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The factory turbo system, cracks, and coatings

Old 12-27-18, 12:52 AM
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The factory turbo system, cracks, and coatings

Hi all,

FWIW I did a ton of searching before posting this, but couldn't find quite the specific answers I was after. Apologies if this has been covered before, and for the mods - please feel free to move this to the new (ish) member technical forum if need be! This'll be kind of a long read, but I want to lay out my research thus far, and questions going forward, as clearly as possible.

I'm researching ways to restore the twins (that's easy - BNRs or new '99/'99 SP twins) but also keep them in the best long-term condition possible, using whatever the latest knowledge/practices/products that can be brought to bear in almost-2019. A number of the threads I found on the topic through searching are over a decade old.

I'm trying to gain a better understanding of historical issues with the factory twin turbo system's manifolds cracking - both the manifold that bolts to the housings/keg (exhibit A):



...and the exhaust housings/union assembly locating the turbos themselves (exhibit B):



I'm aware of the long history of cracks forming in the latter - can anyone clue me into any cracking issues with the former (exhibit A - exhaust manifold bolted to the housings)?

I found a post from turbojeff in this thread alluding to there not being a history of cracks in this manifold:

Originally Posted by turbojeff View Post
The cast iron piece that fits between the turbos cracks and is typically not a problem. The cracks can grow to appear HUGE and still not be a problem. It usually cracks on the side that mates with the exhaust manifold that bolts to the block. The DP side doesn't crack.

I've never seen the exhaust manifold that bolts to the block crack.

Post pics!
...but that was 13 years ago. Are cracks in the factory turbo manifold an issue today, and if so, what are the negative effects?

For the more common/documented cracks in the turbo assembly/exhaust housing manifold, the general consensus in the research I've done is that these are largely unavoidable, and have to get really big to cause concern. Lots of discussion about thermal expansion and contraction across uneven casting thicknesses, low nickel content from the manufacturer being a factor, hairline cracks being discovered on <5k mile twins, and so on. Seemingly, proper welding with a nickel rod can be used to repair these cracks. I get that it's not a huge deal to have them, but if there's a smart way to avoid them in the first place, I'm all ears. There are various threads that allude to the practice of radiusing corners in the manifold/housings, and coatings. An example from this thread:

Originally Posted by jimlab View Post
My manifolds had cracking even when pulled off the car at ~3,000-3,500 miles. They all crack eventually, to greater or lesser extents.

No one has mentioned (at least that I saw) that cracks typically form at hard edges. Radiusing the edges would reduce the chance of stress risers forming and cut down on cracking. The same theory that applies to cross-drilled cast iron brake rotors applies here. Radius the edges of the holes and they won't be as prone to cracking. After that's done, ceramic coat the parts.
That logic sounds...sound...to me, and it appears that Pettit actually offers radiusing (and coating) on their "Hi Flo" twin turbo package. What say you - is this a procedure worth performing?

On the subject of coating - what is the current consensus on coating, well, everything that is cast iron in the twin turbo system (manifold bolted to housings, and manifold adjoining turbos) with something like Jet Hot 2500, Swain Tech, etc. etc.? There would be a benefit to keeping underhood temps in check, but:
  • Would this coating be at all helpful in preventing cracks?
  • Would coating in someway actually encourage cracks to form due to radiant heat being kept/trapped in the metal, while running and possibly during cooldown cycles?
  • Should you coat the interior of the manifolds as well as the exterior?
My going all-out idea currently would be to do something like acquire new '99 twins, have Pettit or a capable local shop do "Hi Flo"-ish work to include radiusing, and then have everything coated for the known benefits (underhood head & ward off corrosion) but only if coating doesn't promote cracking in any way.

Thanks for reading this far, and please provide your input on what it would take to keep new twins happy and healthy for the long term. And before anyone says it - I'm not interested in just going single

Last edited by GDSpeed; 12-27-18 at 01:05 AM.
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Old 12-27-18, 01:31 AM
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Great job collecting your thoughts and information in this this thread. It's well laid out. in the past 6 or 7 years I've been messing with the fd I also have yet to see a manifold cracked or damaged. I think it would be safe to say they are fine as they are and investing any money or time into them to make them "better" would be a waste. The turbo assembly itself where it mounts to the manifold has been found to be the crack homeland, yes. The n3c1 are the most robust I have seen first hand. I don't have any experience with the n3g1. The n3c1 twins I have come in contact with have had very little to no cracking at all. The super ugly ones you see all over are likely the n3a1, not to say the others don't get that bad.

when it comes to improving on the design of or coating the stock set up, I don't think there is any real reason or purpose to do so. I'm sure it may help out some but even with the spiderwebs of cracks on the part that attaches to the manifold and even at the wastegate itself, the twins have shown to give great operation. Turbos by themselves are wear item, It's just the nature of their operation. There are methods to prolong their life but in the case of the stock twins, you're already starting with 25+ year old heat cycled cast iron. You can put a tuxedo on a gorilla but he's still ugly.

