asmall

I am in the process of rebuilding the engine in my 97 Cobra after a slight detonation problem broke 2 pistons. So far I have pulled the engine out and stripped it down. I am now gathering information, parts, and money to complete the rebuild.

I bought the Cobra with 80K miles on it, put a few more on myself, then promptly slapped a 10 PSI, 3 core intercooled, ATI P-1SC on her. After tuning in the Virginia heat, whe put down 408 RWHP. Not too shabby, considering the base was 264. Doesn't 408 seem like a lot? I know that the car goes like a bat out of hell, and doesn't stick to the road in first, or second, and has been known to break free in third... much to my dismay (an the joy of the black camaro) I need to upgrade the rear end.

I put 10K miles on it and the belt shredded. I found that the ATI idler pulley was severly worn. I contacted ATI and they told me it was a known problem... and were happy to replace it, for 40 or so dollars. Put on my new gatorback belt (slightly longer to ease the side load on the idler and accesory bearings) and promptly ripped it to shreds a couple days later. This time I replaced the tensioner and put a new belt on. That is when I noticed that the check engine light was on. I checked the codes and got bank rich and misfire. I found oil on the number 6 and 8 plugs, and had been noticing smoke from the driver tail pipe.

So I got the engine ripped out and torn down to find 2 broken pistons (and rings) but amazingly the cylinder walls are unmarred. No ridges, and crosshatch still visible.


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asmall

Now that I have the engine torn down, here are some pictures of the number 6 piston and 7 piston, side by side. You can see in the 6 the broken ring and broken ring landings.

This is a warning to anyone that uses the Hypereuretic pistons that came stock in the GT and Cobra prior to 03. If you S/C them, it is ok until you get detonation... and believe me, it does not take much. This car had been tuned and everything, but still, I broke the pistons. These pistons typically will fail in this spot. It can go unnoticed for a long time. Indications of this problem will be smoke and increased oil consumption. A dry compression check will show the loss of compression (good cylinders were getting 210, the bad one got 60), where a wet compression check will restore the compression (I got 230 once I added the oil).

Other horror stories include the breaking of the rod. So if you are going to S/C this engine, be very conservative with the A/F mixture in the tune to ensure you don't get any detonation, ever.


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asmall

My wishlist of upgrades to this car include:

Stroker kit to bring it to 298CI (3.75 stroke, std bore)
Forged rods and pistons
port and polished heads
long tube headers
new exhaust (including high flow cats)
new rear suspension
new, wider rear wheels and tires
Functional 6 point roll bar (hard to find for the convertible)

As I delve deeper into the science and art of these engines, I find myself wishing to buy more and more. Unfortunately, the more I wish to spend on her internals, the longer she will sit in pieces. But when she is done... watch out! Of course with out a new transmission to handle the power, you might be watching for a post on replacing a tranny!

I'll keep you up to date. Till then here are some questions I have:

1) Does anyone know where I can find a 6 point roll bar for a 97 convertible?
2) Anyonw with high horse power convertibles ever worry about body flex? Have you added subframe connectors, and what sort of experiences do you have?
3) anyone rebuilt a 4.6 cobra engine (aluminum block) before? Where are the pitfalls I should avoid? What sort of prep should I do for the head gasket mating surface?

97 GSR

Sry, i dont have an answer instead i have a question... I have always wondered this after i had drivin an svt cobra i noticed the low end (pre 3000rpm) wasnt as strong as a gt and noticing a 7000rpm redline, are these motors de stroked for increased revs?????

Dark_Horse

im sorry, but i have to ask... why is this a sitckie?

asmall

The pre 99 Cobra uses a Intake Manifold Runner Control (IMRC) that turns off one of the 2 intake ports. This was Fords way of solving the low torque output of the 32V non Power Improved (PI) heads. When driving it, it creates a sort of lull or dip prior to 3200 RPM when the IMRC opens. Since your base of reference is what happens next, which is a sudden rush of torque and power, you "feel" like it is not pulling hard. The GT has a smoother curve, so it "feels" like it is pulling harder. Here is the Dyno curve of an unmodified 97 Cobra. At 3200 RPM, you can see the dip and rise slightly after. The Torque output, even at low RPM is more than a stock GT makes at full Torque (or fairly close).

So the short answer to your question is: no, the Cobra is not de-stroked (it has same stroke and bore as GT) to make it pull less hard, the unmodified GT does not pull harder than the unmodified Cobra, and the higher rev limit is due to some better head parts, specifically, the valve springs.

As for why this is a sticky... you got me. It started out at the bottom. I am not sure why it made it's way to the top.

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rustyshackle4d

Good luck on the convertable 6-point. Check out Maximum Motorsport for a 4-point conv. roll bar. I think this will be a good sticky once you get going on the build up.

Dan04COBRA

Excellent posts, that's why it is a sticky. It'll be great for reference. Is that okay with you?

asmall

That is fine with me. I'll try to include more technical content in here as well.

asmall

The main goal of this rebuild is to rebuild the 97 Cobra into powerful machine, but at the same time not break the bank. The 4.6 engine is a fairly pricey engine to rebuild and modify compared to some of the more classic engines such as the 302, as many people have found out already, I am sure. I also have noticed people asking "should I buy a Cobra or GT?" for which my answer is always the Cobra. You can pick up a used Cobra fairly cheap and have a good time, but the Cobra engine has more "potential" in it, especially the 03-04's.

