RX-7 boost lag no more?
 

Hey guys, I'm trying to plan my FD build out before I purchase the car to know exactly what I want but I keep reading complaints about the lack of low end torque especially once you go single turbo. Imo I'm not interested in a engine swap because to me that's like pulling the soul from the car. If I'm going to get a LS1 I might as well just get a C5. Now I was talking to Turblown and apparently they're new BW 8374 EFR kit can get between 450-520whp with supporting mods, start boost at 2400rpm, and get to full boost by 3400rpm. In your opinion does this pretty much get rid of boost lag? I'm not sure how it is in the RX-7 but I generally cruise around 2000-2200 RPM in my Miata so that would seem pretty instant to me.

Thanks

I'm not super technical, but I've got a modified sequential setup on my FD. I've read that the new ball-bearing turbos have less lag, so it shouldn't be as much of a problem on a properly built single turbo. The size of turbo affects your boost lag and peak power... fortunately, other people have done a lot of research before you. An FD with over 400 hp is crazy fast... mine is running about 320 hp.

Boost at 2,400 RPM is pretty early. My boost kicks in around 3,000 RPM, and don't forget the redline is 8,000 RPM. Unless you plan on doing a lot of drag starts, you'll be staying in the boost after 1st gear, especially if it starts as low as 2,400 RPM.

I don't know if you have a late-model Miata, but the RX-7 has 5 gears and you might cruise at a slightly higher RPM than in the Miata. The rotors rev pretty fast, so it isn't as if you won't get any push from idling RPM. Obviously, mileage will be worse, but nobody sets up a single-turbo FD to save gas.

You might get better information in the Single Turbo RX-7s section:

Single Turbo RX-7's - RX7Club.com

Just remember that that turbo is only hitting positive manifold pressure at 2,400. On the highway, your gonna need to down shift to have some decent passing power. Your also still gonna need more rpm to build boost from there as it will come on gradually. Not so on a 13b with perfectly operating sequentials as the primary will hit + manifold pressure as low as 2k. Full 10psi will hit around 2,600 or sooner depending on intake and exhaust flow. A perfectly running seq setup running 12psi with all the bolt ons is one torquey mofo and fun as hell on the street.

According to turblown it's about 8psi at 2400rpm. So I'm not sure if it's faster then sequentials?

Thank you

No, with sequentials with full bolt ons you get full boost (10-14psi) around 2,000rpm.

I have a built single turbo FC with a small 400rwhp turbo that 2psi at 2,000rpm, ~8psi by 2,500rpm and full boost around 3,300rpm and it feels laggy as hell compared to my sequential FD with downpipe, catback, intake and IC.

The only thing rotary better than sequential turbo for low end power is displacement and sequential turbo (20B).

It doesnt matter how much boost you get at low rpm. You wont be making any where near full power at low rpm. That's what you get with a rotary engine.

Singles have gotten better for response, but for instant response you can't beat a sequential setup. At the end of the day, how much power do you NEED? You can make 330-350hp to the ground with a sequential setup with all the supporting mods, and that makes for a damn fast car.

You could also do a BNR setup, that will give more power (over 400hp to the ground) but still sequential.

Swapping out to the RX-8 4.44 rear end gears helps response quite nicely, it just makes the car more "eager".

It's really up to how you intend to drive and use the car. My car is a fun street car that sees the Dragon once a year. I have 99 twins running sequential and 4.44's, the thing is CRAZY responsive and very fun to drive.

Dale

I agree! My FD has twins still running sequential and i really dont understand the "lack of torque" many people go on about. It pulls with authority

Yeah maybe on paper it dosent have as much torque down low as some other engines, but it only needs enough torque to overcome the weight of the car, which is less than most cars.:icon_tup:

I'm cross-posting this in two threads because I think the point is valid in both cases.

The FD is old, and so is the 13B-REW. It's port injected. It has fixed port timing and poor combustion. Even in steady state flywheel torque it can't compete with turbo piston engines anymore. I'm not saying this to be a downer, just to give people a little context. I'm going to compare manufacturer rated torque curves for a series 6 FD, series 5 FC turbo, and a vehicle that is in the same price range as the FD was back in the day--the Daimler AMG A45.

For those of you who aren't familiar with manufacturer rated torque and power curves, they are run on an engine dyno in quasi steady-state. Basically, the flywheel is bolted right to the dyno. The engine is held for roughly 30 seconds at each speed and load point, and then the line "connects the dots." So spool is not an issue--it's basically the max torque available at a given rpm.

https://www.rx7club.com/attachment.p...1&d=1395798773

Here's the FC and FD torque curve as published by Mazda back in 1993 in a technical paper.

https://www.rx7club.com/attachment.p...1&d=1395798773

Here's the torque and power curve of the Daimler M133 engine. Note that these are metric units so horsepower and torque don't cross at 5252rpm.

https://www.rx7club.com/attachment.p...1&d=1395798773

Here is a chart I made comparing the two (using a tracer program to dump the torque curves to Excel). It's not even close. There is absolutely nothing you can do to a 2 rotor engine to get it anywhere near that at low speeds unless you drastically cut off the high end torque (and therefore power).

The A45 has a 345 horsepower 2.0 liter turbo with the most advanced gasoline direct injection system available today, and a single twin scroll turbo. It runs a small turbo at about 26psi of boost to get the low end torque. The FD was rated at 255 horsepower using sequential twins, and the series 5 was rated at 200 horsepower using a twin scroll turbo just like the AMG.

