It's been a few years since I began my carburetor modding fun and now I think it's time to start progressing towards my original goal: a blow-thru Nikki. This thread is going to be similar to my Intake and Nikki Modding thread in that it will be me working through the process, posting photos and whatnot, and asking questions along the way. Plan is to begin the work on the car this summer, so for now I'm in the research and planning stage. Boost is something I've not dealt with in the past so pardon what will be some misconceptions and learning mistakes - goal is to make this a somewhat comprehensive turbo Nikki thread, although focused on documentation rather than a "how-to". I also won't be getting into much carburetor stuff besides tuning later on as it's been thoroughly documented already.

Starting things off with the centerpiece of the organization, the turbo. I've seen a lot of older threads that suggest using the S5 turbo because it's simple, effective, cheap, and easy to obtain. Unfortunately the latter two no longer seem to be true unless you get lucky or are constantly scanning marketplaces. More recently I saw a thread (this one) where a Borg Warner S200SX-46 turbo was used, successfully fitting beside the engine. After some looking around it seems like this may be one of the only "budget" name brand turbos that fits the bill. Similar offerings go for far more (I found the S200SX at Banzai for only $590.00 new) unless you start going down the eBay rabbit hole. Curious if anyone has looked into this recently and perhaps found another comparable alternative. I'm also curious what people think as far as the sizing goes. It looks like the S200SX comes in a 46, 51, and a couple larger compressor inducer sizes. Now, I'm just starting to get my head wrapped around a/r, trim, compressor maps, etc. so please excuse a little ignorance on the turbo selection aspect. Again this is me trying to learn some stuff as I make progress with this, so it'd be great if those with experience could chime in.

Moving on from the turbo (for now); ignition! Fully locking a distributor for a street car has never been too appealing in my opinion. I understand why it can make sense for a track car (you're not concerned with a steady idle and it's "more reliable"), but it seems like a poor solution to limited timing advance on a street car. That's where limiting the total mechanical advance came to mind, and to my delight it looks like Jeff has done the hard work already in this thread: Semi-Locked distributor how-to (for boost). Based on what I'm seeing it sounds like total 15 degrees mechanical advance is a good starting place under boost, or perhaps a little lower for safety. I won't bother recounting how the modifications work since it's already been well outlined in the aforementioned thread. I am curious, however, about vacuum advance. I spent a while yesterday looking at some timing maps from the later gen cars and it looks like high advance under vacuum/little boost is quite common. This of course mimics what vacuum advance already offers, and I was surprised to see in many threads that it was suggested to ditch the vacuum advance when in my opinion (coming from NA experience here) it offers significant benefits for light throttle responsiveness. And it's not like it could cause greater than total mech advance (say 15 degrees for example) under boost because it's vacuum advance. Really it would only affect light throttle like it already does for NA. As long as the diaphragms can handle the boost pressure (and maybe they can't, I'm not sure) I don't see why people wouldn't keep it if the option is available. Is there some part of the equation I'm missing here? Of course we can't do boost retard with the stock dizzy, but keeping vac advance seems like it would still offer a safe way to get extra timing at low throttle inputs while letting mech advance take care of the high end.

Those are the major topics for now, although I am a little curious about the turbo oil feed source. Seems like most people either use a sandwich plate on the oil-filter or use the stock oil pressure sender location, although aren't both of those pre-filter? Is that not a concern for turbos, or perhaps I have my oil gallery routing mixed up?

For the time being I'll be working on a few other vehicle preparations, most importantly switching over to an RB Street/Strip HD clutch in a couple weeks so that I won't burn up yet another clutch when the turbo goes on this summer. And thanks to those who read this all the way through. It's probably going to be a bit text heavy until physical work begins this summer.

 

OEM on the S4 & S5 the turbo gets pre-oil filter oil. I use a sandwich plate on my 6 port engine, but my 4 port engine uses the direct feed from the first iron.

Good luck. I've done 40k on my turbo Nikki. Every time it brings a smile to hear the woosh.

 

Is there any practical reason that you've chosen to do this the hard way? You could use aftermarket fuel rails, some cheap injectors, learn a bit about EFI and make it all work quite easily to produce way more power, with much better reliability and spend way less time on the dyno.

