weak 451

[6packCuda]

My 451 just doesn't have the punch that I think it should. This is the engine that was in it when I bought the car and I never was able to find out much info about what exactly had been done to the engine. All I know is it is allegedly and 451 stroker with a six pack. The cam is a Comp Cams 284/507. TTI headers with 3" exhaust. I ran it on a Dyno and only managed 256 RWHP. It was running pretty lean, so went through gave the carbs a good cleaning and bigger jets and adjusted timing a bit. On the second pull it got a little better with 280 RWHP. Better, but still pretty weak for 451 cubic inches. I'm not really sure where the problem is. The engine seems to run great. Doesn't miss, and runs smooth with no hesitation or bogging on acceleration. I don't know what heads it has. It's a 72' block. After reading some of the forums here I've learned I should do a compression test and a leak down test. Can someone tell be the correct procedure for doing this and what kind of readings I should get?

[Oldschool]

While it is a good idea to check the compression and tuning of the engine, it could be other things.  Is it a manual or auto tranny?  If auto, could be eating some power there, or the torque converter could be ready for replacement.  Also, dyno's are limited by the software and the operator.  All dyno's are not created equal.  Check around and see if there are others in your area that get different readings.  Also, check the ignition timing and make sure all 3 carbs are opening fully.....   Just my 0.02....       

[6packCuda]

Oh yeah, it's an automatic. No idea what stall. I'm also posting a copy of the dyno results. It's kind of embarrassing.

[Killer_Mopar]

Mustang dynos numbers are generally 5-10% lower then dynojet numbers, so your would probably be around 300 WHP on a dynojet. Factoring in an 18% loss for the auto and you are looking at around 350-375hp at the crank, which sounds right for a 383 but is quite low for that 451. What was the air/fuel ratio looking like on the dyno? If it wasnt a tuning problem, then the first thing I would do is pull/check the plugs and at the same time do a compression test. If everything else checks out, I would then check to see if the engine really is a 451......

[Chryco Psycho]

I agree the Mustang is a ***** to get a good # on , I would expect to see more in the 325-350 range even on a mustng , you could have any # of issues from bad machining & porr sealing internally to poor cam choice , poor heads , on & on

[Hot_Rodder]

You asked how to do a compression test and leak down.... I didn't see that answered, so here it is.... Pull all plugs out of the motor, hook the gauge up and install the gauge in place. Also, for safety, unplug the coil wire so you don't have sparkplug wires trying to get someone or something..... Anyways, once you got that done, reach in, and spin the motor, and let it make about 3 revolutions..... This way you get a more acurate reading on the gauge (hopefully the gauge works .... Jot down cylinder #'s 1 - 8, and note the compression readings for cylinder, and let us know what they are. Now as far as the leak down, leave the gauge in place after you have spun the motor over, do not release the pressure out of it, walk away, the gauge should still be reading what ever it was, come back and see what it is, typicaly it should still be the same. Here's the fun part though, with the hose that threads into the head and that the gauge hooks into, that hose has a shrader valve in it.... This is so that when doing a compression check, it want let the compression back out.... when doing the leak down (unless there's a simpilar way that I don't know about) test, you need to take the valve out so that way it doesn't hold the compressed air in. Now, another fun fact... The valves have to be closed so compression want leak out, and the piston will have to be at TDC..... Get all that, I hope so, sounds like a pain, but I've had to have done this several times on cars at the shop...

[v8440]

I thought leakdown tests used a setup where you introduced air from a compressed air source, and used a different gauge?

[moper]

Yeah, I think it's low. The A/F mix says 12.2 for the second pul...but we cant see the entire graph. It looks to me like the 6bbl is either just barely opening, or not opening at all. On every six pac I've dyno'd (chassis dyno) there is a momentary drop in the hp and tq  as the outboards start to open. You said you changed jetting and timing..What did you change them to for a final value? What springs are in the pods? If you have iron heads I would be running something like 82s in the primary, with either purple or tall yellow springs in the pods. If possible, you may want to invest in the Pro Max jet plates for them so you can richen the outboards slightly too. It sounds like a fairly stout combo, I'd be looking for at least 350-370hp to the tires with it regardless of dyno type.

As for doing a compression test... That is like was said above. Run the enigne to warm it up. Pull all 8 plugs and I disconnect the dist so no spark. Wire the center carb at wide open throttle. Then test eachcylinder, turning it for 3-4 revolutions per cylinder. Write the reading on the gage. You should have all cylinders within 10% of each other. How high a reading depends on your exact build. But I would say anything lower than 130psi indicates a possible cam timing problem.

