Who cares what's going on a BDC? The exhaust is open (usually) by then.

Using your example, the short rod would be that much QUICKER around TDC which affects intake centerline.


Before I lost some of my OCD, I used to degree several intakes and several exhaust lobes. It was senseless unless you are running Comp or Pro Stock, maybe even Super Stock. But reality settles in and I realized I had to live with what I had because I couldn't afford to get another cam.

Also, the fact that cam twist and flex changes the picture.

I just set the number 1 intake lobe where I want it, and start to test.

A now deceased really good west coast Mopar racer and engine builder told me he degreed every lobe in five degrees increments to compared the lobe lift rate, timing and closing in relation to TDC on all eight cylinders :shock : His motors held the SS/BA, SS/O,SS/N and SS/M records in the time I knew him RIP, Joe

Sadly, most people treat the Stock and Super Stock guys like knuckle dragging, square jawed apes.


In reality, they do less with more than most people could ever even dream of. I have huge respect for those guys.

I think the average person who just goes out to see the fuel, jet, and pro stock / pro mod cars have no idea what stock and super stock is?
I'm fortunate to know a few of these guys.

You lost me here? At BDC intake lobe the exhaust better not be open?
Yes, a shorter rod has less "dwell" around TDC, but this would be the overlap area, not intake centerline.

You can have different cam grinds with various duration and overlap, but the intake lobe (when degreed in) centerline usually falls within a small window when the piston position is around 3/4 stroke length from TDC. I was just curious how this came to be, and if there was significance to the piston position?

I know it is sort of like looking at the cam design backwards, starting with the lobe centerline(s) in relation to piston position, then selecting the opening or closing point and doubling it to get duration. For example, you don't see intake lobes ground with an opening point way before TDC, without a really late closing point (high duration, but normal ICL).
Example1 - 292 duration 108cl, intake opens @ 38 degrees BTDC, and Closes at 74 ABDC

If the closing point was much sooner (lower duration, with same early opening) than we would see intake lobes installed in the 90-100 degree range.
Example2 - Intake opens @ 38 degrees, but closes @ 58 degrees ABDC, results in the lobe center moving to 100 ICL, with only 276 degrees duration.

Example3 - the exhaust duration on both cams is 292, and we want equal exhaust overlap of 38 degrees ATDC, then the first example had the LSA of 108 (cam installed straight up), and the second example has a LSA of 104 (cam ICL = 100, so advanced 4 degrees.)

451Mopar I want to apoligize for taking up some of your post on the wrong subject that you were asking about. Streetwize made me realize about how I was forgetting that the rod does not travel in just linear motion and my brain just kept thinking that it did. So in theory even if you turn the crank 90 degrees half of its stroke the piston wont be dead center in the stroke. I just kept thinking since the crank would be turned 90 degrees the piston was halfway through its travel but I was forgetting there is side to side and linear movement on the rod. Streetwize even gave me the formula about it. So again I am sorry to waste some of your time on your post and I know I did not think it through which made me wrong.

On another note this post made me remember that about 30 years ago I had to put a timing chain on an little Import truck as I think it was a Datsun. But I could not find any timing marks on the gears so I tried what an older tech had told me years ago. And that was to just set the lobes of the #1 cyl in overlap and let the cam relax and center itself in overlap in between the the intake and exh cam lobes. Then I just timed it off the exh and it worked good. I really cant add much to your question that others have already said about the intake centerline other then advancing the centerline can help bottom end and retarding can help up top. I was told that some of the FAST racers were retarding their cams to kill some bottom end and not blow the tires off that they have to use. And so much matters on the overall combo as what works best on some combo's may not work the best on other combo's. Ron

I think I get what your asking now, I hope.

You are stuck with LSA's and ICL's exactly because of what you have pointed out. They are a function of duration and opening and closing events. And the fact that all the lobes are on the same cam.

You can only open the intake valve so, fast and so early because the piston is hanging around TDC. Once the piston is going down the hole, you can let the intake valve literally. chase the piston down the hole. Newer lobes generally open later, open faster, get to max lift longer, stay ther longer and close faster than lobes from 30,40,50 years ago. You decrease the seat to seat duration, while gaining duration at .050 and even more importantly, at .200. That's why intake valve to piston clearance can be as close as piston to head clearance. If the parts are correct, you should NEVER bend an intake valve.

The exhaust is different. It is opening as the piston is coming UP the bore. The piston is chasing the valve back to the seat.