 

Whats hard about it? Why is it not reliable? Wait, you've done it both ways and found the blow through carb setup is hard and not reliable?

More power? So Fuel injection somehow magically turns more power out of the same fuel? Or do you mean to say fuel injection is dynamic so its able to adapt to different conditions as apposed to static like a carburetor?

 

Huh, I could've sworn your message was longer Jon (looking back into my email I guess it was indeed longer). Anywho, on we go.

In my opinion a blow-thru setup is the easy way. Less things to change (no need to redo the ignition system, get a new intake system and whatnot, or do much wiring at all), I already have a carburetor that I spent two years modifying, and I understand the tuning pretty well by this point. Practical? Well no, not for some things, but for my budget it is, not to mention that I enjoy being one of only a few carbureted vehicles and how easy it makes working on the car. Sure it can be a pain sometimes (cold starts), but that's just something I deal with as a trade-off.

Making a lot of power isn't really the goal, rather it's just to make more power. I definitely agree that an EFI setup could make more power since it doesn't require restrictions (venturis) and can be tuned for every little scenario, but again, that's not the primary goal. Better reliability? No...? How is a carburetor inherently less reliable than EFI? It may not run perfectly in all situations (mainly cold starts and significant altitude changes), but that doesn't make it less reliable, at least in my mind. Reliable to me means failure, not an expected and avoidable complication. Also a dyno is quite unlikely to be a part of this process, but even then street tuning a carb doesn't take that much time, plus I enjoy it.

In response to (from the deleted portion):


Not much confidence? It's been done quite a lot, just not documented particularly thoroughly. Qingdao said just a couple posts above that he has over 40k on his setup, that seems pretty confidence inspiring. Tuning doesn't sound like a nightmare to me. And is a blow-thru Nikki something to aspire to? Maybe not as an end all be all, but I think that's it's a cool project that is fairly unique and I like that. Will it make as much power, no, but I'll have fun with what it does make and on the way to get it there.

My apologies if this has come across at all in a negative way, I just find it interesting that carburetor = "why would you?" was one of the first responses I'd get.

 

To satisfy the power/reliability question. In the 5 or so years I've daily driven my FB I've gone through 3 clutches and 2 transmissions. It seems the carb isn't the issue with reliability, and apparently it makes enough power to crunch up the stock drive line.

 

Look up user "bad 83" for details on boosting a nikki. He had one on one of his cars and it ran great.

https://www.rx7club.com/build-thread...-553481/page7/

 

Gimmy a couple of years to get bored with my current car, and I'm gonna run back to the RX7. I'm going with twin turbos on my Nikki.

 

Thanks for the tip on that one t_g_farrell. I did a bit of digging and I found a couple things, but it seems like that massive build thread of his begins after the 12A Nikki blow-thru where it's off to the 13B setup and other work. Admittedly I didn't read every part of the 306 posts, but it seemed like much of the setup was glossed over. I'll take another look at it once I've got a little more time.

I did take another shot at the search function though with bad 83 as an additional param and found that for one, it sounds like he ran 10 degrees advance on leading with a 10 (or 5, both were said) degree split. So far that sounds pretty similar to what I had been seeing elsewhere with 10-15 degrees total leading advance and then some variation on the split. It also briefly addressed one of my other up and coming questions which regarded spark plug choices. I'm already running BUR7EQP's in leading for the DLIDFIS setup and had wondered about what to do with the trailing thinking that the stock 12A plugs might be too hot for boost. A couple times I saw the suggestion to run 9's all the way around, although I wouldn't have thought the leading needed to go much colder than 7's for relatively low boost. That also had me wondering if the stock trailing system could even fire BUR9EQP's reliably (I have yet to find what "bad 83" was running for an ignition system).

On a slightly different note, fuel pumps. The Walbro 255 appears to be the default choice for many, although it seems a bit excessive and requires extra beefy everything just to get all that unused fuel back to the tank. I found the Walbro GSL395 which in terms of pressure seems like a great compromise (I don't see myself running more than 10-12psi boost) but it lacks in flow. Not sure if it's too little flow to be a concern or not, but it probably wouldn't give much headroom. It's also just one example I found while perusing around, so I'm sure there's still other options out there. Does anyone have a "target" fuel flow that they look for in a pump when doing this, or is it more so a "way over build it for safety even if it's excessive" mentality.


