Is porting for power a myth?

Any form of normally aspirated power mods.
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Is porting for power a myth?

Post by PeteH » Fri Nov 09, 2018 10:56 am

Let's kick off straight away with the issue of the subjective comment. Phrases like "it pulls stronger", "it revs faster", it's more urgent in the mid-range", and so on, are inadmissible as evidence!

Now let's deal with the question of whether porting really adds power, and whether it's worth it in general.

It's clear that a well done full bridge can add a small amount of power to a Renesis. From all the dynos that have been done it seems that a good 231 with stock ports will make about 185whp. A good 231 with associated mods can make up to 200whp. A well done full bridge will achieve up to 215whp with associated supporting mods (cat delete, free flow cat back, possibly an AEM or REVi intake, good coils, and a remap). So we can expect a maximum of about 15% more power, some of which will be due to the other mods. This is likely to be at higher revs, and is likely to require a raised idle speed, low speed running could be lumpy, and engine life may be reduced.

As far as I'm aware, no street port has ever been shown to add measurable power to a Renesis. The only exception to this might be the 192, where street porting, coupled with a raised rev limit, might add some power, and close the gap to the 231 a bit.

So what's going on? I think a good analogy is with computers. When building your own computer system you spec all the components to a certain level. If you have one item which is throttling the speed then every other component is wasted. And if you just bolt one fast bit into a slow system then your system won't go faster. The speed of the system is dictated by the speed of the slowest part. This isn't 100% true, but it's a fair starting point for building a system. The ports in a Renesis are just one small part of the gas flow system. You can make them the size of the QE2 and the engine won't get much more powerful if you leave everything else stock. It's all about where the gas flow is actually throttled.

The gas has to negotiate the following major bottle necks:
1) vFAD inlet
2) Air filter
3) MAF tube
4) Throttle body
5) Upper intake manifold
6) Lower intake manifold with all its valves
7) Intake ports
8) Exhaust ports (I know writing 8 ) makes a "cool" smiley, but I still like it)
9) Exhaust manifold
10) Cat
11) Cat back

When Mazda designed the Renesis you can be 100% sure that they sized every one of these items to match every other. No single item in the list above will present a disproportionate restriction in comparison to any other. If you don't believe me, consider this; your throttle actually works....(!) The throttle is the primary air flow restrictor in the engine, all the way up to very high throttle openings. If the throttle body size was over-specified then at medium throttle opening the engine would no longer be air flow limited, and the remaining travel on the throttle would do nothing. And if your throttle is throttling the air flow then your ports can be as big as they like, because they won't have access to any more air. So why do we think that just changing number 7) will suddenly release a gazillion horse power? It clearly won't. With the mentioned supporting mods you can address 1), 2), 7), 9), 10), and 11). Reducing the losses in all these areas is what yields your 15% bhp. And there may be another few bhp by continuing to work on those (for instance, my exhaust clearly has lower losses than anything you can buy off the shelf). But you are still left with 3), 4), 5), 6), and 8) holding you back. If you could address these other five as well then you might start to get somewhere. But that's a purpose built race engine, from the ground up. And you'll still be constrained by engine architecture (capacity, compression ratio, chamber shape, e-shaft strength, rotor clearances). And you would need associated fuelling and ignition mods. Ultimately we are probably always going to be limited by what we can do with the side exhaust port. That is probably our ultimate "throttle". What is that limit? I've no idea. Maybe it's 215bhp.....

I should address efficiency at this point. One reason for porting is to try to increase the volumetric efficiency at specific revs (and maybe make it worse at other revs). The problem with this is that the Renesis is already above 100% volumetric efficiency when at peak torque. It does drop a small amount above about 6000rpm, but not that much. So porting can help high-rev volumetric efficiency a bit, but it can never be night-and-day because the Renesis is already very very good (it's an inherent rotary strong point).

