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Thread: REPU and Cosmo intakes
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Jeff20B
Moderator
Posts: 661
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posted February 07, 2002 05:43 PM |
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REPU and Cosmo intakes
I've been fooling around with carb and intake manifold swaps lately and I figured I'd better write at least the gist of it here for anyone else attempting what I've been doing, if for no other reason than to keep a record of what I've done and plan to do on these two rotary vehicles.
I went ahead and swapped a '74 (normal runner) intake manifold where a '76 reversed runner manifold used to be, on my '74 truck's engine today, but kept the Cosmo's carb on it as an experiment (this carb ran way too lean on the reversed manifold, but should run closer to stoich on this one with its smaller ports). It started easier and reved better from idle, but at higher RPM when I was expecting the secondaries to open, they never did, or at least they didn't pop open like they normaly would have done with the reversed runner manifold. I only drove it a little before heading back home. It's kinda hard to feel how much power you have when all the roads are wet. One leggers in the rain are as cool as your high school buddies. Gotta wait till the roads dry and there's less traffic.
So what's up with my setup?
My truck's engine is basically stock but with a very bad part in it. It has a 12A EGR style intermediate housing with the characteristic tiny ports. Way too small for a '74 13B. I got the engine this way, so don't ask me why the previous owner never complained to the shop that rebuilt it about it. He apearantly thought it ran ok the way it was.
The original manifold had major damage to the gasket surface from some mechanic who used a die grinder gasket removal attatchment and totaly wore down the aluminum in between the two intermediate ports. This caused the gasket to pop out and allowed comunication between the ports which equaled a major power loss on the primaries. And what's more, they didn't even bother to use a new gasket when they rebuilt this engine. This manifold was once used on a tall port 13B and the old indentations were still clearly visible in the gasket even after it had been installed on a short port engine since '95 (heh, 5 years). What a rip-off! I almost feel sorry for the previous owner. This engine was probably only meant to run for around 1 year, not smoke too much, and have just enough power for the average person who is just getting into rotaries and may want to street port his engine some day for 'more power'. Heh, I (being a rotor head) know better. This engine is not the worst rebuild I have ever seen, but still has plenty of used and worn parts on it. I'm not even going to get into how bad the flywheel and pressure plate surfaces are.
So what do I do?
I wouldn't have very good luck street porting the intermidiate housing because it is probably just like the 6 port engines which have way less material to cut into than a regular 4 port engine. My best bet would be to try to find any normal 4 port intermediate housing and port it, or leave it stock (but I'd prefer a tall port from '76 or '83 or something). The exhaust ports are also the tiny '74, (12A sized) ones which could probably be helped with some porting, but we'll see. They look quite out of place on such wide rotor housings. Oh well, the 13B was brand new back then. I suppose the 15A would have had the '76 13B sized exhaust ports at first, if it would have been mass produced. Too bad it wasn't. Ahem, back on topic.
As long as I've had this engine, it has never liked running at high RPM. The best it ever ran was with its carb on the Comso's reverse runner manifold. Very low RPM primary operation was kind of slugish, almost streetported-like in response, due to the large ports, but anything above 1500RPM was very nice, and the secondaries would open quite forcefully at 3000RPM. Too bad the torque dropped off dramatically at 5000 and continued falling untill there was nothing left at 6000. It could almost hold its own against other mini trucks from the same era off line (but less powerful than other REPUs), and could do ok from 30 to 50MPH. But then again, the carb is stock and so is the porting and fuel pump (well, RB Facet, which kinda sucks, er blows). This was all with its carb. Now with the Cosmo's carb, go ahead and subtract power from everywhere in the RPM range and you get the idea. (At least the secondaries still opened with the reverse runner manifold.) I think the highest RPM it has ever attained with either carb was 6200 to 6500 RPM. Hmm, could this have something to do with only one set of ports at the proper size? Darn 12A housing. If I'm only running on 3/4 of my engine, I suppose that makes sense, but I can't account for the vibration I feel in the shifter. Maybe the (over) used flywheel is out of balance? Or maybe the front counter weight is for an auto? Plenty of Cosmo automatic engines have found their way into other vehicles, whether using a Racing Beat light steel flywheel (with rear counter weight) or not. Felix Miata says the front counter weights differ between autos and manuals. Youch!