Previously, I showed the dyno curve of the unmodified 97 Cobra, here I am going to discuss the install of the ATI Pocharger P1SC on the car and the subsequent tuning.

First some TECH... Supercharging 101... Supercharging is term frequently thrown around, as well as intercooler, aftercooler, so on. Supercharging is the forced induction of air into the engine. It comes in two forms... Turbine-driven forced induction, commonly known as Turbocharging, and Engine-driven forced induction, commonly known as Supercharging, although technically, both are forms of Supercharging. Engine driven superchargers are broken down further into Centrifugal and Roots (or a variation of, such as twisted helical). These describe the means of compression. A centrifugal engine driven blower operates exactly like a turbocharger, except it's speed is dependant on the engine's speed, where a turbocharger is dependant on the turbine speed (which is dependant on engine LOAD). As the engine speed increases, the blower blows more air, and boost goes up. A roots style is similar to an oil pump, it is a positive dispalcement pump, using twin vanes with interlocking splines that pump air into the intake plenum. It too is dependant on engine speed, but it is more of an air pump, moving large volumes of air and forming the pressure in the intake manifold, rather than the centrifugal, which compresses it inside the blower and outputs compressed (and heated) air.

Some Superchargers feature intercoolers, others have aftercoolers, some have air to water, others have air to air! First, what is an intercooler? An aftercooler? It is a heat exchanger. It takes the heat from the intake air stream and gives it up to some other fluid (air or water). Technically, an intercooler is the heat exchanger located between compressors. On large engines with a turbocharger feeding a supercharger, the intercooler sits between them. The aftercooler sits between the compressor and intake plenum. So what is the difference on a car? Nothing... they are the same thing. Vortech refers to thier coolers as "aftercoolers" (the proper name) and most everyone else refers to them as "intercoolers" (socially acceptable, I suppose).

Then there is the Air to Air, and the Air to Water. Deciding which is better here is a bit more use orientated. Technically, the Air to Water cooler is and "air to water to air" cooler because heat is transferred to the water, then the water gives the heat up to the air. These systems are most common with Vortech (and work quite well from what I am told). My concern with it is that it has to undergo 2 exchanges of heat to get the heat out of the car. The transfer of heat is dependant on the difference in temperature. So to go from 220F blower air to 75F outside air, I first have to get it to the water, which might be 140F, then cool the 145F water with 75F air. An air to air would go straight from 220 to the 75F cooling fluid, which results in a much better cooling rate. However, if you drag race and have an Ice Box, you can stuff Ice in your box to cool the water, this might last one run, but your water temp might be 50F, but when would you ever get out and stuff Ice in the box on the street? Did I mention how much you need?

My decision to buy the Procharger was because it came with the intercooler as a kit, and it was air to air. So far I am impressed. The install took about 4 days and was not too difficult. I was not too happy with the instruction manual though as it lacked a few key details that I had to figure out on my own. If you do not have good mechanical skills, get a friend who does. If you can handle a wrench and understand how to change a powersteering cooler tube, you should manage, just take your time.

I followed the procedure as they indicated, placing brackets as needed, and so on. I found that I had to grind down one of the timing chain bolts (where the pulley is) to make room for the pulley. It was contacting when I first put it on.

The belt went on EXTREMELY tight. I had the tensioner in it's most open position, the belt wrapped around all the pulleys, and still had to rotate the engine to get the belt ot slip over the main pulley (harmonic dampner pulley). Later on (10K miles) the belt shredded on the outer edge. When I took the belt off and checked all the pulleys, I found the ATI supplied pulley was completely galled out and sloppy. The bearings in the pulley were fine, but the pulley itself was severly worn. ATI sold me a new one and told me it was a known issue and that some people had it, others did not. When I inquired about the very tight belt, he told me they runn them even tighter and that should not have done any damage. I ended up getting a slighter longer belt (gatorback 4061145) as opposed to the original (gates K061140). I shredded that one also, then I replaced the tensioner and blew up my engine.

The kit came with a Fuel Management Unit (FMU). Basically, an FMU works only on return style fuel systems. It senses intake manifold boost pressure and restricts the return flow. The result is an increase in fuel pressure. The increase in fuel pressure tricks the computer into spraying in more fuel for a given pulse. This is how you feed your beast! I opted for a custom tune ($350, including a new SCT chip). The increased air flow over the factory Mass Air Meter will cause it to reach it's maximum output (also known as saturation), therefore it will not sense any further increases in air flow, therefore the computer will not sense the increased airflow and will not increase the fuel delivered. More air... same fuel = lean... which equals broken hypereuretic pistons when you get detonation. That is where the FMU comes in and raises fuel pressure to increase fuel. As I was saying, I got rid of the FMU for a tune. This required a supercharger MAF meter. I chose C&L with a green sample tube, although, there are a lot of good ones out there. I also moved the intake air temp sensor from the intake tube (as ATI tells you where to put it) prior to the S/C (therefore sensing ambient air temp) to the tube feeding the intake manifold, after the intercooler. Now the computer sees the temperature going into the intake manifold. Now, if the intercooler is not doing a good job and intake temps climb, the computer can pull timing to protect the engine.

Also, installing the modified MAF meter required some inginuity since the parts ATI sent did not fit the C&L sensor.

Basically, that is it. There is a ton more technical material regarding S/Cs, and depending on what you want, there is an application to match it. Later I'll talk more about this system and what I would do if I were going to do it again in order to get more power and cost less.

Here is the after tune dyno curve.
Cheers!

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