BC

I have enough pieces now for an engine I wanted to put together for some time.

I want to make a mutt-motor (in the sense of pieces put together) for an 86.5 roller I am getting ready for the track. This will not be a perfect assembly, but I will still be following best-practices.

It is said that because of the smaller chamber on an S4 head, and the smaller ccs of the S3 pistons, that when they are put together, you get very high compression.

That's exactly what I want. Ethanol will work very well with very high NA compression.

What I am thinking of doing is mocking the motor up with old rings and bearings and assemble the head on the shortblock so that I can measure and determine how much the pistons need to be cut (hopefully a small amount) for the usage of S3 cams with the S4 heads and valves.

The backyard mechanic way is to make a cutter of the same size and shape as the OEM valve head on a shaft and install the head. Plunge the valve into the piston just enough to make room for the valve at full lift with the piston at TDC.

The thing is - the piston will never see TDC with the valves fully open. The piston consistently chases the valves up and the valves chase the piston down.

I guess maybe just assemble everything and verify no valve-to-piston contact first? Maybe without a head gasket to give extra room for valve movements at the upper rpms?

123quattro

Get a set of these for intake and exhaust to notch the pistons. My buddy just used them on a LS build with a 0.615" lift cam. If you send them valves they will make you a replica in a cutter.

http://www.ls1tech.com/forums/genera...ing-tools.html

BC

Wow, thanks. If it turns out cutting IS needed, that is just a no-brainer right there.

I could also assemble the S4 head with one set of valves and put the S3 cam (the one that fits) in and just turn it to see if there is contact. Too rough?

FBIII

I've been contemplating the same build. I was going to take an S4 piston to the machinist and have him match the same cuts into the S3 pistons.

123quattro

Not really. Since the lifters are hydraulic they will compress more than normal without the oiling system pressurized. Someone on here might be able to tell you how much a stock lifter compresses under load. Or, you could take a stock lifter and tack weld it so it won't compress. Then you could run some worst case tests. Or measure up some S3 pistons and cut yours to match.

BC

Hmm. Of course, it would not go down as far. Or get a solid lifter of the same type.

BC

That would probably work.

RPetty2

Sounds fun! Good luck on rebuild and be sure to share pics!

dr bob

BC--

A little bit of children's modeling clay, in a pancake on the top middle of the piston, will give you a very good idea where the valves and piston get close. If you "dry fit" without a head gasket, be sure to add that thickness to your clay measurement.


So what is the mathematical compresion ratio of the combo motor before you start cutting pistons? Can you make consistent enough pockets on the pistons to keep the compression even after cutting? I'm pretty sure I'd have the burrete and the plexi deck plate installed to be sure. I think I remember a horror story here about a mechanic's OOOPS, where an '85-86 block was used to replace a damaged S4 block in a car. From that foggy yet tearstreaked memory, symptom was severe knock/ping but not valve/piston interference.

BC

I'd rather cut nothing at all. I probably will need to to make this work, and if the cutting results in anything below the 10.5:1, its simply not worth it.

If, however, even with cutting, I get above 11:1, it is worth it to me, as I feel a high compression 928 motor could be fun.

E85 is at the hear of this plan - there will be no pinging even up to 13:1 compression, static. Dynamic will be technically lowered by the S3 cams.

As I mentioned - the pistons are chasing the valves. Maybe there is not need of alot of clearance.

FBIII

E85 is at the hear of this plan - there will be no pinging even up to 13:1 compression, static. Dynamic will be technically lowered by the S3 cams.

I have a perfect set of GT cams but was thiinking of running a healthier set of Colins cams to do exactly that, lower the dynamic compression.

mark kibort

its simple. the motor will always be interference. there is not enough piston thickness to make it non interference. the valves are a little larger diameter, so hog out the valve cut outs, (or slits) on the 85 pistons. use the 45 degree BTDC to rotate the engine around with cams tied to the belt. if you want a little safey room should you skip a belt or two, like you can on an S4, make the valve cuts a little deeper . you can do this by hand with the pistons in the car, or send them out to a machine shop for valve depth of the S4 pistons, which is very shallow anyway. (sub 3mm)
yes, you are right, nothing will hit. remember valve timing specs? the valves are droping near 2mm with 20 degreemovment of the crank, so they will never touch, even if the pistons are flat. (this is why claying the engine is silly, and time consuiming) better to assemble the heads with a couple of bolts and then lever the valves down and see how far they go before they hit and where they hit on the piston. you willl be surprised.

i think we calcualated the CR to be around 12:1, right???