I should also address lightening and balancing. This adds no power. Let me say this again. This adds no power. It can make your engine smoother. It can help your tuned engine survive at higher revs (if it can breath at higher revs). And it can help low speed acceleration a little bit (reducing inertia is analogous to reducing the car mass). But it adds no power.

It seems to me that lots of people are jumping on the porting route at the moment, but that the main effect is simply to recover some of the power loss resulting from using older housings. As a consequence, people are getting stock power plus the associated problems of high idle speed, poor low speed running, and having to run a high rev limit.

Let me stress, I'm not saying "don't port". Porting is clearly a small benefit if you are chasing power. And there are limited opportunities for chasing power with the Renesis. And modding cars is one of the basic human rights, protected by UN decree* But I'd love to see a more holistic approach to tuning. I'd love to see someone go for it. Take on all 11) items, and see what's really possible with a Renesis. But I suspect it would cost more than a single turbo FD swap, and still give half the power. :( It would be cool though.

Let the debate begin. And let's wait for the first person to say "yes, but my engine still 'feels' faster"..... or "yes, but my engine still 'sounds' faster"...... or "yes, but my engine now 'zings' ". They might be good reasons for doing it, but they are not good reasons for claiming that porting adds power.
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Re: Is porting for power a myth?

Post by Conan » Fri Nov 09, 2018 11:19 am

My engine is faster ;)

I will expand my post later :)
Last edited by Conan on Fri Nov 09, 2018 11:46 am, edited 1 time in total.
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Re: Is porting for power a myth?

Post by kopite72 » Fri Nov 09, 2018 11:21 am

it's not so much the power it adds Pete,its the driveability,The car is sooo much more responsive and just fizzes when its half or full bridge ported.It's totally worth doing,in fact once you compare it to the stock engine its only then you realise how laboured the stock engine really is.

No you wont get big gains with a port but you get pure driving pleasure,just ask Dan,its barely run in and he's grinning like a Cheshire cat.Plenty of cars out there with big power that are pretty rubbish to drive.For me its not about the power,its all about the driveability
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Re: Is porting for power a myth?

Post by SeriousSam » Fri Nov 09, 2018 11:37 am

The main difference in feel on my engine comes in at 6500 RPM, where there is now a transition which never existed before it was ported. I'm happy to put down any gains in the low end and mid range to the fact that it's been rebuilt, and I'm sure those gains will drop off as the car ages and the seals continue to wear. That said the upper end of the rev band is different, delivering an extra shove which never existed before the rebuild. Nothing else in my engine, intake or exhaust has been changed, so I know that the stage 2 porting does add something extra.

Having said all that, I'm also happy to admit that the power gain is minimal, and the only reason I had it done was that there was no reason not to - it needed a rebuild anyway, so an extra bit of cash on top of the rebuild cost for a small gain seemed like a decent idea. I'm glad I did it, but if the engine lets go at some future point (or I get bored and decide to chase bigger power) then I'll be upgrading to an FD lump or one of RRP's PP turbo engines. Realistically that's the only way to go after significantly bigger figures.
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Re: Is porting for power a myth?

Post by cib24 » Fri Nov 09, 2018 11:50 am

I said it in a couple of other threads but the exhaust port will always be the major limiting factor in the renesis. Swap the motor for an older 4-port or 6-port 13b with peripheral exhaust ports and watch your power climb, even without adjusting and tweaking all of the other components in the system to maximise efficiency.

Anyway, do we have a nice comparison chart of someone with a street port or bridge port compared to a standard ported car just to see the difference in power and torque under the curve? Peak power and torque might not be all that different but perhaps there are 10% gains under the curve at various places?
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Re: Is porting for power a myth?

Post by warpc0il » Fri Nov 09, 2018 11:56 am

We shouldn't be focusing on peak power but "area under the curve", and therefore higher power and more torque around the revs that you're more likely to be driving the car.

All engines are designed around a long list of criteria; including power, reliability, noise, economy, drivability, and meeting regulatory emission requirements.

It's this latter thing that's the most artificial and results in all modern engines having a dip in the power just at the point that you really want a hump, just because that's where the bulk of the type-approval emissions (both exhaust and noise) are measured.