The way I see it, best case scenario is to get a low mileage set and use that as ground for experimenting. Do whatever you wish to them as far as coatings and taking the edges off the ports as mentioned above and run them. Take them off after some time and see if it helped any. As crazy as it sounds, I don't think the twins are efficient enough to begin with where the maze of cracks that form on them actually make a difference in their operation..... to an extent.

Last edited by cr-rex; 12-27-18 at 08:57 PM. Reason: Typos
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Old 12-27-18, 08:41 AM
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CR-Rex has some great points. I agree, I have NEVER seen the exhaust manifold cracked on a wide variety of FD's. It's pretty solid.

N3A1 turbos, which are the original turbos, will almost always crack. Some worse than others. Once they crack the cracks will grow.

The N3C1 turbos, which I think were introduced in '95, and the N3G1 "99 Spec" turbos both have a higher nickel content in the cast iron so they are MUCH less likely to crack. I have seen it before but it was very minor.

That said, cracking isn't a big deal as long as it doesn't compromise the performance. It's mainly cosmetic. Cracks that go under where the gasket meets up can blow the gasket out. Cracks around the wastegate "hole" can let exhaust gasses around the wastegate flapper, resulting in a lazy turbo that takes a long time to build boost and may not hit full boost.

Radiusing hard edges as Jimlab suggests may be helpful, but it's hard to say long term if that is a trick that fixes the problem.

I know guys a LONG time back experimented with ceramic coating the turbine housings and manifold and there were mixed results. That technology has come a LONG way. If you are interested in looking into it I'd get with Jet-Hot, they are one of the leaders in the field. I've been sending them downpipes for coating for ages, they do incredible work and the result is damn near bulletproof. But, I don't know if that will prevent cracking or anything, it mainly will just be reducing heat in the engine bay and cosmetics.

Also, Thanks for making a good, informed post and welcome to the community!

Dale
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Old 12-27-18, 12:14 PM
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also never seen the manifold crack.

coatings need some research, they usually come right off.
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Old 12-27-18, 05:00 PM
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I’ve killed multiple sets of turbos via heat from track use. I’ve never seen a manifold crack it’s always the turbine housings. The reason is that the exhaust flow pretty much slams into the secondary turbine housing and over time will erode it. The highter the EGTs and longer the exposure quickens the process.

I had had a set of ceramic coated housings once but found the coating wouldn’t allow a completely flat gasket surface and caused gaskets to fail prematurely. I had to have the housings machined flat which got rid of the gasket woes but then left the housings bare to crack again eventually. I suppose if you machined them flat first then coated perhaps you could have better success? Ultimately if this is purely a street car then the effort is likely not worth the reward. Buy the best set of turbos you can find, put them on, and they’ll be fine for a long time.
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Old 12-27-18, 06:25 PM
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FWIW OP, I'm still using the original N3A1 turbos with ~135k with 10+ years of autocross. When I last had them off about 10 years ago there was some significant cracking on the housing...none on the manifold. And there's been no issues. Not even smoking...yet. And pretty sure there are others here with even more miles on their twins. Still, I realize I'm on borrowed time. When they finally go I'll pop for a new 99 spec. set and probably radius the edges, but doubt I'll sweat bullets over heat coating.

Last edited by Sgtblue; 12-27-18 at 06:32 PM.
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Old 12-27-18, 06:49 PM
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^ What do you mean when you say radius the edges?
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Old 12-27-18, 07:22 PM
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Originally Posted by Johnny Kommavongsa View Post
^ What do you mean when you say radius the edges?
read the initial post
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Old 12-27-18, 08:06 PM
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Thank you all for the replies! I meant to include in in my initial post, but I did have a question about the susceptibility of later-model twin manifolds to cracking, which was answered. It sounds like the ultimate ticket would be massaged & coated '99s for the nickel content & newer turbo tech over the N3A1s. My goal is to have a nice OEM+ car, turbos included, so this is great news to me. Thanks again guys

Last edited by GDSpeed; 12-27-18 at 08:17 PM.
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Old 12-27-18, 08:09 PM
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Take a die grinder and round them over
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Old 12-28-18, 01:59 PM
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Only issue I've seen is an exhaust leak will erode the manifold face - if you're silly enough to let it go on long term.....then you have to get it on a surface grinder. N3C1 and G1 might be slightly better ni-resist, but will crack in exactly the same spots as the early ones, lower potential mileage if buying secondhand, is probably the best you can hope for.

