Blow by issue.

[1993 stanglover]

I have a 93 GT with a Paxton SN92 blower, GT40 x heads and intake, SVO roller rockers and a f303 cam. I bought the car with all of the engine work done. A few months ago the PCV valve blew out of the lower manifold and after some research and a compression test, i figured that I had two cylinders with bad rings in them. I finally got the engine out of the car and started to tear it down. I just pulled the drivers side head off, which is the one with two bad cylinders, and i was able to see the pistons and cylinder walls of the two bad cylinders. The motor has 50k on it and the piston walls still have very visible cross hatching and no obvious marks from a ring scratching into them. My question is. if there is a bad ring, would it definitely mark the cylinder wall, or could it blow by without causing any damage? I need to rebuild this motor on the cheap if at all possible. I would like to take the block to get hot dipped and honed, and I am hoping that I dont have to get it bored out, cause that would mean that i have to buy new pistons, which increases my cost. "not what i want" Would it be ok to just hone the cylinders and put new rings in? what else should i do, without spending too much money? I know this is a lot of questions, but I am hoping that someone on where can lend me there expertise. GOD knows i need it. I am no idiot when it comes to cars, but i cannot say i know everything. I have never rebuild an engine and I dont want to mess up a great car. Thanks in advance for any help

 

[twistedwankel]

I have a 93 GT with a Paxton SN92 blower,
My question is. if there is a bad ring, would it definitely mark the cylinder wall, or could it blow by without causing any damage? I need to rebuild this motor on the cheap if at all possible.

I bought a "boosted" car with "blow by" and found out just how bad when it pumped oil all over the ground out the vent just driving it up on the trailer. It appeared someone had run nitrous without adding more fuel once we pulled the head. Melted things in one cylinder. But it still had nice cross hatching on all the cylinders.

There is no "on the cheap fix" for a (not normally) boosted engine unless you want to rebuild/replace it frequently.

IMO: Personally I would have gone back to the original builder (you indicated was a reputable one?) and at least asked their opinion of why it failed. Sometimes professional pride will get you a decent discount on repairs at least.

I wouldn't go on the cheap unless you are lowering the psi. HP breaks things.

 

[1993 stanglover]

I bought a "boosted" car with "blow by" and found out just how bad when it pumped oil all over the ground out the vent just driving it up on the trailer. It appeared someone had run nitrous without adding more fuel once we pulled the head. Melted things in one cylinder. But it still had nice cross hatching on all the cylinders.

There is no "on the cheap fix" for a (not normally) boosted engine unless you want to rebuild/replace it frequently.

IMO: Personally I would have gone back to the original builder (you indicated was a reputable one?) and at least asked their opinion of why it failed. Sometimes professional pride will get you a decent discount on repairs at least.

I wouldn't go on the cheap unless you are lowering the psi. HP breaks things.

I don't know who built the motor. As far as i know it was a guy in his garage. the blower only produces about 5 pounds of boost, which i think is pretty low. My thought is to spend the money to get good rings and keep everything else the same. What rings are good for a boosted engine? I see so many choices out there.

 

[twistedwankel]

I don't know who built the motor. As far as i know it was a guy in his garage. the blower only produces about 5 pounds of boost, which i think is pretty low. My thought is to spend the money to get good rings and keep everything else the same. What rings are good for a boosted engine? I see so many choices out there.

Below is a good start of your thinking processes (I lifted off a web site). You are adding 5psi and how many cfm(?) to whatever you started with? 9.5-10:1 compression non- intercooled? Heat, heat and lean for more heat. My boosted engines all started life at 8.5:1 to accommodate alot of boost with a moderate sized blower, a great knock sensor, a big intercooler and they lasted a very long time. You need to consult with whomever you are having redo your engine. Don't just show up with a pile of cool name parts and expect it to fit and work for any period of time - if at all. Depending which year block you started with will probably determine how well it will hold up with an extra 100HP. There are tons of searchs. You might simply try asking about the "rings to use" in a thread for your boosted 5.0 if you are convinced you want to try that alone.

RACING RINGS
Like stock rings, performance ring sets are getting smaller and thinner. Reducing the tension on the rings not only cuts friction but also seals better and reduces blowby. This means a performance engine builder can pull more vacuum in the crankcase with dry sump oil pump and gain horsepower.

Many performance pistons today have ring lands that are very close to the top of the piston and use metric sized ring sets. Ring grooves on many of these pistons are also machined to have a small vertical uplift to compensate for thermal expansion as the piston heats up.

