drnick

ive had my GTS camshafts reground with a smaller base circle and different profiles, two questions now arise which im hoping someone who has been there and done that can answer. the old profiles were:
*
*************************** in*** *** *** ex
Total Duration******* 252.8**** *240.7***********
Cam Lift*** *** *** *** .374"*******.335"
*
now modified to the following fast road spec:
*
*************************** in*** *** *** ex
Total Duration******* 254*********252***********
Cam Lift*** *** *** *** .400"*******.368"


1) do i need to check the valve to piston clearance with these profiles?

2) how do i time the camshafts when reinstalling?

after looking in the workshop manual i can see that a gauge is normaly used and applied to the hydraulic lifter. is the point measured simply the time of maximum lift?

worf928

Unfortunately no. The spec deflection is part-way into the opening phase and well before maximum lift. (I just did this on the '89 so it's fresh...)

drnick

bummer, that would have made life much simpler. any clues as to how to approach this?

worf928

Nope. Wish I did. You might get some more comments from other folks on the forum. If not, then I suggest a foray over to the e-mail discussion list. There are still a number of hard-core folks in Pure ASCII land.

Z

With the new cam profiles, is there any overlap period where the intake and exhaust valves are both open at the same time? If so, how many crankshaft degrees of overlap is it? If there is no overlap, how many crankshaft degrees of rotation is it after the exhaust valve closes before the intake valve opens? Finally, are you hoping for more top end power, or more low end power?

John Speake

If you have any overlap, then be prepared for a lousy idle. The backpulse into the inlet at idle confuses the MAF...

87_944s

Nick, I have the precise procedure to time the 944S 16V cams (1/2 of your engine, I assume all GTS models were 32 valve..?)

Are the camshaft sprokets slotted...?

You'll need two dial gauges, mounting brackets (nothing to stick magnetic base to) and extensions for the dial gauges long enough to reach .100 below TDC of piston travel and .100 below max lifter height.
You can actually run a thread die of the appropriate thread size (inside thread at hollow end of gauge plunger) to make a male thread on the end of a length of coat hanger to fashion an extension.

I can scan page text (unless scanner doesn't work haven't used it in ages), possibly fax or even post the procedure for you here tomorrow.....?

If interested send a reminder to
944t@bellsouth.net

drnick

i have the procedure as well from the workshop manual, however it lists the specs for the GTS, GT and the S4 variants only. it gives these in measures of deflection as the cam depresses the lifter, with non stock cam profiles i dont have a reference point so easily.

z, i dont know for sure yet but i suspect i will know more before too long concerning your points! i am assuming the new cam profiles will have been ground symetricaly from the existing profiles ie with maximum lift occuring at the same timing point as previous and perhaps from looking more closely at the timing events as you suggest will be the way forward. i assume that all of the 32 valve cams have there lobes in the same places, just with different profiles?

the intake duration with my custom cam is very close to the stock 86.5 that is 254 vs 253 respectively. the GT intake has an even longer duration with 258 so my intake valve will be opening some time after a GT at 3 deg ATDC and very close to an 86.5 my exhaust has a duration of 252, longer than any other stock exhaust duration but syill less than the GT intake. if i had the same exhaust duration as the GT intake i figure that would take my exhaust closing point to about 2 deg ATDC. therefore no overlap?

naturaly z, im hoping for more power all around!

Z

There are a number of things that could have potentially been changed with modified cam lobe profiles. Even if the duration were the same, that only tells you the period that the valve is open for, and doesn't necessarily tell you anything about the opening or closing points. It doesn't tell you if the cam lobe separation angle may have been changed or not, and there's also the posibility of the lobes not being symetrical in their opening and closing. If you can, try to get the exhaust closing and intake opening points in relation to each other.

