Power Query

[Megaman]

Question for the techies out there...

There are a lot of people who modify their cars putting bigger turbos on and exhaust systems and so on. Is there a way to keep the power that the car has now but extend it on the revs. At the moment it comes in at 3-4krpm and dies off just after 6k rpm. Can you change something in the engine to make it gain or maintain its power futher up the rev range without it dying off?

I thought the difference between an STI PPP and a WR1 of the same age was that the WR1 revs higher. But I dont see the point of keeping it in gear longer if the power is dropping off..

[John Mc]

Add a supercharger

[Megaman]

top idea - can i get one at halfords?

[John Mc]

You could 'adapt' a Dyson and sellotape it to your engine!

[CAVEMAN]

Now a small disclaimer before I start spewing possible bull shit here.....

The revs and power curve of an engine is pretty much a combination of a few different things...

Firstly the ability of the engine to rotate at the required rpm, so balancing the overall displacement (bore and stroke) in the given number of cylinders to have the minimal amount of rotating masses thus limiting the internal forces involved and the engineering to keep it all together. This is why generally a large cc motor has a very low max rpm, as the cylinder capacity has to be quite large in both bore and stroke. But say an F1 engine spreads the 3l over 8 cylinders, and the stroke is very very short. So during each revolution the piston doesn't really move that far of a linear distance, meaning that it can rev like a b*stard. The higher the revs, the more power you can make...however torque on an F1 car is relatively low but it doesn't matter so much cause they weigh so little.


and secondly the VE of the engine at that rpm. So the ability of the design to suck air in, compress it and add fuel, ignite it and spit it out again...all at this higher rpm whilst still making a decent enough amount of bhp and torque.

Going back to the good old days, the normal way to change your power/torque curve was to slap in a new set of cams. This gave you a better optimised profile for high revving engines. Meaning the timing and lift of the valves were such that at this higher rpm range, the engine would happily be able to suck air in, and spit it out again even though it was rotating faster than originally designed to do. Ofcourse it then ran like a bag of shit on idle and up to say 4k rpm, but after that wahoo...on cam and off you go.

Then the VVT came in, to then optimise VE (volumetric efficiency) for both high and low rev ranges. Most using oil pressure to force the engine at a given trigger point to use a 3rd middle high lift cam profile during high rpm's. Honda VTEC and Renault are the ones I know the most about.

Now, given that you want to keep the engine internals standard, I would assume that you can remap it to give you the max power higher in the rev range, but this would effect your torque curve too...as you are essentially delaying the power delivery until a higher rpm is reached...then the engine won't be producing as much torque as it should and your power/torque curves would be very much reduced and spread over a narrower range.

What you are suggesting is keeping that torque and power the same but extending the range it is effective for. To do this, I would assume that you need a greater supply of air to the engine at higer revs to maintain its VE. So the turbo is the obvious component to change. You need to keep the balance between air in, mix fuel and air out. So a change in turbo alone will not give you a lot because all this is doing is increasing your ability to give your engine more air...you then need to be able to squirt in more fuel - need bigger injectors and more fuel pressure. After that you then need to get rid of the air more quickly too, so better exhaust.

Fooked if I know what this all means....but either way I don't think a simple re-map with standard internals will extend it a great deal. Just better optimise what you have already.



Now - where's my pint....

[John Mc]

Just bolt a hoover on

[WR1 Bro]

Great post from The Captain.

So then Mega, what do you think?

[Megaman]

Nice post Caveman. I was wondering if doing something simple to the engine would allow it to rev higher. My mechanical skills come from playing Gran Turismo on the Playstation where doing things like lighter flywheels, ported heads (whatever that is), uprated spark plugs and injectors seem to make it keep the revs for longer.

So my theory was focussing on the engine itself rather than changing the exhaust and turbo around it. If its a case of the turbo running out of steam then thats fine. Just wondered why people dont look at tweaking the engine to rev for longer say 9k rpm based on the fact you wont be putting any more power through it crippling reliability. When people talk of standard internals and bottom ends, is it always the better route to keep standard internals and bottom end and to throw bigger exhausts and turbos at the car rather than strengthen the main heart of the engine?