I see at least one of your vendors over there is selling the newer, better Hitachis as "SP"- and that has infiltrated this forum - possibly again with the OP's mention. I'd say that claim is getting into the realms of deceptive advertising.
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Old 12-28-18, 03:06 PM
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I believe an added reason the turbo manifold cracks on the secondary side is the port is unmatched and the very hot exhaust pounds the secondary manifold, especially when the door is open at WOT. On my just completed rebuild,that is why I hand ported the secondary to match the exhaust manifold and to match the opening size of the primary. The door still closes fine with about 2-3mm of overhang. Also, I smoothed out the turbo manifold just beyond the opening on the secondary side to make the air flow more laminar. I also ceramic coated the inside and the outside of the both manifolds. This all will hopefully help.
Mike
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Old 12-28-18, 10:05 PM
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Originally Posted by billyboy View Post
Only issue I've seen is an exhaust leak will erode the manifold face - if you're silly enough to let it go on long term.....then you have to get it on a surface grinder. N3C1 and G1 might be slightly better ni-resist, but will crack in exactly the same spots as the early ones, lower potential mileage if buying secondhand, is probably the best you can hope for.

I see at least one of your vendors over there is selling the newer, better Hitachis as "SP"- and that has infiltrated this forum - possibly again with the OP's mention. I'd say that claim is getting into the realms of deceptive advertising.
Thanks for your input, it's interesting to read what someone from Australia has to offer given that enthusiasts there have longer, and broader amounts of experience with later-series FDs and their turbos. Assuming the casting process has been kept the same all these years later for the N3G1 '99 turbos, that's a good data point to have.

On the subject of "SP" - I have no idea what that term means, or why it's used, to be honest, in relation to '99 spec turbos. The only "SP" I'm aware of is the '95 RX-7 SP for the Australian market, but that car wouldn't have anything to do with '99 spec turbos. But yes, by using that term (in conjunction with how some shops here use it), I meant to refer to the HT12-3KAI turbos that are further improved on the standard '99 specs.

Originally Posted by mikejokich View Post
I believe an added reason the turbo manifold cracks on the secondary side is the port is unmatched and the very hot exhaust pounds the secondary manifold, especially when the door is open at WOT. On my just completed rebuild,that is why I hand ported the secondary to match the exhaust manifold and to match the opening size of the primary. The door still closes fine with about 2-3mm of overhang. Also, I smoothed out the turbo manifold just beyond the opening on the secondary side to make the air flow more laminar. I also ceramic coated the inside and the outside of the both manifolds. This all will hopefully help.
Mike
Mike, thanks for the info - curious to know which type of coating (brand name and temp level) you went with for your manifolds? You gave me some good ideas.
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Old 12-29-18, 02:35 AM
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GDSpeed,
To answer your second question, there are many different types of ceramic coatings from various manufacturers. Some of the coatings are sprayed on, some can be poured on or in tubing, and almost all the better coatings are baked dried at around 500 degree F. Again some are barrier (block heat transfer) and some are dispersant (promote heat transfer). For example, the compressor covers of the turbos are coated with dispersant to help give off heat build up and on the outside only as to not interfere with wheel tolerance or clearance. The remainder of the turbos and the manifolds are coated in barrier inside and out to keep the heat inside to promote the hot exhaust velocity for quicker and more intense spool and somewhat less back pressure since the exhaust air moves out quicker. All of my intake pipe and exhaust pipes are barrier inside and either barrier outside or dispersant depending on where there are in the engine bay and the need to relieve heat soak if possible. In example, my intake pipe feeding the secondary turbo is barrier along the lower half and dispersant along the upper half. The reason for this is the high heat source(the exhaust and turbo manifolds as well as the downpipe) are directly below which needs to be blocked. Any heat buildup or heat soak in the pipe can then be somewhat transferred away along the upper half (dispersant side) to the underhood area. Lastly, most of the coatings have ranges for best performance. In example, the exhaust manifolds and downpipe are high heat and the coatings are good to 2000-2400 degrees F. The coatings for the intake pipes, LIM, UIM, etc. are made for peak performance in the 250-500 degree F range. I did a lot of research on all of this prior to having all of these parts coated and sent out all of my stuff to be coated by a professional shop in Tampa, FL called Engine Armor Performance Coatings. I had many discussions with the owner, Scott, who has extensive experience in coatings and does work for race teams, professional drag racers, etc. He uses several different manufacturers of high end coatings including TechLine. When ready or to ask any questions, give him a call and tell him Mike the RX7 guy referred you.

