AT Performance upgrade?

[pralognan]

My car has a Dali rice rocket exhaust and I plan to fit Top Speed headers. This should provide about 15 BHP.What is the next step? MT camshafts, or is all this ill advised because of limitations within the AT box?

[Kaz-kzukNA1]

Not much response so I'll try something.

First of all, the camshaft profile below VTEC region is the same between MT and AT engine. Purely from engine dyno point of view without chassis weight difference or gbox efficiency, the engine power and torque curve are similar or even slightly better with AT model below the VTEC region.

The max power figure is only relevant at WOT (full throttle, Wide Open Throttle) and if you are mainly driving on the street, 99% of the time, you won't be at WOT so modifying the engine hardware without clear objectives is not cost effective.

For the street usage, torque is much more important than the max power so it is important to decide on what is the main target for your modifications.


With the effect of the torque converter, AT model can be driven fast if you know its characteristic plus the extra weight involved (as many owners said, one of the best thing you can do is looking after your body weight - not saying you should and hope you don't misunderstand me as I don't know you by just the user ID ;):)).

Changing the final ratio can dramatically improve the acceleration without even changing anything on the engine side. Very popular modification in Japan although you may already have the later spec final ratio if you have the later AT models.


Back to the topic.
Modifying the parts around header/exh box area will shift the torque curve so you need to take this into consideration if you are installing the aftermarket header and exh back box. In order to let more air into the cyl, you need to first extract as much exhaust as possible out of the cyl so you actually need to consider the intake and exhaust system as a complete package but I'll leave out the intake side for this post.


I don't know what kind of exhaust box the Dali one is but if it uses big diameter with short pipe length and no consideration for the flow speed, you need to be very careful with the combined effect of the aftermarket header.
Depending on the diameter and length of the pipe or unless you have shutter valve or some sort of mechanism inside your exh back box, you could slow down the exhaust flow speed too much resulting in shifting the torque curve too far towards the higher rpm band.


Typically, I noticed big diameter header and exhaust on the aftermarket products manufactured in US as there is enough market for the FI application on NSX.


Slow exhaust flow rate at lower rpm band will result in too much loss of torque below 2,000 – 3,000rpm that is important for the launch.
You will feel very slow or sluggish on launch from standstill despite having extra HP by installing the aftermarket header (and the exh box although the majority of the increased power will come from the header).
OEM exh double pipe structure, diameter, lenght including inside the back box were fairly good balance between the requirement for the low/mid/high rpm bands.


This is especially important on AT model because of the extra weight and also you are normally letting the AT controller to carry out the upshift for you during the launch.


AT model is normally equipped with C30A 3.0L engine so wheel size is also important.
While having 18inch at the rear looks really nice, you will notice significant drop in acceleration compared to having 16 or 17inch at the rear. It's the effect of total extra mass of the wheel and tyre but on top of this, you are effectively reducing the final ratio by installing the larger diameter wheel making it slower.


A few AT model owners in Japan have replaced their engine with C32B 3.2L engine and regularly taking their NSX on track. They are aware of the extra stress on the multiple CL discs contained at each gears but also they try to control the upshift with their right foot or F-matic lever to prevent upshift while at WOT.
On street, normally you will start feeling CL disc slip contained inside the 3rd gear when the AT box is start showing the sign of fatigue.
Very important to at least replace ATF regularly to prevent blockage of pump strainer making the issue even worse.
It would be best if you can first test drive another AT model with no modification at all to establish the start line and then ideally, drive another one with similar modifications that you have in mind.


Good luck with your design process.

Kaz

[Rdevelopments]

Thats a lot of valuable info Kaz, with myself having to drive an auto because of a handicap it's a "mandatory thing, as obviously i would prefer to drive a manual. Few Questions;

- I have a 1997 F-matic, does this already have the "later final drive" (is there anyway i can check).

With my car i'm running the following modifications;

- Carbon fibre intake scoop with K&N panel filter
- NA2 Headers, Custom 200 cell Sport cat's, Advance Titanium exhaust

I've seen some Jun cams on Yahoo Auction (made for Auto) do you have any experience with those? Furthermore i know "Level 10" do a gearbox upgrade (500 RWHP proof) where they modify the friction material, the torque converter and valve body. I've been looking into this but the shipping more or less kills the deal.

I've driven mine on track with no issues (please note i'm no Ayrton Senna so could also be my driving). Only once the gearbox went in failsafe mode but that was because of a sensor giving a faulty signal (so the dealer said after checking fault codes).

- Bas

[pralognan]

Dear Kaz,
Thank you for your reply,I was beginning to think only Britlude and I had autos and no-one had any ideas! Driver weight is 75kg so may be able to save 20kg if I give up biscuits!
I will forget about changing camshaft.
My car is UK spec 1991 so has 4.06 final drive ratio.
Member "nobby" posted on 31/1/2013 about the "rice rocket". It is very loud and looks quite a neat ,simple design,probably lighter then OEM.
Britlude fitted "top speed" headers a few years ago and seems happy with them,so I will probably go ahead.
I have 15/16 OEM wheels,with a set of16/17 OEM also available,no plans to change to larger wheels.
I change AT fluid each year,but only 1/3 drains out. Is there any way to drain fully and refill with fresh fluid?
Any ideas about cost/practicality of changing to the later spec. final ratio?

