where are the horses?

[L8apex32]

Compression numbers are within 6% of each other (measured warm).

Next check will be to verify cam timing. On the possible check list are: try other headers; try another dyno.

[Texas RS]

Sorry to dredge up an old post, but I am chasing a horsepower issue with my 1982 911SC. It does not have Buckley headers. The engine sounds good and responsive, but is only making 175hp at the rear wheels. The strong cars in class are making 215-220hp at the wheels. After a very exhaustive study of the engine with people who know these engines I am still nowhere.

A/F ratio 14 at idle and 12 at redline - a little rich and I am still working it up a little at a time
Timing 36 btdc on 98 octane race fuel


Compression is 155psi on five of the six cylinders, but cylinder #1 is only 135psi. Leak down was good with the worst cylinder only 3.8%. I am getting ready to pull the engine to change the pilot bearing and throwout bearing and am thinking about a valve job while the engine is out.

Any ideas would be timely as the engine is coming out this week

[chris_seven]

Is the leakdown hot or cold? and I guess the worst 3.8% is the one with the worst compression?

In general ssn't there some contradiction in leak down and compression measurements.

If the leak down is good and pistons haven't been changed surely the compression must still be OK.

If compression is good why do a leakdown?

We tend to do leakdowns as a diagnostic if there is an issue with the compression test.

Dyno issues are a whole other can of worms.

Chassis Dyno's can be notoriously difficult to understand depending on the individual type.

The most basic inertia chassis dynos have no torque measuring system and calculate torque from the accel of the rollers.

The way the car is strapped down and the amount of wheel slip can have a significant impact on the determination of Horsepower and this can easily differ by several percentage points on a day to day basis.

A system with a retarder and a strain gauge torque measuring system may be more repeatable but only if the cal is checked in a reasonably regular time frame. I would think at least once a year.

The 'Cal' button on these systems isn't a fat lot of use as all it does is put a resistive offset on the bridge and doesn't take any non-linearity into account.

I always worry when this type of dyno gives identical results on the same car from one year to the next.

On a race cars we build we fit an accelerometer and look at peak acceleration and any drop of indicates a power loss.

A dyno may then be helpful for looking for issues and comparative results from the same session would be valid.

Having spent more than 10 years supplying torque measuring systems for military helicopter engine testing using a variety of very high quality brakes I have always been suspicious when chassis dyno results seem to be more repeatable than we could ever achieve with a precision water brake.

The brake to brake variation on a Turbomecca Makila Dynos is around 2% when you compare around 25 dyno's with a 'Golden Engine'.

I realise I am cynical but I just find it difficult to believe the repeatability claimed for some of these fairly crude systems.

[Texas RS]

The leak down was done by the first mechanic I took the car to. The compression check was just done by another mechanic.

The chassis dyno was confirmed by the "track dyno" - I am losing more than 10 car lengths on the straights when we enter the straight together with me having the benefit of a draft. No question there is a large differential in hp.

Recently in created a math channel to calculate hp based on longitudinal acceleration and drag and at full throttle down the straights it correlates very well with the chassis dyno.

My real issue is on the track, I really don't care what the machine says as long as I have what I need when I get to the track. ...I don't, so I search and search....

[chris_seven]

I know it sounds obvious but have you checked the cam timing as I think this can have a significant impact.

If the chains have worn and have effectively elongated it can significantly affect cam timing and with good tensioners, particularly hydraulic you will never hear any rattle.

New chains always have an early and rapid amount of elongation as the bed in and while the engine is out it must be worth a quick look.

On the 2.0 FIA engines we build we always use fixed mechanical tensioners so when we hear the chain we know its time to adjust and re-time.

They are a real PITA for initial set up but generally we only adjust them once a year.

[Texas RS]

That is one of the reasons I took the car to the expert this week. The cam timing was checked this week and was good as were the chains and tensioners. The cylinders are going to be scoped next and I am going to change the exhaust from a two into one to a dual out exhaust without mufflers.

I am also thinking about doing a valve job while the engine is out for replacing the pilot and throw out bearings.