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Old 08-24-2009, 08:29 PM
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Blower motor blowby and piston damage

I made three runs at the strip, third run my car layed down at about 1,000 feet, as if it had run out of fuel, cam or something, at about 5k rpm. It's a SBC 383, 8-71, two Demon 750, Lunati full roller.

My first two runs were strong all the way through the lights. The only known issue is my BTM was in for repairs, I had put an MSD 6AL on just for the day, ran it at 38 degrees total with about 10 #'s of boost.

The damage is on seven pistons, #1 is perfect. Also, all damage is on the outboard side of the pistons. The Cylinders look pretty good, no negligible scratches or gouges. The pics seem to show damage, but it's smooth.

I"ve run this motor for two years on the street, hard, with no problems. I'm thinking piston rock, excessive P/W clearance maybe?? I think washed cylinder walls would have taken out the rings and lands, they're intact. I'm not sure what it is. The piston tops are fine, the plugs are rich, but no metal. No metal in the oil.

Figure someone has seen this exact thing. I also experienced massive blowby out the valve cover vents, I wasn't using a PVC setup. I drove it home from the strip, it wasn't fun.
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Old 08-24-2009, 08:52 PM
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Classic sign of detonation from too much timing and too fat, too much timing and fuel in a nitrous engine does the same thing. 38 total at 10 #'s of boost , 28 would have been on the high side. I bet if you hardness check the piston tops you will find them soft.

What do the upper half of the rod bearings look like??

Last edited by ericnova72; 08-24-2009 at 08:53 PM. Reason: added ??
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Old 08-24-2009, 09:23 PM
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I've never detonated a motor, I realize the timing was aggressive though. Here's pictures of all the top half bearings, I didn't beat them to death.

You still think detonation? And why only on the outboard top edge? I expected to see a ventilated piston or two, I had very low compression, about 70 all around.

So, with the rings and lands still in place, what caused all the blowby through my valve cover vents, I had nothing out the exhaust, but so much blowby it made it uncomfortable in the car driving. This is a new ball game to me, and my first blower car. I was also running gapless rings, not sure if I had a flutter issue or if it would have any effect here.

I'm hoping to ge by with a light hone, new pistons, rings and bearings, depending on my p/w clearance. The heads look good, Dart aluminum. You have me thinking about the soft piston head surface though.

Thanks for the quick response Eric.
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Old 08-24-2009, 09:39 PM
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Instead of being too fat, maybe you are too lean. The piston tops got too hot, expanded, and scuffed the cylinder walls.
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Old 08-24-2009, 09:45 PM
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I wouldn't be worried about the heads, the are water cooled, the pistons got much hotter, probably so hot that they grabbed the rings and then released them when they cooled off. Probably why they feel okay now.

Are you running the pistons to the high side of recommended piston to wall clearance, such as is recommended with blowers/nitrous/turbos and/or filled blocks?? More clearance is needed to deal with the added heat, but your skirts don't look badly scuffed in the pics, so it's not too too tight.
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Old 08-24-2009, 10:01 PM
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I agree with the consensus of it running lean, and running too hot.

One thing I noticed from your pictures, is that the center, or origin, of the damage is not in a straight line with the bottom of the piston skirt. The picture of the cylinder wall you posted, shows the damage started in the centerline of the cylinder head stud.

This makes me wonder, if the block was 'torque plated' when it was bored, and honed.
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Old 08-25-2009, 08:55 AM
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Welcome to the world of blown gas racers. Used to happen nearly every run in the Top Gas dragster back in the 60's. You came pretty close to making a "nice mess".

You have too much advance. 28 to 30 is about it today. Get it all in by 2000 to 2200 maybe a little less. Also if those pistons are just flat top and no dish or down the hole comp height you have too much CR for the boost level you are running. Check this chart out from BDS.
http://www.blowerdriveservice.com/techcharts.php

I think you are too lean too. Possibly it is going lean on the top end, ie over 4500 or so. Make sure your fuel pressure is holding all the way up the rpm scale. What do your plugs look like??

Running hard on the street is not like running at the track where you can get a sustained blast.

Don't run the air cleaners at the track. The little ones have a hard enough time on the street but can't cut it at all at the track.

Consider switching to E-85. It's the miracle fuel for hi po hotrods. There are a couple of co. that make the conversions. Here is one.
http://www.raceone85.com/
It's right around $2.00 per gallon here in Minn. Readily available too.

I'm just putting together my new blower set up for my car so I'll probably be posting my heart aches too haha. The more fun you have the more it costs.

To re-direct to the questions;
Doesn't matter how hard the pistons are now. If it were mine being a cheap skate I'd probably clean up the pistons and carefully check the ring lands. If they were reasonable I'd put some moly rings on them and re-use them after a light hone on the walls. If they worked, fine if not I'm only out a set of rings, bearings and some labor. I'm always wasting time anyway.

I don't think I'd worry too much about the torque plate. This is from detonation. It will start from the hottest part of the combustion chamber. It can vary. Interestingly the very first hit of the throttle at the start of the run can be too lean and cause initial detonation that lifts the top ring land in the first second or two of the run. Now because of the oil leakage it only gets worse as the run progresses.

A little extra piston to wall clearance wouldn't hurt. You will probably have a little extra now. It's compromise. The pistons will definitely rattle when cold but if you run 180 TS they should quiet down on the street.

"I've never detonated a motor..." You can drop this from the list of "nevers" I realy don't think gapless rings are necessary.. nice and expensive but a 1/16. 1/16 1/8 or 3/16 oil are fine. You could be fancy and use Dykes top rings but again not necessary. I like a stainless chrome top ring and moly second rings. We used that combination for years on the blown cars even the alcohol funny cars.