While some of the bureaucratically enforced craziness can be addressed by a re-map, I believe that this can be further enhanced by an appropriate adjustment to the porting. This would result in the subjective reports of "it pulls stronger" though these should also be supportable from dyno curve results.

I, and others, would really appreciate true back-to-back analysis of ;
- a standard ex-factory just run-in engine
- a standard high-mileage (but undamaged) engine
- a blue-printed just run-in engine
- a blue-printed plus street ported just run-in engine
- a blue-printed plus fb ported just run-in engine
With all else being equal, sufficient to prove the benefits (and losses) right across the load/rev range.

Repeating the process with a good bespoke remap rather than just the generic out-of-the-box, one-size-fits-all factory map.

However, it's just not going to happen, as no one has the resources to replicate this sufficiently accurately.
Picking cars that most closely fit the above would give (has given) some approximation but there are still too many other variables from car-to-car.
Dyno results are also known to be fickle, even engine test beds without any transmission etc can be impacted by changes in barometric pressure and humidity, which is why manufactures have full climate control in their engine labs.
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Re: Is porting for power a myth?

Post by Dr. FrankenRex » Fri Nov 09, 2018 12:00 pm

I'm sure most people who have read or will read this on here have seen my thread about my build, but I have to say that I agree with what PeteH has said mostly. Realistically the power gains from porting aren't huge. I will give an honorable mention to a well ported 192, though. I never dyno'd my old 192 before or after, however that had a large street port and the 0-60 time as read by the OBD was the same as a 231 after the build (it had a 6 speed box fitted so the engine was the only differentiator) - long story short, if you have a 192, port it.

That said, if an engine is being rebuilt anyway the drawbacks of porting are generally small for any potential benefit they may give. Ignoring my experimental porting, a full bridgeport engine is reliable enough (Adam Galtress' has hit 30k for example) and the idle speed can be as low as ~1200rpm, so not as much of a problem as people may expect. MOT time may be tricky, but that's another question and not something that is really a part of this discussion.

The other thing is that Mazda haven't designed everything to match everything else perfectly. To say so is an assumption. Mazda built the renesis engine very well given the brief that they had, but they had strict targets to hit for efficiency and for emissions - restrictions that are (maybe) less important to someone rebuilding their engine.

If you look at well built bridged engines that have been dyno'd (ones without other issues) the whole torque curve has been bumped up across most of the rev range. For something with minimal drawbacks, both financially and reliability wise, I'd say do it.

The theory of Pete's post is bang on, but there are dyno printouts that give tangible gains and the old reliability argument isn't applicable now building has got to a point that it's nice and strong.

When you factor in the experimental stuff that's on my car - which will be refined and improved over time - then I'd say porting is still a worthwhile consideration. The main thing, though, is that owners need to have a realistic expectation of what it will deliver - it's not going to make it a 400whp monster. There's modest gains to be had, but they are gains - and nowadays without the drawbacks people assume they will have.

TL;DR - Porting is a good thing if your engine needs a rebuild anyway, but be realistic about the power expectations :thumleft:
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Re: Is porting for power a myth?

Post by warpc0il » Fri Nov 09, 2018 12:09 pm

cib24 wrote:
Fri Nov 09, 2018 11:50 am
I said it in a couple of other threads but the exhaust port will always be the major limiting factor in the renesis. Swap the motor for an older 4-port or 6-port 13b with peripheral exhaust ports and watch your power climb, even without adjusting and tweaking all of the other components in the system to maximise efficiency.
It's curious that people think that it's the exhaust port that is the limiting factor in the Renesis, compared to earlier engines, when Mazda made a big thing about the "RENESIS an exhaust port area twice that of conventional designs" Okay, the exhaust path may be more convoluted but the area of the port is critical factor.
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Re: Is porting for power a myth?

Post by warpc0il » Fri Nov 09, 2018 12:19 pm

The closest comparison to porting a rotary engine is fitting a piston engine with a performance camshaft.

