Rear mount to Top Mount (CXracing top mount) (yes some of you were right lol)

[acbx]

Hello! I prefer to lurk most of the time because usually my answer is already on here and also I know most of you weren't really too fond of my rear mount setup so I just kind of kept it in the dark on here for the most part lol, but I still love going here ofc because there's lots of great information on this board, I love reading the stories of trials and tribulations and the setups people run and all sorts of things. Its great. Anyways...


Backstory I made a rear mount setup and it worked as it should but honestly the lag was just terrible, made full boost 10~11psi at around 4500 with a small t3t4 turbo. BUT having ridden in a local car with a bottom mount spa mani I was quite aware of how much of a different animal it was compared to my setup. After trying a couple of different methods trying to make mine spool better (smaller piping to the turbo, heat wrapping, turbo blanket, etc, even did over the subframe charge piping, not that would help with spool lol but i was tired of the piping scraping everywhere) I eventually gave in and switched to top mount.. Lesson learned, lot of money wasted, but I don't really regret it, I was boosting, kept my a/c, and everyone that I've met in person actually loved it.. holy shit was probably the most common response lol.

Anyways I kinda got fed up with the laggyness, switched to a top mount few days ago, got a cxracing top mount manifold, using the same t3t4 turbo by twisted motion, ebay wg with 8 psi spring, I used n54 copper studs and nuts (was still a HUGE pita, if you ever get this stupid manifold and need to install it with the engine in the car, I wish you the best. lol), draining to drain plug, removed the a/c. Had 3" downpipe done all the way to back of the car, venting wg to atmosphere.

I love it, spools fast as hell, when the wg opens it just screams! But I've hit an issue, for some reason ever since the switch to top mount it's running stupid rich, afr gauge is maxed out at 10! It idle hunts and it just bogs everywhere since its running so rich. Thought maybe I broke the stock o2, replaced it same thing, did a twice over on all the places exhaust could've been leaking and re tightened a lot of the exhaust bolts, etc. Still nothing. Vacuum reads fine at idle -19 so I don't think its a vacuum leak? o2 sensor is in the same general location as before. I tried reseting the battery to see if its adaption but nada so far... new knock sensors, new camshaft pos sensor. ngk 4554 plugs gapped to .22. bov is located pre intercooler so I don't think its affecting the maf.. I'm really stumped..

So far I've just been driving it with maf unplugged, afrs are good driving and in boost with it unplugged just little rich all around. Especially at idle, 12 afr and idle hunts sometimes. So is there something I'm missing here? Everything is relatively the same apart from removing a/c. Is there like a sensor that's related to the a/c that I might've tripped? Tune has remained the same, it was never tuned for my rear mount setup and it worked completely fine then so I'm not sure why it would decide to change now. I have a Nick G / technique tuning tune package I bought used, blow thru maf, 42lb injectors.

Anyways any info, ideas, what to look for, let me know, thanks! I would love to get it fixed soon so I can finally move on to replacing the clutch and adding just a lil bit more boost!

[vollosso]

Charge pipe issue id guess.


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[acbx]

Thanks for the suggestion! Just tried changing the cold side piping around, no luck so far. Ran it with piping unplugged just the hell of it. Same thing. Did find that my tps wire was being pinched so fixed that real quick. Unplugged a couple of vacuum lines to see if that could cause it(to gauge, bov, fpr) nothing.

With maf unplugged it runs like it should.. sigh.. I'm about to just put the a/c compressor back on and plug it in see if that fixes it

So does the o2 sensor have to be a foot or so away if its a top mount? Mines currently around the stock location. I have an autel at work that I could use, it has a connector for the diag port in the engine bay, maybe I can try using that to see if that'll show anything on my obd1 car?

[pbonsalb]

Is the HFM located on a straight section of pipe with several inches straight before and after? It might not read the air perfectly if on a curve or near a curve or the BOV is too close.

Have you cleared adaptions?

[acbx]

Is the HFM located on a straight section of pipe with several inches straight before and after? It might not read the air perfectly if on a curve or near a curve or the BOV is too close.

