mark kibort

where is the overlap? the exhaust closes before the intake opens, right?

mk

BC

Good convo with Todd. He mentioned a few things as we both looked at those specs. He also mentioned someone had "thunked" it up before me, obviously, and had phasing issues (the mixing of the S3 and S4 cams, or GT and S4).

I will get a quote back hopefully on the base circle cutting. If its some huge monstrous number, I will just slap this thing together the way I intended.

Dennis K

Right, according to the factory event timing specs, which are at 1.0 mm of lift. In reality there is some overlap but it's happening at somewhat less than 1.0 mm.

Vilhuer

To change exhaust and intake cam timing a little one option could be to move tensioner system up or down in head. This would make chain on fixed side little shorter or longer and if my idea work like I think it does also change intake cam timing. Together with stock adjustable cam gear both exhaust and intake would be adjustable separately.

Jim Morton

Expanding on Dennis' last post... Rememeber the factory cam specs are stated at 1mm lift. This is similar to many of the US cam companies citing cam durations and timing at 0.050 inches. These duration numbers are good for high level comparisons of different cam lobes, but cannot be used for much beyond that.

If folks were to measure the install height of the 928 lifter on a stock cam base circle diameter, you would see that the mechanical operation of the lifter is reduced to something less than 0.020 inches. With this in mind, different engine builders will choose a lifter clearance for determining operating durations for what the valve sees for a given cam lobe. When the valve opens and closes, affects the broad range engine tuning a lot more critically than duration events at 1mm.

Using a clearance number 0.020 inches or less, there is overlap but still not very much as compared to BMW's or Honda;s 4V's. Applying the amount of overlap as compared to these other pent chamber 4V engines along with other parameters like the intake system in the 928 engine explains a lot of the observed performance of this engine as compared to other 4V's.

Enjoy the quest ! .... I surely am.

john gill

Kepp in mind that , if you decide to go to more radical cams later on , & if they are higher lift than the gt or s3 , you will need to fly cut the pistons (been there done that ) so I would reccomeend that if are comtemplating camshafts , do it now , valve meeting piston is not a happy event as many of the ngine builders will atest to here

mark kibort

Oh yeah, i forgot about all that work i did to see that cam push the valve down at 10degrees after top dead center! it was about 1mm, just like the spec. then, you would expect at the 5degree before TDC closing at 1mm of the exhaust valve, I guess it wouldnt be closed until near 5 degrees after TDC . so , i guess there could be a little slight leakage over the 0 to 5 degree ATDC range between the exhaust and intake.

mk

drnick

john, how much lift and duration can you have before the pistons need the valve pockets deepened?

here are the specs on the modified cams in my GTS engine:

stock -
in ex
Total Duration 252.8 240.7
Cam Lift .374" .335"

as modified by piper -

in ex
Total Duration 254 252
Cam Lift .400" .368"

im currently waiting on a set of modified 86.5 cams from dema at elgin. my instruction to him was to reduce the base circle in the hope that this will give me something more aggressive than the current modified GTS cams. i did initialy have concerns about valve-piston contact with the current set of cams but so far there hasnt been a problem. im hoping i can run the cams that will come from elgin without taking the engine to bits.

the latest news from elgin is that the cams are ground and now awaiting nitriding - i have not had confirmation of the numbers lift and duration wise, yet.

Ketchmi

That's funny, we have two sets of 86 cams with Dema for the same thing at the moment...

Mike Simard

Nick, I've calculated clearance needed for a cam of more duration and lift than yours and was surprised at how little notching was required. I dare say that if you have any measurable releif in the piston already it might be enough. Of course it's better to be careful and verify but don't worry that you'll need some really deep notch.

Just a little optomistic outlook for you, chin up!

john gill

I went to larger intake valves oso we had to widen the existing notches ,not increase the depth , had we kept standard valves , we probably would have been ok , which I thought at the time raised an interesting observation , that there was scope at least with the piston notches for more duration & lift ,how much? I couldnt be sure ,as with a multi valve engine how much margin for error do you leave ? as for rapid accelleration & decelleration & stretch & ****** in the timing belt o allow for float .

One possible problem for larger cams , I observed with mine was that the peak of the cam was past the eedge of the lifter , as it rotated with the ramp it was ok , with larger cams another possible modification problem . it runs wells no problems to date ,will be going on my finished dyno this week , will post results .,

Jim Morton

FWIW. I have the detailed lobe specs (IN and EX) that was used by Dema for all of the Elgin cams mentioned in the last few posts. The lobes look ideal as a good "hot rod" cam for the S4 engine with or without a slight bump in static compression if used in a 5.0 liter application. I would list the duration and lift no.s here in this post, but I have that spreadsheet at home... sorry.

The cam lobe velocity should wipe well on the stock lifter and gives a good amount of intake valve lift at 75 deg ATDC as compared to the OEM cams. The exhaust cam lobe is well suited for the valve size ratio used in the S4 head. Both lobes are fine with stock valve reliefs in the piston tops.

If anyone want more details, feel free to e-mail me at:

jmorton@morton-engr.com

FWIW, I can run the seat to seat duration and lift numbers for what ever your choice is for lifter lash.

Regards

drnick

ketchmi, mine are the very clean straight set that look like new

jim, thanks for the feedback on the cam grinds - did you post the numbers already in another thread? i understand that dema has been ocupied and havent pressed for details from him rather relying on his reputation to do what was needed.

Dennis K

Dr. Nick -

Did Piper say at what lift those duration numbers occur?

drnick

dennis, thats the sum total i got from piper so i assume its at the same measuring point as for the stock cams. the engine makes peak power at around 6400 rpm on the dyno, im hoping the cams from elgin might be a bit more pointy using the 86 profile to begin with.