In both cases you can modify the timing, duration and flow (to some extent) of the charge into and out of the combustion chamber.

Manufacturers choose the valve timing, lift, etc of their cams to meet a wide variety of requirements and engine tuners have been replacing these with "performance cams" for nearly as long as the internal combustion engine has been made - with a wide range of successes and failures.

No one has ever said "performance cams don't work" but most people are prepared to accept that the wilder the cam, the more compromises there are outside of the area of benefit.
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Re: Is porting for power a myth?

Post by PeteH » Fri Nov 09, 2018 1:12 pm

warpc0il wrote:
Fri Nov 09, 2018 12:09 pm
RENESIS an exhaust port area twice that of conventional designs
Do you believe that Dave? I know it's from an official Mazda document, but visually that statement doesn't make sense to me. A peripheral exhaust port that was only as large as a single Renesis exhaust port would (I'm guessing) be a very small exhaust port. I'd like to see some numbers. I've got the area of the Renesis exhaust ports, so has anyone got an FD housing and a ruler lying around? It almost looks like Mazda said "there's 2 ports instead of 1, so that makes twice the area".
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Re: Is porting for power a myth?

Post by PeteH » Fri Nov 09, 2018 1:26 pm

warpc0il wrote:
Fri Nov 09, 2018 12:19 pm
The closest comparison to porting a rotary engine is fitting a piston engine with a performance camshaft.

In both cases you can modify the timing, duration and flow (to some extent) of the charge into and out of the combustion chamber.

Manufacturers choose the valve timing, lift, etc of their cams to meet a wide variety of requirements and engine tuners have been replacing these with "performance cams" for nearly as long as the internal combustion engine has been made - with a wide range of successes and failures.

No one has ever said "performance cams don't work" but most people are prepared to accept that the wilder the cam, the more compromises there are outside of the area of benefit.
Agreed, but we have a fairly fixed relationship between the port timing and the port area. The maximum area is basically defined by the point at which they open, the point at which they close, and the geometric constraints of the rotor path itself. These are all independent variables in a piston engine, as is the valve size (I'm not saying there are no constraints with a piston engine, but they are less). The net result is that rotaries generally run much more conservative port timing than most piston engines, and our only resort is to introduce lots of overlap.

Even the 26B runs less aggressive port timing that a typical Ford BDA.

Porting does work, just like changing your cam works (or fitting bigger valves), but our constraints are so much more restrictive that we just don't have the scope that the piston engine tuners have.

I'm not being anti-rotary here. With the MSP Mazda did something quite special. I'm just suggesting, as Dan says, that expectations should be managed if people choose to modify.
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Re: Is porting for power a myth?

Post by PeteH » Fri Nov 09, 2018 1:28 pm

kopite72 wrote:
Fri Nov 09, 2018 11:21 am
it's not so much the power it adds Pete, its the driveability,The car is sooo much more responsive and just fizzes when its half or full bridge ported. It's totally worth doing, in fact once you compare it to the stock engine its only then you realise how laboured the stock engine really is.

No you wont get big gains with a port but you get pure driving pleasure, just ask Dan, its barely run in and he's grinning like a Cheshire cat. Plenty of cars out there with big power that are pretty rubbish to drive. For me its not about the power, its all about the driveability
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Re: Is porting for power a myth?

Post by kopite72 » Fri Nov 09, 2018 1:41 pm

Isint that what it's all about at the end of the day? I'll leave all the technical jargon to those that know ( or pretend to) for me and I'm sure the vast majority of people on here it's like I said, the driveability that matters, in fact it's all that matters....
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Re: Is porting for power a myth?

Post by warpc0il » Fri Nov 09, 2018 1:50 pm

The trouble is that the butt dyno is very easily fooled.

Increase the level of noise = more power
Make the power delivery much less smooth by actually removing power at particular revs, and the resulting recovery will seem like more power.

