mark kibort

The cams vs the sprocket teeth are almost identical. the index keyway on the cam vs the lobs and sprocket are close. im going to verify that first. ive already been able to detect the 85 vs holbert cams and measure the 5-8 degrees timing difference. (85s are 5-8 degrees retarted)

Ill try to make some kind of fixture to verify the cams before i do any mixing.

Mark

mark kibort

Thanks for the advice!

I think the B1 cams would be a no brainer if we had a set to drop in now, and didnt require the spring changes . (you have sub-B1 cams available with only 10.5mm of lift? ha ha)

anyway, i was told that fixing the cams had some risk, S4s will probably give up 15hp or so and GTs are not available anywhere. so, its all about whether Elgin will come though tomorrow as promised and have the 85s fixed up!

as far as the car being stock, sure there are many of the preproduction parts on it, but i have a hard time believing there is much of a difference, given that the cams are close to GT spec, and the hp numbers with the same headers give close to identical RWHP. Im sure there could be subtle differences that have given the car better durability, but im willing to bet there is not dramatic differences in most of the components. Someday, we will find out, but hopefully not for a long long time (knock knock!) my only point is that the 928 can perform like it does. any stock GT with headers can do what the Holbert car can. not too many cars you can bolt on a suspension, put on some headers and some big tires and run 1:40.xx at laguna. do you know any???

Thanks for the off line advice too. Hopefully, we can put some smok'n cams in that engine someday soon and see it really move! The job doesnt seem as bad as it looks, but talk to me after its running on sunday! (knock knock again)

Mk

mark kibort

I forgot to ask earlier. How did they modify the 85 exhaust cam to fit the S4?
there has been a couple of ideas tossed around, and someone in the states can do this and HOPEFULLY today at noon, ill know if Elgin can do it (or have done it!!!)

MK

mark kibort

Just saw the finished modifications of the 85 cams from Elgin cams. Like art work!! to the untrained eye, you can barely tell there is a mod. even did the two little additional oiling rings in the cam. also made the entirely new bearing surface, so the entire bearing is still being used. (i just thought there would be a valley and then the thrust bearing surface.)

anyway, Ill post pictures tonight and the holbert car should be running tomorrow! (knock knock!) now, i wonder what the effects of slightly retarded intake cams will be (approx 5 degrees or so vs the holbert cams)

So, it looks pretty darn good. Elgin is willing to do this mod for anyone else that has a set of 85 cams and wants them basically made into GT cams. $400

Going racing!!!!

MK

Louie928

The '85 and GT intake cams have the same duration (219 deg @ 1mm lift). The '85 cams are set for 1.6/2.0 mm lift @ 20 deg ATDC for opening at 11 deg ATDC with 1mm lift. The GT cams use a setting of 2.8/3.1mm lift @ 20 deg ATDC for opening with 1mm lift at 3 deg ATDC. Use the '85 setting for better top end and the GT setting for better low end, or choose something in between.

mark kibort

Thanks Louie!

Is that a function of the lob shape or a purpose timing change for different performance? I mean, i measured the holbert cams at 2mm both sides (probably .1mm +/- due to my measuring technique, but pretty consistant)

so, i wonder why one side of the engine is .3 mm different vs the other ?(#1 vs #6 both at TDC and advanced 20degree and measure) (yet S4 is 1.8 vs 2.0mm)

i understand that the GT goal was to advance the cam opening with the same duration , giving more torque i guess, but where does this "more torque" usually happen? is it a peak torque 4krpm thing, or something lower?
certainly want more in the 5k range, so maybe just use the stock 85 settings?

thanks a mill!!

again, you guys are the best!

Mark
PS im thinking the 1.6mm vs 2.0mm setting is based on the advance of the intake cams. (since the GT cam are more advanced, it stands to reason, that there would be more movement down the lifter with 20degrees ATDC, right?
so, if i use diferent numbers to advance the timing , then the poor old exhaust cam is along for the ride and it may not like the extra advance. am i confusing myself here??
Oh, but then again, the sprockes are all aligned the same , so maybe the GT cams like the advance on the exhaust cams too. Now, is that how they determine when the valves open??? GT and 85 cams look the same as far as the exhaust lobes, so if one is 3mm at 20ATDC and the other is 2mm, then one cam is advanced vs the other. AHHH, my head is hurting!

BC

YES! Elgin comes thorugh. Good luck Mark. I'm sure they will have at least a few customers.

