13 replies to this topic

[22068]

If i was to give my MPI a port polish what sort of increase of power would this give me?
what other mods do you think would be useful to do?
also i have wonderd what the little black button is on the top right hand side of the engine bay? anyone got any ideas?

[minimoz]

not sure bout port polish,but that button i believe is the fuel cut off switch so you dont need to touch that!!!

[chappy]

yup,

its an inertia switch for the fuel pump so if you have a crash it stops fuel from goes everywhere. (also a good place to start if your car wont start randomly)

[Ethel]

the button is an inertia switch and it is designed to turn off the fuel pump if you have a bump. If the engine cuts out after a close encounter with a pothole or similar it's worth giving it a push in case it tripped.

Polishing with do nothing for performance but you can do a lot to improve the ports in an A series head. Tuning the A series by David Vizard is the book to read.

[mykweb]

that is a magic button....... its does Nothing.... unless you smack it lol then it cuts all the fuel off to the inlet...... not good well uless u have a bash.

MPi tuning.... expensive. I am planning on having a bash at modifying a mpi head. Going to buy a spare and replace the valves and springs.

Mike

[bluedragon]

I've read that a mirror polish on the intake ports can be a negative. A slighly rough finish is claimed to help keep the fuel droplets atomized in the mixture, whereas a mirror polish tends to allow fuel to "stick" to the port wall. I'd tend to believe this, if only because it means less work.

[Cooper1999]

Did the same thing myself - ended up with 74bhp - up from 60-63.

Here's my effort:
http://forum.minicoo...read.php?t=9731

[22068]

cooper1999 how come it made such a difference????
what mini you got???

[bluedragon]

Did the same thing myself - ended up with 74bhp - up from 60-63.

Here's my effort:
http://forum.minicoo...read.php?t=9731

Perhaps I misunderstood - to me, a port polish refers to the finish of the ports. You can't "port" an intake with just a polish job!
If one is talking about actual porting work (grinding away substantial amounts of intake material to improve flow) that's a completely different issue. But ported or not, many feel a rougher finish to the port is superior to a mirror polish, especially for a street car. If the car spends most of its time at high RPM, a mirror polish might be beneficial according to this theory, since the high port velocities help keep the mixture atomized.

[Cooper1999]

cooper1999 how come it made such a difference????
what mini you got???

22068, it's a 99 MPi. According to Neil Slark (whose rolling road I used after fitting) this is the kind of improvement (i.e. 20%-ish) that he would expect from a head/manifold change. Basically I produced something as good as bought off the shelf (for the spec) .

bluedragon, yes -you're right, a highly polished surface can be detrimental to the fuel atomisation - as I understand it a less than mirror finish does impart some turbulance which helps to keep the fuel in suspension. But if you look at the casting marks in the MPi inlet manifold for example, it's dog rough!
I'm suspect you would get very little improvement in just doing a polishing job on the head, but it's not too difficult to make decent improvements with a little grinding, and if you're going to the trouble of removing the head, stripping it down etc why not go the whole hog?
ETA - port velocities have almost as much to do with shape as size, as I understand it. All heads (i.e. road or race) would benefit from a high port velocity. That's where the skill of an experienced head tuner and the use of a flowbench, and lots of time/heads to experiment with, comes in. It's a balance between getting more air in (volume/bigger port) and velocity (smaller port/shape) - Vizards book explains it better than I can.

Question for the masses: - I've got my old MPi head and would like to build an injected, supercharged engine. If I've read vizards good book correctly, basically the same process applies to forced induction as naturally aspirated. Before I start gringding, is this your experiences?

Edited by Cooper1999, 01 October 2009 - 09:42 AM.

[bluedragon]

cooper1999 how come it made such a difference????
what mini you got???

I think 10-12 hp is a pretty consistent increase for a good port job on an MPI. The head is where substantial power increases will be found on an MPI. You can't really do much with the cam due to fuelling restrictions, and the intake/exhaust is actually quite good for a stock Mini - not a limiting factor until exceeding the 80hp range at least.

Most of the "Stage 1" kits bring about 75 hp on average (comparing from a 63 hp base), and it's mostly in the head. The so-called "90hp" kits rarely, if ever, deliver that much. I have read many people say the very best they'd ever seen from such kits was perhaps 85hp, and that was very rare. The vast majority of MPI uprate kits deliver in the 75hp range, perhaps 80 if you are really lucky. Fuelling is the limitation.

[Cooper1999]

Yeah, that sounds aabout right - 10-12bhp.
I had intended to use a 'magic pipe' on the exhaust and keep the standard manifold (which as you say is just about the best fitted as standard to a mini), but the cost of a maniflow lcb wasn't that much more.
Another lesson learnt was the 52mm TB - although I got it new, at roughly half price from ebay, I wouldn't advise fitting/buying onbe unless your plastic one is fractured.
With the MPi the ecu is the limiting factor (max revs, max fuelling), so yes there is limited gains to be had from fitting a cam without changing the ecu - this could be my next development (there's no easy 'plug-and-play ecu though, that I'm aware of). This was also the reason why I went for 1.3 ratio roller rockers, as opposed to 1.5:1's which can harm the emissions on some cars (but not others apparently ). This is really the only difference between a '90bhp' kit and where my car is now, but I really don't think fitting 1.5's would get me anywhere near an extra 16bhp! It might break 80, but at the expense of emissions.

So, 22068, what are you going to do?

[22068]

Yeah, that sounds aabout right - 10-12bhp.
I had intended to use a 'magic pipe' on the exhaust and keep the standard manifold (which as you say is just about the best fitted as standard to a mini), but the cost of a maniflow lcb wasn't that much more.
Another lesson learnt was the 52mm TB - although I got it new, at roughly half price from ebay, I wouldn't advise fitting/buying onbe unless your plastic one is fractured.
With the MPi the ecu is the limiting factor (max revs, max fuelling), so yes there is limited gains to be had from fitting a cam without changing the ecu - this could be my next development (there's no easy 'plug-and-play ecu though, that I'm aware of). This was also the reason why I went for 1.3 ratio roller rockers, as opposed to 1.5:1's which can harm the emissions on some cars (but not others apparently ). This is really the only difference between a '90bhp' kit and where my car is now, but I really don't think fitting 1.5's would get me anywhere near an extra 16bhp! It might break 80, but at the expense of emissions.

So, 22068, what are you going to do?

Well i already have the exhaust, manifold and air filter part of the stage 3 kit, so i am thinking about just getting the rest of it! but with the 1.5 rockers. Where can you get a rolling road tuning done in the midlands? also with the emmisions how would you sort them out, if they did become over the limit as such?

[Cooper1999]

Try here:

http://www.rollingroadregister.com/

Speak to therolling road people about what adjustment is available to alter the emissions - the car'll need adjusting back again afterwards to run as near to required fuel/air ratio as possible. Some of the kits had an uprated fuel regulator, which basically fooled the ecu (as I understand it) by suppling extra fuel I think by increasing the fuel pressure. This extra fuel spoiled the emissions. Not running the higher pressure regulator means the engine runs even leaner at higher revs.