I built a little DCR calculator that you can use: https://dcr-calculator.silverlock.workers.dev/

For comparison, my in-progress 3.2SS build — a 3.0SC with Mahle 10.2:1 98mm P&Cs, DC 993SS cams (a bit more aggressive than an M1!), etc — is about 7.8:1 DCR. Staying single plug + running pump gas.

Thanks! I'm going to play around with that, seeing how changing each input impacts the result will help me understand the variables. What are you running for cam advance? That's something I have yet to research for these motors.

Henry was nice enough to run some DCR #s for me in another thread I started, looks like a 9.2:1 piston resulted in a 8.25:1 DCR with dc30 cams. He also suggested 8.35:1 as a safe upper limit to run on pump gas so the 10:1 rated Wossners (actually 9:1) would be below that but the 10.5:1 (actually 9.6:1) rated would exceed it.

So I quit debating and took the plunge...just ordered the 10:1 84mm set. I'll update this thread on my findings when I build the motor in June.

As an FYI...they are currently on sale on Vivid Racing and code MarchMadness takes another 5% off. $865 for a set of 4032 pistons is a pretty spectacular value!

Typically zero. Very few folks run adjustable cam sprockets and instead just advance ignition.

I updated the calculator to simplify a little and it’ll get more accurate if you have rod length (easy) and seat to seat (advertised) duration.

That's what I thought, hadn't seen many/any using adjustable cam gears, wasn't sure if there was some sort of intrinsic advance built into the cam to be accounted for though.

I'm not sure if I'm doing something wrong or what's going on but I'm getting slightly different outputs with your calculator than what Henry calculated for me in another thread. At 9.2:1 compression with 84mm pistons, 70.4mm crank, and dc30 cams (242 duration, 102 separation), Henry calcuted 8.25:1 DCR and I'm getting 7.68:1 with yours. The Wossners I bought calculate out to be 9.1:1 static with 68mm head volume so either way they are definitely safe to run on single plug and pump gas.

If you’re using advertised duration, plug that in. I get 8.26:1 with your numbers, which is what I would expect — my 10.2:1 CR with more overlap (114) and duration (260 deg) has a 8.14:1 DCR despite a higher SCR.

Do you use the duration at 1mm as the "advertised duration" on dougherty cams?

I just added the advertised duration as the 1mm duration and that gave me the 8.24:1 you and Henry both got. These Wossner 10:1 84mm pistons come out to actually be 9.1:1 assuming 68mm head volume, which gives me 8.15:1 DCR with dc30 cams...perfect!

"That's what I thought, hadn't seen many/any using adjustable cam gears"

All 911 have adjustable cam sprockets. Thats why you have to set them up vs just aligning some timing marks.

john

I didn't realize that, I haven't had the pleasure of assembling one yet. I did read setting up the timing was a little tricky, now that makes sense.