3900 Info

[CNCguy]

I posted this in another thread but wanted to put a copy here to keep all the 3900 info together. I made a quickie template of the 3900 port and layed it on the 3400 LIM for a visual comparison. I put some red layout die on the manifold for contrast.

[CNCguy]

They did increase the base circle to 35.5mm diameter and IIRC, the old base circle was 32.0mm. The main tube size also increased so the wall thickness of the lobe only increased to about .5mm thicker (5.0mm vs. 4.5mm). Also, the 3900 has a slightly higher lift than the 3400 (.287 vs. .2727).

I wonder what Bens software shows for 20, 30, 40, etc degrees retard since the VVT can retard up to 54 degrees from the base numbers. Curious about the effects of such a large LSA and VVT.

You said the base circle was larger than on the earlier cams so shouldn't that help provide room to bring the LSA back into something more reasonable for natural aspiration? I would like to stay the course having found a good price on an engine but I certainly don't want to risk it given that I'm probably in pretty good shape with the cammed 3500 being a bit closer to the 3900s hp output. The problem is the valve size and runner design in the 3900 may yield an astronomical overall output under boost relative to the 3500 at the same boost pressure. That's something I don't want to miss out on.

[CNCguy]

As an alternative, I am going to check into how much the cam can be reground. Since it has more duration than the older stock cams, perhaps it can be more easily reground to reduce LSA. Other options would be to make cam bearing spacers or make some camshaft blanks. With bearing spacers or regrinds, longer pushrods will be required since the base circle will be smaller.

If it weren't for cost, the best solution would be new cam blanks that use the old style chain setup. Although it would eliminate the VVT feature, the engines could more easily be swapped into older vehicles, stock pushrods could be reused and more cam profiles could be offered.

I had hoped the stock cam could be used since I had a good idea on a mod that allowed the stock cam and gear to be used and had an adjustment for cam timing built into it. I also had a solution to limiting the VVT actuator travel for those wanting to toggle the cam timing.

Oh well..... I guess it's better to know the problems now before spending time and money making prototypes that can't be used. Time to do some more measuring and searching.

Actually, at 10 degrees retarded on the timing, the cam specs will show good power from my 3500 simulation. I don't know that I would be turned off from the 3900 just yet.

I'm still weighing the cost vs. the benefits. My current 3500 cammed against the 3900 and .4 more litres with complication. Not quite the difference between the 305 and 350 small block chevy. I believe the 6spd with more torque handling capacity would be the better investment, but it has its baggage as well; dual mass flywheel, different splines on the input shaft and axles requiring a custom flywheel and clutch to insure holding capacity on high output applications, and combining 6spd specific axle parts with the Fieros axles. Perhaps I should focus on using what I have; 3500 with an apparently low mileage Getrag from a 92 Beretta and spend on stronger valve springs, the Zeitronics datalogger and the Moates Ostrich for the best tuning approach.
I would like the oil squirter on all 6 cylinders but I believe my 400 hp capacity oil cooler will still lend a good internal cooling effect.

[SappySE107]

Actually, at 10 degrees retarded on the timing, the cam specs will show good power from my 3500 simulation. I don't know that I would be turned off from the 3900 just yet.

I think I'm going to stick with the 3500 and get the 6spd transaxle instead. It would be crushing to swap the 3.9 only to discover a moderate improvement if that, due to the cam grind. Someone in GM is working against us regarding the cam journal size preventing interchange with non VVT cams.

[SappySE107]

Ill need some flow numbers, min port area in sq in for intake and exhaust, exhaust manifold inlet diameter, exit diameter, intake manifold upper intake port area, gasket area, max lift, length from the valve seat to the plenum opening (runner length), and maybe some more info.

A new cam will probably need to be ground if you want to make it worthwhile without the VVT. 120.5 LSA is quite a bit. I could try it on a 3500 and see what the power looks like

[CNCguy]

I am curious if this cam is even usable with fixed timing or if it can even be reground to use without VVT. If not, swapping this engine will have to wait til VVT control is worked out.

[SappySE107]

Hmm, probably but I just wanted to see how the cam looked. I need a lot more specs to run anything meaningful for the 3900 simulation.

[CNCguy]

Dynomation won't accept -29 for the exhaust close. Oh well. 120.5 LSA.

Can you bump all the numbers 10 degrees back and run it? 20 degrees? Just curious.

[SappySE107]

Dynomation won't accept -29 for the exhaust close. Oh well. 120.5 LSA.

[CNCguy]

Here you go Ben.

The vavle timing checked at .006 lift is as follows:
Intake Open: 25 BTDC
Intake Close: 63 ABDC
Intake Duration: 268
Exhaust Open: 85 BBDC
Exhaust Close: 1 ATDC
Exhaust Duration: 266

The vavle timing checked at .050 lift is as follows:
Intake Open: 1 ATDC
Intake Close: 30 ABDC
Intake Duration: 209
Exhaust Open: 59 BBDC
Exhaust Close: 29 BTDC
Exhaust Duration: 210

The vavle timing checked at .260 lift is as follows:
Intake Open: 71 ATDC
Intake Close: 41 BBDC
Intake Duration: 68
Exhaust Open: 15 ABDC
Exhaust Close: 80 TTDC
Exhaust Duration: 65

I did the .260 lift to find LSA. Usine this method, it comes up to 118. Still alot.

Wide lobe separation angles are good for boosted/blown engines but I think 120 is a bit much and very likely can only properly be taken advantage of with VVT or boost. The most I've seen recommended with boost is about 116.

[SappySE107]

Can you get the measurements for .006" lobe lift for intake and exhaust open and close, and then the .050 numbers.