[britlude]

someone mention me??? :)

yup, i've got the top-speed headers, do exactly what they say on the tin, and seemed to give better torque at lower revs... no solid numbers, but felt better after! there have been reports (on NSXPrime) of top speed headers hitting cross members/etc, but mine went on with no issues, maybe it was a duff batch. the price i got them for i did expect to have issues, but they fitted and cleared everything without any tweaks.

subsequently mine gained a home-brew exhaust but still kept the stock cats, just to make things easier at MOT time.

K&N air filter in stock airbox, and 17/18 inch wheels cos they look pretty! no idea what the diff ratio is, but its JDM stock.

i did read somewhere that you can buy the rebuild parts kit from level 10... obviously that means you have to find someone over here to rebuild the box...

[Kaz-kzukNA1]

Finally, managed to finish the service on the NSX so going to write some topics on this subject.

The OEM 4.4 final ratio on AT model was introduced from 95 (F-matic) on JDM with -A01 series countershaft and final driven gear around the DF but went back to the original ratio of 4.0 later so best to check through the parts system.
Interesting enough, just looked at the UK parts system and looks like it never had the 4.4 final ratio even at the time when the F-matic was introduced so it clearly depends on the country model spec.

In Japan, we have very narrow road with lots of traffic lights and stop and go traffic. The speed limit is just above 60mph even for the motor way (99% of them are toll road) and for the A-road, it goes down to 30 – 37mph so the average speed is very low. Addition to these and due to other factors, the annual mileage is also low that having 4.4 final doesn't affect much on the fuel cost.
On the other hand, almost everything is the opposite in UK. Much higher average speed with not much traffic jam (M25 morning traffic is nothing for me, it still moves) and much higher annual mileage so looks like Honda opted to keep the original final ratio for entire year models for UK market.

Whether you are going for the OEM or aftermarket ones, you need to be bit careful when changing the final ratio on NSX with EPS and all AT models came with it from the factory.

EPS controller is checking the VSS1 (the rotational sensor driven by the metal gear ring on the DF case) and VSS2 (differential input MPU, counting the pulse generated by the final driven gear teeth itself) and if the difference between the two goes outside of the specified window, it will trigger the EPS warning light (but keeps the power assist until you switch off the engine).
This only happens at really high speed range when using the 4.2 final ratio with the later spec EPS controller when Honda introduced tighter window (99 or 97, I forgot) so you are fine if your EPS controller is still the same as 95 spec (and very likely to be fine if using higher ratio such as 4.4).


I never dealt with Dali so don't know the build quality of your exhaust but on their website, they do show the comparison dyno chart for the torque and power.

Please note that unless you have proper engine stand dyno with driving environment controllers, the dyno figure from the vendors/tuners are just a reference and not absolute even measured on the same day on the same dyno.
For me, these dynos are more of diagnosis or tuning tools than power measurement device.

So, the direct comparison between before/after can't be done but from their website, it shows that you will loose significant amount of torque at lower rpm band (until about 3,000rpm, if we simply ignore the above facts and use the data from their website) so it's slowing down the flow speed too much.
Again, important to consider the combined effects of aftermarket header + exh back box as well as the effect on the intake mod.


For C30A, unless you change the engine hardware such as the cam shaft, there is almost no gain in the torque/power by modifying the intake design (inc. the TH body diameter) unless you can speed up the air flow.
The engine is simply not requesting additional air into the cyl (no bottle neck) even with the aftermaket header/exh back box.
OEM intake is enough including the air box and OEM TH diameter is just perfect for C30A from the driveability point of view.
TH is a power controller so you want the minimum TH diameter that outputs 100% full power only at WOT.
You could go with the larger TH diameter but with the same TH pedal stroke, you would achieve max power before reaching the WOT resulting in worse driveability. Also, you are slowing down the flow speed by using larger diameter.

You need to be bit careful when measuring the intake manifold pressure against the atoms pressure at WOT. Some dyno people claims that the TH body or intake system is causing the bottle neck if the two measurements are different but it's not that simple when it comes to the pressure within the flowing air before/after the TH butterfly even at WOT.
There are many SAE papers on how to predict the manifold pressure depending on the distance from the TH butterfly and if you look at our intake design on NSX engine, it has dedicated tiny chamber where the #4 MAP senser tube is inserted.


Personally, I will stick with the OEM air filter.
As mentioned above, you won't gain anything in power by using aftermarket scoop, filter, air box on C30A engine.

When you disassemble the intake manifold and investigate the VVIS plate, you will see much more sand like debris inside there with aftermarket air filter than the OEM one.
You may get louder intake sound though if that's what you are after....


The OEM scoop/snorkel design is very cleaver.
The air enters from the left side air opening and most of the debris and water that have some weight will be at the lower stream and being guided towards behind the rear wing panel.
Only the air from upper stream can enter the air box.
So, if you have aftermarket air scoop without any such cleaver method, you better check what's happening with the debris especially with the moisture on high speed long right corner/bend.


It's important to consider the entire system as a package and decide exactly how you want to design the power and torque curve considering the characteristic of AT driving style.
For example, AT 2nd gear covers wide range, unlike MT model you must upshift several 100s rpm earlier before hitting the rev limit due to shift lag, you need to understand the software oriented shift inhibitor at high rpm band on down shift, etc.


Most of the time when you attend the driving skill course, AT NSX owner performs better on consistent lap time than the MT ones.


Power is never enough... You just get used to it...
However, even after tracking at full speed with more than 550HP car and then jumping into the NSX cockpit on the way back on the public road, I feel the power is just about right for street usage but would love to have better acceleration with different final ratio.

Again, good luck with your design process.

Kaz