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Old 08-05-2008, 04:13 PM   #21
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I didn't read all of the responses to your question cause I don't have the patience. Anyways if you are going to stay dpfi I will recommend one thing from a FE standpoing. Put in the restrictor plate from a std civic. I just did and my fe just jumped, just go look at my gas log. The restrictor plate didn't hurt the drivability. It is slightly sluggish at take off if you floor it but to me this isn't a problem. This tank I am now going to try the std civic ecu (PO9). Stay tuned to my gaslog if you want to see what happens when the tank is through. I figure about a week and a half and I should fill up again.
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Old 08-06-2008, 02:49 PM   #22
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Would the MPFI conversion change where the engine operates best by very much?
It's possible, but not likely. The design of many engine components can affect where it's power band lies. While the biggest contributor is the camshaft profile, the intake manifold also plays a role. Longer, thinner runners will move the power band down the RPM range, while short, fat runners will move it up.

The reason I don't think it will make much difference is that the DPFI manifold doesn't seem to be designed with runner length, or even flow dynamics in general, in mind. First of all, the air/fuel mixture has to take a 90 degree turn when entering and exiting the throttle body, resulting in flow restriction.

Then again, those right-angle turns in the airflow are nothing compared to the restriction the tandem valve presents:


It's basically a vacuum-controlled choke flap near the top of the throttle body, whose function seems to be to maximize air flow over the main fuel injector nozzle. The ECU has some control over the flap via a solenoid valve, but that's limited to switching vacuum from a venturi on and off. I guess if the flap was presenting enough restriction, manifold vacuum could play a role there too.

Really, I think the most significant issue here is that the manifold's four runners don't seem to be equal in length. The runners for cylinders 2 and 3 have a shorter path to the head than those for 1 and 4. That would mean that the center two cylinders would have a higher RPM power band than the outer two.
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Old 08-07-2008, 07:38 AM   #23
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Sooo you're saying that the DPFI system is a hogs ear in terms of performance.... But if the MPFI system doesn't have those same shortcomings, couldn't it result in a more revvy engine? (Unless I'm reading things wrong and the tandem valve is actually part of the MPFI system... but I doubt that)

Usually I wouldn't mind that, but if I do install the HF final drive, I want to make sure that I don't do any thing that makes the engine lose low end torque or moves the power range up in the rpm range. If anything, I need to keep it the same or maybe even lower it by a few hundred rpm.
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Old 08-07-2008, 11:05 AM   #24
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Sorry, I can't give you any really solid assurances one way or the other. I've never put an MPFI swapped D15B2 on a dyno to check its torque curve, so I can only give you an educated guess and how I came to my conclusions. The only way to really know is to try it and find out.

That said, the D15B7 is close to what we're talking about here. It's an OBD-1 D15B2 with MPFI and likely a different cam profile.
The D15B2 makes 92 HP @ 6000 RPM and 89 ft/lbs of torque at 4500 RPM.
The D15B7 makes 102 HP @ 5900 RPM and 95 ft/lbs of torque at 5000 RPM.
Peak HP is nice, but we're after torque here. The additional 6 ft/lbs of torque will help you, but coming 500 rpm later will not. It's really a question of the shape of the two curves... Are they more or less identical, except the 'B7 continues increasing beyond 4500? Does the 'B7 sacrifice low end to get the higher peak value, or is the 'B7 just a more efficient setup, yielding an overall increase in torque?
Anyone have dyno graphs? :/
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Old 08-07-2008, 11:42 AM   #25
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Originally Posted by bobski View Post
Sorry, I can't give you any really solid assurances one way or the other. I've never put an MPFI swapped D15B2 on a dyno to check its torque curve, so I can only give you an educated guess and how I came to my conclusions. The only way to really know is to try it and find out.

That said, the D15B7 is close to what we're talking about here. It's an OBD-1 D15B2 with MPFI and likely a different cam profile.
The D15B2 makes 92 HP @ 6000 RPM and 89 ft/lbs of torque at 4500 RPM.
The D15B7 makes 102 HP @ 5900 RPM and 95 ft/lbs of torque at 5000 RPM.
Peak HP is nice, but we're after torque here. The additional 6 ft/lbs of torque will help you, but coming 500 rpm later will not. It's really a question of the shape of the two curves... Are they more or less identical, except the 'B7 continues increasing beyond 4500? Does the 'B7 sacrifice low end to get the higher peak value, or is the 'B7 just a more efficient setup, yielding an overall increase in torque?
Anyone have dyno graphs? :/
its likely that the b7 makes 89 ft/lbs at 4500rpm and it just doesnt stop there. you are right, you need to look at the curves of each to really know
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Old 08-07-2008, 12:48 PM   #26
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I've always had bad luck trying to find torque curves or dyno charts for stock engines (nobody ever gets their car dyno'd until after they mod it...). I was really hoping for a seat of the pants opinion.

Maybe I'll check over on honda-tech.com to see what those guys have to say.
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Old 03-06-2009, 02:58 PM   #27
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Hate to bump this old thread but I'd like to get some discussion going here again

In the past few months I put in a brand new (rebuilt) D15B2 into my '89 DX, did an MPFI swap, and mated it with a new (rebuilt) Si transmission. I did alot of reading on the swap before hand and it seemed the general consensus was that, overall, FE would be about the same (assuming the Si transmission would potentially offset some of the "gain" from the MPFI swap).

Now that I've got a few hundred miles on the setup I'm beginning to be suspicious that FE is/will be decreased enough to irritate me.

My best tank so far with the new setup was 36 MPG, and I regretfully must admit that this was 90% highway driving. I haven't had to fill up again yet, but judging by the gas gauge (I realize it's not linear) and trip meter I would say I'm not going to get above about 32 MPG on this tank.

I also will point out that I haven't even had to run the A/C yet and judging from my previous experience, heavy A/C usage (unfortunately a must in Dallas) severely taxes the little D15B2 and really hurts MPG.

Before my swap, I could easily get 40 MPG. During the dog days of summer, however, the heavy A/C usage sometimes dipped me as low as 33-34 MPG. If this is the case, I fear many sub-30 MPG from my new setup in the summer, which just won't be worth it (as I can get 27 MPG all day long in my Maxima).

One advantage of the MPFI swap with Si transmission is that highway merging is no longer so nerve-wracking, and I would gladly take this trade-off if I could eek up to about 38 MPG consistently with the swap, but that's not looking too realistic.
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Old 03-06-2009, 03:56 PM   #28
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How did you break it in?
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Old 03-09-2009, 08:30 AM   #29
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... and mated it with a new (rebuilt) Si transmission. I did alot of reading on the swap before hand and it seemed the general consensus was that, overall, FE would be about the same (assuming the Si transmission would potentially offset some of the "gain" from the MPFI swap).
[...]
One advantage of the MPFI swap with Si transmission is that highway merging is no longer so nerve-wracking, and I would gladly take this trade-off if I could eek up to about 38 MPG consistently with the swap, but that's not looking too realistic.
The Si transmission is hurting your highway milage... It's likely costing you 8-10 MPG compared to the DX gearing, but that doesn't mean there's no room for improvement. Which intake manifold and ECU did you use?
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Old 03-09-2009, 09:30 AM   #30
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I saw your garage on how you said "Virtually a brand new car!" It's great that your saving a EF since they are getting way more scarce these days. If you invested so much in restoring this car and you are doing 90% highway driving, my suggestion is swap the SI 5th gear to an hf 5th gear. You'll keep your first through fourth for acceleration purposes but have lower rpm's for cruising... best of both worlds!
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