I assume those are forged pistons if not they should be for as hard as you seem to be running.

good luck on the rebuild.
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Old 08-25-2009, 09:43 AM
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Hi wings, the motor was built right, 8.2:1, all forged and I knew the timing was aggressive, I had it all in at 2,500. I didn't read the plugs after the last run and drove the car 50 miles back home afterward, they're rich from that.

It's not the end of the world by any means and I was wanting to re-cam it soon anyway. I'll spring for new Pistons, freshen everything and see how it goes. I bought a dual channel wide band, an Innovate, I need to figure it out, it'll help a lot. We have one chassis dyno in town so it's hard to get time on it.

Thanks for the input, everything I've gotten here was pretty much expected.

Larry
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Old 08-25-2009, 10:13 AM
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I have a Innovate single wide band too. It's great. I'm just learning to use it and to tune with it. They have great site forum too. The main guru is from Holley 15 yr exp. Really good. I really need to get a lap top just for the data recording features.

Hard to believe you made it home.
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Old 08-25-2009, 04:58 PM
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Its unfortunate you trashed the motor over the neglect of re-adjusting the timing to reflect what would have been the correct total WOT timing under full boost with the Boost timing master connected (around 28-32deg) and pump gas.
For the cost of two electrical cable ties and 3-4 minutes to temperairly lock out the distributor advance mechanism (easily reversed) and set the frozen distributor at 30deg fixed timing, $$$all this carnage could have been avoided$$$

Open up the piston ring end gaps a bit on the new motor.
Consider a water/methanol injection system.
Allows lots of timing and boost on pump gas with big power without busted pistons.
Otherwise your cr should be between 7:1 and 7.5:1 for max performance on 92 pump gas and 10psi or more. (most amount of boost with the least retard required. Maximum possible boost, timing reliability and power on pump gas.

http://www.oddysracingengines.com/ Althou they are far away from you, these guys are some of the best in the supercharger game. They may be able to help you.

Last edited by F-BIRD'88; 08-25-2009 at 05:10 PM.
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Old 08-26-2009, 12:33 AM
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Thanks, but there was no 'neglect' with the timing, I knew I was on the high side and richened the motor thinking I could still pull it off. An error in judgement for sure. The motor isn't trashed by any means, carnage??
The damage isn't that severe, I'm probably going to put $2k in it, that's fully freshened. My ring end gaps were fine. I could have recurved the distibutor, I chose not to, it wasn't neglect.

I appreciate you're thoughts, it's all good, thanks.
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Old 08-26-2009, 11:14 PM
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Ring gaps have nothing to do with this. If the gaps were too tight you would have either broken ring lands and or major gouges in the cylinders from the rings butting.

Timing is the issue and I don't know if the A/F mixture had any effect on this. Bottom line 38* of timing is a LOT for a roots blown engine, that's what killed the pistons. 38* is even a lot for a pump gas N/A 383 SBC they usually like right about 34*. Always start conservative with your timing and sneak up on the tune up, read your plugs and make small adjustments. I highly doubt you were lean or your pistons probably would have had a lot more damage. Lean and 38* of timing wouldn't have made it 3 runs, plus you should have felt surging. I'm actually surprised you didn't feel it detonating. I think the scuffs on the pistons is from the piston getting banged around from detonation.


Next time out make a few easy passes, read your plugs, run it a little harder, read your plugs or watch your new A/F sensor.

The reason your compression readings were so low and you had so much blow-by, is due to the rings no longer sealing. I promise you that your top ring lands are all tighter than they were when you put that engine together. the ring lands were causing the rings to stick. You are checking the rings at room temperature, that will change at operating temp.

As far as compression goes anything between 7.5 -8.5:1 is fine. More goes into it than just static compression. Also lower compression will cost you some bottom end, usually not an issue with a roots blower.

Hang in there, it could have been a lot worse. Hopefully other will learn from this post as well, thanks for posting with the details.
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Old 08-26-2009, 11:29 PM
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You're right on with the top ring, it's tight. I'm surprised I didn't feel something as well, the surge you mention. I did feel it lay down at about a thousand feet the last run. I watched the videos and didn't see or hear anything remarkable either.

Yes, lesson learned. I had been running my BTM, but for these runs I only had a new 6AL, last minute buy, to get me in the program, remember, the BTM was being repaired.

Thanks, I'll talk to you later about the Buick, anxious to see it in paint.
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Old 08-26-2009, 11:35 PM
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As far as compression goes anything between 7.5 -8.5:1 is fine. More goes into it than just static compression. Also lower compression will cost you some bottom end, usually not an issue with a roots blower.


What you think you'd give up with a seeming low cr (7:1 to 7.5:1) you more than make up for by being able to reliabily run a higher blower drive ratio and more resulting boost that comes in sooner. You make more engine power and torque, go faster on pump gas with less spark retard without damaging the motor.
The net result is more power everywhere.

slightly larger ring end gaps even thou they cost you a slight slight bit of compression seal go a long long way to avoiding losing a ring land or scuffing a cylinder when the thing gets a bit of detonation and things get hot. Supercharged motors are by nature much more likely to experience detonation and ring butting.

By adjusting the ring end gap larger you build in a big engine damage safety factor and get to live to run another day from a brief indiscretion.

In both cases you give up a very little little bit and gain a lot lot lot and go faster- longer.

Last edited by F-BIRD'88; 08-26-2009 at 11:46 PM.
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Old 08-26-2009, 11:59 PM
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68NovaSS ,
Im wondering what was wrong with your BTM, I had a brand new one and I had to crank my engine quite a few times before I would get any spark. Finally figured out the BTM was bad. I sent it back to MSD for repair and after they sent it back I had the same problem. After a long talk with the tech at MSD he decided the BTM was still bad. I never got around to sending it back to them again, still sitting on the shelf.

Duke
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