Thanks to all who responded. As I mentioned, this was a theoretical question, not a practical one. For example, it the connecting rod was infinity length, then the piston position would follow the crank position, but because the rod ratio (rod length to crank stroke length) is in the 1.5:1 to 2.0:1 range, the piston position(s) and speed is different than crank position.

I was just curious why most cams "degree in" with a centerline around 106-108 and if changes in rod ratio would (or should) affect the intake centerline?

Every cam grinder I use asks me how long my rods are, and what the stroke is. If they don't, I call someone else. So I say, yes, R/S ratio will affect ICL and LSA. I still think that the 340 should have had a 6.500 rod. That would be a 1.96 R/S ratio. I was collecting parts to do it with some Groden rods and W-5 heads. Then I sold all that crap and quit trying to outrun my 15 second budget.

Now I regret not doing it. I have some thoughts on seat angles and such, and how LSA and ICL for ratios that high.

I think you have that backward ie they do MORE with LESS.

u


Damn pain dope. You are correct.

OK explain this--you put the old McCandless .650 solid in a 440--it usually goes in straight up depending on timing set used in the 110-112 range using centerline method--like that it won't pull a greasy string out of a dead cats azz--you advance it to about 101 centerline method and it will yank the slober out of your mouth. ( I put it at about 104-105 in a 500 in for same effect) This is all based on old iron heads ported until the grinder blew up--and seems to remain the same with good heads including Indys etc
FWIW I always had best results in bracket small blocks using iron heads with a 106 in at around 102-103 for 1/8th --no matter what that seemed to always work the best -- I "inherited" a footlocker full of small block cams from Herb one time--it was years worth of testing centerlines--there were cams with 98-108 and most were 101-105 range have to remember these were probably used in 4 speeds leaving on the wood with tunnel rams and shifting right before the rods exited so.....
school me smart boys

It has very little to do with school. It has to do with Max lift occurs if you are talking intake center line. Sometimes it has to do with gearing. How well the engine recovers RPM on gear changes. If you decrease the centerline you open the intake earlier, therefore closing it earlier.

Better heads will allow wider LSA's and later intake center lines. I have never been a fan of sticking a cam in 4 ahead and calling it good. It should be tested on the dyno.

All the stock head Mopars, SB,BB and 426 Street Hemi motors I've tested ILC on at the track and on the dyno liked at least 4 degree advance on the intake lobes fromm the LSA, no matter what LSA
I have ran some of the bigger C.I. street stroker motors with no advance or a max of 2 on the ILC to kill some bottom end to keep the tires from melting The bottom line is you have to test to get results and valid information

I though advancing a cam just moves the torque lower?
Seems advancing the cam, on the intake side would close the valve sooner after BDC which would build more dynamic compression ratio, but may reduce the amount of air/fuel captured at high RPM (unless the duration is very large to start with.) On the intake opening side, the valve would open sooner before TDC which may pull some air/fuel through the head and into the exhaust, but the exhaust valve closes sooner too, so I'm unsure if this would help or hurt overlap? The exhaust valve also opens a bit sooner, but not sure how much that would take away from total output because the volume above the piston at exhaust opening will have increased many times reducing the combustion pressure. I read a good article on this where they compared the expanding combustion volumes on a low compression engine and a high compression engine, where the exhaust opening timing was more important on the low compression engine?

Yea it does make a lot more cranking cylinder pressure advanced

I am just poking the bear--folks spend weeks picking cams when building an engine--my experience is that if you spend more time on a really great set of headers and exhaust, getting distributor curve perfect for your combo then it will be time better spent--cams really get guys excited though
I have gone really fast with "too small" of a cam and can honestly say I have never had one that was just a dud--don't put too big a cam in a 318, put bigger than you think in a slant six , never use a hydraulic in a Hemi, and lay the lobe to it on drag only big blocks and you can't go wrong
>590 Mopar grind for entry level drag in a big block is hard to beat either Mopar or better yet Racer Brown 590--after that go roller --get springs just right for the application use titanium retainers on everything you do and keep on keeping on
Think and study about how to win races not how to get a few more HP from this grind or that one--- That is time and research well spent--not calling every cam guy on the planet

Yes, there is alot more to to making up the entire package. The original question was if there was some specific importance to the cams centerline, or if that is just where it has worked out best in testing over the years.

Dynos seem to really like the wider spread and drag cars usually like the shorter ones in my experience

Is that because dynos don't shift gears? I think the narrower LSA gives a higher and peakier torque curve? Might work better when recovering from gear shifts compared to a longer/flatter torque curve? Just my guess FWIW?