The leakdown test... That is nothing like above. Typical compression gages do not "bleed down"...you relase the pressure with the schrader valve. So they will not indicate a loss unless the gage itself is bad. You need a leak down tester. That is a small manifold setup, with two gages. You connect a good compressed air source to one side, and the other goes to the spark plug hole like a std tester. The piston needs to be at top dead center on compression so the cylinder is sealed. Then, the shop air source is set to typically 100psi. That's your reference gage and pressure. The second gage reads what the cylinder is actually holding. So if the reference is 100psi, and the cylinder shows 93psi, you have a 7% "leak". If the pressure is a lrage amount, you can hear where it's leaking to...sound in the intkae means intake valve seats...Sound in the exh is exh valve seats. Sound and vapors from the breather hole of filler cap indicate bad rings or damage to the ring sealing surfaces. A typical engine, in good shape, with standard rings that are seated will leak down in the 5-8% range if the engine is not fully warm. A gapless ring set, or similar will leak down around 2-3%. Anything greater than 8% in a healthy motor to me shows a possible issue. Many times it's just the rings havent fully seated yet if it's like 10%.

[6packCuda]

Thanks for the advice guys. I made the attatchment of the dyno sheet above smaller so you can see the whole thing.
The jets are 65s, power valve is 3.5, outboard plates are 34, adn 34-36 degrees on the timing. How can I check that the outboard carbs are opening all the way (safely)? The linkage seems to move freely.

[Chryco Psycho]

you should Hear the outboard carbs open , you can use a plastic zip tie on the rod by the vacuum port if the carb opens the zip tie will get pushed down the rod as the carb opens up

[moper]

Witht the traces superimposed it's hard to tell..But what I see there is the outboards starting to open at 3400 or so, and not fully open at the 5200rpm test peak. Also, as they open, the mix gets progressively leaner. My fix would be first, replace the pod springs with purples. Careful not to tear the diaphrams. Then, replace the non-adjustable jet plates witht he plates from Pro Max that allow you to replace the jet plate with real jets. run 2 jet numbers higher than the direct conversion from the jet plate size. I think they convert to 72s as I recall. So you would want to install 74 jets in the ProMax plates. See how it responds with that. I'll bet it wakes up a bit...lol. IMO you should be making peak hp in the 5500 range with peak torque around 3800-4K.

[Hot_Rodder]

Moper, you said: The leakdown test... That is nothing like above. Typical compression gages do not "bleed down"...you relase the pressure with the schrader valve. So they will not indicate a loss unless the gage itself is bad.

That is true... What I was saying is, with the gauge that we have, on the hose attatchment that screws into the spark plug hole, if you unscrew the valve out of it, it will not hold the compressed air in place...... If you watch the gauge as you turn the motor over, you get to watch the needle  .... The way we had done it, is a pain in the rear, but it does work...... But it is a pain, mind you that these were done on fairly newer cars, or on motors that only have a few miles on them, but not on brand new cars or fresh motors.... We measured compression drop over time, and determined how to go from there.....  aint it

[BIGSHCLUNK]

Pardon my question... With all this talk of compression / leak down tests. Does your car have  any oil usage issues? 

[6packCuda]

Pardon my question... With all this talk of compression / leak down tests. Does your car have  any oil usage issues? 

No, it doesn't use a drop of oil.

[moper]

Hot, I see your point. But what I was getting at is, you are looking at a loss over time. That's not a steady amount. And I dont know any that will hold pressure when the engine is motionless. That's not what rings are meant to seal against. So you are seeing a drop in PSI over say 10 seconds from the cylinder. But really, it would leak then anyway. All you can say is whether or not it's leaking faster or slower, and there's no baseline to compare to. A true leak down gives you a baseline, and a reading of the amount of loss. That ratio becomes the percentage leaking. As an example, lets say a cylinder pumps 150psi, then you pull the gage's valve, and watch it. It then pumps say 150, and in 10 seconds, it drops to 20psi. That's 130psi/10sec. What is bad about it? More important, what is good about it? If they all do it, you know they are all in the same boat..but is that good or bad? Using that same cylinder: You plug in the air line, and set the main gage to 100psi line presssure. the gage that is reading the cylinder reads 90psi. You know that cylinder leaks 10%. The next cylinder is done. It shows 100psi and 60psi. You know it has 40% leakage. Think of it as reading a tire tread as opposed to sticking it on an alignment rack. You know it's bad, but it cant tell you why until you get the right equipment. You're test is the right track, just not quite there.