X-post: Cam Shaft Selection, and ITB's vs Single Throttle Body

[Fly911]

I'm in the process of rebuilding my 1987 Carrera race car, and need advice for cam shaft selection. The engine will be taken out to 3.8L (3,746cc), and every single moving part will be changed out. The car is a dedicated track car (DE, so far...), but I will drive it to/from the track and some week end back road driving. But I don't mind to have a pure-bred race engine, and not a street compromise.

The cam shafts kind of determine everything from here on in the project. I have been looking at DC80 and DC100, bot fairly aggressive cams. Those cams will dictate ITB's, I believe I will have severe reversion problems with a single throttle body.

I have also looked at the RSR 3.8L cams. They have about the same duration as the DC80, but a 109 degree lobe separation angle compared to 100 degrees on the DC80 (and DC100). So I will assume that reversion issues are less with the RSR 3.8 cams (confirmed by my engine builder and ECU manufacturer).

I have equal length headers and a Y-pipe exhaust with straight through resonators, so the exhaust extraction should be fairly good to prevent reversion. I also have a 997 GT3 Cup air intake plenum (with two resonance flaps rather than the usual one flap), and 50mm ID runners for each cylinder.

If I go for a single throttle body rather than ITB's, I believe that this solution will be less restrictive for air flow to the engine. I can have a 100mm throttle body, and the air flow path will be very unrestricted all the way to the cylinders. I believe that ITB's (not slide valves) will cause more restrictions.

So my big question is: Will I get more power (HP and TQ) using the RSR 3.8L cams and a single throttle body, or use DC100 cams and ITB's?

[petevb]

Originally Posted by Fly911

So my big question is: Will I get more power (HP and TQ) using the RSR 3.8L cams and a single throttle body, or use DC100 cams and ITB's?

Forgetting the cams, the correct ITBs will make more power and give you better throttle response. Modern GT3 RSRs and Dayton Prototypes, etc go this route for a reason.

Single plenum intakes can offer resonant options that allow for more low end torque, so ITBs push you towards higher RPMs (and hence shorter hours between rebuilds) in order to take advantage of them.

I might start with a target engine lifespan between rebuilds and work backwards from there. There are many changes that ripple backwards from the choice to use high overlap cams, including pistons (valve pockets), compression ratio and hence gas, etc.

[Bill Verburg]

Originally Posted by Fly911

I'm in the process of rebuilding my 1987 Carrera race car, and need advice for cam shaft selection. The engine will be taken out to 3.8L (3,746cc), and every single moving part will be changed out. The car is a dedicated track car (DE, so far...), but I will drive it to/from the track and some week end back road driving. But I don't mind to have a pure-bred race engine, and not a street compromise.

The cam shafts kind of determine everything from here on in the project. I have been looking at DC80 and DC100, bot fairly aggressive cams. Those cams will dictate ITB's, I believe I will have severe reversion problems with a single throttle body.

I have also looked at the RSR 3.8L cams. They have about the same duration as the DC80, but a 109 degree lobe separation angle compared to 100 degrees on the DC80 (and DC100). So I will assume that reversion issues are less with the RSR 3.8 cams (confirmed by my engine builder and ECU manufacturer).

I have equal length headers and a Y-pipe exhaust with straight through resonators, so the exhaust extraction should be fairly good to prevent reversion. I also have a 997 GT3 Cup air intake plenum (with two resonance flaps rather than the usual one flap), and 50mm ID runners for each cylinder.

If I go for a single throttle body rather than ITB's, I believe that this solution will be less restrictive for air flow to the engine. I can have a 100mm throttle body, and the air flow path will be very unrestricted all the way to the cylinders. I believe that ITB's (not slide valves) will cause more restrictions.

So my big question is: Will I get more power (HP and TQ) using the RSR 3.8L cams and a single throttle body, or use DC100 cams and ITB's?

Any of the 3 cams mentioned will require ITBs, I'd go w/ ITBs and the plenum, all run by Motec, the closest to street-able will be the 993 RSR cams
911RSR spec are 104 104 100 80, duration @1mm 290/262, lift 12.1/11.6mm timing 6.2mm
993RSR spec are 80 11 4 104 65 duration ~1mm lift 280/264, l1ft 12.5/12.5mm timing 4.6mm
DC80 specs are unknown to me but duration @1mm 283/265, lift .505/.471" timing 6.2mm
DC100 specs are unknown to me but duration @1mm 292/275, lift .530/.495" timing 6.5mm