Cheap (and now not so cheap) Miata mods to increase HP
I get asked this question all the time from 1.6 Miata owners 'what mods can I do to get more horsepower without costing a lot of money?'
The following is based on my actual experiences and my own dyno runs and is not intended to be a complete laundry list of available mods. If what you're interested in isn't on here then I probably haven't done it. The 'cheap' information is intended primarily for a 1.6, however I have found that the 1.8 responds to basically the same mods.
A motor is nothing more than an air pump that has a system to ignite a fuel to keep it spinning. To increase power from stock levels you have to remove any restrictions to pumping the air in and out and then optimize the fuel/air ratio and spark timing for best efficiency.
There are many products on the market that claim to increase power. Based on my personal experiences, I recommend the following to the home tuner on a tight budget:
1) The simplest and most common thing you can do to the Miata is advance the initial timing a little. 14 degree BTDC instead of the stock 10 helps with about 2 more HP and also moves the power band lower in the RPM range by about 400 rpm. Cost $0.
2) Replace the cat-back exhaust system with a performance system. This removes the largest restriction in the exhaust system. The OEM muffler and OEM 1 7/8" exhaust pipe are very flow restrictive. Replacement with a 2" or larger system is good for about 4-5 HP across the entire rpm range. Cost about $300-400. For the really budget conscious, a generic 'turbo' muffler (cost about $60) can be installed in place of the OEM, but you'll still have the restrictive mid-pipe in place.
I do not recommend removing the catalytic converter but you can expect another 2 or so HP above 6000 rpm with a performance unit.
3) Replace the flowmeter in the intake tract. If you will be using the motor above 6000 rpm frequently this can provide another 5 or so peak HP.
A lot has been said about airbox mods, these are OK because they can shorten the intake tract for better throttle response and reposition the intake to breath outside ambient air instead of the hot underhood stuff. The problem is that they typically do not add any horsepower in a 1.6, they just help keep from losing any. What I mean is that they aren't really increasing flow because the flowmeter is the real bottle neck and, while the cold/dense air is a good thing, it is only making up for the fact that the stock motor can't breath.
The flowmeter is very restrictive and limits HP above 6000 rpm, replacing it eliminates the bottleneck. The OEM flowmeter flows 165 CFM while a bone stock 1.6 motor at 7200 rpm flows 178 CFM, this means that the OEM unit is undersized for high rpm usage, the change probably won't be noticed below 6000 rpm. A 86-88 RX7 unit plugs in and flows 20 percent better (200 vs 165 CFM). Dyno Proof. This change will require custom intake plumbing since the RX7 unit will not bolt to the OEM airbox. This is a good thing since you can now create that shorter cold-air system and really take advantage of it. A junk yard RX7 unit should cost between $35-80. See also my flowmeter replacement page and testimonials.
If you do replace the factory intake plumbing with an aftermarket or a DIY system then make sure to keep the (anti)resonance chamber. It has been dyno proven to provide 4 ft lbs of torque from 3000-4000 rpm. If you cannot use the OEM piece then use the stand-alone unit from the 90-93 Protege/91-93 Ford Escort GT.
4) Replace or modify the exhaust header. If you need more horsepower above 6000 rpm, an aftermarket header can provide another 3-4 peak HP. Useless unless cat-back exhaust upgrade done first.
Just because the factory headers are stainless steel tubes doesn't mean they are any good. The Miata OEM header is better than most OEM exhaust manifolds but it is not tuned and has lots of restrictive welds and crimps while most of the aftermarket Miata headers have equal length, crimp free tubing. You probably won't notice any HP improvement until after 6000 rpm with a header. See the Racing Beat dyno comparison. Cost $300-500. I recommend a ceramic coated or stainless steel model to eliminate rust through.
There are three places where the OEM header is restrictive because of welding and fitting. The head flange, the inside of the collector, and the downpipe flange. Both flanges are welded on the inside and really restrict diameter and flow. These two are easy to grind away. The biggest restriction comes from the collector. The tubes are crimped down to nearly 1/2 their diameter and they have lots of welding protruding into the exhaust flow, too.
If you are handy with a grinder you could remove the welds at the flange and collector of the OEM header and achieve nearly the same results (remember to weld some beads on the outside to keep it structural), but that is a lot of work and, in my opinion, not worth the effort.
5) Now, with the above mods, the motor can breath and is letting more air into the motor above 6000 rpm. More fuel is needed to keep the fuel/air ratio at optimum at WOT since the factory WOT settings are fixed (open loop) and do not take into account the O2 sensor. An aftermaket programmable ECU, larger injectors, a resistor in the ECU coolant sensor or a variable rate of gain fuel pressure regulator can all let more fuel into the motor.