I also have an MG Midget that has been modified to fit a 13B. The only manifold that will fit is the normal runner '74 style. This means I'll need to rebuild my truck's engine with the correct intermidiate housing if I was ever going to use it in the MG. Otherwise it'll be a slow dog.
Unless...
My Cosmo has been sitting since last year when I got it running with my old damaged intake manifold and truck's carb. I sealed the bad gasket surface with gasket sealer and it didn't seem to leak. The Cosmo ran smooth and seemed to accelerate well above 3000RPM. Well, now I have a good '74 manifold that I can use if I want to, which will let me keep on using the Cosmo's manifold on my truck so I won't have to rebuild the truck's engine just yet. However, the last time the Cosmo had its own carb on it, it didn't run good at all. But strangely enough, that same carb ran really well on my friend's '77 REPU engine. I'm assuming the ports are similar from '76 to '77. This means that the Cosmo should run ok with its own carb, now that I've found and fixed all the vacuum leaks that previously had gone unchecked.
I've toyed around with engine swap ideas (truck to Cosmo) so the Cosmo could get a known good runner at low RPM which should work out ok with the Cosmo's automatic tranny. Autos usually shift before 6kRPM anyway. And if the Cosmo's engine could run at high RPM on a stick shift like my friend's does (8kRPM+), then I'd be dumb not to do the swap. But then there is the flywheel issue with heavy stocker VS RB light steel in an REPU. Some say the 4.6 rearend is low enough for even an aluminum flywheel. Well, my other friend's REPU engine had an aluminum flywheel for a time and it sucked for trying to do any burnouts or anything fun like that, even with 4.6. Sure you could tow with it, but again, an engine that simply stalls on you, even at 4k RPM (no spin down to death, it just went from running to instantly stalled), is a bitch when trying to start out on a hill, especially when towing. The 'old grandpa' driving technique became quite a regular occurance. No geriatric driving skills for me. :)
Well, I have a '77 flywheel which would probably be a good match for the Cosmo, but Felix pops into my head and reminds me of the front counter balance differences. Well, my friend installed a light steel flywheel *with* rear counter weight from RB on his '77 and he said it doesn't vibrate. Hmm... The Cosmo has a weird vibration that comes and goes at idle which is possibly coming from the power steering pulley, which is slightly tweaked, and the fan. They seem to find a peak and null point during their rotations which could actually possibly allow the engine to feel even smoother than a any other rotary for a brief period, then on the flipside, it vibrates more than most (even my truck). It is less noticeable at higher RPMs, so it is an engine speed related issue. I'll have to keep an eye, or rather a hand on it. Other than that, it seemed to be a good smooth runner. Should I attempt a flywheel to rear counter weight swap? I wouldn't be using my truck's flywheel on the Cosmo's engine since I have the '77 which looks to be in better shape (less visible heat spots and cracking, but it still needs a good rust cleaning just to be sure). I wonder if my truck's vibes would go away if I went with the Cosmo's rear counter weight?
Does anyone here know if the '83 to '85 225mm clutch disks will fit the REPU 9 bolt flywheel? I know the '71 to '82 215mm will fit and work within reason (my friend did this for about a year, untill it lost its lining hehe), but it is smaller than the REPU disk (and much lighter duty). I dropped a used '83 225mm disk onto the the '77 flywheel sitting on my garage floor and it seems to fit well enough, and there is some space between the pressure plate and flywheel mounting surface for the bolts to tighten into; this is also a good sign. Too bad there is so little meat left on the 225mm disk, otherwise I'd seriously consider trying it.
The other option is to get the RB light steel flywheel because I already have a good slightly used 215mm disk and pressure plate. However, this disk is the same duty rating as the one my friend grenaded. The only difference is that it would be used in a real 215mm environment. The same load will be applied however (towing, etc).
I've already got my sports car (rotary MG) and I may get an RX-7 some day when the emissions tests no longer include the '79 model year, but that is still a few years off. So for now I'll just make due with what I've got. The truck will be used for truck-like things (towing, hauling... ass). The Cosmo will be used for family sedan type things, probably. Well, if what Cal said is true about the unibody being able to handle 450+HP, it'll be more like sport sedan type things.
