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.

I heard the ring lands are weak after 250hp. And bust. Best bet call Corkey at Begi. he will shoot you straight.

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.

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.

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.

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...

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.

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.

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.

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. ;)

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!

I've been told it is very close. John@M-Tuned 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.

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 :)

If you change anything in the bottom end you should balence it, its better for everything.

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.

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.

You will be fine my friend... Don't regret it!

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

rharris19:

I have a set of stock sized rings I got with a gasket kit and didn't end up using. (got wiescos) I'd be willing to let these go on the cheap. PM me if interested!

Hi my name is Josh I'm new to this group. Regarding this post. I have seen people running stock pistons with cheap H beam rods and having no issues at all pushing 300+. I have also done this myself and I'm still driving it on the street daily. Its all in the condition of the engine first of all make sure you have a nice healthy platform to start with. Go through it change the gaskets clean any carbon from the valves pistons and so on. Change the bearings there cheap enough. replace the head gasket with a good quality multi layer gasket I used ARP head studs same again cheap enough and worth it. File the ring end gaps so there's more room for expansion the rings get hot they expand if the ring end gap make contact you will shatter a piston and that might take out the cylinder. I did a quick home hone. I got some thermal tecline on top of the pistons to help with heat dissipation. The main things to keep the car safe and reliable is a GOOD tune by a pro or someone who really knows what there doing. Make sure you have lots of fuel and a good oiling system also make sure you can keep the engine cool the more power you make the more heat its going to make so a bigger rad is a good idea but not essential some good fans will still do a great job cooling. Intercooler you don't need a huge over sized monster intercooler a small to mid size will be perfect there cheap too you wont have to chop the car up loads to fit it ether. Plus an engine is like a big air pump its all well and good forcing all the boost in but it needs to be able to push it out or your going to have restriction issues a good exhaust system helps but that's why you need to make sure the valves are clean and working great you can also look at cams and valve work if you wanted but its expensive. I used my dremel to port and polish the cylinder head intake side and exhaust side out it helps flow and you get a quicker spool time but the main thing is your engine can breath it can expel all that boost. Good fuel is a must a good ECU is a must and a good tune is a must. retarding the timing will really help knock and pre detonation. A good fuel pump and lines get some bigger injectors you don't need to go crazy with injectors at all you don't need 1000cc injectors for 250/300hp at all. All the stuff I have mentioned there its all just supporting mods and all cheap some stuff and some free and it's simple to do on your own in your garage with basic tools. You don't have to do all of that but it all helps and it helps keeping it healthy and alive It's all elbow grease and time really. Get a wideband and a boost gauge so you can see the air fuel. I find is the best AFR making the most power for you NA will be the best AFR under boost your tuner will do that. Also when your making considerably more power it's a good idea to upgrade your brakes it's usually over looked but last thing you need is enjoying some power and then crashing because your brakes are not good enough. You may need a better clutch to handle that power I did my stock one was fine for a short time but it soon started slipping I hove a twin disk now it gives me room to grow in future.

That's just a ruff guide you don't need to do all that its not set in stone but it's what I found out to be a good recipe for a budget reliable 300hp daily driven car.

It all depends on what you want to do with the car also your budget if I were you I would just read everything I could to find out as much information as you can theirs people on hear who have all done different things in different ways with different results you'll learn loads if you look about and ask. Good luck with it it would be dope to see some pics when your done.

11 year old thread bump for a wall of text.
10/10

Is that a record?

Wow, on his first post Starkiller killed me dead.
What is interesting is that 11 years ago patsmx5 had it together.
I'm guessing he was in his early 20s then.
God, I wish I was as advanced as he is at his age.
There was NO incorrect information in his four posts at all...
His suggestions still ring true today.
His suggestion on increase of bore sized is dead on.

And he was a punk kid at the time.

And I'm leaving cats he should have gotten back then...

Would have been even more stellar (in a negative kind of way) had he left out capitals and periods.
DNM

My "rods only" build engine has last over year now with 390whp (only using E85). Around 30 track days with no problems. :jerkit:

That's what I like to hear!

Mines going strong on much lesser power (~250whp), about 20k miles, and 5 trackdays. I put the rods in for insurance, and the ability to crank the setup to ~300+whp on e85, though I have yet to do so.

I also enjoy driving the car in the winter and frequently drive the car to work when it's below freezing. OE pistons are great for this purpose.

Doing a build just like this currently.
94 pistons and ebay rods. New bearings and arp head studs
Kraken with GTX2871R Gen 2

We'll see how it holds up!

While I have zero personal experience with "rods only", after 2 years of reading this forum, I'm convinced I didn't need to go all out with my BP build.

Haven't heard much to suggest a build with just rods, rings, bearings, and perfect tune or e85 wouldn't have a long life at 300+ given it wasn't an endurance track car or the likes.