Edit: stock trailing system meaning it goes through the cap in the leading positions, fired by a J-109 and an MSD high vibration blaster II coil. So not really stock, but stock in operation principles.

 

I've been doing a slightly limited advance travel. About 7 degrees.

I use a direct fire through stock coils, and more recently I switched to some off the shelf coils (no difference in power; it was just a try).

I have been running stock trailing through the cap. The idle changes a bit without them, but I never noticed a power loss or anything with or without them.

The 255 is gonna be WAAAAAAAAY too much fuel. WAAAAY too much. MSD 2225 has worked for a while. Ground it good, and run a dedicated hot wire to it. The stock wiring for the fuel pump is sub-par for the MSD pump. You do 100% need to put a larger return line on the car. You cannot keep the fuel pressures down without a larger return line; I run a -10AN line to the back.

Best fuel pump is the COMP140. But its EXPENSIVE, and being a racing pump it requires CONSTANT maintenance. I ran it for a while, good pump but after I had to rebuild the thing the second time I was done with it.

You will, if you don't already know, need a boost referenced fuel pressure regulator. Mallory makes a good unit.

 

I'd probably start with 10 degrees mech advance and see how that fairs. Seems to be about what people were running with locked dizzy's and some with the limited advance. Still plenty of time to decide that however.

Noted on the ignition bits. Still curious about what plugs for trailing considering it's not DFI. I figure some BUR9EQP's would be the best bet, even if they don't fire 100% reliably - it's better than causing pre-ignition.

The MSD 2225 was another one I saw commonly used so it's nice to hear another proponent. Pressure still seems higher than needed, but perhaps these "in-between" carb and efi pumps don't really exist. I also already have my Carter running direct from the battery through a relay, so wiring shouldn't be an issue there. As far as a larger return, yeah, I had planned on that as well, although I figured I could used the stock feed line (5/16") as the return and just run a new 3/8" feed; -10AN seems really large but maybe I'm crazy. What size is your feed? And yeah, I definitely will not be going for a race pump. Any thoughts on what GPH/LPH is too low to be worth considering (is 34GPH unrestricted too low)?

Yep, I'd already factored that in, but thanks for mentioning it. I've seen a few from Aeromotive, Mallory, and even a Summit brand one that doesn't seem too bad (I currently run a Summit branded fpr and it's fine). Also am aware of the difference between rising rate and boost referenced, so we're good there as well.

I'll continue looking around to see what's out there for options on everything. I have a tentative spreadsheet with most components in it, sans piping and fittings since I haven't gotten to figuring out where exactly it will all route. I also already have a wideband installed (which will need a new bung for the downpipe) so tuning won't be completely blind.

 

You're gonna want direct fire ignition. I ran through the cap for a while when I first got the car up and running. Direct fire made lots of difference. You've got more fuel to combust compared to stock so you need more ignition.

10AN I think that was the number. Its about 3/8" return line I think thats -10AN?? IDK could be wrong. Stock feed line is fine. It doesn't suck anymore fuel than stock*, but when It dumps the unused fuel back into the tank you need a less restrictive line. * I got as high as 20MPG on the highway, but that was a lame 75mph drive/test run. Thats about what I'd expect from a stock Nikki N/A. In town its like 12-15MPG; I stomp it on a regular

Are you going to use some kind of charged air cooling? Like an intercooler or just run strait from turbo into nikki?

 

Do you have direct fire on the trailing? I already run DFI on leading with some HEI igniters; I've always understood that it's not worth the effort to run DFI trailing (since you can't run them wasted spark) as they do so little. I've seen it done it with two MSD boxes on trailing, plus one for leading, but that seems a bit unneeded. Just wanted to clarify what you meant.

10AN should be 5/8" if I recall correctly, which seems quite large for this setup. 3/8" would be 6AN. Also we might have different sized stock feed lines, or was it only the SE that had the larger lines and if so maybe it was just the return? Not sure. Regardless, my stock lines are 5/16" feed and 1/4" return. And yeah, it doesn't suck anymore fuel than stock** except when you're in boost . I usually see 14mpg on the hogged Nikki around town with a heavy foot. 25mpg is the best I've seen on a trip of 99% highway, still romping on it some.

