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Old 02-15-2009, 08:28 PM   #1
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Default Swapping just rods for 300hp

I know I have seen mutiple posts in the past with people swapping just their rods but doing things like ceramic coating the piston and such. I was wondering if anyone has ever just changed the rods out some some H-beams and left in the stock pistons.

I know people have done 300hp for a little on stock internals, but the rods ineveitably bend. I couldn't come up with much in the search and i really am on a budget for this, as it is going to be a dedicated track car.
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Old 02-15-2009, 08:33 PM   #2
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I heard the ring lands are weak after 250hp. And bust. Best bet call Corkey at Begi. he will shoot you straight.
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Old 02-15-2009, 08:33 PM   #3
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If I could only do one thing to my engine, it would be new forged rods. In an extreme budget case, this could be done without any machining if you reuse the stock rings and pistons, though I would advise at bar minimum to hone it a good bit and install new rings.

If I could do two things, it would be forged rods and forged pistons. Then the bottom end is DONE. But this will require more machine work and more cost.

But yeah, rods will get you to 300whp, but having a set of forged pistons sure would be nice if you could get those in the budget. Especially since this will be a track-only car.
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Old 02-15-2009, 08:55 PM   #4
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I dunnoo... For the cost and trouble of putting in the forged rods, with still the possibility of blowing a piston up, I'd not want to do it. Like Pat said, I'd want to at minimum go in for the rods and new pistons too. Otherwise, I'd take the rod money and put it towards a spare motor.
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Old 02-15-2009, 09:33 PM   #5
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I understand where you guys are coming from on that i could just do the pistons and rods and be done with it, but it triples the cost of the engine work. I am not one to just find the cheap way out to do things, because that tends to come back and bite you in the end. If there is a way to get this more reliable and spend only about 500, then in my mind it is justifiable on this car. If not, that really puts it past the threshold of what i should spend on it. It is a slippery slope.

I haven't really heard too much about piston failure though. Paul is pushing 300+HP on his stock block and everything is fine, for now. My car will obviously be getting abused pretty well, but I almost think that would take it with just new rods.

I know most people don't do this and it's understandable why, but if anyone has done it, then i really want to know.
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Old 02-15-2009, 09:39 PM   #6
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actually piston failure is pretty common. every bit as common as bending rods. the problem is detonation destroys the ringlands. if you're lucky, you only loose the 1 cylinder but chances are debris will damage other areas too.

you can roll the dice and hope that you never detonate.

but of course new pistons also come with new rings (and good pistons come with rings better than stock). not to mention you can get them oversized and in your choice of compression ratio...
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Old 02-15-2009, 09:51 PM   #7
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Here's the thing. Factory engine is clearanced to run factory HP, more or less. About 1/3 of the energy from the fuel goes into HP, another 1/3 out the exhaust and another 1/3 into the block to be carried away by the cooling system.

When you increase the HP, you increase the amount of heat that goes into the engine. Even if you have a badass radiator and cooling system that can carry heat away fast enough to keep up, the temperatures of the pistons and cylinders will still be higher than normal.

Engines are clearanced so that under a certain temperature and load, they will work reliably. IE- when fully warmed up the piston-cylinder clearance on a stock motor may be 0.002" and when beating on it on a track at stock power, 0.001". That's ok.

But you want to put 300whp out and do that on a track. If you make more power, there will be more heat to dump and overall high working temperatures. Higher temperatures means more thermal expansion which causes working clearances to reduce. If they get too tight, you get excessive friction, which causes..... more heat, and the problem compounds itself until things get so hot it detonates and you break something.

I could very easily put together an argument that all rod and piston failures are caused by detonation, which is in turn sometimes caused by insufficient clearances for the loads put on the engine. Very possible. Mazda clearanced these engines pretty tight from the factory and the coolant routing is not ideal.

Last edited by patsmx5; 02-15-2009 at 10:11 PM.
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Old 02-16-2009, 11:50 AM   #8
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Man you guys suck.. I was really looking for someone to just tell me it's ok so i have someone other than myself to blame for it still blowing up.

Pat, your explanation is exactly what i was looking for. Thanks much.

I really want to build the engine, but it is hard to justify it in a track car that will only be used once or twice a month. At the tracks where i race, having a miata with 225ish hp is still pretty damn fun. Especially when it is stripped to nothing. Maybe later i will build it up, but i guess i will just try to work on being able to drive better for now.
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Old 02-16-2009, 12:17 PM   #9
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Well, if you got more time than money, you could do this...

Buy an ebay complete engine gasket kit, a cylinder hone, a new set of ring for the stock pistons, valve grinding compound.

Pull everything apart. Buy a snap gauge and micrometer or borrow them from a machinist friend. Clean everything up. Assuming the bores are in good condition with minimal wear and within factory specs, you can hone them out a bit bigger to increase the clearances. This wouldn't be a good idea for a motor that will be daily driven, as it will wear faster. Probably only last 50k miles. But if it's a track car only, mileage doesn't matter and clearances do.

