odd 2.7 engine torque curve on dyno

[larrym]

need to hear your 2.7 engine experience regarding a '75 911 with modified engine

- a recent rear wheel dyno test - chart below - is for a 2.7 engine with a weber conversion; the numbers look way below what should be expected, even allowing +15% for the rear wheel results

I am wondering if this low torque could be an artifact of stock 2.7 CIS cams with a weber conversion? (I seem to recall that Bruce's book mentions that a weber conversion should include a cam change to the "early E" spec)

cams were claimed to be Motormeister "performance" regrinds, but there is no paperwork to verify - cams are stamped <.143 and .144> on the nose and measure 0.445" total lift; valve clearances are correct;

CR is stock 8.5, and compression tests 150 psi cold on all cyls; the webers have 32mm venturis; a Halmeter shows the AFR to be slightly rich under acceleration, and in the green at cruise;

Engine has MSD ignition with Pertreonix and plugs set at .040, Ignition Timing is 7 initial, 35 total; exhaust headers are early style with sport muffler,

on the road, the engine performs well enuf and starts to "pull" at about 2800 rpm and continues to feel strong all the way to redline; the car delivers about 22 mpg on pump gas.

The big question - ?? - would a simple cam change be likely to have any significant impact on this engine's performance, and if so what cams or grinds would y'all recommend?

or - Any other ideas?

Can anyone can post dyno charts of modified and unmodified 2.7 cars for comparison?

[Walter_Middie]

The best 2 ways to make power are compression ratio and cams – you have a real compromise using the CIS pistons, compression ratio, and cams.

Did you say the motor was rebuilt by Motormeister?

Here’s some stock CIS specs:

1975 911 150 hp @5,700 RPM 173 lb/ft @3,800 RPM (911/41)
1975 911S 175 hp @5,800 RPM 175 lb/ft @4,000 RPM (911/42)

49 state emissions for USA
1975 911S 165 hp @5,800 RPM 167 lb/ft @4,000 RPM (911/43)

California emissions
1975 911S 160 hp @5,800 RPM 162 lb/ft @4,000 RPM (911/44)

I'd like to see the fuel ratio along with your results.

Remember that different dynos can have a wide range of results.

It's not uncommon for mis-matched "performance" parts to have a detrimental effect on actual horse power.

[whalebird]

what he said^^
Pistons are different for CIS cars, and unknown cams need to be identified. I don't know the whole story on MSD ignition, but that may need some sorting as well. Is the fuel system adapted for carbs? The total package will hinge on the proper combination of components which was proven decades ago...before your car was even built.
I have driven many 2.7 "mid year" 911s properly assembled with bone-stock components and they are very lively motors that make good power. If the dyno is correct, you have a problem.

[larrym]

only the cams were claimed to be "Motormeister regrinds" which I learned by contacting the original owner, who has only responded via eMail messages from his wife;

apparently the engine work was done by a local who fixes all kinds of porsches out of his home garage, but I could not get much other info, & he (she) has quit responding

c'est la vie

- the deal is that a '75 911 or earlier is smog-test-exempt here, so putting webers on one is a "cheap fix" (maybe)

the engine is not a bad performer, it's just not a kick-in-the-pants experience compared to some I've driven - I recently also had a bone stock '76 911 for a short while and my feel is that it had more low-end grunt - (I should have kept that engine)

mis-matched parts may certainly be the culprit - I was just hoping it was "only" a cam issue since that would be relatively easy to deal with

If it isn't just a cam issue, I'll likely not invest any new money in this engine

[jpnovak]

That lift you report is less than an SC cam. This is not exactly a performance cam. Of course, without knowing the seat duration or overlap there is no way to compare.

The torque curve looks like a CIS cam. I agree the numbers are a bit low.

Are you sure this is a 2.7? Is this dyno corrected for ambient conditions?

Contact John Dougherty (DRCamshafts). He has some excellent grinds for CIS pistons.

[JeremyD]

Motormeister and performance should not be used in the same sentence