The key to the modern direct injection turbo piston engines is the direct injection in combination with variable valve timing (and sometimes variable lift). So the piston engines dial in overlap to spool the turbo and increase mass flow, while being able to run good spark advance at full boost. The rotary needs this kind of technology to make torque close to what a piston engine can do.

As arghx states, it's not possible to match the newer tech....

However....

Do it artificially with DIFF GEARS.

Changing the diff ratio will alter the ease and speed at which the rpm will rise to your boost/positive manifold pressure point. Engine will rev quicker with less gear load to pull, increasing your acceleration and getting you to boost sooner.

There are drawbacks but that is what I have done in the past with cars that had small engines and or not a lot of torque to play with.

A lightened flywheel will also enhance rev-ability and boost spooling. But once you're under way and you get to a hill, you may find yourself going down a gear due to less rotational mass inertia.

ali123, I will tell you what the BW is like in about a month, turblown's kit is sitting next me right now, might even start a build thread.

Someone has this torque thing figured out. ;)

Ok that would be awesome if you could. For reference the car I'm talking about had the following mods,

"vmount, 3 inch full exhaust, bigger fuel pump, full fuel system including rails, regulator and Id injectors. It has a street strip clutch, ls2 ignition coils and a haltech ecu."

I wonder if it'll still feel laggy if it was a full 3.5" exhaust with a aftermarket flywheel. Not sure what other mods could help with faster spool. Reason I want to go single turbo is because eventually my goal is between 450-500whp. I want to make sure I buy all my parts step my step correctly.


Thank you

Arghx, while I enjoy your posts and the use of data to back them up and I agree that our out of date and under-funded engine platform can't keep up with a lot of newer tech, I do believe you're comparing apples to oranges to skew numbers in your favor.

ali123, are you planning this build for street driving or the track (drag/drift/road racing)? As stated by other posters, the sequential system has decent push down low. If you want all-round flexibility, the BNR sequential turbos will take you into the power envelope you are discussing (400+ hp).

People go single turbo to simplify the system (and tuning) and make big peak numbers, but the limitation is the flexibility of the turbo and supporting hardware (fuel, engine porting, cooling, etc.). There is always going to be a tradeoff in your build, especially if you are on a tight budget – for example, emissions and exhaust noise. If low-end torque is a must, you should think about a 20B engine swap.

http://img600.imageshack.us/img600/8...ompsondyno.jpg

from https://www.rx7club.com/single-turbo...d-dyno-956766/

I think thats a nice torque power curve . for a 20 something year old engine

It doesnt matter how much boost you get at low rpm. You wont be making any where near full power at low rpm. That's what you get with a rotary engine.


While this is true, there is quite a bit of difference running 10psi on the primary turbo and making 150rwhp @ 3,000rpm and running 12psi on the primary turbo and making 175rwhp @ 3,000rpm.

It isn't near the peak 350-400rwhp you will get when the 2ndary turbo is online, but it is enough you have to modulate the throttle in the lower gears to put the power down even before the 2ndary turbo come on.

Lack of torque is subjective term. I personally hate driving "big torque" engines where the torque drops off hard after 5,000rpm. I like torque at the wheels, not on paper. Gearing, rpm, work (hp) gets you torque where it matters.

However, the rotary is "under square" in design and so makes more power with rpm and to compound the problem the poor sealing grid is most apparent at low rpm. Additionally, it has little rotating mass.

This does lead the 13B to be a dog from idle to 1,500rpm even with boost, even supercharged. While this should not affect performance, it does affect driving feel.

If you really really dislike this feel, you should put another engine in the car or get a different car or truck. Just don't get a race car, even V10 Viper Cup cars are so cammed up its easy to kill the engine off the line.

:icon_tup:

ali..
my setup when it gets warm (it snowed again)
-vmount, bosch 850cc/2000cc, "streetable" clutch, PFC with new harness.. fresh rebuild from Howard C... Have everything ready to except Ihor is building fuel system right now...

on the other hand, the grass in the 3 rotor lawn looks greener... torque low end and power up top, and sweet redline! but that is the thing with cars, nothing is going to be sweet enough...

I hear people talk about diff gearing a lot. I don't understand how it's different than just choosing a different gear in the transmission. The end result is the ratio of engine rotations to the tire rotations. How is gearing at the diff different/better than gearing at the transmission?

I hear people talk about diff gearing a lot. I don't understand how it's different than just choosing a different gear in the transmission. The end result is the ratio of engine rotations to the tire rotations. How is gearing at the diff different/better than gearing at the transmission?
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Its about 1/10th the cost, legal in more racing classes and faster to change to suit different courses.

Ideally, you have both.

This can be taken 2 ways. If you're talking about simply downshifting, yes, that will get you in the powerband. But, changing the rear gear will reduce the effort needed to move the car forward in any gear at any RPM all the time.

This makes driving just far more responsive.

2nd answer would be putting different ratio gears in the transmission itself. While possible, this isn't common. I don't know of different gears that could be swapped in (save for changing 5th gear) and most of the gears are a LOT of work and require many special tools to get to and change. Or, you could go with a totally different transmission, which is HUGE bucks and lots of fab.

Or get a used RX-8 diff for $200 :).

Dale

This is what I'm talking about. I'm aware there's a major difference but I don't understand the physics of it. I shall make a new thread rather than hijack this one.

Howstuffworks.com search for differential gearing. Should pop up.