Hi all,

I'm slowly starting to piece together what goes into a turbo build, and almost everyone recommends improving axial rigidity of the core. So far i've learned that the stiffer it is, the less stress there is on the dowel landings during high engine loads and/or a knock event, preventing cracking and catastrophic oil leaks, correct?

Now, there's a couple of solutions to achieve this, from what i've seen including;

Increase axial rigidity - Adding a single extra dowel like Brett did. Brett if you read this; do you bolt into that dowel from both ends using shorter tension-bolts or does the dowel allow a OEM bolt to run right through?

Increase axial rigidity - Adding multiple (4 to 5) extra dowels by machining out tension-bolt holes. Again has me wondering how you bolt into/through those.

Increase clamping force - Adding a few 10mm studs in strategically picked locations. These don't need machining of the holes, the drawback being they don't provide the same axial rigidity increase as a tight-fitting dowel.

Increase clamping force - Adding a complete 10mm stud kit all around. This again means relying completely on friction between plates to prevent micro-movements and stress on the original dowel landings.

Increase clamping foce AND axial rigidity - Adding a few / a complete oversized stud kit, (1/2"). This would require some serious machining as alignment of the holes between plates plays just as big a role as the accuracy of the hole diameters.

So now i'm kinda on the fence about what option to pick, and i'm hoping you guys can help me along a bit on this. Is just a decent increase in clamping force paired with properly prepared mating surfaces "enough" or would adding a dowel / dowels / oversized studs be a preferable option?

Thanks in advance!

 

Hm, as per RX7Club;

thing to remember is the irons are not trying to move away from the rotating assembly but the rotor housings are trying to twist away from the combustion forces, and the rotor housings have almost no way of supporting themselves via an outside brace system. aluminum is also a soft and easy to move material so even the decking brace system may not work, although it would definitely help, but so do the following systems.

1)the easiest way is larger tension bolts without reaming the block, for more clamping force. not ideal but it is better than the factory tension bolts/dowels alone.
2)the more difficult but better way is to ream the tension bolts for snug fit tension rods which will not allow the engine to twist against itself, since the tension bolts act as 16 induvidual dowels plus the factory 2. can that really be argued against? technically you do not need to ream all 16(or 18) but i usually do 6 oversize tension rods.
3)about the same as above is adding more dowels, but dowels are hollow and i have seen them still warp/crack and also still allow irons to crack. ideally additional dowels would be solid but the only place you can use solid dowels is in the factory locations due to tension bolts passing through the rest. this also means external oil plumbing to the front/rear iron.

So what's *actually* needed for a Renesis making anywhere between 300 to 400 hp?

 

This:

1/2 studs everywhere. Or if you have really precise machining then 5-6 of them will do.

 

Why? Seems excessive.. I see fully studded blocks in Kiwi dragrace builds, but for a street 8?

Also half inch studs need machining anyways, so what do you mean by "precise"? It's not like you'd just drill 13mm holes and have the big studs still be loose fit through them, right?

 

Double post.

 

I have 4 extra (2 piece - not 1 piece) dowells in my engines in the combustion area much like is shown below:
Studding & Dowel Pinning - RotaryEngine.com | RX-7 Specialties

 

Because:

 

Yeah so what I was wondering about; do you omit the tension bolts where those dowels go? Or are they bolted trough/into.

 

Stock hollow dowels are used so tension bolts go through.

 

Gotcha. I found a machine shop who can do either/both, so depending on their rates I might go with what @ciprianrx8 said, otherwise it's gonna be a couple of dowels.

Thanks both for chiming in, much appreciated

 

Yeah there is alot of info around this topic that makes the topic kinda cloudy. For instance there are kits that dont require machining and only have higher yield bolts that let you torque more. Then there is the dowelling which lets you insert extra factory dowels to help distribute the shear force away from your corner dowels that often crack. Then there are the enlarged higher yield bolts like REC does which go to 0.5in(12.7mm) studs with nuts on the end for a more true torque reading.

Howard Coleman from the Rx7 forums has some interesting info on the stock tension bolts LINK . Basically what he found was mazda specced the stock torque spec for theses bolts specifically so they would not experience yielding when the engine heated up. People who overtorque the factory bolts may very well push their tension bolts into yield and lose compression. "Tighter is better, RIGHT?.....no" Seen plenty of youtubers like Rad continue to do this playing with fire and potentially getting other people to play with fire.

Keep in mind the Rx8 has 3 extra long tension bolts so don't just buy any Rx7 kit if you go the route of larger 0.5in studs. Then things to consider are highest yield strength. Material choice for thermal expansion. Corrosion. Whether it has cold rolled threads or just machined threads. Whether it requires larger threads be machined into the front iron. Whether they are thru studs that require a thru hole to be machined in the front iron. Then what torque to even go to?
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IMO ideally you'd do the high yield 0.5in studs which get all the benefits. Or do a few 0.5in studs in key locations which many people have proved on Rx7s to go 800+hp. Doing dowels with the stock tension bolts wont let you go higher torque for clamp load, but at least they can help distribute the shear forces to avoid cracking the oil passages and lose pressure.

Worst I think is the simple loose fit higher yield bolts. Sure the higher torque may increase clamp and therefore rigidity enough that the irons/housings prevent issues. Personally I would rather have the dowels to distribute any shear events away from your oil passages and not solely rely on clamp alone. The only notable benefit of these are that they typically use the stock thread, so you can technically do this on a built engine in a car slowly replacing them one by one.