I do plan on running an intercooler, and I'm hoping I can get away with not relocating the battery in the process. Should be possible to get both pipes through the passenger side radiator support, but I may need to relocate the coolant bottle. I've seen setups done both ways, so it's definitely a possibility, I just need to do some planning in the engine-bay.

 

Oh, you got it right. Direct fire leading and trailing through the cap.

I guess it was 6AN. IDK its been years since I've had to mess with fueling. I did have to poke a big hole in the top of the tank to get the return working.

I used the HEAVY single side intercooler. Hot in through the top and cold back through the bottom and into the carb. That intercooler is quite heavy though. If you wanna go all technical use an air water intercooler, but air water seems like too much for an otherwise simple situation.

 

Ok cool, glad that I don't need to worry all the fun of making trailing DFI.

Huh, ok well that makes more sense. I'll look into it some more; there's got to be a chart that someone's made regarding fuel line size vs flow vs pressure drop vs length... etc. If not I'll probably just do some calculations myself. And I'll still be on the lookout for fuel pumps as well in case anything slightly better suited pops up.

I do plan on an air to air setup. Air to water doesn't seem worth it to me either with the extra room it would take up (street car) and greater number of parts (money) to get it all sorted. As far as the arrangement I've been debating between a single side in/out (like yours) or an opposing side in/out and then running the far end back to the passenger side under the core. I honestly haven't looked into the aspect of it much yet and so I need to do some investigating both on the car and on what options are available before I go further with that decision. Realistically the latter option doesn't make as much sense, but we'll see. Hopefully within the next week or two I'll finally have time to install my electric fan and also do some needed transmission work. At the same time I should be able to look into routing and space requirements.

 

A bit of time later and I've run across some more questions.

First off, is it possible to still use the OMP with a boosted Nikki? Many people say ditch the OMP even on an NA car, but I really appreciate not needing to add oil to my tank (we can't pump our own gas here so it would make things a bit more convoluted). In fact I just rebuilt the OMP last summer and it's working great and leak free. I feel as though even if the OMP could handle the backpressure, the lines wouldn't take so kindly to it given they're old brittle plastic. Although maybe not, I don't know, if I did I wouldn't be asking . Running a little premix may be smart anyway with the greater stress on the engine, but I still like the simplicity of the OMP. Thoughts? In Jeff's boost prepping thread the OMP nipples go back on the carb, so perhaps it's possible?

Second is the turbo manifold. I have a lead on a JDM 12AT exhaust manifold that has been modified to clear the Nikki intake as well as having an external wastegate port added to it. Besides that, the only other options would be modifying a 13B turbo manifold or going tubular, both of which will take more time of course. There isn't a whole lot of info on the 12AT manifold, but I assume it works pretty well with lightly modified aftermarket setups...? If anyone has an opinion regarding that I'd love to hear it; seems the forums are rather scarce on the subject.

 

I'd be afraid the pressure from the boost would push the oil retrograde (back into the oil pan). You wouldn't know it was doing this till you pull the engine apart to rebuild it.

Why not just pour the oil in before someone else pumps your gas. We aren't a complete **** state yet.

I stacked "build your own header" flanges to space out the turbo away from the Nikki intake. You could do the same with a 13B turbo re-drilled for a 12A.

 

Yeah, that's definitely a concern. I might try and think up some sort of test rig, but that wouldn't give me much of an idea for the longevity. Looks like I've got some more investigating to do on that one. Is the OMP fed the full oil pressure the rest of the engine sees? I don't see why not, but I haven't looked the oil system for a while.

Yet... But yeah, pouring it in before hand is what I'd have to do, I was just referring to the fact that it complicates matters. Nothing impossible to deal with, but it'd be nice to not have to.

Ok, cool. A couple days ago I actually wrapped up a deal with someone for a 12AT manifold. It's probably not as efficient as a tubular setup might be, but I like the novelty aspect as well as the simplicity. That said, it's already been modified with a spacer to clear the Nikki manifold (at least in theory). Once it arrives in about a week I'll know more.

