Studs or NO Studs? (Confused)
 

Hi guys. Last week we went to the Dyno, and we tune the car (FD series 8) with good results until the clutch couldn't handle the power anymore. We made 430 bhp at 1.3 bar boost pressure, and now we will proceed with a clutch upgrade, since we look forward to raising the pressure up to 1.5 to 1.6 bar. I would like to ask if it's a must or not to stud the engine. I was looking for turblown stud kit that no modifications are needed and that can be installed with the same Mazda stock bolt pattern by uninstalling one by one the stock bolts. Me engine is series 8 REW with all the clearances done on rebuilt before 2k KMH. Also, we install the RA Black super seals. I read a lot of discussions about studing with people being confused if studing is needed or not for engines with 550 bhp+. Some say is needed, some say is a must, some say to not do anything until 600+ bhp and some even say to stud an engine is a step backward. I would like to know your knowledge and some honest reviews on this since we will replace our clutch soon and if It's worth it to stud our engine for 550 bhp goal. Also, please some advice if our stock transmission with a kaaz lsd differential will hold this power goal. Thank you all.

You're all over the place....

The motor should have been studded during the rebuild if POWER was your goal. The stock trans will hold but who knows for how long. A stronger clutch with a lot of power will shorten the lifespan by a lot. The stock diff will hold so long as you're not launching it and doing burnouts. A sticky tire will jeopardize its lifespan as well.

Very simply, you're entering the realm of a high horsepower car. The necessary mods to make it work are universal across all chassis. Unless drag racing is your goal, there's no real reason to have an FD with that much power. The car is FAST FAST at the ~450HP mark and for the most part will hold itself together.

There is a such thing as too much and the car just becomes a hazard at a certain point. If living on the edge and fighting to control the car in a straight line is your thing then you're on the right path.

Dear fdauto,
Thank you for your response. The rebuild was done many years ago, and we didnt be familiar back then with studing. So we proceed with ra black super seals and big street porting both for intake and exhaust. We also using ra oil adapter kit to supply 2t premix to the omp + premix using the same 2t oil. About the chassis we have installed autoexe brace in th front so to hold the trans and engine to the chassis. We also using solid engine mounts. Also autoexe brace to the rear for the diff. We installed gredy type s coilovers with motorised computer for better handling and 18" rear wheels and 17" in the front with yokohama advan semi slick tyres all arount. After speak with turblown they told us that studing can be done one by one after drain the coolant. What we are asking is if it is necessary for our powe goals. Thanks

You're doing everything you can to break the car and the parts attached to it. All that bracing and solid mounts WILL cause more damage to EVERYTHING the higher the power goes. To stud or not to stud will be the lowest of your issues.

As I mentioned in the first post, the engine should have been studded if power was the goal.

good advice from FDAUTO.

your clutch choice will be important in relation to your transmission's health. ACT just released a double disc with a sprung ORGANIC lining. do not buy a clutch w a cerametallic lining as it transmits too much shock.

Turblown studs increase the clamp 28% V Mazda thru bolts. the primary challenge to the block integrity is radial not axial. simply put, if you encounter sufficient knock the studs will do little to avoid breaking the front iron at the upper dowel. one key re our motors and the stock thru bolts is not to over torque them. the block has over 6 inches of aluminum (rotor housings). aluminum, when heated, expands approx twice as much as most other metals. if the thru bolts are over-torqued, and then stretched as the motor expands, the thrubolts can get into yield. a fastener in yield looses its clamp.

for more on this...

TENSION BOLT TECH
.
bottom line: studs are a plus but if you really want to do it right you build the motor w 16 1/2 inch stud dowels... if that sounds too over the top consider the motor was offered at 215 rwhp. build it once build it right.

Hey man, congrats on your build and surviving dyno day. However, 1.3bar boost on a non studded engine is already high on the risk curve. Obviously, its upto you how much risk you want to take. IIRC Mr Dahm on a bone stock 13b made 550hp at 31psi, so you will need to push the boost up real high on a stock non ported 13b to achieve a real 550hp. Also it is also very likely you'll have engine failure on a non studded stock port 13b engine.

Now to your main question — should you stud the 13B for your 1.5to 1.6bar boost or a 550hp goal?
Short answer: 100% YES, if you plan to run above 1bar boost....just stud the engine for peace of mind.

