Brain dyno - trying to decode lobe centers and durations into HP
 

I was just looking at a post on the Early S Board from a 906 engine owner. His car, running the same ‘69S heads as mine, using 906 cams, returned 210 HP on a dyno at 7800 RPM. My car is a 2 liter S that is being seriously tarted up.

I just bought some cams from camgrinder that met the drivability and rev range characterstics I gave him to work with. I am trying to use the brain dyno to figure out how much power to expect on top of the power band. I am not concerned about anywhere else in the rev range, as these cams are going to make significantly more HP and Torque sub-4000 RPM than 906 cams due to having very fast ramps.

Looking at the 906 cam, it generates significantly less lift than the custom DC44:
906= .463"I/.460"E
DC44= .490"I/.490"E

The 906 runs on a lobe center of 96 vs. the 102 of the DC44.

The 906 runs durations of:
290I/262E @ .040" and
283I/255E @ .050"

The DC44 runs durations of:
264I/254E@ .040" and
258I/248E@ .050"

Since top end HP is essentially a function of how much and how fast you can get air into the motor, I am wondering if anyone with more experience than me has a guess as to how much HP this motor is going to make at 7800 RPM on the DC44 cam, using the 906 info as a baseline. A real dyno run will come at a much later date.

Thanks!

BTW, assume the same CR.

Kenik:

Before I'd venture an educated guess, more information is required.

Heads: stock '69S ones?

Stock-type heat exchangers?

Muffler?

'69 MFI?

Twin-ignition?

CR?

Answer all that and I'll offer something based on similar combinations.

Stock HEs for now; eventually going to headers by George
Muffler is stock for now; something VERY special later (assume megaphones, coupled with the above headers)
'69S MFI
Twin plug: 10.3 - 10.5:1

Feel free to email me on the "muffler" if you are curious. :D

George's headers are too short for these motors,...... :)

Megaphones without a crossover kills bottom and mid-range manners. The effects are exacerbated by longer duration cams.

Any other internal mods? Boattailing? Knife-edged crank?

George's headers are too short? I was assuming he could make them to spec. Let's assume a crossover pipe; since the new exhaust is going to custom fabricated, all is possible.

I wasn't planning on boat tailing or knife edging this motor.

George makes one length in these 1.5" headers but he does do a custom version for us, exclusively. You can use his standard one, but you give up some mid-range power.

All in all, I'd expect somewhere around 190-195 HP if you have good flowing heads.

Sounds like I will be buying my headers from you then. :D Thanks for the feedback. It is actually almost exactly what I hoped.

Let me get you a copy of the dyno overlay that shows the HP/torqu differences between the short (normal) and long-tube headers on some 2.0 litre race motors that are configured closely to what you are building.

You'll see what I mean,....:)

Sweet! I can't wait!

This matter of headers is interetsting, Steve.

I'm assuming you are using 1 1/2 inch pipes?

I wonder whether , with somewhat more rev-oriented cams, eg DE 60's, 1 5/8 with shorter pipes may give a little more in a racer?

Also must decalre an interest!

Kind regards
David

must...subscribe...:D

Hi David:

Yessir,....these smaller engines; 2.0-2.2-2.4, prefer 1.5" pipes.

Larger diameter pipes simply kill the low & mid-range power without a payback on top (above 6.5K) and they generate uneven A/F ratios which can be observed in the AFR plots. The torque curve mirrors the A/F curve so as mixtures vary, so does torque.

IMHO, larger but shorter primaries do not pay dividends.

There is a possibility that some kind of stepped header might work pretty well too, but that would require some detailed and lengthy dyno tests to prove that. :)

Great advice, Steve.

I will pm re headers.

Kind regards
David