My bro and I are planning on a major engine improvement in the Cosmo sometime soon. Some sort of TII and SC. I'll be performing most of the labor, while he'll be purchasing the parts. I won't ask for any kind of reimbursment for my trouble other than the Cosmo's engine itself. Then I could at least get my MG running with virtualy no out of pocket expenses hehe. That is, if it'll run good with its carb and the '74 manifold, leaving my truck to its own carb and the Cosmo's reversed runner manifold. It's either that or an engine rebuild, which won't be attempted untill I get another car running around here.
So as of this moment, I think my plans are as follows:
Test the truck on a dry road with the '74 manifold and Cosmo carb and see what it takes to pop the secondaries open. If the results are not satifactory, I'll probably try the truck's carb as a last ditch effort. If still not satisfied, I'll be reinstalling the Cosmo intake manifold and keeping the truck's carb on the truck. This means the Cosmo's carb will need to run well on the Cosmo. I have faith in it because it ran pretty good on my friend's '77 engine with the '74 manifold. I've got my fingers crossed that it will work out ok enough so it won't die when the gas pedal is depressed and what-not (it seems to have a bad accelerator pump on my truck, but it was fine on my friend's). If it works good, I'll have an engine for my MG and then I can go back to dreaming about a TII and SC in the Cosmo as soon as my bro can afford it. What comes after that? Either an RX-8 or a 20B. Decisions!
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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Jeff20B
Moderator
Posts: 661
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posted February 07, 2002 07:42 PM |
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Here is an update
I was able to test the acceleration of my truck with the '74 manifold and Cosmo carb today. I had good and bad results.
The good: my engine finaly, for the first time since I've owned this engine, finaly hit 7000RPM!
The bad: it took forever to get that high, while floored, and only in 2nd gear. I couldn't really test it in 3rd because I got up to 60MPH in 2nd at 7k RPM in a 45MPH zone. I can't really do any speed stuff around here, which is just as well because it still ran really rough and stumbled when the gas pedal was pressed etc (all the usual problems) so I'm pretty sure it's still running lean.
One more good: I now know beyond a shadow of a doubt that if I want performance out of my engine, I'll need to rebuild it with the correct intermediate housing. That high rpm problem was only caused by a leaking intake gasket (from the damaged original manifold) and the 12A intermediate housing.
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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Klaus43
Rotorhead
Posts: 1259
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posted February 08, 2002 01:05 PM |
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Congatulations!
Congrat's on finding 'yet another' vacuum leak...didn't mean to be patronizing, but your troubles sounded like that might've been the case! I'd venture a guess that you could get a whole lot more from your current motor/intake, despite the shortport housing, depending on all those 'other' factors: carburetion/exhaust/ignition... Of course, all else being optimized, the porting will still be the limiting factor. But, even my stock japan-port (teeny!) 4port 13B with the stock carb, RB header, and 1st-gen electronic ignition--which makes most of it's power between 3500 & 5500 rpm--will rev past the redline if allowed to.
Keep on tinkering & tuning!
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Jeff20B
Moderator
Posts: 661
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posted February 09, 2002 08:19 AM |
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Hey thanks!
No, that vacuum leak was found early last year when I first did the Cosmo manifold to truck engine swap. That's when I discovered the truck's original manifold was bad. I also was able to prove my theory correct about the how small the ports were in my intermediate housing at the same time.
I had seen an EGR style housing before and couldn't believe how small the ports were, so it was good to prove that my engine had one of those housings. All these experiments I've been doing have just been to prove beyond a shadow of a doubt that my high RPM problems were related to the housing and nothing else. The good '74 intake manifold I installed the other day was just another piece of evidence against my housing. Infact, I'm so sure of its guilt, I don't think I'll even bother trying my truck's carb on it before rebuilding my engine. Er, well maybe not. I still haven't driven far enough to see much color change in the plugs. I also need to get my Cosmo running which only needs a battery. No point doing carb swaps at the moment.
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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Jeff20B
Moderator
Posts: 661
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posted February 13, 2002 11:31 AM |
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I got a battery for my Cosmo, but I still need to do some rust work at the tail lights to make it rain worthy. Otherwise I'll get a wet trunk.
I drove some on the highway and checked my plugs yesterday. They were still a bit light, but had some tan on the porcelin. Much healthier for my engine to use the '74 manifold, but it is so slow now. Oh well, I'll pretend it's a 4 boinger and try to get good gas mileage for now.