Another trend has been to drill gas ports in the ring grooves behind the rings. Compression rings typically require .002? to .004? of side clearance so combustion pressure can blow around the ring and force it outward to seal against the cylinder. By drilling tiny gas ports in the back of the ring land, less side clearance is needed and ring sealing is improved. There is also less ring flutter at high rpm, which is where most performance engines spend a large percentage of their running time.

Gas ported pistons work best in high rpm applications and with thin, narrow compression rings. But the high pressure sealing that works well on the race track does not work so well on the street because the added gas pressure also increases ring wear.

RETHINKING RING GAPS
The old school philosophy of engine building said the end gaps on second compression rings could be tighter because the number two ring is not exposed to as much heat as the top ring. The new school of engine building says it's better to open up the second ring gap a bit so pressure doesn't buildup between the rings and cause the top ring to lose its seal at high rpm. The result is better compression, better piston cooling and reduced oil consumption. Any pressure that builds up between the rings will blow down into the crankcase, keeping oil out from between the rings.

Getting rid of the end gap altogether can also improve sealing, cooling and horsepower. Some engine builders who have switched the rings they use to a set that includes a "gapless" top compression ring say they've gained three to five percent more horsepower with no other changes. Gapless rings are available in popular sizes with various wear-resistant face and side coatings. On some engines, the second compression ring can be eliminated if a gapless top ring is used. Getting rid of the second compression ring cuts friction and adds horsepower, too.

SMOOTHER, FLATTER
Another trend that seems to have additional benefits is the use of smoother, flatter rings and pistons with precision machined grooves. Once ring supplier says their racing rings are manufactured to within 50 millionths of an inch flatness and parallelism, with a finish that is typically 4 Ra microinches or less. This allows tighter assembly tolerances for better performance.

With some low priced pistons and ring sets, there is a certain amount of waviness that concentrates contact between the rings and lands. This encourages microwelding the groove pound-out at high rpm.

Friction-resistant coatings on the sides of the rings and/or ring grooves in the piston can help prevent this from occurring.

CHOOSING THE "RIGHT" RINGS
The right ring set can not only make more horsepower, but also improve the engine's durability. Both are just as important on the street as on the racetrack. The best advice here is to follow your ring supplier's recommendations. Use street rings on street engines, and performance ring sets on racing engines.

One of the newest trends in piston and piston ring selection is specifically matched components, delivered together in the same box. Leading manufacturers say this takes the guesswork out of selecting rings, makes for better weight-matched rotating assemblies and includes top-quality products for ease of assembly and maximum performance.

The type of ring materials and coatings that work best in a given application will depend on the engine's compression ratio, the type of fuel it is burning (gasoline, alcohol or nitro), how much horsepower per cubic inch the engine will hopefully make, and the engine's rpm potential.

For example, plain cast iron rings should never be used in an engine that burns alcohol because alcohol cuts lubricity. Coated rings are a must with alcohol.

For high boost turbocharged and supercharged engines, and engines using large doses of nitrous oxide to add power, ductile iron or steel top rings are a must. Many racers prefer to use nitrided rings made from steel wire because they can handle higher loads and thermal shock better than other materials. The nitriding penetrates into the metal and won't flake off like other surface coatings.

Another factor to consider is the type of racing. Off-road and dirt track engines often survive best with chrome rings that can handle dirt contamination better than moly faced rings.

RING SEALING
No ring will work well if the cylinder walls are not finished properly. Most ring manufacturers recommend a plateau finish, which typically involves a two-step honing process.

For plain cast iron rings in a stock motor, #220 grit silicon carbide honing stones are the best choice, followed by a honing tool or brush.

For moly faced rings in a stock motor, hone with a conventional #280 grit silicon carbide vitrified abrasive, then finish by briefly touching the bores with a #400 grit stone or giving them several strokes with an abrasive nylon honing tool, cork stones or a brush.

An average surface finish of 15 to 20 Ra is typically recommended for moly rings. Anything less than 12 Ra can result in glazed cylinders and the rings may not seat. If the surface is rougher than 20 Ra, the rings and cylinder will scrub excessively as the rings seat.

For moly or nitrided rings in a performance motor, hone with #320 or #400 and finish with #600 stones, cork stones, a honing tool or brush.

If the cylinders are honed with diamond, they should be finish honed with a finer grit diamond, a fine grit vitrified abrasive or a honing tool or brush to plateau the surface.

Bore geometry is also important. Many late model blocks and most high performance engines should always be honed with torque plates bolted to the block to simulate the distortion created by the cylinder head bolts. Crosshatch provides lubrication for the rings.

Most engine builders prefer 30°, but some use as much as 45°.