Just wondered what the costs would be as most people dont think twice about putting new exhausts and turbos on to imprezas spending 3-6k quite easily.

[CAVEMAN]

I think bolting on new exhaust and turbo's etc.. are the usual starting points because it is easy to do even for a home DIY'er. Undo old bolts, take part off...bolt new part on...remap and hey presto.

Actaully balancing the engine spec as a whole involves a lot more choices and give people a headache just thinking of the possibilities.

I would assume the starting point is how you want the car to behave or drive to suit the application. Then build a spec based on this so that it will be used in the right way to minimise the risk anything of blowing up on you.

You can get compressor maps for turbo's to give you an idea of airflow and spool up - this data can then be used to configre the internals to cope with the forces involved and get a feel for your power curve/delivery.

Its a system overview rather than a stack of individual parts...but as the stock unit could take up to say 450 bhp, the weakest link right now is turbo, injectors and TMIC. Hence why these are changed first, then more power requires internals to be upgraded.

I am not an engine builder or even claim to be a 'tuner' so don't really know what to change if anything for you to get what you need. I am just an engineer that knows the theory of combustion...once you understand the air,fuel, spark variables...the rest is pretty simple physics.

[Megaman]

So when people say the engine is good for 450bhp then you have to uprate the internals, what do they uprate and by uprating are you simply strengthening everything by increasing various tolerances or by uprating do you gain some more power?

[CAVEMAN]

Uprating or building a 'forged' engine is basically replacing the major internals to cope with the increased forces you are likely after. I mean why would you want to build a new engine if you are not after more power. The built engine doesn't give you more power as such, but it will be better able to withstand the bigger bangs you need to get more power.

So the more common parts are the ones likely to undergo the increased stresses. So pistons, conrods, crank and sometimes the main bores and block - so going to a CDB too. Changing and porting heads would only give you better breathing so you can get more air into the think to give you a bigger bang.

These are made form a better material and undergone manufacturing treatments like queching or drop forging. Making them super strong and having a way higher yeild stress than the OEM units. These parts are also lighter, better balanced, tighter tolerances and sometime have a optimised design for better power too.

Like the piston crown can be a different profile to help with the combustion and burn cycle. This helps burn more of the fuel/air mix to give you a bigger bang even though the actual volume is the same.

I am not sure of the actual limit of the OEM engine, but I have heard it is around 400-450bhp, but I am sure people have squeezed more out of it, but for how long I am not sure.

The old cliche, a chain is only as strong as it weakest link. If you blast lots of air and fuel into a standard unit, eventually something will go pop. So make it strong to cope and you'll be OK.

[Micktype WR1]

Cheers for the good read fellas.

[Foley]

Great info caveman, Enjoy reading your posts

[Megaman]

So from what you have said the WR1 has 316bhp, but has a safety tolerance rating on the engine of 450bhp.

How come when all these tuners quote you for a power upgrade that they quote a turbo, exhaust, injectors, intercoolers, clutches etc but none of them seem to ask you to uprate your internals. In theory if you get a power upgrade to 400 to 450bhp then wouldnt you want to uprate the internals to cope with 500 to 550bhp to build in some safety tolerances. Surely when giving the car some stick a car rated to 450bhp running at 450bhp will not last very long?

I suppose also that other components have their own ratings too like the gearbox, what would that be rated to? the clutch, the driveshafts, even the differential must all be "power rated" with some kind of strength rating. When planning a power upgrade that you want to be reliable why dont people check the ratings of the OEM installed subaru parts and then uprate the other parts to be in line with their exhaust / turbo style bolt on power upgrades?

I know if i was to invest a lot in a power upgrade that I would want to ensure it is reliable in stop start traffic just as well as being reliable on the track or whilst being driven in anger. I would be cautious of driving a 450bhp modified wr1 to its limits going by what you have said so far. Is this theory wrong?

Keep the interesting posts coming Caveman, its a good read...

[CAVEMAN]

Well, I can't really talk from a massive amount of experience of 'tuning' subaru's.

