JörgenK

I played around with the supercharger settings in EngineAnalyzer standard edition and found Eaton MP90. I didn't understand that this was a 90ci supercharger, that is a bit to small for our engines. with a 2.2 ratio it spins at 14300rpm (14000 is max) when the engine is at 6.5k. Engine analyzer showed 355hp at 6500rpm.

The MP112 is a much better match. around 6psi boost and 395hp at 6.5k with 2.0:1 belt ratio.

These numbers should be taken with a grain of salt, the supercharger simulation seem a bit lame.

ryoung

using my previous model with a 2:1 belt ratio also yielded 6.0 psi boost.

I also get the warning that the cam has too much overlap resulting in 3.7% short circuit @ 2500 rpm and 0.9% short circuit @ 6000 rpm ... air:fuel mixture goes straight thru the combustion chamber and out the exhaust valve. Another warning reports detonation likely.

ryoung

I have zero first-hand experience with forced induction, but the short circuit problem seems serious. Without cams with less overlap, I can easily imagine that any significant quantity of unburnt air:fuel mixture is likely to melt the catalytic converters (to say nothing about emmisions). I ran a simulation with the S8 motor, and as you'd expect with it's more radical cam, it gets even worse.

Since the problem is worse at low rpm, is this why MTM and others use centrifugal superchargers on A8/S8 motors? These SC's don't provide much boost at low rpm?

Any thoughts on how PES addresses this with their 2.8L superchargers? Their kits don't include new cams.

Am I missing something?

KaMaKaZieP -ALUMIN8-

Im no genius with s/c's but would a blow off valve solve the added air at said rpms?

ryoung

We definitely need an expert here!

As is, we're modeling running with only 6 psi of boost. Though short circuiting is worse at low rpm, it occurs over essentially the entire rpm range. Reducing boost further will quickly make the project barely worthwhile.

Overlap is the number of crankshaft degrees that both the exhaust and intake valves are open simultaneously. The 4V ABZ motor has 82 degrees of valve overlap; and the 5V AYS, 126 degrees. You do want enough overlap to allow the incoming fresh air:fuel mixture to displace the exhaust gas, but anything beyond that and you're just blowing fuel into the tailpipe. Overlap in time is longer at low rpm, 6 times longer at 1000 rpm than 6000 rpm.

Note my <A HREF="https://forums.audiworld.com/a8/msgs/61139.phtml">Earlier Post</a> on the intake pressure that results from just natural resonance tuning in the intake manifold runners, it's less than 0.5 psi below 4000 rpm but hits 4.3 psi @ 6000 rpm, and the result is zero short circuiting over the entire range. I'm sure there's some room left for additional boost, but it looks like not much. With a supercharger, I believe our motors will need new cams with less overlap, when you can just pack the mixture into the cylinders.

JörgenK

Can someone post the IVO, IVC, EVO and EVC of the ABZ and the AYZ engines?

Duration and lobe centers (or lobe separation) is also ok.

I use the ABH specs for my simulations... Assumed that they were similar to ABZ.

ryoung

Nuts, I need to redo all the modeling. Three mistakes,

1. IVO opening is after TDC, I assumed before.
2. EVC is before TDC, I assumed after.
3. The Bentley contains a typo. A 2nd column that I assumed was the AYS motor is actually the AKB/AUX motors listed twice, and I don't even have valve timing for the AYS motor :-(

The valve timing below is at 1mm valve lift.

.......................... ABZ ...... AEW ... AKB/AUX
IVO after TDC ....... 10 ......... 10 ....... 25
IVC after BDC ....... 40 ......... 25 ....... 45
EVO before BTC ... 30.5 ........ 30 ....... 38
EVC before TDC ..... 2 .......... 10 ........ 8

Overlap .............. 58.5 ........ 35 ....... 50

What do you have for the ABH motor?

JörgenK

The ABH cam that I have used for my simulations is about as mild as a cam can get. I guess that the ABZ cam is similar.

For the ABH I have (at 1mm):
IVO: -7deg BTDC Note that the cam is mild enough to have a negative IVO reading!!! That is very unusual.
IVC: 29deg ABDC
EVO: 31deg BBDC
EVC: 2deg BTDC

The interesting numbers are EVC and IVO. Measured at 1mm the overlap is -9degrees. I would say that there is no shortcircuit at all...

Check the cam specs that you use again.

JörgenK

Hehe... ABH timing in 'That can't be a problem.'

ryoung

I didn't expect IVO and EVC to both be negative.