mrvedit

This is an LS2 with 243 ported heads, nearly new LS7 lifters, dual valve springs, TI retainers, stock rockers.

I just switched from a custom 231/237 .612/.607 cam to a 231/239 .617/.624 (Comp 54-459-11).

As best I can measure it, the new cam is reducing my Intake PTV to .070 (previously .090) at 5-ATDC.
My exhaust PTV is OK at 0.097 (previously 0.098) at 10-BTDC.

Is this asking for trouble? I like to rev to 7000 rpm.

Here is how I am testing PTV:

1. I have Comp test springs installed on Cylinder 1 Intake (1I) and Exhaust (1E).

2. I have a steel bracket mounted to the head for my magnetic Dial indicator.

3. With 1I and 1E at base circle (TDC for firing stoke), I install stock pushrods,
push them down firmly, install the rockers and begin tightening them down.

4. With the dial indicator on top of the rocker above the valve, I tighten the
rocker until it just barely pushes down the valves by .001. I figure that
takes up all the slack, regardless of how much the lifter is compressed.

5. I turn the crank 360 near TDC for the overlap area. I measure from 15-BTDC
until 15-ATDC in 5 degree increments. At each crank position, I zero the
dial indicator and then push down on the retainer with my fingers until the
valve hits the piston, measuring the distance.

I got this:

Intake
------
10-BTDC - 0.160
5-BTDC - 0.115
TDC - 0.078
5-ATDC - 0.070
10-ATDC - 0.070
15-ATDC - 0.080

BTW - I degree'd the cam and came within 1 degree of specs
BTW - I don't believe my heads are decked. At the firing TDC (cam on base circle), my PTV clearance is .220.

Question - Is the the right way to measure PTV?

Question - Again, is this likely to cause trouble by 7000 rpm?

Question - Would less lifter preload be recommended (e.g. 0.030), or is that a myth?

Thanks.

-TheBandit-

iTrader: (1)

Your method is similar to what I used and seems fine.

What are the full specs on the cam?

-TheBandit-

iTrader: (1)

On step 4 you should put the indicator on the spring retainer, not the rocker. That will measure acyual valve travel instead of whatever the top of the rocker is doing. It will also free room so you can push down right above the valve. Try and see what you get. It may be slightly better.

hymey

thats heaps just limit preload t0 20-30 thou otherwise it will pump up and hit

427

iTrader: (3)

You should be using a solid lifter as the puck may drop and cause your readings to be off. I took an old pair of stock hyd lifters apart and removed the spring and put a spacer inside to hold the puck solid. I do all my testing with these cheater lifters. I put silver paint in the band and on top so they look funny and I don't accidentally install them!

Kurt

mrvedit

Thank you everyone. I will reply to each and then give an interesting update.

427: When I swapped cams on a SBC, I did install solid lifters for testing by modifying two old hydraulic lifters. However you cannot change lifters on an LS engine without removing the heads, which I didn't want to do now. I would test with solid lifters if I was swapping heads and cam.

TheBandit: Thank you for confirming my method and suggesting an improved way of measuring the valve movement. I will give that a try (but see below). The "full" cam specs are at: http://www.compcams.com/Company/CC/c...csid=1403&sb=0

hymey: I recently read another thread that claims "a tight PTV clearance requires less preload" is a myth. The oil pressure is only enough to close up any looseness, not enough to change the preload or valve timing; at least not until the valves float. Nonetheless, I share your concern and plan to run minimal preload of about .030; I already have shorter push rods if needed.

Update:

During my previous PTV testing, it seemed that the spring inside the lifter was fully compressed by the test spring on the valve, but I wasn't sure. Therefore, I thought of another interesting way to measure worst-case PTV without using rockers and with minimal pressure on the lifters.

1. Without rockers, I measured PTV at +/- 15,10,5,0 TDC using a dial indicator on the valve stem. Since +10 and -10 TDC should then have the same PTV value, it also let me fine tune my degree wheel. (.194 at TDC, .202 at 5-ATDC, .234 at 10-ATDC).

2. I then inserted a push rod into Intake-1 and put my dial indicator on the end of the pushrod. (Fit nicely into the PR hole.) I then measured lift at +/- 15,10,5,0 TDC. (.070 at TDC, .084 at 5-ATDC, .104 at 10-ATDC)

3. I then multiplied those lift values by 1.7 (rocker ratio) and subtracted that from the base PTV values in step 1. That gave me theoretical PTV clearances at each crank position.

From that my tighest PTV clearance was only .057 at 10-ATDC (.075 at 0-TDC and .059 at 5-ATDC). This seemed scary low, but in actual operation I suspect that the rocker bends a bit, the pushrod and lifter compress a bit. Also I think the rocker ratio is a bit less at low lift. Still it just seemed too low, until...

The custom cam I removed had been individually measured and I had lift values at many different positions. The measured value at TDC on the old cam was 0.070, exactly the same as I measured in step 2 for my new cam.
Since both cams intake duration and lift are the same, the identical .070 lift at TDC indicates they are advanced the same, I am confident I was running the same intake PTV with the old cam. (I degreed both the old and new cams and verified they matched their spec within 1 degree.)

Therefore, since I ran the old cam many times to 7000 rpm, I believe my PTV clearance is tight, but OK.

BTW - The new cam has more advanced exhaust lobes (higher LSA and less overlap) which I measured to result in more exhaust PTV clearance than the old cam. I am hoping for a bit more mid-range torque. Mostly, I just like to experiment and learn things.

67SS509

iTrader: (25)

I do this also but use orange paint for the chevys and blue for furds.....

RonSSNova

iTrader: (3)

Ted,

My intake PTV came out at .075" and I left it. Been to 7000 on the burnout a few times. I let my heap shift at 6600.

The actual running clearance is likely larger due to deflection of the pushrods, rockers etc.

Fun process eh?

Ron

A.R. Shale Targa

iTrader: (2)

Another thing to consider is the valve springs' pressure loading against the
cam and accelerating the engine...both of these factors do retard the cam
by as much as 1.0 to 2.5 degrees depending on spring pressure and timing
chain type or looseness. This is not a theory as I have measured ICL both
before and after on 18 degree roller cam road race stuff.

mrvedit

Interesting; makes sense. It is clear that the actual dynamic timing in an operating engine, especially under load and high rpm, is very different than what we statically measure at rest.

TheBandit: I tried measuring on the retainer with test springs, pushrod and stock rocker. I then measured around .075 PTV at 10-ATDC. However, I again felt that the spring in the lifter was compressing a bit during the lift. I'm beginning to feel that the suggested .080 value is very conservative and accounts for some compression in the lifter and other "slop" in the way it is typically measured.

I going to run what I have and report it it "blows up". I have a long Michigan winter to then rebuild it.

mrvedit

Yes! In a few old SBC builds I had used much more modest cams (XR-268H) and never really worried about PTV. Just a quick/rough cam-degree check and I was good to go. This time I spent a good 4 hours reading other threads and another 4 hours measuring things myself; I was fascinated and wanted to think about how advancing either the Intake or Exhaust lobes (changing LSA) would affect PTV. Next time I rebuild an engine I will get flycut pistons.

The only un-fun part was getting the harmonic balancer off - as I emailed you, I broke 2 pullers until I finally wised up and put a torch on the balancer. Next time I rebuild an LS engine I will get the crank keyed. Otherwise I would look forward to experimenting with other cams too.

I know lots of people read these threads without participating. For those, I found this extremely detailed how-to thread on ls1howto.com useful as a check-list:

http://www.ls1howto.com/index.php?article=23

(F-Body Cam and valvespring swap)

Ted.