ITT: Your ideas for increasing torque output
 

Ok, before I get into it.. please leave the "go turbo" comments out of here. I am not at this point going turbo. I may in the future, but right now this thread is to brainstorm ideas for my current Rotrex'd set up.


So as most of you know I have a Rotrex'd 1.6L Miata. While I'm happy with where it's at right now and could realistically drive it for years on the track and be satisfied... a lot of the fun for me is the build side. I love to tinker and change things, make it better.

Now, one of the poor things about a Rotrex is the amount of torque. I make a dismal 194wtq peaking at redline (8000RPM) but silly amounts of HP (297whp @ 8000RPM). The car hauls more than any car I've built before, and up high the lack of torque is not a huge deal with the HP numbers. But on hill climbs the car bogs out of the corners a bit due to the lack of torque. I need to keep the motor above 4000RPM to really go anywhere while going up hill. This is not so much a problem on my local track since it's flat. I don't want to be lighting the tires up out of the corners, I jsut want a bit more "oomph."

What kind of things would help squeeze some more torque out of the motor? 3'' exhaust?, smaller/bigger IC? smaller/bigger IC pipes? Water injection?

I have done pretty much everything one can do to a motor so I need to think outside the box. The bottom end is build, head is ported, OS valves, cams, GotPSI intake manifold, large TB, LS2 coils, Maxim Works header (modified slightly) but only a 2.5'' exhaust. I am also running a small 2.5'' thick core IC that is 7''x 22'' or so. 2.5'' piping all around. Could going to smaller IC piping pre intercooler help? A 3'' exhaust? Should I drop my RPM limit down to say 7600RPM or so and spin the Rotrex harder to make more boost down low?

I am running 15's next year with hopefully a 6pd. I have not chosen a final drive yet but right now I have a 4.1 in the diff.

My dyno plot for reference.

https://www.miataturbo.net/attachmen...ine=1312059371

Help me boost gurus.

You didn't list camshafts specs, but you have "cams" in you sig. Details?

Did you go bigger or smaller cams?

IIRC (without the spec sheet infront of me) they are 207 IN*/214 EX* duration at .050. 8.8mm in 8.9mm ex lift. Valves are 1mm OS. I don't remember the overall duration but I want to say they are in the 290*-300* for both.

Ideally I'd like to not have to touch the cams. With the solid lifters it would be impossible to find an OTS cam that works with the shim sizes available and I really don't want to go through the cam grinding process again.

figure out a way to get full boost at 3K.

It might mean installing a turbo.

.

:giggle:

Honestly, you reap what you sew. You switched to a 1.6 and used a rotrex blower. That combo does not produce torque. There is no magic tuning tweak that will suddenly add a noticeable amount of torque to your setup. Since you aren't willing to go turbo, or change the cams, this thread is pretty much pointless.

If you want torque, get new cams or (better yet) build a vvt motor. Or get a bigger rotrex and restrict the top end. It will still never make big torque at 4000rpm, though. That is not what the rotrex is good at.

I would enjoy the car as much as you can for a while, then wise up and build a vvt turbo motor.

I hear you. I just don't want to dig into the motor any longer. Hence the reason for not wanting to touch cams. I think since my resonator is blown out on the 2.5'' exhaust now is a good time to just get a 3'', drop the RPM down a bit for reliability reasons and call it a day.

Then maybe add lightness.

With an 8000 RPM redline, why are you dropping RPMs below 5000?

What kind of torque gains are you looking for?
You need to look at power at your minimum expected RPM, which is typically 60% of redline. e.g. power at 5000 RPM when you have an 8000 RPM revlimit, or power at 4300 RPM with a 7000 RPM revlimit. And then the way to maximize torque and spread of torque when you have a centri blower is to maximize the engine's VE at the bottom of that RPM band. With a centri blower that suggests milder cams than stock - this has the same effect as "restricting" the topend, while maximizing the torque. Having said that, you can only increase VE at 4000 by ~10% compared to where you might be now.

Stock cams have a torque peak at 5500 RPM which would work well with your 8000 RPM redline.
I guess indeed the only thing left is to use a smaller pulley and restrict the top if the top end might break your motor.

Yeah I don't really go below 5k most of the time but some corners on a few hills if i stay in 3rd i bog but if i downshift im at 6k and the motor revs so fast that im upshifting almost right away. I'm thinking i need to see how the car is with 15's over the 13's i was running.

Or get a 6-speed, which tightens up the spacing between gears by around 30%.

I have my eye out for one.

How much power/torque do you think I could realistically get from moving to a 3'' exhaust and dropping the RPM down 400 or so and at the same time gearing the Rotrex a bit higher? Say 2PSI more at 4000RPM, 3PSI more at 5000RPM with the 3'' exhaust?

Worth it?

And I'd likely go 6spd if I can find one with some 15x9 6UL's

At maximum VE:

Torque (ft-lbs) ~= 72 ft-lbs * displacement_in_L * (1+boost/14.7)

Power (hp) = torque (ft-lbs) * RPM/5252

Some of that torque is used up to drive the s/c
When you're not at max VE you will have less torque than predicted
This is why with a centri s/c you want peak torque at the bottom of your powerband. i.e. stock cams show peak torque from 4000-5500. You will want to gear the car so that you never drop below 5500 rpm. As VE diminishes above 5500 rpm the centri increases boost.

