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Which turbo for Auto/HPDE/Weekend Street

Old 01-11-17, 11:01 AM
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Which turbo for Auto/HPDE/Weekend Street

Folks, I need help choosing between EFR 7670 and 8374. I am building my FD to do autox (for fun not for SCCA national placement), HPDE days and weekend hoonage.

With regards to autox my good friend and I have a healthy competition going on between me and his S2k. The arms race has begun and he is slapping on a Rotrex with a proven 400 to the wheels setup. That has become my target HP goal to keep things even. Both turbos can get me to my goal, but which is the best for a well rounded car? I do plan to run flex fuel. I want the most reliable setup, but I also want something with as little lag as possible. This is why I chose the EFR Turbo to begin with.

My ultimate long term goal is to take this car in the Optima Ultimate Street car challenge AND Tire Rack One Lap.

If I run E85 and the 7670 I get the least lag, but am I running that turbo too hard and too much boost for my longevity/reliability goals? How much more does the 8374 lag if I'm trying to make only 400whp? 400 will probably be our pinnacle of whp. Neither of us want to fix our cars every year and are going to have our hands full handling this much power.

Which turbo and why do I go with to give me that S2k spanking Ultimate Street car dominating One Lap monster?

Thanks in advance for your input everyone

Disclaimer, I've already bought into the Turboblown group buy so I am not open to switching to other turbos/setups. This is meant to just flush out the merits of both EFR turbos and why I should run X size)
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Old 01-11-17, 12:05 PM
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I had the same question and after talking to several people decided to go with an 8374. What I found out in layman's terms:

7670:
Less lag than an 8374
Better for short autocross courses and the street
Will work harder to make more power (smaller compressor)
Will run out of steam on a road course/track and wear out faster due to higher demand

8374:
About a 300 RPM difference in spool time
Lag over the 7670 is not very noticeable on a longer or faster autocross course
Capable of more power with less work/ Will last longer at 400WHP
Will make power longer on a road course or track


I went with the 8374 for longevity and the potential for more power in the future without major changes. My understanding is that it would be easier to reach the 400WHP level around 1 Bar with no AI.

A good place to look for info if you haven't ventured into it yet is the "Single Turbo RX-7's" section here.
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Old 01-11-17, 01:14 PM
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Tough choice!

There are enough people on the fence that you should be able to trade with no loss to either party once you try the turbo no matter which you go with.

I will say if you go with the the EFR 8374 and someone is trying to talk you into 3" downpipe/exhaust to stay at your low 400rwhp goal- don't do it.

Do 3.5" or 4" to get the best response out of either turbo.

If the boost creeps to where it will be over your power goal on the EFR 8374 put a restriction at the very end of the exhaust (muffler tip or last gasket before the muffler).

Intake- run a velocity stack to 6" or 8" base size filter. Makes a big difference.

You can choke out a 7670 so it spools slower than a 8374 and you can choke out a 8374 so it makes less power than a 7670.
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Old 01-13-17, 09:36 AM
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Stepping up to a 3.5" means no more Gandor Titanium exhaust that I love. This makes me sad. I know I can flip turbos if I change my mind, but then I get to pay for a retune which I don't want to do with how much it costs.

I'm kinda of leaning towards 8374 for reasons Molotovman lists and it looks like going 3.5" full exhaust. Are there many 3.5" exhaust choices out there or will I have to go custom? Is 400 whp really that low of power for an 8374?

With the exception of the local Miata club, most autox around here are of the faster/larger type.
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Old 01-13-17, 10:19 AM
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A 3.5" or 4" downpipe/midpipe work very well with a 3" exhaust- there is no conflict.

On my car I tried various combinations and found the 3.5" downpipe/midpipe to 3" catback had all the spool of the full 3.5" exhaust and did stop boost creep.

I preferred the deeper tone of the 3.5" cat back section, but it was much louder.
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Old 01-13-17, 10:46 AM
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The EFR kit supplier can supply you with 3.5" downpipe and possibly midpipe to fit the FD.

I have a made in Korea OBX-R ebay catback that is 3" for a short section and then 3.5" on my FD (full 3.5" was on my FC). It is a big oval muffler and fairly quiet, but deep tone.

HKS used to make a 3.5" midpipe and cat back, but no 3.5" downpipe for the FD.

There are various low production 3.5" exhausts of Ti for the FD, they are all hard to find and $$.

SMB the supplier of the exhaust on the Bathurst 12hr winning Mazda RX-7 SP makes a full 3.5" exhaust available as components and various configurations as well-




Though it is set-up for stock twins (downpipe for stock twins).

Their standard 3" exhaust includes a 3.5" downpipe from their race experience on stock twins.

Full 3.5" components and systems also available.

SMB Full Exhaust Systems-Mazda RX-7

Don't have a heart attack the prices are in AUD

I have an SMB 3" catted, muffled midpipe on my FD I picked up used and it is VERY high quality materials and workmanship.

SMB famous bent tip exhaust as on RX-7 SP




So, you can have your EFR kit manufacturer supply you with the 3.5" downpipe to fit single turbo and buy SMB 3.5" midpipe of your choice (muffled, catted or muffled and catted) and use your 3" Ti cat back or buy 3.5" SMB cat back.
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Old 01-13-17, 11:15 AM
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Awesome suggestion. My EFR kit is coming with 3.5" DP. Thanks!
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