Here’s why:

• Anything over 450–500 bhp and especially pushing 1.5–1.6 bar of boost, you're asking a lot of the stock bolts. Studs give you way better clamping force, hold torque better under heat/boost cycles, and stop the housings from lifting, separating and seals from failing.
  
• Brand new 13Bs cost almost 10k+ USD/GBP now from Mazda. Rebuilds aren't cheap either. So dont risk a failure, unless you have spare engines and lots of monies.
  
• You're already pulling the car apart for the clutch swap so it could be a perfect time to do the stud kit. Turblown’s kit is good, no machining needed, just swap them out one at a time and torque properly.
  
• The stuff about "studs being a step backwards" is just bad info and internet misinformation. Only becomes an issue if someone installs them wrong (wrong torque, wrong thread engagement, etc).
  

As for your stock transmission and Kaaz LSD for 550hp:

• Stock FD trans is pretty tough. My stock box survived 325+ 100-200km/h runs. It eventually died on a track day but it did great for nearly 5 years of consistent abuse at a true 500-580hp levels. I had a tilton flow valve installed to prevent driveline shock combined with an aggressive competition stage 4 clutch. If you want max box reliability, go with a stage 1 organic clutch which will provide a bit of slip.
  
  However, your weakest link is your stock engine which is more likely to fail at 550hp before the gearbox or diff!!

Dear friend,
Thank you very much for all the precious information. Just to point some miss information. My engine is not stock. It was rebuild 2000 kmh before, with used rotor housings in good condition and using new rotors straight from Japan. All seals new with all clearances and torque specs according to mazda catalog. Also RA black super seals where installed to handle high boost and also the irons where ported using large street port templates. Housings also being ported using Pineaple racing templates. So the engine is not completely stock. Tuner was also great and achieve to almost max out the 1000 primary and 2000 cc secondaries and stayed at 3.5ms dwell settings for the ign1 kit we purchase and installed from sakebomb. Next step is after change the clutch (we will only uninstall the transmition from the car and not the whole engine with the teansmition apart from the car) we will raise the boost but with the help of wmi 70%methanol and 30% distilled water. So do you all believe that when the transmition is out off the car to proceed with stud kit?

https://cimg7.ibsrv.net/gimg/www.rx7...256a9639b7.png
this clutch package will do more to save your transmission than any other mod. it is brand new as of November. the key is the organic lining. it will not slip yet is easy on the driveline. combine w solid diff bushings so as to minimize chatter and you have your best shot at transmission survival. clutch lining matters.

it sounds like you are on a pretty good path. the addition of the studs is a plus and they are easily swapped in with the motor in the car.

Whenever I build a highly modified RX-7 for someone, the first thing I ask is: How much power do you want to make, and what will the car be used for—street, drift, track, etc.? When modifying REW engines, there’s a point where certain upgrades become necessary not just for performance, but for reliability.

For example, engine studding helps prevent the block from twisting under high boost, which can otherwise lead to cracks in the front or rear iron. In places like Florida and Puerto Rico, where high boost is common, you also need to address other vulnerabilities—like adding external oiling systems, ensuring proper side clearance, doing dynamic balancing, and most importantly, getting the tuning right.

At the power level you’re currently at, I’d say studs aren’t strictly necessary as long as everything else stays dialed in. But keep in mind, a fuel system issue or an overboost event could still end your day fast. Making 550+ hp is already more than double the stock output, so I’d personally add studs just to build in more margin for error.

That said—what are your goals beyond that power level?

Dear friend. Thank you for your response,
My goals are easy. 550bhp with reliability for street use. Not hard launches just normal ones, not drifting just rolling drag acceleration next to german cars that want to play. That's it. Will have low boost settle at 1.2 bar and high boost at 1.5 bar. My clutch upgrade is going to be from os giken twin 185mm disks to the same type 215mm twink disk

Thank you for clarifying the details of your engine. My apologies for the initial assumption that it was a stock-port engine based on the previous context. The information changes things significantly and improves your chances of achieving 500-550hp. What turbo are you using?

Following the previous discussion where it was advised that pushing boost to 1.6 bar on a non-studded engine is risky, and acknowledging there's still risk even on a studded engine. Interestingly, my engine is also a 13B with a large street port (the biggest my engine builder does), running RA Super Seals, balanced to 10,000 RPM, and it uses his custom 1/2" stud kit. On my setup (studded engine, street port, RA seals, WMI, 99 RON fuel), I've successfully completed around 160 runs from 100-200 km/h running between 1.4 and 1.6 bar boost without experiencing issues. I don't know if the same level of reliability or success could be achieved on a non-studded engine. I think this will be a call for your tuner/engine builder and yourself to discuss and move forward from there. Keep us posted if you decide to chase 1.6bar.