 

Just found a thread where Jeff mentioned that the OMP can supposedly work at up to 10psi. Unfortunately I'm pretty sure this was in reference to the S4 13B OMP system, but perhaps they are similar enough that it's a non issue? Not sure how it works as I've never encountered one, although I assume it was still injecting into the housing like it would stock, unless he changed it to run up to the Nikki. So many unknowns.... Anyway, here's the thread (post #25): https://www.rx7club.com/1st-generati...o-12a-1031457/

And here's the quote:
So again, not certain if that applies to a 12A OMP since it was in reference to a 13B engine, but perhaps some more research will bring it up. Hopefully Jeff will chime in here at some point and clear things up, although it looks like he's been off the forum for about a month now.


EDIT:
So I've found yet another post (once again by Jeff20B) with the "don't delete the OMP it works up to 10psi" spiel. Thread: https://www.rx7club.com/1st-generati...nikki-1089118/

Still not sure which OMP is being referenced, and I haven't found any of this info outside of Jeff's posts. Not to say that I have doubts, I'm just trying to tie up these loose ends. The OMP system is one of the couple major factors (to me) for the turbo Nikki. Other than it everything else is pretty straight forward, although I'm not looking forward to having to add a new line into the fuel tank. Realistically not difficult, but I still don't fancy it. On that subject I've also come across enough people saying 5/16" feed and a 3/8" return is the recipe that now I'm convinced. Yes Qingdao you already told me that, but for some reason it hadn't clicked, now it does.

 

Don't FC omp's inject oil directly into the housings? FB's push it strait into the carb. Too risky a game to play IMPO. Full stock setup I'd trust the OMP all day long, but boosting an engine that engineers determined would be NA is a different scenario. Really risky when you have no way of testing the OMP's effectiveness at full boost; you'll just have to assume its working right. Or tear down the engine in 10 000 miles and look for scoring in the housings.

 

They do, at least as far as I know. I recall reading that a 12A OMP run with the housing injectors will fail (someone learned it the hard way) due to the ______? Restriction of the injectors, back-pressure? I'm not sure. It sounds like it's possible to fit an -SE or S4 OMP on a 12A front cover, although it requires some drilling. I'll have to take a closer look at the OMP to better understand how exactly it works. Once I get that down it should be more straightforward whether it's just a risk or if it's an instant no-go.

I figure that unless someone chimes in with personal experience here the only way to see if it's even possible is to build a test rig. Make a pressure vessel for the OMP to pump into, a plate to bolt to that will feed the oil supply, and hook up another pressure vessel with oil in it as the supply. Redneck engineering means this would probably amount to a steel plate and a couple soda bottles with valve stems, although I have yet to look into it. Key would be figuring out what rpm to spin the OMP at. Theoretically the OMP can operate with very little oil pressure, at least based on the few people that have rigged it to run off a gravity feed tank. Seems like all that would need to be ascertained is whether it can handle the pressure from the opposite direction (ie. boost). 10psi isn't that much, although for seals that were designed to see zero it could be a death sentence.

Maybe my "keep the OMP" mentality isn't worth it. Seems like nobody has tried it, or at least documented it. I'm up for experimentation, but not if it means I blow up my daily's engine - I have spares, but that's not a reason I'd like to use for replacement.

 

I've talked with a couple other people now and it seems the general consensus is that using 12A OMP under boost would be a pretty iffy idea. I still felt like it may have been worth a shot but.... perhaps not. That said, now I'm looking into fitting an S4 OMP to the 12A front cover. I could also just find an -SE front cover, but this was "supposed" to be a budget build so... we'll see. The -SE front cover would require no modification, but it sounds like not much is required to fit an S4 OMP on a 12A front cover either. Plus I'll be tapping it anyway for a turbo drain. Given that, I've done some brief searching on eBay to discover a sale of two S4 OMP's for not too much. I expect to rebuild them if they get used, but the main thing is figuring out how it would all fit together.