To help answer your first question, the different type twin turbos from Hitachi for the FD. There is a common misconception being circulated on this forum. Let me try to clear it up. There are 3 types of turbos direct from Hitachi. All 3 are a direct fit and require no modifications, if you so choose. These are the original HT12, the '99 spec HT12-3B, and the SP HT12-3KAI. There is modified version that is sold by a guy in Australia that he calls the Bathurst SP, which are modified by him or someone else. I don't know much about them other than what he claims on this forum. I will leave it at that for those. Here is what I know or remember about the 3 true factory Hitachi sets.

1.Original '92-'98 HT12- 265ps(metric HP) rated in JDM's, lower nickel content(greater cracking), 25.5 mm wastegate, 270 degree seal, 41.5 mm compressor wheel, 57 mm turbine wheel, 0.6 kg/cm2 actuators, small shaft, wear out quickly when constantly used at or above 14 psi
2.'99 spec '99-'02 HT123B- 280ps(metric HP) rated in JDM's, high nickel content (less cracking), abraided housings, 25.5 mm wastegate, 270 degree seals, 41.5 mm compressor wheel, 57 mm turbine wheel, small shaft, still wear out quickly when constantly used at or above 14 psi
3. SP '14-current HT12-3KAI- 27mm wastegate (more controlled higher boost, less boost creep), redesigned compressor housing with same AR, 360 degree seals (higher boost capable, no oil starvation like the previous versions), 43.2 mm compressor wheel (12% more air), 57 mm turbine wheel (same), larger shaft (40% greater, capable of higher boost), same 0.6 kg/cm2 actuators (I had custom Forge actuators made that I mounted using modified stock brackets with 1.0 kg/cm2 springs for better higher boost control), these turbos were designed for 14 psi or greater (I will boost to 18-19 psi once fully tuned)- IMO I don't know why anyone who not use these KAI's or BNR's for any rebuild if you want to stay twin and desire greater boost, greater turbo longevity, or simply better turbos- these will last longer and boost higher with no downside other than the slightly higher cost of around $300-$400.

Here are some pics of my coated parts and some turbo specs on the KAI's.







Mike

Last edited by mikejokich; 12-29-18 at 02:38 AM. Reason: Added one thing
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Old 12-29-18, 11:49 AM
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Originally Posted by mikejokich View Post
1.Original '92-'98 HT12- 265ps(metric HP) rated in JDM's, lower nickel content(greater cracking), 25.5 mm wastegate, 270 degree seal, 41.5 mm compressor wheel, 57 mm turbine wheel, 0.6 kg/cm2 actuators, small shaft, wear out quickly when constantly used at or above 14 psi

Mike
i know this is nitpicking, but the high nickel housings were a production change:

1a. original HT-12 255/265hp, N3A1 stamped on the compressors. factory installed 1992/1993
1b. same turbo updated, stamped N3C1-13-700, 94-95 factory installation.
1c. final update, N3C1-13-700A, high nickel housings. in the US it was the service replacement (and we sold a lot of them), factory installed on the 96-02 265ps cars.
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Old 12-29-18, 12:28 PM
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What drove the creation of the KAI version. Is it an aftermarket upgrade by the OEM supplier or is it a new part superseding the old ones?
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Old 12-29-18, 03:30 PM
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Not sure. From what I remember from on an Irish rotary website where one of their member first tried these several years ago, Hitachi engineers were looking to try to modernize as much as the could with the older design of the "99 spec turbos. They obviously knew everyone was boosting the old ones above what they were designed for and knew that they were failing at sustained higher boost levels. Hence, the larger wastegate, bigger shaft, 360 degree seals, and 12% more air. They were also obviously limited by the overall size and design which had to fit without modifications. The KAI's did not supersede the "99 spec as far as I know since both are still available. Again, with my recent rebuilt it was a no brainer to me to get these after comparing the two and my goals of still pseudo stock twin sequential turbos.
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Old 12-31-18, 11:19 PM
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mikejokich, thanks so much for all this great info! I'll definitely keep you, and your coating source, in mind going forward. Thanks again and happy new year.
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