OK, last weekend I enjoyed time alone with my car and measured my camshaft lift and duration because I had no idea what cam was in it, and i did not want to remove the radiator, etc just to find out. This was the 3rd time I measured lift on this cam, and my measurements are consistant with those collected previously. My exhaust lift is about 60 to 80 thousandths more than intake. Yes, a little error between the reading dates, however still within the "tell the big picture" range.

Please tell me a little about this camshaft grind (what lash, approximate RPM range, etc.) It's in a 406 small block with TH-400 tranny, 3.73 rear end.

The lifters are mechanical. I took measurements from cylinder #1. All lifts are measured then multiplied by 1.5 (my rocker ratio) Now, I marked the harmonic balancer with timing tape in 10* intervals, and collected 92 lift-degree readings for the entire 720* crankshaft duration. After collection, I entered the data into Excel, compensated error by calculating against my known degrees (0, 360, 720), and plotted the curves on a 0-720 x axis. Here's what I found:

Starting at TDC on the compression stroke, the exhaust lobe begins at 55* after TDC, and lifter is open 0.050" by 108* after TDC. It's total lift of 0.496" is achieved at 265* and stays fully open until 275*. It closes to 0.050" at 380* after TDC. Thus the exhaust duration is approximately 272* at 0.050"

Now, the intake lobe begins at 250* after TDC, and is open 0.050" by 337* after TDC. It's total lift of 0.417" is acheived at 465* and stays fully open until 483*. It closes to 0.050" at 590* after TDC. The intake duration is approx. 253* at 0.050"

Both lobes are equally open 0.150" at 355* after TDC, and both are completely closed from 60* BTDC to 55* after TDC.

Thus, this cam would be advertised something like this: solid lifter cam, duration @ 0.05 253/272 int/exh, lift 0.417/0.496 int/exh

So,.......what do informed minds think? I'm going to run this cam for the time being, however I would like your opinions :cheers: :cool:


[Modified by thejaf, 3:17 PM 9/10/2001]

 

I can tell you that a cam with 270/295 duration at .050 would not run at or if it did you would have a 3000 rpm idle.

Generally split cams are a good idea if they are kept in the less than 10 degree range or typically 6-8 degrees Also normal cams have a small variation in int/ex lift.

If you go to any cam vendor web page anything above 240 degrees at fifty they will include statements like ruff idle low gears manual tranny or 3000+ stall with headers.

The hottest cam I ever owned was something like 268/276 and it wasn't streelable.


[Modified by gkull, 2:32 PM 9/10/2001]

 

Is it possible that you have different rocker arm ratios on the intakes than on the exhausts? Might be better to get direct readings to measure cam lobe lift.

 

Yes, these last measurements were collected off the cam lobes, with the heads and intake removed from the block.

I'm changing the heads because compression was too high. I did have vacuum problems and idle was fairly rough, but manageable as long as it was kept above 700 rpm. There are ceramic headers on the car, and when put in gear and foot held on the brake the car can be held back until about 2000rpm, then it can't be held back anymore.

 

Correction (thanks Lars ) The 0.050 duration measurement at the lifter (not the rocker arm) is 253 intake/272 exhaust. I corrected in the original post as well

(hey, I'm learing :yesnod: :crazy: )

 

Starting at TDC on the compression stroke, the exhaust lobe begins at 55* after TDC, and is open 0.050" by 100* after TDC. It's total lift of 0.496" is achieved at 265* and stays fully open until 275*. It closes to 0.050" at 395* after TDC. Thus the exhaust duration is approximately 295* at 0.050"

Now, the intake lobe begins at 250* after TDC, and is open 0.050" by 330* after TDC. It's total lift of 0.417" is acheived at 465* and stays fully open until 483*. It closes to 0.050" at 600* after TDC. The intake duration is approx. 270* at 0.050"

Both lobes are equally open 0.150" at 355* after TDC, and both are completely closed from 60* BTDC to 55* after TDC


To really picture what your saying I looked up cam of mine on the crane site

Crane gives advertised lift at .004 inches. So if you put a magnetic base dial indicator on top of your lifter and rotate the crank clock wise a typical .004 reading would be Exhaust opens at 80 deg. BBDC and closes at 30 ATDC So if you take a piece of paper and draw a circle with a 0 for TopDead Center at the 12 position and 180 at the 6 o'clock for the Bottom Dead Center.

so if I draw it out the cam has 290 degrees of advertised duration

The intake on the cam card says 26 BTDC and close at 72 ABDC plot it out and you have 26 degrees before TDC + 180 degrees to BDC + 72 degrees for after or ABDC which equals 278 degrees.

But the .050 thousands timing is only 224/244 with card event numbers of I/E of 4btdc 40abdc & 56 bbdc 2 btdc

Your plot for intake is (.050) 30 degrees btdc so you still have exhaust going out and then your closing it at 60 degrees after ABDC. You can plot your cams events and your exhaust is opening way early in the power stroke 100 degrees atdc and it closes 35 degress ATDC on the intake stroke. Your motor would have very bad reversion issues and no vacuum. If these were advertised duration numbers and not .050 for a mechical solid cam I would say that you have a hot running machine that revs very nice to 6500+ rpm with good heads and exhaust.

