I've started a project, which I'm not going to call a 20b project since, while I have purchased a rolling chassis, I haven't purchased a 20b yet. Nevertheless, I'm trying to decide on what gear ratio would be ideal for cruising at around 75mph.
My ported 13B is happiest cruising at 3000rpms, which equates to approximately 75mph. The engine seems like it's suffering a lot more at 2,700rpms (65-70).

For those of you with ported 20bs, what is the lowest rpm I can expect to cruise at on the highway without having to downshift everytime I want to moderately accelerate to navigate through traffic ect?

Best,

Chris Carlisi

 

more to do with what will be on it for turbo's
the stock twins start making boost at about 2000 rpm
a big single will be higher.

 

Very true. The faster spool you get in the low range, the more power you will have in that low range. Of course the added torque of the 20b will help. My stock seq Fd has no problems accelerating from 70mph in 5th. At 70 if I floor it, my boost will shoot up to 10psi fairly quicky. The non seq guys take longer to boost and aren't as lucky in this situation.

 

Assume the turbo won't spool up until 4,000. The goal here is to get the best gas mileage possible. EG: If the engine can cruise at 2,000 rpms I will use a gear set that will put the car at 75mph @2,000. On the other hand, if the engine lugs at 2,000 rpms I will use a different gearset to raise the cruising rpm.

I'm my previous post I said the minimum cruising rpm for my car was 3,000. I think that was a little high. 2,700 is probably more accurate.

 

Chris,

I have seriously considered converting my FD to a 20b. In fact, I have already aquired much of the hardware to do it and if not for the smog laws out here, not to mention the gas situation, it would have already been done. Moreover, I have been considering the same questions you raise in this thread.

You might be interested to know that with some tuning, I was able to get very good low end and a very respectable cruise out of my 13b rew in the 1800-2000 rpm range. Keep in mind that in stock form, it was terrible below 2500+. Now it will pull moderate grades comfortably at 2000 in 5th and I'm getting 20+ mpg in mixed driving. This is important to me as I do a lot of highway crusing at 80 - 85mph, not to mention the price of gas here in Southern Cali. If I could have just one more thing on the FD, I would opt for a 6th gear to put that 80 mph cruise at about 2000 rpm.

That being said, I would expect that the 20b would--or could--pull a 2000 rpm cruise better than, and certainly no worse than a good 13b, even with ports and a big single. The reward is not only in gas mileage, but a signifcant (30% or so) reduction in engine speed, mileage and wear if you do a lot of highway driving.

Just my thoughts...

 

This is the main reason I would love to have a six speed with my conversion. I would keep gears 1-4 stock. 5th would be a little shorter than before but the 6th gear would be strictly overdrive like in a GM ls1 tranny. That would be near perfect and a 20b would get excellent gas mileage on the highway.

 

is there anything like that out there ..... a six speed like that ??

 

I agree. I tuned my car all the way down to idle with my TE WB2. However, while it is much smoother than it was with the base map at 1500-2000, it doesn't have enough power to navigate through light traffic without downshifting; it feels much stronger at 2700rpms.

The transmission I'm looking at has a .56 6th gear. The best dif ratio I can get is 4.1:1. I'm planning on using 335/30 18s (overall diameter of 25.9"). With this setup the car will cruise at 76mph at 2,250rpms. If I could find 335/30 17 tires I could reduce the overall diameter by approximately 1" and raise the cruising rpm at 76 to 2350.

I really would like to get some field data from 20b owners before I start throwing money at this part of the project :o

 

Makes me laugh.

Wants a 20B RX-7 with a large single turbo job & your concerned with fuel consumption.................

Gotta love the irony in that.

 

This is an "it depends" question. It depends on your expectations.

When I first picked up the RXX-7 from Peter Farrell, we went for a pre-delivery test drive. As we were approaching a standard, 90-degree, city interection turn, Peter said, "Leave it in fifth."

In its 2-rotor days, I would have downshifted to second for that sort of turn. However, I did as he said and marvelled at the way the 20B pulled us smoothly through the turn at about 1200 rpm.

Torque is a lot of fun.

Another time, when I was crossing the bridge over the Thousand Islands area between New York and Canada, I got busy rubber-necking and wasn't paying attention to my speed. As I swung my gaze from one side of the bridge to the other, I noticed, in passing, that I was cruising comfortably at 850 rpm with no trouble at all.

To be fair, I didn't try to go to WOT in either case, but simply allowed the motor to spin up smoothly using about 1/4 throttle.

Hope that helps.

 

same here my stock port NA 20b moves below 2000rpm on the freeway. not at 800, my haltech hasnt been retuned since my 4" exhaust upgrade. But it moves in 5th at 2000. Not real pulling power, but it accelerates. Im only making ~200lbft.