Have you cleared adaptions?

So I tried clearing adaptions except this time looking it up first on how to do it on an obd1 car. Yeahhhhhh so I thought clearing adaptions just involves disconnecting the battery for like 5 seconds. Fully disconnected it this time (pos + neg) left it alone for an hour or so, reconnected it started the car idled funny at first but got better more it warmed up, drove the car to work and back idles fine now with maf on. 14.6~ afr lol. that was easy, felt kinda stupid but oh well it works now.

Also there is a slight bend almost directly before the maf so that probably doesn't help lol. When I do 3" cold side piping I'll get the right piping to fix it.

[vollosso]

Also check your wiring. It should run like shit unplugged.

Also who did you buy the tune from?

[acbx]

Also check your wiring. It should run like shit unplugged.

Also who did you buy the tune from?

I mean it runs, just runs rich as hell with maf unplugged. It's my go to solution when something isn't running right and I need to get somewhere. lol.

It runs fine now though. afrs are 11.5~ at full throttle, 14~15 afr at idle. hopefully it stays like that *knocks on wood* lol. Now I just need a clutch. I have an s50 but I'm still running my old getrag trans (I know i know), fx stage 4 should still work correct? Thinking about getting that one since that seems to be the go to budget deal. Already looked it up but just need reassurance lol

Also its supposedly nickg tune, bought the whole tuning package used from Rally Road roughly a year ago. Obd1

[328iFun]

Rear mount has always been kind of interesting when setup properly on the right car.

I could maybe see one being beneficial on e36s due to limited space and for those who want to keep AC. Maybe in a twin package? One smaller, lower AR off manifold and then a larger, higher AR in rear. Not in in trunk though, would have to be on the underside somewhere...

[acbx]

Where the stock muffler sits on the underside would be ample space for the whole setup to fit I think, I was just reckless and decided to have it in the trunk lol.

That would be sick! Just sort of like cause they can sort of things. Would be very cool. There's a local here with a miata and a compound turbo setup, small turbo off the manifold in the front, big one in the rear, and that thing is pretty sick. His reasoning was he wanted a broad power band and massive power or something along those lines with his tiny 4 cyl engine. Thing looks bone stock from the outside, apart from the slicks. It's great.

[acbx]

Since the last post I got the FX stage 4 clutch + ltw flywheel in, 255 lph pump and mbc came in today raised the boost up to 10 psi. Man, its fun!!! I'm still in a rush lol burned through 1st and 2nd on heavy 18x10.5s with 255s. I was surprised. Chopped the soft top off as well so lost like 150 lbs there. (hardtop vert life) I'll try to get used to this first before I go to 15 psi lolol. But I ran into a hiccup on the way home from beating on it. Hit 2nd and it boosted great with afrs and right when it was about to spin the car shuddered and afrs went to 14 so I let off. Tried to boost in 3rd and 4th and the afrs were fine... Thoughts? I also noticed that at cruising my afrs oscillates a lot. Like goes from 13-15. Is that normal? Maybe that could be part of it? I've replaced most of the sensors, the only thing I haven't is the crank position sensor. knock sensors, camshaft pos sensor and so on i have replaced. Could crank pos be the cause?

[acbx]

Update: Well it was pretty eventful today. Though I'd take the car out, raised the boost up to 14 and stutter / lean issue was still there. So I lowered it back to 7, and it went away. Raised it back up to 10 and it was still good. So idk what happened. Thought I'd take a look at it again later.

Anyways I went to a friends place since he wanted a ride in it (mind you this guy has broken 2 getrags) literally the first pull i did at 10 psi from 1st to 2nd, bam 2nd gear gone, and its clunking like mad. Lol soo I'm going to buy a Zf now. Sucks because I just had the trans out to do the clutch! I heard I'd need the driveshaft too?

Its crazy because the whole time I had it supercharged on m50 and s50, no issues with the getrag. Goes to show how much more torque a turbo can make lol and how weak a getrag is.

[acbx]

Well I bought two parts cars, one has a zf in it. Hopefully I can get it done by the end of the month!