Don't get me wrong, I totally recognise that drivability, including the "fatter" mid-range power, can be very real and can make a massive difference to the driving experience; as I'm convinced that I see from my Revi Ram, though others disagree.

Now if you always drive the same route and know that the maximum speed you can hit by a specific marker is usually N, and after a modification you're now consistently hitting N+10 at the same point then you really have more power.
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Re: Is porting for power a myth?

Post by kopite72 » Fri Nov 09, 2018 2:22 pm

Problem is that you could FB 10 cars and assuming they all put out the same HP ( I know I know but humour me) you'll probably have 10 different opinions on how they drive.

It's entirely subjective and so imo the power is much less important as opposed to how the damned thing feels. Nice to have 220 horses under the bonnet and also nice to say you have but much more important for me is how it feels
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Re: Is porting for power a myth?

Post by Dr. FrankenRex » Fri Nov 09, 2018 3:47 pm

PeteH wrote:
Fri Nov 09, 2018 1:12 pm
warpc0il wrote:
Fri Nov 09, 2018 12:09 pm
RENESIS an exhaust port area twice that of conventional designs
Do you believe that Dave? I know it's from an official Mazda document, but visually that statement doesn't make sense to me. A peripheral exhaust port that was only as large as a single Renesis exhaust port would (I'm guessing) be a very small exhaust port. I'd like to see some numbers. I've got the area of the Renesis exhaust ports, so has anyone got an FD housing and a ruler lying around? It almost looks like Mazda said "there's 2 ports instead of 1, so that makes twice the area".
The Renesis engine has 4 exhaust ports (if I recall, as I'm 99% certain that there's 2 on the centre iron...?), all of which do add up to quite a significant surface area.

If you compare that to an RX7 engine (as pictured below) then I can fully believe that the total combined surface area of the RX8 exhaust ports is greater than a 7. That said it does completely ignore the fact the exhaust gasses have to do a hideous 90 degree turn etc. which impacts the flow.


Image

Port size is not directly proportional to flow in this example.
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Re: Is porting for power a myth?

Post by PeteH » Fri Nov 09, 2018 4:33 pm

The other complication is that the FD port opens to full area very quickly, stays fully open for a long time, and closes very quickly. I think the Renesis port opens and closes much more slowly, and the full port area is only available for a short time. I'd need to do the calcs to prove this, but I suspect it's true.
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Re: Is porting for power a myth?

Post by 13Black » Fri Nov 09, 2018 5:07 pm

From my bewk that graphically shows port area on y-axis with crank degrees on x-axis, dis is fact ^
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Re: Is porting for power a myth?

Post by ChrisHolmes » Fri Nov 09, 2018 5:51 pm

Conan wrote:
Fri Nov 09, 2018 11:19 am
My engine is faster ;)

I will expand my post later :)
Depends an the definition of "faster" I suppose but if you can post your dyno graph perhaps all would be clear.
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Re: Is porting for power a myth?

Post by 350matt » Fri Nov 09, 2018 6:18 pm

I think we've already have a sniff of the potential here, as V8 power 's car with the Peripheral exhaust port and renny inlets is promising to make some large numbers and there must be others already out there

yes the renny has more area but as Pete has alluded to, the opening times / time to max flow is very long and the flow path is very convoluted so all that area isn't working that hard when its only exposed for a fraction of the time required to get all the gas out

to my mind fixing the bad exhaust port flow is the key to getting more poke here

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Re: Is porting for power a myth?

Post by ChrisHolmes » Fri Nov 09, 2018 6:24 pm

The views and experiences posted so far are varied and I am sure many more will follow.


I am not going to offer any opinion on the attached graphs but comments are more than welcome.

My original engine ran out of breath at 87k and just would not start. I had an angine lying in the garage, like you do, which when stripped was found to have good condition housings, irons and bearings so after a though cleaning and extensive measuring to confirm bearing and rotor clearances I set about my home cut set of RB Street Ports and I assembled the engine with all new springs, oil seal rings, side seals, corner seals, Goopy Apex seals, rotor bearings and every joint and O rings possible including for all the external items.

