I am trying to create a model for the 292 in Engine Analyzer. I have hit a couple of snags with getting all the specs right for the 292. Perhaps the most difficult is specifying the intake ports which are shared. My best guess right now is to take the area of the port and divide it in two for the intake port area.

The intake and exhaust manifold port lengths are also problematic in that they are much longer for the outboard cylinders. I don't have great numbers for either the intake or exhaust port area or port length for either the manifolds or head.

If you can, help me fill in some of the blanks. Thanks, Cris

Here's what I am using for specifications. I've culled these from manuals, forums, and (a big help) Tom's website 12bolt.com:

Stock 292 Engine Specifications:

Bore: 3.75”
Stroke: 4.125”
Deck Height: 10.875”
Rod Length: 6.76”
Piston Pin Height: 2.01”
Head cc’s: 74
Piston cc’s: -30
Gasket Thickness: .04”
Calculated Compression Ratio: 7.6:1
Head Intake Valve: 1.72”
Head Exhaust Valve: 1.5”
Head Intake Port Length: ????
Head Intake Port Diameter: ????
Head Intake Port Volume: ????
Head Intake Flow Numbers:

..05” 27.3CFM
.1” 52CFM
.2” 103CFM
.3” 149.5CFM
.4” 164.6CFM
.5” 168.5CFM

Head Exhaust Flow Numbers:

.1” 49.7CFM ????
.2” 93.1CFM ????
.3” 125.2CFM ????
.4” 147.2CFM ????
.5” 163.7CFM ????

Carburetor CFM (Assume Rochester Model B): ????
Intake Manifold Runner Diameter: ????
Intake Manifold Runner Length: ????

Exhaust Manifold Runner Diameter: ????
Exhaust Manifold Runner Length: ????

Cam Specs:

LSA 111 Degrees
Intake Duration 188 Degrees @ .05”
Exhaust Duration 188 Degrees @ .05”
Gross Valve Lift: .399”

The stroke of the 292 is actually 4.120", not 4.125" and the bore is 3.875" not 3.75". Small difference but it can skew results. Also, you will find that software doesn't accurately predict results from changes done to siamese port engines. So this may be an exercise in futility and frustration. However, there has been a lot of dyno testing done to these engines that can show very closely what can be achieved with various components.

W/r/t "take the area of the port and divide it in two for the intake port area"
Efforts to divide the intake ports vertically (making 6 ports) largely show that the reduced size is more restrictive than the shared port.
Each of the pair of cylinders that draw from a port use it in rotation, not simultaneously. However, the time intervals are not common to the three ports due to the firing order.
The 1 & 2 ports are on intake stroke separated by 240-480-240° of crankshaft rotation, the 3 & 4 have 360-360° separation, and the 5 & 6 480-240-480°.

Unless the program has specific input for siamese ports the results will not be useful.
Have you read Santucci's book?

CNC Dude:

Whoops, yes, thanks for the catch, bore should be 3.875". I was not able to get consensus on the stroke. Santucci lists it as 4.120, but the Chevrolet Truck Shop Manual has it at 4 1/8". Other sources were equally split.

CNC and Panic:

I agree, the modeling of the siamesed ports is a sticking point. Santucci (agreeing with Panic) related that they found each cylinder would not share 50% of the port, but because of firing order, each port could use more of the plenum. (Page 102 of Santucci)

I am talking to Performance Trends (Engine Analyzer) to see what they recommend.

I am hoping to get close enough with the model to see trends as I change the cam, carb, exhaust, etc. Even if this is modeled as a non-siamesed port engine, do you think that would give useful information? Preliminary runs show similar torque and horsepower curves as the literature, though.....

I really could use an estimate of the flow of the Rochester Model B. And I'm also a little stuck on the unequal runner lengths for the center cylinders.

Also port areas if anyone has them would be helpful.

I a doing this for fun as much as anything else. I have had excellent results with EA on a high performance SBC and as I start my 292 project was going to use it to help make parts decisions.

Thanks, Cris

Sorry

Good luck with your project

I thought I would post my results on modeling the 292. I am not 100% sure I have a representative model, but I think it is getting close to having something that I can make changes to and see effects.

To get curves that match a stock 292, I had to model the siamesed input ports as a plenum and the intake port lengths as (very) short. Basically the port length is the length after they split from the siamesed section. (Modeling the siamesed ports as a plenum has been suggested by others.)

Unfortunately, I can't post pics, but here is some tabular data of torque in lb-ft.

The "stock" GM data is something I dug up from a GM publication.

RPM GM Model Delta
1600 280 260 20
2000 276 263 13
2400 272 260 12
2800 264 251 13
3200 252 241 11
3600 236 224 12
4000 212 201 11

The model curve follows closely with the exception it is not predicting the torque peak quite as low as the actual data. The data peaks around 1600, the model peaks around 2000.

Hi Cris,

Are your or GM's numbers gross or net? I did not look it up but I think the GM numbers you list are gross.

Regards,
Rick

Rick:

Yes, those are gross HP numbers. The GM data was taken to the old standard of 60 degrees temp and dry air. The EA model is using the same conditions.

Regards, Cris

Cris,

Did you include or exclude the bolt boss when modeling the 'plenums' (intake ports)?

Have you tried adding virtual spacers to the 'plenums'?

One more note. I have a 1973 service manual. It shows the the lift for cam pn 3848000 (292 cam) is .405 and the compression is 8.0:1.

Also, the manual shows the stroke at 4.12 not 4.120. The 1965 gm truck document shows the stroke @ 4 1/8, which of course converts to 4.125. So GMs own documents have conflicting information.

Maybe back then the tolerance on stroke was +/- .005"?

Rick:

I am using Engine Analyzer Plus. The model is not sophisticated enough to allow specific port geometries. Inputs for the head are valve size, runner length, cross sectional area, head flow table, and basic combustion chamber shape.

I am guessing that a model that would analyze a 3D rendering of the input port would be pretty expensive.

Regards, Cris

What happens if you change the compression to 8:1 (the static compression in the gm manual), lift to .405 (manual) and use 4.12 for stroke (manual). It's not much but it should alter your results.

Rick:

If you bump the compression to 8:1 you see a modest improvement across the RPM range. Maybe 3 to 4 lb-ft or torque. I does not move the torque peak appreciably.

Changing the stroke by 0.005" or lift by 0.006" had miniscule effect.

Regards, Cris

Engine Analyzer Plus The Pro is 499. It better was my car as well LOL But good software.