[NOTORIOUS VR]

Update: Well it was pretty eventful today. Though I'd take the car out, raised the boost up to 14 and stutter / lean issue was still there. So I lowered it back to 7, and it went away. Raised it back up to 10 and it was still good. So idk what happened. Thought I'd take a look at it again later.

Sounds like you have spark blow out.

What plugs and gaps are you running?

If the plugs and gaps are good (try around 0.022"), then you probably need a new set of coils/boots.

[tptrsn]

I'm curious, what size tubing were you running from the exhaust manifolds to the rear mounted turbo? And then what size charge tubing back to the front?

[pbonsalb]

I agree with spark blow out. Go a step colder than stock on the plugs (down a number if Bosch up a number if NGK) and gap the plugs down as noted above and try new coils and connectors if gap alone does not do it. On the ZF, you need the guibo and DS from the donor car or at least the front half of the DS. The output flange on the ZF is bigger. Also, the shift linkage changed so get the ZF linkage as well. The ZF is more robust.

[acbx]

Sounds like you have spark blow out.

What plugs and gaps are you running?

If the plugs and gaps are good (try around 0.022"), then you probably need a new set of coils/boots.

Currently I'm running 1 year old ngk 4554 gapped at .22 funnily enough lol. I will definitely look into that! I've replaced two coils but I remember one of the boots wasn't the best in shape. So I'll double check. Boskey (has turbo black 4 door) suggested I should gap it down some more as well when I do get a new set of 4554s.

I'm curious, what size tubing were you running from the exhaust manifolds to the rear mounted turbo? And then what size charge tubing back to the front?

At first I had a single 3" running to the turbo, and 3" downpipe. Later on I switched down to 2.25 to help improve spool. Charge piping was 2" up to the intercooler. Then 2.5". I love what you're doing btw, looking forward to see how long the auto lasts boosted. Lol

I agree with spark blow out. Go a step colder than stock on the plugs (down a number if Bosch up a number if NGK) and gap the plugs down as noted above and try new coils and connectors if gap alone does not do it. On the ZF, you need the guibo and DS from the donor car or at least the front half of the DS. The output flange on the ZF is bigger. Also, the shift linkage changed so get the ZF linkage as well. The ZF is more robust.

Excellent, I will definitely check on the plugs and coils. And ok cool! I'm hoping I can keep my DSSR, I have a hard mounted shifter that removes the top part of the shifting assembly. We'll see. I got the transmission out today from my car, definitely was a lot easier than the first time. Fun fact my getrag has a dot-r sticker, so it's been replaced before lol. Not that it matters I just noticed it today when I pulled it and thought that was kinda interesting.

[tptrsn]

At first I had a single 3" running to the turbo, and 3" downpipe. Later on I switched down to 2.25 to help improve spool. Charge piping was 2" up to the intercooler. Then 2.5". I love what you're doing btw, looking forward to see how long the auto lasts boosted. Lol

Thanks for the info!

I have some theories about the sizing of the exhaust from the engine to the rear mount turbo based on some of the stuff I've seen on yellowbullet and whatnot. It seems like 2" would have been the very biggest you would have wanted in order to have good turbo response, and possibly even smaller than that. There are some really complex calculations that can be done to figure it out, and basically it involves keeping the exhaust velocity as high as possible without having it go supersonic. All very strange. I keep wanting to try a rear mount setup, and I feel like I should have done it on one of my current cars...

Thanks for the encouragement on my nonsensical projects! FWIW, the automatic is already beyond its depth. Because of the way the DME is programmed with a hard load limit of 1389(mg/stk? I can't remember the units off hand), and the fact that I need the DME to send a properly high load signal to the trans controller for proper shift firmness, I'm stuck at about 8psi of boost currently. The car rips pretty decently like that, and the trans will hold the power in 1st, 2nd, and 4th, but will not hold WOT shifts or boost in 3rd gear.

Since I drift it in 2nd, it works well enough, but I'd like to push the engine further. Overall the situation is kind of a mess haha..