EFI to SU's

[MC75Z]

Iam planning on putting a P90 on my L28. It has been bored to3000cc with flat top pistons. I have a mild Schyder cam. I am working with a limited budget. The motor now has an N42 head with EFI. It's a 75 so its smog exempt.I have about 3 sets of SU's to work with and thinking about trying them. I would like to get some opions on the advantages or disadvantages and overall performance improvement if any.

Thanks,

Michael

[Bruce Palmer]

We have had several who have carried this off very succesfully. It's a bolt on that requires only low fuel pressure, and unhooking only the FI portion of the harness....

Linkage, fuel rail and chokes along with a fuel source and away ya go.....

Pretty straight forward deal....

[Bruce Palmer]

Oh and the thing I'm hearing from these guys as advantages is the simplicity of SUs and having a system they can get in hit the key and drive off with no leaking injectors or other degrading parts etc.

[Arne]

Been there, done that, wrote an article on it. A simple swap if you do your homework up front. Check this article for the basics:

Fuel injection to SU carb swap

[=Enigma=]

Are the stock SUs going to be enough to feed his 3000cc motor w/cam, or would he be better off getting a set of bored out SUs from Rebello? Just curious.

[Mike]

Moved this to the SU Central forum area.

[sblake01]

Oh and the thing I'm hearing from these guys as advantages is the simplicity of SUs and having a system they can get in hit the key and drive off with no leaking injectors or other degrading parts etc.

I've been doing that for the last ten years and I have EFI. I know you're in the business of selling carbureators but there is no need to repeat other people's bad mouthing of the EFI system. People who don't understand the system or don't know how it works or don't have the desire to learn, see carbs as a way out. I don't. I've worked with both (SU's and EFI) and they both have their plusses and minuses. Without proper knowlege and maintainence, carb systems can leak and their components can become degraded just like EFI. So what? If SU's were so much better than EFI and never suffered the problems that they do, you'd be out of business. I'm not saying that one is better than the other. It's all just a matter of personal opinion.

[Arne]

I love my SUs, but I certainly agree with Stephen as well, there's nothing wrong with the factory injection for normal use, assuming it's in good repair. And that applies to the carbs as well.

But the other part of this is that the original poster asked about carbs on a seriously modified motor. The stock injection might not be able to flow enough air to feed that. A pair of stock SUs might not be able to supply it either. I should think that a 3.0 stroker with a cam and more should really be running triples of some sort.

[Bruce Palmer]

... that my statements were taken as bad mouthing FI systems on Zs. It was certainly not my intent.

When customers come to us saying "I'm tired of spending the time and money involved in keeping my 20 something year old FI system functioning safely and reliable, do you have a suggestion for another way I could go?" Our answer is, "SU carbs are a very simple, functional solution to your relability issue".

And believe me we don't need the FI conversion business to stay in business.....

[sblake01]

Bruce, I too apologize. The 'out of business' statement was more tounge in cheek than anything else. I just don't like to see the EFI system put down for the wrong reasons. It's an antiquated system, yes, but realisticly, so are S.U.'s. That's really all I was trying to say. I actually find the EFI system simple to understand. This 'reliability issue' is something I've never really had to deal with. I guess that puts me in the minority. I have used your carbs in the past and I've not come across anywhere else that does the fine job that you do with them and I wasn't attempting to undermine what you do in any way.

[sblake01]

I love my SUs, but I certainly agree with Stephen as well, there's nothing wrong with the factory injection for normal use, assuming it's in good repair. And that applies to the carbs as well.

But the other part of this is that the original poster asked about carbs on a seriously modified motor. The stock injection might not be able to flow enough air to feed that. A pair of stock SUs might not be able to supply it either. I should think that a 3.0 stroker with a cam and more should really be running triples of some sort.

Strokers can be done with injection. To quote Steve Golik (zcarnut) from another site:

For the engine upgrade I installed a 3.0 liter displacement engine by boring an L28 block 1mm over and using the Maxima diesel crankshaft, 9mm 240Z rods and milled Z22 pistons. To handle the increase in engine size I will be using the 240SX throttle body, N42 intake and a the mass air flow meter and ECU from a Z31.

Not mentioned here but I would assume he also used the ZX turbo injectors.

[jmortensen]

I'd be willing to badmouth the factory FI. Let's see...

The factory FI uses a very restrictive AFM. It also has "batch fire" where all 6 injectors fire at the same time, regardless of whether they're firing on an intake stroke, and exhaust stroke, power stroke, whatever. Not good. Later EFI systems are sequential so that the injector fires at the end of the exhaust/beginning of the intake stroke for every cylinder. The computer on these old systems is a joke and has a set fuel map, so it can't handle too much out of the ordinary. A decent sized cam will make a factory FI'd car run worse, and the stock cam is very small. The manifold has small runners and a small throttle body, limiting it's potential. One can pretty well assume that the connectors are junk on the old cars, so most driveability problems on the factory FI stem from that.

SU's have their problems too, don't get me wrong. Leaky throttle shafts, the taper on the needles might not be ideal for a given motor, spring rates, oil weights, etc. I also believe that they do not flow enough air to feed a big L engine. The Mikunis that I'm running aren't perfect either. Again leaky throttle shafts, rough idle to power transition, poor atomization of the accelerator pump jets, etc. One could argue that they flow too much air, and hurt bottom end performance.

I just get tired of people dogmatically (is that a word?) trying to make the point that FI is always better than carbs. Usually this comment is backed up with some ignorant point like "carbs are just a metered fuel leak" or some such thing. A really good example of this is the new GM LS2 crate engine. You can buy a carbed version that actually makes MORE power than the factory FI, but if you look online all you'll see is all sorts of people stating how inferior the carb is.

Don't get me wrong. If you aren't looking for performance and just want to drive a car that is period correct, the stock FI is great once you work out all the bugs. It's when you try to make more hp than stock with a normally aspirated engine that it's limitations become immediately apparent.

Modern FI is definitely the way to go. Unfortunately it's not smog legal in most places. If it is legal, a good modern FI is the best solution.