 

253/272 Is a cam on the way hot side and I wouldn't understand the low intake lift number unless it was made for 1.6 ratio or better on the intake side.

 

Thanks Gkull.

I'm not sure what reversion sounds like, but I did measure 8" Hg vacuum from the intake manifold at idle (1000rpm) when the car was running (actually, the vacuum gauge bounced between 8-10" Hg) From what I have learned thus far, manijfold vacuum should be around 15" Hg in a street car. I needed to install a separate vacuum canister to get my power brakes back.

 

When you are measuring the lift on a cam @ .050, is that done before or after the rocker arm ratio? Perhaps it's .050 on the lobe, which means your cam is a little smaller than 253/27x. Dat's-a big-a-cam! Especially with that moderate lift.

Rob

 

Seems to me that you could have had the cam out, and back in, or at least read some numbers on the end, by now, instead of all the second guessing...... All the "guess work" in the world will not identify the cam!

Even if you arrive at a "guess" what good is that? It sounds to me that you want to know for sure what it is or you would not be trying so hard to identify it.

 

It took me a couple hours to get all the data, but I wanted to "map" it out to see what I had first before jumping in and getting too greasy :smash: I'm not going to change anything yet, I'm going to see how the new heads with lower compression and better timing helps drivability first.

I may change the cam at a later date, but I am mostly curious what benefit or lack thereof, is to running a dual pattern cam? And with the low lift compared to long duration, if there's a reason or application for this setup? Also, at what point is the benefit return diminished by increasing the exhuast lit and duration away from the intake?

I'm pretty green at this stuff, but I'm trying to learn :crazy: What dictates the limitations and characteristics of a cam? Intake, exhaust, combination, is one more important than the other?

For instance, in designed remediation systems, then business end is the intake, that's what is sized for the application. As long as the effluent (exhaust) is sized a little larger, you have no backups or problems. In most instances, the larger the better.

Does a camshaft design work in that way? Calibrate the intake for a specific use, and make sure the exhaust does not get in the way by making it even bigger and stay open longer?

I'm trying to learn :cheers: (Ever since I was 3 years old, my dad used to say I asked too damn many questions. I'm not changing now :D)

 

I just reread the thread. What is the possibility that you are measuring intake lift on a wiped-out cam lobe on cylinder #1? Did you try checking other cylinders too?

The other thing is why do you think you have too much compression? What's the symptoms?

 

I really didn't think about that possibility. It may be a good idea to check another cylinder. I removed a couple lifters, and they look brand new. Actually, almost everything in the engine looks very new, not a lot of miles on this rebuilt engine.

My compression resulted in massive amounts of detonation once my timing was up to par. I had my distributor rebuilt by Lars when it broke. He told me that the previous owner had wired the advance springs in place, and that it had no advance, which I'm finding now masked the high compression problem.

The engine is a 406, TRW flat top pistons with 2 valve reliefs, the block has been zero-decked. The tops of the pistons actually come out of the block a hair. And the heads that came on the car were 64cc heads. From what I've learned, the compression was somewhere about 11.5 to 11.8

Lars is in the process of rebuilding a set of 76cc heads from a 400 block for me, and this should bring my compression down below 10:1

 

Not to belabor the point. But, you already have the intake and heads off and it sounds like you "might" suspect something is wrong somewhere. At this point wouldn't you feel better if you went ahead and pulled the cam to make sure it is ok? Sure it is a little more stuff to remove but at least you could sleep better at night. I would hate to think you buttoned it all back up and drove another couple thousand miles and had to yank everything again.

 

hey jaf remember i had the same motor in mine,had the same detonation problems,till i took it out..but im sitting here reading you thread and if the cam did have that much duration at .050 it should have helped out with detonation...

 

Thanks for the replies. The lifter bottoms and cam lobes look great, so I don't think there's any damage to the cam. Talked to Lars a little last night, and while the duration is a little long, the lift is moderate. From what he says, different cams like different engines. It may be that someone custom ground this cam based on the engine setup. Lars told me there's also some fine cam tuning that can be done with solid lifter cams by adjusting the lash. Increasing the lash a little will decrease the duration, if needed. Too much will cause excessive wear on the valvetrain.

Some things I'm considering. There appears to have been a decent amount of professional (and hence, expensive) machining done to the heads I removed. I'm finding that every part in this engine is top notch, if not precisely setup for the street. Almost appears that whoever directed the engine assembly process, picked out the most expensive pieces in the Summit catalog and said "I want that, that, that...."

While not built for a 100% streetable car, I doubt that the cam currently in the car is of poor quality. I don't mind too much tearing the engine down again to get the cam out if I have to. I figure it's just more hands on time learing how to do this stuff :D :chevy I want to get the new heads back on, see how the car performs, and then if needs be, do some research and pick out a cam suited for my engine, based on a comparison of data. I think that would take some time :yesnod: :crazy: I may be a little nuts, but I want to take it one step at a time, and see how/if each step makes an improvement on performance.