Dyno vs Reality, Big Single vs Twins
 

After spending some time reading and digging through dyno graphs a few things are pretty clear...

The big single setups running high boost build more "horsepower". They peak around 7000 RPMs and on a very good setup have a powerband from 5500 to 8500 RPMs.

The modified stock twin setups build less "horsepower" but they have a wider power band that starts lower. They peak around 6200 RPM and have a band from around 3000 to 7000 RPM.

For drag racing it looks like the big single should win hands down since you spool it up once and keep it packed until its over.

For autocross it looks like the twins should win hands down since you'll be slowing and having to spool the boost back up. Is it possible to stay in a lower gear and keep the big single spooled? You'd have to downshift coming into the corner to keep RPMs up? Can this be done well with some practice or is it a traction nightmare?

For track racing I'm leaning towards the big single unless its a really tight track (see the autocross paragraph) since it seems with more time to setup your corners you'll be able to keep the RPMs up.

Ok, so enough for my theory...words of wisdom from you folks with seat time in both would be greatly appreciated. ;)

I think the single is easier to drive on the track and on the street (aggressive street driving) because you don't risk that annoying 4500 rpm spike sending the tail out. I have no problem modulating the throttle and holding exactly 0, 5, 10, 15 PSI through a corner. WIth the twins, it was easy to get a spike that would change the balance of the car.

Track : single
Road : single
Road (driving like a granny at 1500 rpm) : twins
Drag : single
Easy-to-fix : single
Road course : single
AutoX : maybe this one does go to the twins... but a really small single could be better.

I'm with Wargasm on this one. If you want to drive like a granny, keep the twins. Otherwise learn to downshift. :hah:

I would always pick power band over peak hp. Peak hp looks good for bragging rights but on anything but a dragstrip the car with the wider power band will kick its butt easily even if it's down some on hp.

Tuning for peak hp is for dyno queens. Total power is the area under the curve on the dyno sheet and that's what counts.

This sketch represents a simple dyno plot. Both motors make the same peak hp, but the one at the bottom has a much wider power band and is preferable in every case.

https://www.rx7club.com/attachment.p...postid=2554867

Especially on an autocross course where downshifting is the LAST resort. Ideally, you want to shift to second and leave it there, even if it means you're slightly below the powerband. You'll lose more time trying to downshift and rev match than you will just dealing with the lag.

So what happened to the GT35 set-ups? :)


-Ted

Damon, you mean you wouldn't want to drive the 810 "Horsepower" monster with the attached dyno chart? :D

Has anyone done an upgraded twin system/kit other than the '99 J-Spec from Mazda? It seems to me like all of the turbo kits out there are single setups (and most of those big singles).

The problem with this poll is that you forgot "street" where most everyone really drives their car. This is where the twins really rule.

On a auto-x course you can't downshift back and forth from 1st to 2nd. You've really got to stuff it in 2nd and leave there. That is why I'm going with the 4.33 gears. Hopefully they will help keep the second turbo on full boil.

I like how my twins perform on and off the track just fine. However, I have heard that a good quality single turbo conversion will make your engine last longer on the track because the twins somehow transfer more heat to the engine and cook the coolant seals after a few track days. Does anyone think this is true? I've done a total of five days and about 8 hours racing on tracks with no bad results yet on my stock motor stock turbo car (w/ M2 Stage III package)

-John

The only head to head I've had with other FDs at Autoxs were with cars with big singles, and they both had much slower times because of the loss of rpms on the tight course and having to shift. They were both about 2 secs slower than my stock twins.

Why does it have to be a big single?
How about a medium single that makes more overall power than the twins but also has a great powerband?

re: RETed's post above

I hate to say it, but this comparison isn't quite right without more data. I don't doubt that it is possible for this to happen, but cars with significantly more horsepower in autocrosses generally click off much higher times than the rest of the field. Your comparison assumes that you and the other guys are equally good drivers and are equally competent in driving their car as it is setup.

When you start comparing numbers with competition/driver skill factored in, you depend on the driver and not the dyno sheet (well, it is similar to compared a tuned car to an untuned car, or apples to oranges, both fruit, but....).

I can probably click off a better time in my 125fwhp Miata than a newbie in a FD with 255fwhp or someone who just had their car converted to a single (probably leaning toward saying, a newbie in a single).

I say probably, because I've managed to do that to an E36 M3 on my lousy street tires and I'm sure I could have widened the gap even more with stickier tires. And I'm not that good of an autocrosser...