So you would hone it out to get the bores a touch bigger. I don't know what the factory clearances are from memory, but add at least 0.0015 to them or maybe a bit more. Then put new rings, gap them, reinstall, put your new rods in. Reassemble engine.

That would be a cheap, hack-job way of building an engine, emphasis on the hack-job. But it would work. I wouldn't daily drive it though because it would only last 50k miles or so. But it might be an option for you if you have more time than money.
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Old 02-16-2009, 12:24 PM   #10
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Quote:
Originally Posted by rharris19 View Post
Man you guys suck.. I was really looking for someone to just tell me it's ok so i have someone other than myself to blame for it still blowing up.
I've got a few local friends who are doing just what you want to do with a set of our M-Tuned Rods. I have also sold sets to quite a few customers for the same purpose.

I have seen stock pistons break, but it has been due to BAD TUNING and not too much boost. *yet*. Ric (Minimonster) used to run stock 94BP pistons with Crower rods in the 400+ Range.

My next motor is probably going to be Stock Pistons with M-Tuned Rods. I'm actually just waiting for the Coolant re-route to be finished on our the mock-up 94-97 engine then I plan on stealing the bottom end for my build

Your call my friend, but at the 300hp range you should be fine.
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Old 02-16-2009, 12:28 PM   #11
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Quote:
Originally Posted by [email protected] View Post
I've got a few local friends who are doing just what you want to do with a set of our M-Tuned Rods. I have also sold sets to quite a few customers for the same purpose.

I have seen stock pistons break, but it has been due to BAD TUNING and not too much boost. *yet*. Ric (Minimonster) used to run stock 94BP pistons with Crower rods in the 400+ Range.

My next motor is probably going to be Stock Pistons with M-Tuned Rods. I'm actually just waiting for the Coolant re-route to be finished on our the mock-up 94-97 engine then I plan on stealing the bottom end for my build

Your call my friend, but at the 300hp range you should be fine.
Was said 400+ hp engine rebuilt? IE- clearanced and assembled for a 400hp goal? There aren't too many stock engines operating in the 300+ HP range.

And hurry up with that reroute already.
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Old 02-16-2009, 12:38 PM   #12
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Quote:
Originally Posted by patsmx5 View Post
Was said 400+ hp engine rebuilt? IE- clearanced and assembled for a 400hp goal? There aren't too many stock engines operating in the 300+ HP range.
I've ran 2-3 stock 94-97 in the 300+ range, and IIRC Ric told me he just remove the stock rods, slapped in new bearings and crower rods and was done.

Only thing I can think of was that the motors had 80,000+ miles and therefore were not as tight as from the factory. Just a guess.

Take a look at the honda world. People used to say a stock B18 was good for 6psi. Once people started working with proper engine management and not an FMU/AFC Hack we started seeing 15+psi and 350-450hp on a stock engine.

Tuning and good fuel is the key! Detonation is the enemy!
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Old 02-16-2009, 12:42 PM   #13
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And hurry up with that reroute already.
I've been told it is very close. [email protected] is in China/Taiwan this week and has told the machine shop here in Toronto that he wants a working version for the 94-97 when he returns as he plans to then test it on my 2000 to make sure it fits also. Damn EGR is what has slowed the whole process.

Plus he will start making the traction bars then also.
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Old 02-16-2009, 01:32 PM   #14
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I guess my poor old '96 with 125k mi and noticeable cold piston slap should handle the extra heat of my little turbo without getting too tight in the bores.

Or it will render itself into tiny pieces and I will have to build a forged bottom end. Also a
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Old 02-16-2009, 02:03 PM   #15
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If you change anything in the bottom end you should balence it, its better for everything.
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Old 02-16-2009, 02:33 PM   #16
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You could do it and get away with it, but you will certainly regret it later.
Have fun at 225ish and become the better driver while saving up on the side to do it right the first time.
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Old 02-16-2009, 03:43 PM   #17
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Geez, threads like this almost make me regret running a set of tri-coated OEM pistons. Oh well, here's to doing things differently...
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Old 02-16-2009, 06:24 PM   #18
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Quote:
Originally Posted by RotorNutFD3S View Post
Geez, threads like this almost make me regret running a set of tri-coated OEM pistons. Oh well, here's to doing things differently...
I almost went that route, but then I learned that my walls were in bad shape and needed to get bored out in order for me to reuse that block...after that I only had one option, oversize pistons. Turns out to be only about $300 more than the tri-coating the stockers route would have.
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Old 02-16-2009, 10:27 PM   #19
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Originally Posted by RotorNutFD3S View Post
Geez, threads like this almost make me regret running a set of tri-coated OEM pistons. Oh well, here's to doing things differently...
You will be fine my friend... Don't regret it!
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Old 02-17-2009, 01:28 PM   #20
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When I snapped a rod from det running high boost and nitrous, the top of the #4 piston had det signs (pits), the rod snapped but the ring lands were fine.

I'd like to see a bit more proof of failed ring lands == broken rods in Miata motors. I can only remember 1 or 2 pictures ever of broken Miata ring lands.

I think for the budget rebuild for 300+ whp, the stock pistons with good rods is fine.

Frank
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