Tried messing with intake cam timing?

Yeah we did a bit on the dyno. Now I don't know a lot about cams/cam timing and going from memory IIRC my motor builder said they were set at 110* something or other initially (before the motor went in the car). Maybe those of you who know, will know what he was talking about.

On the dyno both the intake and exhaust cams were tinkered with and really showed not a huge amount of gains in either direction. They seemed to like it withing 1-2* of where they were initially set up.

I also don't have a ton of room to play with for the cam gears because my motor is interference with the higher lift, OS valves and decked head/block. I can only go about 4* either way from 0* on the cam gear before getting too close for comfort.

Probably LSA (lobe separation angle). More LSA = more overlap IIRC.

That's what I would expect. You may be able to find some low-end torque by advancing the intake cam, but you will likely sacrifice a disproportionate amount of top-end power in the process.

You've already done everything else, including all the expensive improvements - MW header, LS TB, headwork, etc. IMO, you're not going to find much more than what you have with your current motor/blower combo.

Yeah I'm starting to agree. I think if I go back to the dyno it will be with a 3'' exhaust, RPM limit slightly lower with a smaller pulley and try to play with the cam timing some more. If I loose a bit of top end it's not the end of the world... but how " disproportionate " are we talking here?

Come to think of it, I actually only messed with cam timing on the first tune., before the got PSI intake and before the smaller pulley.

Impossible to say. Maybe 2whp per 1wtq gained, but that is pulled straight out of my butt.

A larger exhaust is not going to give you more lower end torque. There is a reason road racers use smaller exhaust systems than drag racers.

In saying that however, it is possible that the 2.5 inch exhaust is not the ideal set up with your blower. It seems to be with all out N/A applications on the road coarse. Only experimenting will know for sure.

If you have the money to do this, doing the baller mods like electric water pump, evac system, or lighten the load on your valvetrain/powertrain will give you some power and torque across the entire power band. It is not cheap, and it is not going to net you a ton of power. Typically last resort methods when all other options have been exhausted. Just throwing that out there.

Put your car on a diet. That will be like adding power and torque.

backwards. LSA is the angle distance between peak of exhaust lift and of intake. More LSA = less overlap.

It's a pretty meaningless spec by itself. Overlap, intake closing point, and exhaust opening point are more useful. You can't derive overlap from LSA without knowing the opening and closing profiles. LSA is just easy to measure when you're holding an old-school single-cam engine's cam in your hands, thus the measurement's popularity.

Yeah I have all that info it's just sitting on a paper in my car folder at home. Out of reach for the time being until I get back to Canada.

falcon, did the new cams increase or decrease your torque at 4000 RPM?

Don't know.. the motor was built and installed with the cams from the get go.

I'll guess they decreased torque at 4000. You may be better off with stock cams.

Perhaps... I still have stock cams however it took a lot of work to find and modify all the shims for the lifters. I don't know if I want to go through that again.

Sav's right... I think I'll just do a few easy things and the car needs a re-tune anyways and leave it be until I build myself a stroked out VVT motor :P..

I thought that was mainly because road courses often have sound limits and drag strips usually don't?

--Ian

Mill the head?

Weld washers to the pistons?

Who are you and why are you posting here?

Who are you talking to?

You.

Weld washers to the pistons? Really?

Aaron, what about finding a set of HLAs and stock cams, taking out your current cams along with the lifters, substituting HLAs, and seeing what it does for low-end power? You don't need solid lifters to do a 6500rpm dyno pull for low-end torque comparisons. If it makes a big difference down low, you can spend the time setting up a solid valvetrain for stock camshafts so it will rev again. If it makes no difference, or an insufficient difference, you can just drop the Maruha cams/lifters back into place and forget about it.

That's a good idea... I didn't think of that. I should still have my lifters kicking around somewhere in a bag. And I know I have the cams. BTW, not that it really makes a big difference but my cams/lifters aren't maruha :P... they were way to baller for me. I have mazspeed lifters w/ supertech shim and ground cams done locally.

Falcon you missed it sorry buddy.

Dann

Milling the head will increase cylinder pressures which will increase torque.

Welding washers was a joke. Old timers used to try it when they couldn't order custom pistons.

Smaller pulley on the sc, bigger one on the crank (unless you're over spinning the sc right now), a wastegate set at your correct psi level to prevent you from over boosting at high rpms...yes wastegate will be open to remove extra air at redline.

Wait a minute here. Are you seriously suggesting to install the stock cams in place of your hotter cams?

If you are having a problem with power delivery, it sounds to me like you need a final drive gear change. Get your rpms in the range they need to be before doing something stupid like making less power.

Drag racing is always foot to the floor. Road racing spends a lot of time at lower rpms and part throttle.