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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Jeff20B
Moderator
Posts: 661
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posted February 14, 2002 07:39 AM |
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I upped my fuel pressure to 5.5psi (from 5.0) yesterday which seemed to help quite a lot. But the funny thing is the engine runs perfectly with the choke on. Yes perfectly, untill the choke opens. Then it has its usual stumbles etc (even when fully warmed up).
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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Klaus43
Rotorhead
Posts: 1259
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posted February 14, 2002 10:37 AM |
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Yep--rotaries like it rich! In the short-term, anyway, until your plugs foul... Sounds like that's just a symptom, though. Yesterday I helped my friend tinker some on his '74 rx4 he just fired up for the first time... has a later model 4-port (japan?) 13-B, (no 'eyebrows', nitrided side housings, lower-case 'mazda' on rotorhousings)... He's been working on this project darn near forever, everything totally clean and beautifully detailed... so the ignition's retarded for starting ease, and of course it smokes a bunch at first...after warmup & going around the block, he says it's not wanting to rev--and the secondaries aren't opening...we pop the hood, and the first thing I notice is a vacuum leak on the intake...that plugged, greater smoothness the immediate result, of course...next it's a question of the 'best' timing for this particular motor...I convince him to advance it 'some', and it runs somewhat better already.
At this point, we are out of time for tinkering for the moment...but it lives! The point of all this: as far as I know, 'optimum' timing specs/recommendations vary greatly as per particular 13B incarnation... and even a slightly 'iffy' ignition system component could cause stumbling that a richer mixture might mask. Of course, a fuel system idle-circuit problem might also be the culprit...
Next on my friend's agenda: Time/tune it to a 'T', power foam it, and go blast the crap outta it!
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Jeff20B
Moderator
Posts: 661
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posted February 14, 2002 05:33 PM |
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That's cool that the RX-4 is working good. I turned my idle mixture screw a tad counter clockwise to see if it's going to do anything helpful. I suppose I should do it while the engine is idling, but it never 'hunts severly' unless it's still kinda cold right after the choke pops in. I'll go for a drive in a bit to see if it made a difference, and if not, I'll screw with it some more.
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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Jeff20B
Moderator
Posts: 661
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posted February 17, 2002 11:42 AM |
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I got my Cosmo running and aside from some smoke, it didn't stumble on acceleration. It did however bog a bit when floored as the secondaries opened (this is a '74 carb on stock '76 ports and exhaust). The next step is to drive it a little and check out the plugs. If they're blackened, then on with the Cosmo's carb.
The fuel pump is dying in my truck. This may be why it is running lean. But that doesn't explain the rich condition I used to have, unless the pump suddenly decided to take a turn for the worst as soon as I swapped carbs (hey, anything is possible). A good pump rated up to 6psi with high volume should do the trick. Too bad RB no longer sells Facet (the Facet on the Cosmo pumps strong and healthy).
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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Klaus43
Rotorhead
Posts: 1259
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posted February 18, 2002 12:18 PM |
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...bogs at what RPM???...
Why not rebuild your carb(s) and ID the jetting, etc. while you're at it--instead of just swapping back and forth? Then you could tune the right carb to your specific motor... If you've never done one, they're not THAT impossible--just be super diligent about cleanliness and parts placement, as well as any adjustments...no hurries, no worries. Finding a carb kit that actually has what you want in it may be the real challenge... Of course, what your motors really want is a header and free flow all the way back... none of that stock s--t,--THAT'd go a long ways right there!
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Klaus43
Rotorhead
Posts: 1259
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posted February 18, 2002 12:24 PM |
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...oh yeah, I almost forgot...:
..so RB doesn't sell the Facet anymore... and Paul Yaw doesn't like it, according to his site... wonder what that means. Only truth here I can decipher is the right pump is absolutely, 100% essential for peak performance, all else being equal... so what is 'the right pump', and where do we spend for it? Yaw probably had one for sale he recommends, I seem to recall... Good Luck! --K.
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Jeff20B
Moderator
Posts: 661
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posted February 18, 2002 11:27 PM |
 Edited By: Jeff20B on 18 Feb 2002 23:28 |
Yaw has no need for a pump that goes up to 5 psi because he only ever deals with Nikki carbs. Maybe he just doesn't like the clicking sounds :p
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'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7
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