Once assembled the engine was fitted by Rotary Motion with a brief to swap all the ancilliaties from my engine over onto the newly built unit.


So the only assumption in all of this is that my old engine and the rebuilt engine being standard were the same specification and had the same tolerances. As I have lost my paperwork with all the measurments on, much gloom for doing that, I cannot measure the now stripped old engine to compare.


Some Dyno Graphs are attached showing that pre death my original engine produced 169.7whp and the rebuilt engine after 5k 172.8whp a whole 3.1whp gain BUT the torque has dropped from 126.3lbs ft to 116.8 lbs ft which we think may be due to ignition issues. With 4 year old 585s???
Attachments
Street Port RR Hub Dyno.pdf
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Street Port Horsham Rolling Road Dyno.pdf
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87k engine RR Hub Dyno.pdf
(1.36 MiB) Downloaded 13 times
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Re: Is porting for power a myth?

Post by ChrisHolmes » Fri Nov 09, 2018 6:26 pm

350matt wrote:
Fri Nov 09, 2018 6:18 pm
I think we've already have a sniff of the potential here, as V8 power 's car with the Peripheral exhaust port and renny inlets is promising to make some large numbers and there must be others already out there

yes the renny has more area but as Pete has alluded to, the opening times / time to max flow is very long and the flow path is very convoluted so all that area isn't working that hard when its only exposed for a fraction of the time required to get all the gas out

to my mind fixing the bad exhaust port flow is the key to getting more poke here
I think you will find that Grahams engine is peripheral inlet as well as exhaust
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Re: Is porting for power a myth?

Post by RobinPZ72 » Fri Nov 09, 2018 8:13 pm

Wankels need YPVS or Exup system!!!!

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Re: Is porting for power a myth?

Post by 350matt » Fri Nov 09, 2018 9:26 pm

ChrisHolmes wrote:
Fri Nov 09, 2018 6:24 pm
The views and experiences posted so far are varied and I am sure many more will follow.


I am not going to offer any opinion on the attached graphs but comments are more than welcome.

My original engine ran out of breath at 87k and just would not start. I had an angine lying in the garage, like you do, which when stripped was found to have good condition housings, irons and bearings so after a though cleaning and extensive measuring to confirm bearing and rotor clearances I set about my home cut set of RB Street Ports and I assembled the engine with all new springs, oil seal rings, side seals, corner seals, Goopy Apex seals, rotor bearings and every joint and O rings possible including for all the external items.

Once assembled the engine was fitted by Rotary Motion with a brief to swap all the ancilliaties from my engine over onto the newly built unit.


So the only assumption in all of this is that my old engine and the rebuilt engine being standard were the same specification and had the same tolerances. As I have lost my paperwork with all the measurments on, much gloom for doing that, I cannot measure the now stripped old engine to compare.


Some Dyno Graphs are attached showing that pre death my original engine produced 169.7whp and the rebuilt engine after 5k 172.8whp a whole 3.1whp gain BUT the torque has dropped from 126.3lbs ft to 116.8 lbs ft which we think may be due to ignition issues. With 4 year old 585s???
Hello Chris

don't suppose you have the data in an excel format so we can plot it all up on the same graph?
and is this SAE corrected in all cases?

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Re: Is porting for power a myth?

Post by Velocity_Dan93 » Fri Nov 09, 2018 9:58 pm

Mines quite a old FB now but made awesome power back in the day and still does. Id like to think with a very few small (and expensive) modifications I could breach the 220 mark without taking the engine back out. Porting makes a good difference, but then I see people with a FB running a standard header, gutted decat and oem inlets wondering why they don’t make 200whp. Most people’s perception of a ported engine comes after a rebuild. The difference would be huge from a low compression engine to a fresh one with a port. Funnily enough a few times now I’ve been in completely stock rx8s and to me they felt like they were street ported. Only to find out later it’s a low mileage engine with high comp.
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