I do not recommend 'chips' for the 1.6 motor since the 1.6 Miata ECU does not have a replaceable chip to program, the values are hardcoded into the logic. A 'chip' for the 1.6 basically tells the ECU that it sees cooler ambient air than reality, so the ECU runs the motor richer. Timing cannot be altered on the 1.6 with a 'chip', the only real thing a 1.6 chip can do is up the rev limit.
I used the Downing/Sebring fuel regulator unit myself. It is a variable rate of gain model so it can be set to change the fuel pressure at different vacuum levels. I went this route so that only the WOT fuel was altered and not idle or mid throttle, where the O2 sensor will compensate if needed. I increased the WOT fuel pressure from 43.5 to 50 psi based on my fuel/air ratio gauge readings.
This mod doesn't necessarily add more HP itself, the Miata 1.6 ECU setting are actually set quite rich in stock form. But more fuel will keep you from losing any of your torque gains since, with the airflow mods, the motor will probably be running a little leaner at mid-rpm WOT. (hint: this is great for autox and the street where you'll want lots of torque, but for track days remove the AFPR because a slightly lean motor gives the best power at rpm above the torque peak)
6) Shaving the head. Compression = power. Shaving the head .010" will increase the compression ratio about a 1/4 point. I dyno'd this and it was good for about 4 HP and 4 ft lbs of low rpm torque. It is very noticeable across the entire rpm range. (I don't know what the shaving limit is but I have not heard of anyone going beyond .025". FYI, the shop manual limit is .008" for 1.6 and .004" for the 1.8). As long as you have the head off you might as well freshen up the valves and seats, too.
As you can see from my HP estimates, the muffler and the flowmeter are the two most restrictive things on the stock motor that can be easily replaced. They flow enough for about 6000 rpm but anything more and they will impede flow.
It is very difficult to increase power in the lower rpm ranges. Torque is what you feel then and it is mostly limited by displacement in a stock motor. The timing mod and head shave will help that out a lot.
I recommend doing all of the above items. I personally cannot feel anything less than a 5 HP increase myself, so doing just one thing might not make a noticeable difference. All done together they make for a nice increase in power above 6000 rpm. All the above together on a 1.6 motor will get you about 115 or so peak HP at the rear wheels, which is about 135-140 crank HP. There is much more you could do to the Miata motor but this should keep you busy for a while. Total cost, if you do the installations yourself, should be in the $1000 ballpark.
The only variations to the above for a 1.8 are that the flowmeter is probably more in line with the airflow requirements of the 1.8 motor and my guess is that it doesn't need to be replaced. Also maybe the TB could be overbored a bit for the 1.8 (which is covered on the flowmeter Q&A page). The 1.8 could also benefit from an exhaust that has a pipe diameter of at least 2.25". I have not dyno'd the individual items on a stock 1.8 like I did for my 1.6 so this is just a guess on my part based on experience I gained from building my 1.8 CSP motor (see below)
Q Aside from F/I, how do I get the good HP out of my 99+?
**THIS IMMEDIATE SECTION UNDER CONSTRUCTION **
Basically a header is a good starting point since the 99-00 factory cast exhaust header is less than desirable. While an aftermarket header on a M1 is only good for a couple of HP on the top end, the 99-00 M2 motors came with a heavy and restrictive cast iron exhaust manifold and replacing it does wonders. Upgrading to an aftermarket header on the 99-00 M2 motor is one of the most cost effective mods you can make. With just a header and cat back exhasut system it is not uncommon for a 99-00 to dyno 120-125 rwhp, which is significant over the typical stock numbers of 109-114.
A good cat back exhaust system works the same wonders on a M2 as it does on the M1.
Unlike the M1 which has a restrictive air intake system the M2 is actually quite good. Most people report minimal gains for an air intake system on a M2
Just like the M1, setting the ignition timing a few degrees advanced still works wonders. The issue is that the procedure requires drilling and dremeling to make the crank sensor adjustable which can be intimidating to some.
If I owned a 99-00 I would get a header, cat-back exhaust and a stronger intake cam of about 256 and then call it a day. Good for about 140 rwhp.
The 99-00 already has a better intake manifold, better intake plumbing with a less restrictive MAF, solid lifters, higher CR pistons, strong EX cam and better ECU tuning (w/ knock sensor) that we M1 people had to replace for lots of $$ to get to that same level. The M2 people already have a strong foundation so the 140 rwhp level can be obtained for less incremental money.
UPDATE 11/29/98--------------P H A S E II----------------------
I get asked a *lot* about what is the max street HP that a normally aspirated Miata 1.8 motor can make. So here we go onto PHASE II or "not so cheap".
