[snaranjo]

I have a rebuilt 289 that is set up with Dart aluminum head (1.6 intake/ 1.94 exhaust, 170 cc, 62cc chamber), dual-plane air gap manifold, 600 cfm Edelbrock, Comp 260H cam, Pertronix II ingition, and shorty headers. I was told it runs about 9.5:1 to 9.7:1 compression by the builder, but I have been very disappointed with the HP and torque behavior on the street and decided to have a local shop do a quick compression and leak down test. The compression was around 125 PSI and adding a few teaspoons of oil to the cylinder brought the compression to about 175. A leak down test showed about a 30-35% loss but no real obvious blow-by and it is not burning oil. The advice I got was that 125 is a little low but nothing to be too worried about and 30-35% leak down is in the "normal" range, but at the bottom of the normal range.

A little more history on the engine. The top end has about 1500 miles and the bottom end about 11,000 miles. It seems happiest with about 14 degrees of timing BTDC and about 52 total running about 15 inches of vacuum.

Are these numbers something I should be concerned about? Are they the source of my frustration with the lack of power this motor should be generating? I am no expert and am just looking for some advice on where to go next. Bigger cam, advancing the existing cam, etc.. or just starting over with the bottom end? Thanks for any help.

[PaulS]

How many miles on the rebuild?
The rings may not be seated yet.
I would expect to see more compression but it depends on the cam you are running.
Shorty headers reduce the power and torque an engine produces - if you get a set of long tube (34") equal length headers you will notice a big gain. (as long as you don't go too big and have the rest of your exhaust system fit the headers.

[jspagna1]

Originally Posted by PaulS

How many miles on the rebuild?
The rings may not be seated yet.
I would expect to see more compression but it depends on the cam you are running.
Shorty headers reduce the power and torque an engine produces - if you get a set of long tube (34") equal length headers you will notice a big gain. (as long as you don't go too big and have the rest of your exhaust system fit the headers.

I agree with PaulS, I am not a fan of shorty headers. Long tubes are the way to go and depending on your exhaust, I would go with 2.5" diameter with matching H-Pipe or X-Pipe.

[snaranjo]

I'm running a Comp 260H and the bottom end has 11,000 miles on it - plenty of miles to seat the rings. I'm pretty much stuck with shorties (Hedman Elite) given my R&P set-up, but maybe I need to go with 2.5" pipes - got 2" on their now with Flowmasters.

So, you guys don't think 30-35% leak down it too high?

[Ivy66GT]

You didn't say what kind of rings it has but after 11K miles if they haven't seated I would say they never will.

Leakdown is a test without a lot of standards. What is really being measured is the amount of leakage out of the cylinder at 100 psi. There is a small orifice in the hose, maybe 0.060" in diameter, that the leaking air has to pass through. When it does, it generates a pressure drop which is the reading they use. 20 psi (out of 100 psi in) = 20% leakage. The problem is that different testers use different sized orifices so the numbers are only really useful to compare to other engines tested with that, or a similar leakdown tester. For aircraft use there are standards about what size orifice to use. Larger bore engines need a larger orifice since larger pistons have more ring area to leak. For auto use its whatever the makers thought should go in their tester. 10 or 20 liters per minute is a good leakage for an engine in good condition but few of the testers convert their leakage 'percent' to actual leakage rates.

Is 30% high? It would sound like it but what you need to know is how other engines have tested on this same tester. Was the leakage past the rings or valves? One of the good things about leakdown is you get to know whether its rings,valves or both.

[snaranjo]

That's great information Ivy66GT. I have no idea what other engines have tested on that system, but maybe that is perhaps why the shop was not all that concerned about 30%. He did put a few teaspoons of oil in the cylinder and it brought the compression up to 175 so it looks like the rings which are likely moly as the pistons are forged. The heads have only 1500 miles on them so valve issues seem remote - and they were just adjusted about 200 miles ago.

Still seems like I should be getting way more power and torque out of this engine the way it is set up and its frustrating. I think maybe a air/fuel mix reading is next and maybe a couple of dyno pulls just to see where it is now.

Any thoughts on advancing the cam 4 degrees? The tech support at Comp says I would likely get about 10-15 PSI increase in compression, but are there any cons to this set-up?

[Tominator]

Was the compression test done with the motor at operating temp? If not that could explain the 'low' numbers though it is much more important that 10% or less variation between all cylinders be observed than total pressure.

You say you are disappointed in 'HP and torque.' Have you been on a dyno?

That Edelbrock carb may need tuned. The off-the-shelf metering leaves a lot of room for tweaking.

Never ever install a cam on a street motor retarded or advanced!

No mention of gear ratio?

[snaranjo]

Compression test was done with engine at operating temp and there was less than 10% variation between cylinders. And no, it has not been on the dyno - I think that is next. My gauge has been other Mustangs of a similar vintage with similar motors and gears with a lot more punch. Hell, I got beat off the line by a Honda Element the other day!!

I've played with the carb quite a lot but never done an A/F ratio measure, so that is on my list as well. Need to get a bung welding in beyond the collector.

Got your message on advancing the cam loud and clear! The shop says it may be worth a try and I know nothing about the theory behind it - why would you not do it on a street engine? I have to admit, I was very leary of the idea when it was offered. Seemed like snake oil instead of addressing the real problems.

Oh, I'm running 3.5 in the rear with a T5. Thanks to all for the advice so far - keep it coming Steve

[Tominator]

Got your message on advancing the cam loud and clear! The shop says it may be worth a try and I know nothing about the theory behind it

Sometimes a certain category of racing might spec the cam be of stock configuration. By playing with the cam timing one might realize a competitive advantage without breaking a rule. In this case drive-ability is not an issue but a very narrow RPM at maximum power is. The other instance is to make up for a part that is not in spec. It gets very complicated after that.

I've not seen you state the EXPECTED HP level?

[snaranjo]

I was thinking I would get somewhere around 300-350 at the flywheel with my configuration.

I read somewhere on another forum that some cam set ups already come with the advancement build in, such that setting it up as usual at zero degree actually results in a 4 degree advance. Maybe this is bunk.

[Tominator]

some cam set ups

That's like saying peas are the shape of beans....

IMHO, your HP should indeed be where you expect it.

Keep in mind that just because it doesn't feel like it is there does not in any way say it isn't. Dyno or ETs are the only way to tell.

Is the Petronix configurable? Altering the timing a bit, as opposed to just advancing the distributor, could get 20 or 30 HP easy.

[snaranjo]

Originally Posted by Tominator

Is the Petronix configurable? Altering the timing a bit, as opposed to just advancing the distributor, could get 20 or 30 HP easy.

Off to the dyno the first opportunity I have. Not sure about configuring the Pertronix - will look into that - thanks for the advice!

[snaranjo]

Been to the Dyno - finally - and pulled 175 HP at about 50K and 225 TQ at 35K after 5 runs and some jet adjustments. The AF ratio looked good. Using 30% loss through the drive train that puts me at about 250 HP at the flywheel - a bit far from the expected 300-350 with my configuration. Not sure where to go next.

[snaranjo]

Opps - that should have been 5K and 3.5K!!!!!