FD Hitachi Twins re-designed??
 

I own a 93 that has the type of Hitachi Turbos assembly that incorporates a large snap-ring to hold the Bearing and Compressor Housings together. I've noticed on some FD Hitachi assemblys that Mazda did away with the snap-ring and re-designed the Compressor Housing to have four (4) bolts & retainers to secure it to the Bearing Housing.

Is this design a year specific change?? Was it re-designed because of boost leak related problems associated with poor snap-ring seating??

I have 5 sets of turbos. I have never seen what you are talking about. Can u take some pics?
John

A brand new set is being offered on eBay for about $2,500.00. Do a search for rx7 turbos and they'll come up. Look at the Compressor Housing and you'll easily spot the bolt/retainers, NOT a snap-ring.

Also, I recently bought a Turbo Rebuild Kit for my twins and the instructions had a diagram illustrating the Compressor Housing to Bearing Housing bolt/retainer set-up I am referring to.

I'd like to know the specifics and reasons for the re-design.

Actually they do not flow more at all. The compressor of the 99 specs is 1mm SMALLER than the previous ones because they wanted them to spool a bit faster as they do, and there are some other mods on them as well that make them more efficient and reliable like the abraided housing. My 2000 type R makes 280ps BUT at a stock pattern of 12-10-12

With that amount of money i would go with something that flows more than the stockers.

So all Fd Turbos incorporating the bolt/retainers on the Compressor Housing, and not the snap-ring, are 99 spec??

"make them more efficient and reliable like the abraided housing"

The abraided housing is recognized as containing a design fault - apparently the 98- twin turbos will 'not sustain power' and cannot be used for 'endurance-style racing' (I have no idea of the precise details there - that's a direct translation from a japanese report)

Here's a photo of the Turbos I was referring to.

Check out the bolts and retainers situated on the Compressor Housing.

Aha, that would explain the extra 30hp. So if the Efini y-pipe kit adds 1lb then does the efficiency of the smaller wheels add the other 1lb? That's pretty deceiving, didn't everyone think it was 280PS on 10lbs?

The gain is 25HP, not 30 (280-255=25). The redesigned air intake is supposed to contribute 10HP and the remaining 15 is from the extra 1.7psi of boost (9.1 vs 10.8) and the efficiency of the new turbos.

So any thoughts and/or hard data explaining why Mazda ditched the large snap-ring in favor of the bolts/retainers??

This is always going to be a problematic argument - given that it is highly unlikely that 280hp IS the actual output. Remember that this was a 'gentlemans' agreement' between manufacturers - none of them have been very gentleman-like about it in reality

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I also thought the exhaust was freed up a little?

off the top of my head - there's the y-pipe update, better exhaust, 'bigger' dp (thinner metal = more volume), upgraded turbos, better ecu (whether or not that lends itself to better performance, I don't know), better air flow from revised bumper, extra boost

The 280ps (hp) is most likely, underquoted. I can't find the suggested 'real' ps figure at the moment - I believe it to be in the low 300 area (303 or something like that). I've heard that toyota (with the JDM supra) was the biggest cheat in the 280ps agreement - though I'd say the r34 GTR is up there too

How much additional HP gain using JSpec 99 Turbos in a bone stock 93-95 FD??

FYI: 255hp at 6500 rpm w/manual tranny
255hp at 6200 rpm w/ auto tranny.

Possibly just a manufacturing change requested by Hitachi, and/or slightly improved metallurgy in the bolts/retainers that allow them to expand/contract better in relation to the exhaust turbine housing. Just a guess...

But the bolt/retainers I'm curious about are on the Compressor housing side. A totally different design from the snap-ring configuration. Manufacturers usually don't make design changes unless it improves the ergonomic or mechanical function of the component/devise.

That's what I meant by "improved metallurgy to allow the bolts/retainers to expand/contract in relation to the exhaust turbine housing". The bearing housing is still attached to the exhaust turbine housing, transferring a lot of heat from that area. Perhaps Hitachi or Mazda felt that the snap-ring setup could possibly be susceptible to vibration problems that would cause wear/clearance problems after time.

Copy That.

Perhaps the bolts were part of the change to abradable housings. It seems that it would be more important to keep the compressor housing as precisely located as possible relative to the compressor wheel. Maybe the snap ring was not precise enough. Those turbos in the picture do look like they have abradable housings as evidenced by the black stuff inside the housing visible though the compressor inlet.

-Max

I agree. I rebuilt my Turbos a few weeks ago and noticed that the Compressor Housing had a little play once it was placed on the Bearing Housing. This tells me that the Comp-Housing can shift a little if the snap-ring becomes weak or if the snap-ring is not seated properly. Makes me believe this instability can cause boost leaks.