Dear friend,
Thank you for your quick response. What is your current hp at 1.6 bar of boost? Do you use wmi or just pump fuel? I will use wmi with 2x 400cc injectors and my turbo is journal bearing gt4088 with greddy 60mm wastegate and 3.5" exhaust

Dear friend,
Can you explain more about tilton flow valve.? Thanks

It restricts the speed at which the clutch pedal can return and clutch engage.

Dear all,
Just to close this thread,
Is anyone that done this before or all that havent try this i would like to ask if its safe to replace the stock studs with turblown studs one by one without harm the coolant oring seals. I desice to go for it with turblown direct studs replacement but i need to know if it's safe to do this while the
​​ Gearbox will be out of the car for car replacement without disassemble the engine just drain coolant and replace the studs one by one with new oem washers and rtv. Thank you

Yes multiple people have done it without issue.

When I was running 1.4 bar (20 psi), the car was dyno’d at 510 hp. The dyno operator was adamant that their dyno readings show rear-wheel horsepower (rwhp). However, based on consistent 100–200 km/h times in the 6.5 to 6.6 second range, it’s clear that the actual output was around 510 hp at the flywheel, not at the wheels. If only dyno operators knew you cant lie about power in a real world 100-200 test.

Now I’m running 1.6 bar, my 100–200 km/h times have improved significantly to 5.9 to 6.1 seconds, which supports an estimated output of 560 to 580 hp at the flywheel, depending on conditions. Not rwhp.

I always use 99 RON pump fuel with wmi as there is no pump E85 available and race E85 is ridiculously expensive. That being said I might use E85 at 29psi or 2bar and do a 100-200 run to see what it does vs pump fuel and wmi.

Guys does anyone know if i need to readjust the endplay after replacing the studs. Just wondering since the turblown studs will be torgued downt to 40 lbs. Since this is more than the stock torque doecs do i need to readjust endplay? Thanks

The front stack play is independent of block "length".

Just out of curiosity, what failures are you all seeing with factory tension bolts and studs? On a piston engine, insufficient clamp force between head and block pretty quickly results in a pressurized coolant system or blown HG. Are you torching housings and coolant seals?

It's funny you ask that because I have had the same question.
who has come to the point where the clamping force of the stock bolts is a problem and the bolts either stretch or snap?
sure, the stud maybe be stronger with a higher clamping force but i have yet to experience what has been mentioned. my stock bolts hold fine going strong for around 13 years now... 30psi on the 8374 for years and currently 28psi on the 9280 and my compression is still great, and the car fires up every single time without the slightest hiccup.

How is this possible to run 2 bar of boost on stock studs and dowels? You are just lucky that you didnt came across an event. Maybe the stock studs and dowels can hold this boost but clamping appears at high rpm so maybe you were lucky?

Lucky for 13 years is a bit of a stretch...and i have many thousands of track miles on my car. and its not like we are cruising at the track, 1s quicker than a mclaren 720 per lap with the same driver and my tires were worn is far from being just lucky.
I came across plenty of events but at the end of the day, i would like to see actual data where the stock bolts and the dowels are the limiting factor...
I dont think luck is a contributing factor for something that lasts for so long . it is something that it has been built properly

I thought studding without actually machining a slip fit was just snake oil? Unless I am misunderstanding what OP said, it sounded like the plan was to take the flywheel off, drain the engine and just swap out the tension bolts.

Without a tight fit stud you are just getting better clamping and not torsional stiffness in the block which is far more important. This assumes your "better" studs actually provide a better clamp. I thought this was well understood by the rx7 community, or am I off my rocker?

Yes you are right about what OP said, the plan is to take the flywheel off, drain the engine and just swap out the tension bolts. This will be done one by one acording to mazda pattern. The goal is to raise the lbs the bolts can handle up to 195000 psi as turblown specifications for the specific studs which is much higher than the stock bolts. So i believe this will be very helpfull at high rpm and high egts where the aluminum housings tent to expand more than the irons.
Though i didnt get your point what you mean about Without a tight fit stud you are just getting better clamping?