I know the theory as it was my previous job to analyse this crap for an ex-top level motorsport team - as part of a driving simulator supplied to them.

So what you are saying is absolutely correct. Any given engine has the potential for more power, how much you can get out of it and how long it may last is anybody's guess. Even the best tuners would not be able to 100% guarentee the lifetime of even a hand built engine. The most highly strung engines are those of the drag racers in the US...these are re-built not between each race weekend, but between each run!!

If you really want to put your mind at rest then yep, increase in power by whatever means you see fit - new turbo, new fuel etc.. then it would be wise to uprate every component in your drive train too. So internals, cooling, injectors, fuel pump, oil pump, gearbox, driveshafts, diffs. The list is pretty much endless and you can go on to replace every part of the standard car with an upgraded item. I am sure there is nothing that can't be upgraded.

However, Subaru made several models and luckily the model we have is pretty strong as standard. I know of several rally cars using standard items and the only bits changed are the clutch and suspension. Beleive it or not these cars can take one hell of a pasting!

If you are going to go mental then the starting block is the Spec C, with closed deck engine block and revised intake maifold, twin scroll etc, etc... but even the standard 6 speed unit can take a lot of torque through it - but can get a dog gear kit for them or even straight cut gears for even more durability - but sound like a bag of spanners at idle.

The driveshafts can be changed for GKN units, but I haven't heard of any major failure with the standard units unless you really go silly.

I guess it all comes down to budget, and what your personal prefferece is...more power with risk on standard internals....or do a forged engine too but add another 6-10k to your budget. Or go the whole hog and upgrade everything.

Its no womnder people take the easy route to push it upwards, then spend extra cash on the internals when they have too. I would rather do it all in one go to be safe.

[WR 1mposter]

Agreed with Caveman, far cheaper to do it in one hit, as I have found out over the years, also make sure you use someone who is reputable not some knobshite as I found out

[Megaman]

Okay so if you get a new exhaust, turbo, injectors, fuel pump and get the car up to around 400bhp. Would it be the case that the parts that are more likely to fail can be replaced when they fail for example a clutch burns out. Or when putting that kind of power through the car would you expect something in the engine to go wrong having a knock on effect to a lot of other components.

It sounds like a lot of people take the gamble when they get 400bhp conversions then that the internals will cope okay for a decent length of time. Having said that people love to boast about their cars and power gains but rarely come forward with any problems stemming from such upgrades i find...

I suppose the other thing you can give some input on is when is the best time to modify a car. Would you say a car with 50,000 miles on the clock is way too old to modify reliably or 100,000 miles..what is the rough guide?

I know when i got my GGR 300 conversion on the ST that they suggested running the car in first and getting 3000 miles on the clock before uprating any power.

[CAVEMAN]

I would always try to do as much as possible within the budget you have...so rather than blowing it all on the easy stuff, spread it around and try to do strengthen things too. I would assume 400/400 is a relatively safe bet with standard internals providing the car has a good history.

If something goes, then ofcourse you run the risk of damaging other items...such as a conrod or piston coming through the casing or bending a few valves. However the whole thing is effectively tied together with a rubber belt and how often do we panic that this might break?

I guess the time old question of giving a new engine death from day 1 or getting some miles under its belt before giving it the beans is personal choice. I know of many instances where a new build is given full treatment right from the off - good ol rally days again. Then others that prefer to put some mileage on them.

I would guess that 99% of all people prefer to put miles on them to be safe - as it their new pride and joy and want to do right by the dealer and warranty. I don't see it like that - give it full on attack and if something happens then it is still gets fixed for free

The main benefit of those miles however is a proven benefit. To get those miles then you have to start and stop the engine - unless you put it on the rollers in 1 hit. That means that the engine and all items get to go through possibly hundreds of heating cycles. So the new bare metal parts get warmed up, get cooled down and so on. This then will get them all into a fixed state of being....so a lot more mechnically and thermally stable.

Once you know it won't blow up on you, then you can give it death.

[Megaman]

So what did you do to your car in the end to get some more power from it. I thought you were running 360 to 380 horses on the standard turbo?