 224, exh 220
Lobe Lift int .3420, exh .3420

Billiumss

iTrader: (11)

Thanks!!!

What is the true overlap with that cam, -10 like I figured before?

Does the formula (I have above) work with these specs? Help me out with my math. I feel dumb asking but can you go step by step? Please teach me/us...

Bill

PREDATOR-Z

{(DUR int+DUR exh)/2} - 2xlsa= degrees of overlap at .050

(224+220)/2 - 2x116= (444/2) - 232= 222-232= -10

Billiumss

iTrader: (11)

I know that formula, but is overlap always measured at .050? I just figured it might be a different formula at .006.

DAPSUPRSLO

iTrader: (14)

You don't need any pics to compare lobes. What you need is the duration at .006" (advertised), .050", and .200". These numbers are commonly available but will not tell the whole story. You need duration numbers at .100" increments all the way up to peak lift to really know the difference between lobes, try getting that out of a sponsor though.

Billiumss

iTrader: (11)

If you had all the values, you could "plot" the lobe and compare to others so you could see if you get more mid lift than other cams?

So when a cam manufacture states is has an "aggressive ramp rate", does this mean it just opens and closes faster than other lobe designs?

Would a "aggressive ramp rate" cam have less overlap?

What lobe design does a stock cam have?

I know, I know, a lot of questions, but you guys are really helping me understand some of this stuff in more detail....

I greatly appreaciate it!!!

Bill

PREDATOR-Z

in general a cam with a 49 or lower ramp rate is an XE-R lobe or similar (Xtreem Energy Race) Comp Cam.
53 ramp rate is XE or similar. Comp Cam.

DAPSUPRSLO

iTrader: (14)

Do you mean a lobe intensity of 49 or 53, i.e. the difference between the advertised duration and at .050" duration?

DAPSUPRSLO

iTrader: (14)

Agressive means you have a faster opeining and closing.

Overlap can be determined at all lift values of the camshaft, therefore, a more agressive ramp rate camshaft may have more ovelap at .200" and less at .006". There is no general answere to the question of does more agressive ramp rates equal less overlap. All numbers must be examined unless you only want to know the overlap at a specific lift.

Who knows what design the stock cam uses, probably a very soft one for years of longevity out of the stock valve train.