Louie928

Mark,
You are right that the exhaust cams just tag along with whatever the intake cams are set for. We can't easily change the relationship between the intake and exhaust cams. Well, Stirling can. The difference in cam timing between the two sides of the engine is that the cam timing is done with the engine at room temp. When the engine heats up it grows more than you want to think about, and the cam sprockets get further apart. The belt isn't supposed to stretch when hot, and if it doesn't, then the passenger side cam sprocket will come around and be more advanced in relation to the driver (left) side sprocket. That's why the right side cam (cyl #1) has a lower lift reading than the left side cam (cyl #6). The 0.4mm difference between sides is about 2 crank degrees. When the engine is hot, the timing should be the same on both sides. Advancing/retarding the intake cam has more effect on the torque curve than the exhaust cam. The variable cam timing arrangements usually only affect the intake cam. On my GT, changing cam timing seemed to rotate the torque curve around the 4000 rpm point. I have a dyno graph that I made with the timing advanced and retarded 3 degrees from stock. That's 6 crank degrees total difference. You can see the graph here.

http://www.performance928.com/cgi-bi...ss_parent=1125

I should have used more timing difference to get more change for a better illustration, but you can see the effect. Retarded, you get extended upper end torque and an increase above 4000 at a loss below that. I would speculate that the retarded timing on the '85/'86 is to get better top end with the smaller intake valves and longer intake runners as compared to the GT and S4. Just a thought. I don't really know.

mark kibort

Did some double checking again tonight, and it seems all the exhaust cams are pretty close, the intake cams definitely are retarded a bit (85 vs the holbert cams) I suppose the 3mm timing setting for the GT cams determines the opening point at 1mm lift (spec) If i was to change the 1.6 or 1.8mm number for the 85 cams, to 3mm, that would advance the intake cams, but the exhaust cams would be advanced too, and that may not be the right move! anyway, i understand what you are saying about the stretch of the belt and pulley distance space due to to heat expansion.

Im thinking ill just use the 1.6 /2.0 mm number and go with it.


Interesting, i started thinking about the S4 spec being 1.8/2.0 and was trying to figure out why that was. both the S4 and 85 intake cam open at 11degrees ATDC, so, why would the S4 cam be further open at 20 degrees ATDC? maybe it has a steeper cam profile and gets to the lower lift faster and then falls off faster too, due to its 10 degress less duration. ????????????

checked the index keyway and it seems to be right on for the exhaust cams vs the holbert cams, and the intake cams are just slightly retarded .


Mark

mark kibort

this is the new 85 exhaust cam with mods. in the background is the Holbert exhuast cam and the 3rd cam is a stock S4 cam exhaust
(all cylinder 5-8 exhaust)

they even did the little oiling rings around the bearing surface.

very nice!

mk

928SS

elgins did a nice job on my custom gt stuff too - they have a special indexing jig setup for the 928 that gives perfect results... nice guys too.... my only concern w/messing too much with the timing would be screwing up the intake pulse, which could lead to some strange behavior... IIRC, I heard a story about a custom cam that had a 150RWHP HOLE btween 5k and 5500rpms... made a few folks rather crazy for a while/till they used different stuff.....

Louie928

I resemble that remark!

mark kibort

The cams are in . set the cam timing, starting from aligining the pulleys with at the original marks from the holbert cams. turns out to be a pretty good way. driver side was just under 2mm at 20degrees ATDC. (spec for 85 cam is 2mm) and passenger side turne out to be around 1.8mm. (spec for 85cam is 1.6mm and S4 is 1.8mm) since the holbert cam was timed at about 1.9mm for the passenger side cam, i figured id split the diff. and leave it alone.

covers and distributor caps on tomorrow am and hopefully it fires and runs like it did!

A little worried about the bearing clearances, as the exhaust cams when installed, seem to stick in the front bearing journal. (but, this was before the cam was clamped down) however, when clamped down, and then unclamped with the clamping tool, the cam kind of stuck in the bearing. I HOPE its just do to such tight tollerances, and when the exhaust cam is unclamped, its at an angle and the thrust bearings are wedging the cam in place. so many things to worry about, and im worrying about all of them!

hope it runs and runs for a while

by the way, the new cam chains are much tighter than the chains originally on the Holbert car. I bet the additional 4 teeth broken on the driver side was due to the chains being stretched and the tensioner having a little binding /jamming, that ive fixed. (little crimped spring retainer on the tensioner piston, was ajar and now in place and recrimped )

Mk

lorenolson888

[QUOTE=mark kibort]I bet the additional 4 teeth broken on the driver side was due to the chains being stretched and the tensioner having a little binding /jamming

Sounds like a good guess... I know from riding bikes that worn teeth was a consequence of chain stretch.. maybe cracked off teeth is symptom of bad tension and stretching...

Good luck with getting it ready for racing... let me know if you need a hand putting it all back together... I am about 15 minutes from you...

John Veninger

Well did it run???