Looking at the FSM the relevant OMP specs we see are as follows:
- 1979 to ?: 2.0-2.5cc [per six minutes]
- 1984/85: 1.8-2.2cc (12A) 13B unknown [per six minutes]
- S4 (1988): 5.2-6.6cc (turbo), 4.2-5.6 (NA) [per five minutes]

All measurements taken at 2,000rpm with the OMP lever at max. The unknown with the S4 spec is whether it is per two lines or total for all four (you only test two at a time). I didn't see any indication of which it was, although reading as it's put it sounds like the spec is per two lines, though that'd be a lot of oil.

Regardless, it looks like if an S4 pump can fit, that's the best option. The pumps I sourced off of eBay I assume are NA units as it was not specified. Does anyone know if there are major changes between the S4 turbo and non-turbo OMPs besides output? Perhaps that would be a better question for the 2nd gen section. I feel like the bump in output over the 12A pump is sufficient regardless if it's the turbo or NA one, just want to make sure the turbo OMP doesn't have some significant changes to handle more pressure. The other question would be what to do with the extra lines. I suppose they could be merged together, or blocked off (although I'm not sure yet if that would cause a problem). New lines will be required either way and I'm trying to think of a good way to go about that. Pex came to mind, but I don't recall that being chemical rated. One end would be banjos and the other nipples, so... yeah. Still need to do some thinking there. I'm certainly open to suggestions.

This has been more of a brain dump than anything I suppose. Still doing a lot of research to see what of this may have been covered before. So far not much is coming up, but we'll see.

EDIT: Found a thread regarding diy OMP lines: https://www.rx7club.com/2nd-gen-arch...-lines-348068/

 

Well like I mentioned in the first post, this is partially a place for me to think and post short updates, and here's exactly that.

I was talking with a friend about the OMP line material and how PFA would be difficult to use on the carb OMP nipples if I want them to be removable (aka, they need to be removable). To that he suggested Tygon/Versilon tubing. Compared to the PFA's durometer of 55D, the Tygon's 65A is much more friendly to removal and barbed fittings in general. Most likely will be using that unless either something better comes up or a flaw is found with it.

Price is also a benefit (looking at McMaster anyway) where the PFA is $4.08 a foot and the Tygon is $1.91 a foot. Not to mention that PFA is a bit overkill for the application anyway.

EDIT: Hold up, may have found an issue. The temp rating isn't so great.... and the search continues.

EDIT 2: The idea is to find clear tubing so that the oil flow can be visually confirmed without removing the carb hat. It seems like PFA can be used with the OMP banjos even though it isn't designed for barb fittings. Given that, PFA could be used up to the carb at which point it hits an elbow and transitions to a short section of Tygon so that it's removable from the carb. At that location the Tygon should be fine temp wise, or so one would think - the carb runs cool to the touch and the oil would have had time to cool.

The other option would be to use the right kind of fittings, go with Viton tubing, and give up the ability to see oil in the lines. Material wise this is the most ideal, although of course it's not clear. Curious if someone knows of a tubing material that would somehow fit all the ideal criteria.

 

Well the first major piece to the puzzle has arrived. It's a JDM 12AT manifold that has had a spacer welded on to theoretically clear the Nikki intake. The welds aren't beautiful although certainly functional, and this spacer also adds a spot for an external waste gate as you will see below.





I am not familiar with the stamped numbers myself (I assume it's just the part number) but perhaps someone else will recognize something I don't. But anyways, yeah, it might not be the most efficient way to strap a turbo to a 12A, but as I've said before I like the simplicity (assuming it fits) and the novelty that it's a somewhat uncommon unit.

 

Won't the tube be full of oil? Unless there are air bubbles in the oil it'll just look like a clear tube with brown fluid in it.

I don't doubt that it'll deliver oil during idle, when you can watch it, but you won't be able to see at WOT. You are going to be dealing with a pressure gradient under boost.


IF you are dead set on metering oil. You could put a fuel pump on a small tank. Fill the tank with oil of your choosing (2 stroke oil or just motor oil). Then have the pump turn on. Use a jet of some sort to regulate the oil to your liking. You'd still be dealing with a positive pressure gradient, but at least now you can put more pressure on the oil side to press the oil into the charge piping.

^^^^ all this seems like a poop ton of work and adding extra unnecessary points of failure (catastrophic ones at that)