First, If you are concerned about costs, then consider a turbo or supercharger kit. It is my belief that a boosted motor is much more cost effective in the long run for a good HP/dollar ratio. Improving HP with a 90-97 engine beyond the 140 crank HP level using a normally aspirated route can be very expensive because now expensive things need to be replaced since they will need to perform past the operating window that Mazda set for them. In NA trim, the factory 90-95 ECU tuning is good for about 140 crank HP, '90-97 cams 150, 99-00 cams 160, 90-97 intake manifold about 145, and the relatively low 90-97 compression ratio will support about 150.
Following my own 'Cheap HP' advise, my 94 1.8 using stock cams, .004" head shave, gasket matched ports and manifolds, factory 1.6 ECU w/larger RX7 flowmeter, and custom intake plumbing, adjustable fuel pressure regulator and header netted a total of 115 rear wheel hp and 107 ft lbs torque on the chassis dyno (rear wheel hp).
When talking about max hp on a production based engine, engineering (head design), machining tolerances and physics limits come into the equation. The Miata motor's engineering is limited by the head design (port routing and size, combustion chamber size, valve angles, etc) and stock camshaft profiles, which are both designed for good velocity and efficiency and not max airflow. The stock head does not support ultra high HP levels without sacrificing "streetability". Having a motor that is "streetable" is just as important as max power in my book.
My definition of streetabe is:
A primary characteristic of a "streetable" motor is having good power and torque below peak rpm. Based on my dyno testing and the dyno testing of numerous MiataPower listers, a Miata 1.8 motor in a "streetable" state of tune can achieve 180-200 crank hp, which falls into the generally accepted 100-110 hp per liter rule of thumb (crank) for a non-VTEC DOHC production based street motor. At a minimum, this level of HP requires the enhancements of a programmable ECU, aftermarket camshafts, and an increase in the compression ratio.
The 90-97 1.8 compression ratio is 9.0:1 which would need to be increased to the pump gas limit of 10.5:1 - 11.0:1 via pistons, shaving or welded combustion chamber. The stock 94-97 cams only flow enough air to support about 150 crank hp, aftermarket cams with conservative intake (streetability qualities of good vacuum, idle quality, emissions, etc) are required for more. A programmable ECU is then needed to recognize and optimize the fuel and spark for the new level of airflow.
The 1.6 and 94-95 1.8 factory ECU settings are really not all that bad but they cannot recognize the substantial airflow increases needed for those HP levels (adding cams and such that take the ECU out of its operating range can actually lower HP). In addition to adjusting for more fuel and air, the fuel ratio settings need to be tuned a little more rich below 4500 rpm and leaner after 6000 rpm. This enhancement is impossible to do with standard fuel pressure regulators or larger injectors without affecting the other end of the rpm scale. The timing also needs more advance below 5000 but the 6000 and over timing is about right (which leads to the compromise, but free 14 before mod above). The programmable ECU lets you do all of this without compromise and you can also adjust for each individual motor as all are slightly different.
Matching components is important, too. You could easily spend lots of money on a motor that actually produces less power than the stock motor does. For example, if you decide to keep the stock intake manifold and TB size for good low rpm torque (velocity) and then install a large tube diameter 4-1 header for high rpm HP (flow), you'll probably get neither.
When I started Phase II for myself I simply just added the programmable FM ECU, I realized a total of 15 extra HP after fine tuning (I'm using the 1.6 FM ECU so it was not calibrated to the 1.8 requirements out of the box, just plugging it in was worth 10 additional hp on the dyno and fine tuning got to 129.6 rear wheel HP/109 ft lbs).
My new "street" motor with the above plus 11:1 compression (10:1 2001 +.020" pistons, .026" deck and .010" head shave), lightened/balanced/knife-edged crank, 260 degree cam, adjustable cam sprockets, ported intake manifold, +2.5 mm throttlebody, aluminum underdrive pulley, programmable ECU, and using 4.778:1 rear gears just netted 160 hp at the rear wheels with 125 ft lbs torque at just 2500 rpm. This level of power was not plug 'n play, I tuned the ECU and cam sprockets for weeks. Additional costs beyond the 'cheap' level was about $3000 for an incremental 50 crank HP. So, for a total of about $4000 (not including the original 1.8 swap, my installation and tuning time, or the over $600 in dyno time tuning everything), I have nearly the same power as a supercharger or mid-level turbo kit. See why a turbo or supercharger kit is much more cost effective.
If you are willing to compromise any of the 'streetable' characteristics then you could get even more power. A full race Miata 1.8 motor (typical SCCA GT3 state of tune) would probably be in the 250-260 hp range. For reference, the NATCC 2.0 grenade motors got about 300 hp and the 1.6 Formula Atlantic Toyota's get just about 240 hp.