Just some food for thought.... :)

As for the track comparisons, I'd like to hear from an experienced track driver who does at least one event a month who has had the stock twins for at least a year and switched to a single turbo or a car with a single turbo to see how they feel. All things being equal, you are going to get better laptimes with more horsepower on a track, so I think they would have to give it a big thumbs up, until they ended up eating a motor...;)

--Ashraf

I totally agree. I've experimented with downshifting / upshifting on autox courses and found that even though my FCNA has less power in the 3500 rpm range (2nd gear coming out of a corner) than if it were in first gear at 5500, the time lost in the downshift / upshift and distraction while making the turns eats up the gain in power for me. A couple of the top timers around here just leave it in second the whole time, too.

The problem with going bigger twin turbos is you most likely won't be able to run them sequentially meaning you will have a lot of lag, the stock twins run non sequentially don't reach 15psi until around 3700rpm generally and thats with all the bolt ons, if you go with twin T28's or something all your gonna do is make 50 more HP at the same boost level but have even more lag. Why isn't anyone running smaller turbos, such as the GT30R or even GT28RS, you can make 280-350RWHP on pump gas still and get excellent response reaching full boost under 3000RPM. Even the GT35R has really good response.

Well, not EASILY, but it could be done. Going with two larger turbos will require some "interesting" manifold work and even more "interesting" if you wanted some sort of sequential setup. I think sequential works fine for a stockish car, but once you start dumping money into it, they become a bottleneck for many reasons that have been discussed ad nauseum.

One interesting thing I haven't kept track of or seen very much of, are folks with highly modified stock sequentials re-tuning the transition point due to having so much more airflow so they don't get that big spike of power when the 2nd turbo comes online.

Not true. You have two smaller turbos (compared to a big single) that can spin up quicker than one large turbo because of rotational mass and lots of other physics I don't think we want to rehash here. There are other things at play and it just isn't that simple to compare two medium apples to one honkin' big orange. :)

I think Kevin Wyum is the person to speak to about such a setup, because he actually did it, and from what I remember reading, the results were quite impressive in terms of power delivery and spool up time. It wasn't cheap, though (which is why so few people have done it). The one thing a larger single has going for you, is that you only have one rotating assembly to rebuild versus two.

autoX = small single, like an apex rx6. it matches the twins spool without the transition issues. it's much simpler than running seq too.

how does evryone feel about staging turbos? the exhaust from the larger turbo runs into a smaller turbo in effect spooling the small one up for a more usable powerband but at say 4500 rpm's the big turbo hits and kicks in with a huge decisive peice of horspower and carries it through the rest of the rpm scale, you could also design a gasoline injection system directly before the turbos that injects gas into the burning exhaust gas right before both turbos during lower rpm's -you could get a tourqe curve flatter than kansas !

burning gas, or exploding it in the turbo manifold to spool the turbo is commonly referred to as an anti-lag system. they are great but really hard on turbo's.

your other idea doesn't seem like it would work at all.

This violates the laws of physics. If it worked you could build a perpetual motion machine :)

You gotta ask yourself if the smaller turbo is already easier to spool why don't I drive it directly instead of removing a bunch of energy from the exhaust gases by passing through another turbo first???

could you have a setup where you have a smaller turbo in front with a very large wastegate on just one of the exhaust ports(a BIG turbo port) set to open when the smaller turbo tops out then run a pipe from the waste gate back into the exhaust coming out of the smaller turbo going to the larger turbo that way you keep the smaller on from over spooling i know its not cost or time effective just would like to know if it might work?

they do it at tractor pulls, so it is possible, the first turbo does not spool up the second turbo-the second turbo spools up first because it is smaller, there does have to be a bypass for the gasses from the first turbo so it doesnt get choked out by the exhaust restriction from the smaller turbo

Just to throw a wrench into the works.. here is my dyno curve:

http://home.comcast.net/~mhaun5/rx7/13psiDyno.jpg

Hi Mike,

What gear was that in? Seems somewhere between third and fourth, lol. I hate it when they dont list rpm on the x axis......

take it easy--

Heya.

4th gear. They started to record the dyno when we shifted into 4th, thats why you see the initial 3rd gear rpms build, then slouch off while 4th gear was engaged and recorded.

Re: Dyno vs Reality, Big Single vs Twins
 

Big single for AutoX and drag racing? That's a hard pill to swallow. You could stick with the twins for a more a better average power gain. After reading from Paul Yaw's webpage, average power gain is what you're looking for. Maximum power only works at an instant in time, since power is a time derivative of work. A moderately sized turbocharger can get the job done. I'm not going to throw out any brands. The choice is up to you. Consider how much power you are willing to make on both tracks. Obviously, since they're different, it's going to be hard to balance. It is possible though. Happy tuning :).