I'll use the factory GT cam specs as an example of what I'm trying to get at. The published exhaust valve closing point for the GT is 5 degrees before the piston reaches top dead center. The intake valve opening point is 3 degrees after top dead center. That means that there is no valve overlap, and that the intake and exhaust valves are both closed for a period of 8 degrees of crankshaft rotation. The center point between the closing of the exhaust valve and opening of the intake valve would be 4 degrees after the exhaust valve closes, and 4 degrees before the intake valve opens. (8 degrees divided by 2) The result is that the center point would then be at 1 degree of crankshaft rotation before the piston reaches top dead center. The relation of that center point to the piston top dead center position of the crankshaft is what you change when you adjust the cam timing using the slotted cam sprockets at the front of the engine. Setting that center point in a more retarded position in relation to the crankshaft will tend to shift the power band higher in the RPM range. Setting the center point in a more advanced position in relation to the crankshaft will tend to shift the power band lower in the RPM range.

Once you have the engine together and running, you may adjust the cam timing one way or the other, depending on what you're specifically looking for in how you want the engine to run. There is really no one "right" setting, since it's kind of a compromise and depends on what the desired characteristics are. To maybe give you a little more of an idea of where to start, in addition to the GT cams spec of that center point being 1 degree before piston top dead center, the stock specs for the '85/'86 cam timing place that center point at 7.5 degrees after piston top dead center, and the S4 specs place the center point at 6.5 degrees after piston top dead center. Maybe part of the reason for the GT's lower bottom end and higher top end as compared to the power curves of the S4?

Once you know the exhaust valve closing point and intake valve opening point in relation to each other, (how many crankshaft degrees apart those events occur) you can get an idea of where to start with the cam timing. There may very well be other ways of doing this, but that's how I would start if using non-stock cams of unknown running characteristics for the first time. When you do this, it's better to set the cam timing using an old timing belt. Cam timing changes when a new belt goes through it's initial stretching. Set it with a new belt and 2,000 miles later it'll likely be slightly retarded, with the passenger side being off more because it's further back along the path of the belt. Another thing that may be worth mentioning is that the cam timing changes as the engine warms up. The cam timing you set with the engine cold is not the same as the cam timing with the engine running at normal operating temperature. On my car I measured the cam timing advancing by three crankshaft degrees as the engine went from running cold right after startup to running fully warmed up.

87_944s

Yeah, from what I'm understanding you'd start at timing the #1 cylinder intake valve opening with the #1 piston movement just as standard procedure, the cam work would be symmetrical within the 360 degree of 1 revolution for all intake or exhaust lobes. If you were not to time the cams as per factory spec then I don't see how the machinist could have had a point of reference to insure the work would be effective at achieving his/your goal......

UKKid35

I suspected this when setting the cam timing with a new belt buring my engine repair, but I found no mention of this issue in the WSM. It would have been nice if there were different specs for brand new belts to deal with this issue, particularly the imbalance between cylinder banks. Sadly it wasn't possible to use my old cam belt...

FlyingDog

If you assume (or find out from your machinist) that your maximum lift is at the same point as the stock cam, you could try this. Have somebody with a stock cam take a measurement of the cam timing at maximum lift. Then set your cam at maximum lift, and set the cam timing the same. After that, you can measure your lifter at the same point as the manual states for adjusting timing and use that measurement in the future.

drnick

i am sending an email to piper cams to confirm the cam specs... dont go away though, im still not sure ive got this clear.

drnick

z, just re-reading your post. it seems that the cam lobe seperation angle did not change for any of the 32v cams, hence the reason people are now mixing and matching to optimise performance?? assuming my cams have been ground symetricaly and without any change to the lobe seperation angle and presuming i can find out the crank angle between my exhaust closing and intake opening, how does one then set the timing - practicaly? it seems i could choose somewhat arbitrarily between the GT and 85-86s 1 crank degree vs 7.5? what i mean is, is it practical to determine physicaly these points on the engine?

drnick

does anyone have any insight into the piston to valve clearances, i see phil threshie reccomends a grind not too dissimilar to mine as part of his level one package. anyone care to comment?