Drivetrain choice dilemma

[DaveBonds]

I'm building an early '72 that currently has the completely stock L24 and 4spd.

I'm losing the air pump and emissions garbage if I keep the SU's and stock drivetrain. I may dump a mild cam and header in it if I keep it NA.

I've been beating around the idea of going with a custom fuel injection and boost setup with a five speed, in gain of fuel economy and performance. I'm just not sure if I should take the plunge.

I know of a Celica GT with a 7MGTE that gets 30mpg, running a stock engine and pulls mid 13 second E.T.'s, which sounds awful tempting.

I've heared of people using Mikunis and Webers for throttle bodies. I'd like to go this direction, but I'd like to see this setup first.

If nothing else, I may just swap in a five speed, but I don't like the idea of pulling 3 grand up at 80mph. I don't know if that just seems too high to me, but I typically average 55mph to work. That seems like it would be borderline torque abuse in 5th gear. Something like 2000-2400rpm maybe, which seems like downshift territory under that kind of speed.

My brother has an '80 ZX 5speed. When I drive it, I don't get into 5th gear until I'm at up to around 60mph. It just seems a bit out of my usage and I'm not sure if I will benefit from a fifth gear without entering a 4.10:1 rear ratio.

What are the factory rear gear ratios of the datsun 5 speed cars with R180's? I know some of the later ones used a Borg Warner, but I'd rather keep this as simple as possible. I don't mind swapping differentials too, but it would be nice if I could get a good final ratio as easily as possible and keep my speedometer accurate.

Any experience or advice is much appreciated.

Thanks!

Dave

Edited June 2, 2010 by DaveBonds

[Pop's Z]

Dave, If you are going to be into serious mods this is not the site for you. Try hybridz.com and read, read, read before asking questions or they will roast you.

Cheers, Mike

[Gary in NJ]

How many miles a year do you put on your 240 that gas mileage is such a concern?

Even if you could improve the figure by 5 mpg (say from 25 to 30 mpg), and you drive 10,000 miles a year, you're only gonna save 67 - about $200 a year. That's a lot of effort for such a little return. Even less a return if you drive less.

[Bruce Palmer]

We had 3 2.8 Zs from here knock down 28+ going to and from LA last year with 5 speeds and our standard production carbs.

I do know the ZX 5 speed likes torque to pull that tall OD gear or it will fall on it's face.

[240ZMan]

I ran triple webers for a while and never, and I mean NEVER, got over 20 mpg on a tank, even with steady freeway cruising. They sound fabulous and make great power though. Since you seem to care more about fuel economy, I'd take those off your list.

You should think of the drivetrain in terms of the transmission, the diff, AND the cam. A stock cam is pretty much out of torque by 5k while a regrind that's still very street friendly will pull to 6500 or more. With that type of cam, 3k rpm for cruise at 75 mph is no big deal. Add programmable fuel injection and you can still get 25 - 28 mpg at a steady 75 mph.

You should also assess what type of engine you like: torque vs. revs. If you prefer to shift sooner and keep the revs down, then stay with the stock cam and choose the gearing to match (ZX 5 speed and 3.36 or 3.54 diff is a good way to go here). But if you like shifting a lot and turning revs, then the ZX 5 speed with 3.9 or 4.11 diff will be more fun and will do best with a new cam. I prefer the latter, but it's more about what you like best that should guide you here.

You also should take into account how difficult it might be to find the higher ratio diffs. It's pretty hard to find a 3.7, 3.9, 4.11 diff these days for less than $500. 3.54s in both R180 and R200 are common though and you already have a 3.36.

In short, you can get good acceleration AND decent mileage if you choose carefully. And if you haven't already found this site, spend some time there with various combinations to see what revs you'd be turning in each gear at various speeds: http://webspace.webring.com/people/cz/z_design_studio/transmission.html

[DaveBonds]

I've been a member here for a while as well as over at hybridZ. I'm more or less just fishing for ideas and experience with engine/ cam combos against final ratios.

The Mikuni and Weber carbs would only be used as a throttle body, without any functioning parts other than the butterflys. Clifford states on their website that they have them available, along with cannon manifolds for various I6 engines, but they don't cater to Nissan cars for manifolds. I was more interested in the FI 3x2 Weber carb throttle bodies that they were mentioning, but I don't see anything solid on their site. I have no idea what they were using for injectors, a rail or management.

I stumbled on it when I was helping a friend with his slant six and it got me thinking. I would have all of the asthetics of them without the drawback of having three carbs and no choke. It wouldn't be hard to just make from a used set of carbs with a few extra parts.

I would love to just run a set of webers, but the fact that they are chokeless kind of kills it for me. I live in Denver and this is going to be a daily driver in most weather, along with a weekend road track on occasion. This also explains my concern with fuel economy. I don't mind getting down near 20mpg, but more is always better. I drive from Lakewood to Littleton every day.

My primary concern with the 5 speed that I drove in the '80 ZX was what Bruce Palmer mentions about the torque.

I don't mind swapping an L28 and bumping up the HP, but it seems kind of a shame, as I already have the L24 that runs. I also understand that the L24 works better in higher RPM, which is where I would like to leave it. This is really the root of my interest in an extra gear and taller R180. I'd really like a 3.9 final, if I go five speed.

If anything, I'd like to get a little more power out of the car with a 2.4... An L28 isn't entirely out of the question, but I like how the 240 is catered for higher RPM.

[Walter Moore]

I have a L24 (stock SU's understand) with a ZX 5 speed and 3.36 differential. My experience is that if you do a lot of 70-80mph cruising, 5th is great. But if as you say you typically run 55 you will never be able to use 5th. I generally ignore it completely below about 60mph.

A different cam setup, or FI might change the equation a little, but from what I have read, the L24 isn't a torque motor no matter how you set it up. For low speed torque you need more displacement with a longer stroke.

If you keep the L24 and go 5 speed, particularly if you never get above 55, you need 3.9 or 4.11 final gears or you will just have a 4 speed with "that other gear" as a tag-along.

Just my opinion. (There are people in the local club that keep "suggesting" I use a 4.30 or something even more radical, but I have been holding out so far.)

[240ZMan]

I'll just add/clarify 2 things:

- the L28 can rev just as well as the L24. The shape of the torque curve is mostly a function of the cam you choose. The L28 with the same head/cam combination as an L24 will just make more torque at any given rpm.

- When I ran triple webers here in CO I never had any problem starting them cold, even in 25 degree weather. I never used the choke function - it's really not needed with a cam that's reasonable for the street.

[DaveBonds]

That all sounds right to me and sort of confirmed my concerns.

I have completely restored my intakes, linkage and SU's... I suppose I should just install them on the L24 after I check the cylinders for runout and possible re-ring.

If I can find a 3.91 and 5 speed with the right speedo gear, I'll roll with that for a lower first.

13 second ET's and 30mpg from the same car just sounds awful tempting, as slightly more complex as it would be. I think I'll just try what I have for now until I get a reason to build another engine or something.

Somebody start reproducing these, please-

Problem solved.

I always warm my engine up before driving, so I would probably be ok with a set of those carbs, matched with the 5 speed, taller gears and a good cam, I'll bet it would be pretty fun. I'll just have to see what my fuel budget would look like at 17mpg.

Edited June 3, 2010 by DaveBonds