Squeezing the exhaust down gives you more torque at the lower rpms and part throttle. Smaller exhaust is a compromise that pays off on the road coarse, but penalizes the drag racer.

errr..,. my head has been worked over already and "decked" if that's what you mean by milling. otherwize i have no idea what youi;re talking aboutj.

"decked" insures its flat, just taking off the minimum. Milling removes actual material, and will raise compression.

My head was decked once before and I did it again to remove some detonation damage. At this point it's already past the factory limit and with the motor assembled I can't move my cams more than 2-3*. So yea, I'm not taking any more off.

If I feel like it one day I may put some higher compressions slugs in it. It's running 8.7:1 supertechs which isn't helping.

Can you show a dyno plot of a 4-valve head with streetable cams comparing a big bore vs smaller exhaust showing gains in midrange torque?

falcon

DO you have the boost curve of your dyno plot?

If you have the data, then generate this plot in Excel, vs RPM:

torque divided by MAP
or
torque * 14.7 / (boost_in_psi + 14.7)

This gives you a number proportional to VE and cylinder pressure (assuming the timing is near MBT). It will show you at what RPM your engine is breathing well. If it's low around 4000 RPM and highest around 6500 RPM then you should go back to the stock cams.

No I do not have access to such dyno plots. I am not speculating however, I have read about this many times over, and thought it was pretty much common knowledge.

Don't forget, I am talking about part throttle not full throttle applications.

What's the use of having more torque at part throttle? Why not just step on the gas more if you need more torque?

Jason, I have no road race experience what so ever. So I offer little explanation to the subject of on and off throttle transitions on the road coarse.

I do specifically recall a motorcycle team that put butterflies in the collector in an attempt to have just a tad more punch coming out of the corners. This was a few years back.

It has nothing to do with throttle application and everything to do with exhaust velocity. Nobody sizes exhaust/intake/anything for part-throttle power.

My statement about why road race cars typically having slightly smaller exhaust was a bit speculative and maybe a bit misguided.

Would you agree or disagree with the statement that road race cars have smaller exhaust tubing than drag race cars for a reason?

No, I won't agree with that. It's far too broad a generalization to be true or false.

I think the answer to Aaron's problem has already been answered and probably the easiest solution. A 6 speed with a 3.9 rear end. I just made the switch this year and the change is quite drastic.

5 speed 4.1 225/45/15 50mph 4000 rpm
6 speed 4.1 225/45/15 50mph 4940 rpm
6 speed 3.9 225/45/15 50mph 4700 rpm

This will put Aaron back into his powerband and the tighter gearing will improve his acceleration.

Probibly a stupid suggestion since no one else has suggested it. If your not on MBT and are knock / temp limited why not give water/meth injection a try. I would have thought the easy answer is more timing, and add what u need to to allow that. i could be totally wrong though.

I got my aquamist HFS3 kit for under AUD$500 so its not a super expensive thing to test and resell. Woudl would likly need something like the hfs ( scales water flow on fuel injector duty cycle )though so u could control the delivert better since a simple boost activated switch without liniar flow will just dump in water later than were u want it then not provide what u need in upper rpms. Worse case u would be getting some cooler intake temp and in cylinder temps.

What is your valve angle?

Sav is being intentionally short I think.

Essentially its the exhaust gas velocity that needs to be kept up for good scavenging.

If your exhaust is bigger than you need to eliminate a restriction then the cross sectional area of the exhaust is too large and the speed of given exhaust is slowed down, we all know that momentum is mass x velocity, and so if we slow it down we lose momentum.

Less momentum = less scavenging.

A slightly smaller at lower rpm helps because at lower rpm the car produces less exhaust gases, therefore the smaller exhaust is not a restriction to power, however at high rpm we need an exhaust big enough to not restrict the engines breathing. Too big however and we lose exhaust gas velocity/momentum and lose scavenging.

So essentially while a 2.5 inch exhaust might be perfect for 5-7000rpm on a given engine, it will be too large for max power at 3-5000rpm on the same engine.

Dann

Wow that wasnt very well written..

Also Ill add that drag cars use huge exhausts often with no collector in an effort to reduce torque at lower rpm and get traction out of the hole.

Dann

If he's not on MBT from idle to redline with 8.7:1 slugs and a Rotrex, there's something wrong.

Finally someone who makes sense of all this. And yes, I thought it was very well worded.

However, as a drag racer myself, you want all the torque possible off the line. don't forget, you are not only racing to the finish line, but to the first 60' as well. And a very well prepared drag car will have the traction issues sorted out. The more torque, the better 60' time.

The difference between the drag car and the road race car, is that the drag car with the rpm range you posted above, will always be in the 5000 - 7000 rpm range, and never in the 3000 - 5000 rpm range. This is the reason you will typically see a slightly larger exhaust on a drag car with the same motor configuration as a road race car.

Analysis/Regurgitation Alert:

If this was a poll I would vote gearing change first, cam change second, higher comp pistons third, outright setup change fourth.

I know 5 speeds can handle 250WTQ, but 8000RPM shifts and 200WTQ is not exactly easy on one. May as well get ready to replace it and upgrade at the same time with shorter gears. Which the OP is doing.

The 6sp will help a lot. +1 to sparetire's list.