my 200mph rx7 build

[turbotoaster]

There\'s a fundamental flaw in your logic. There aren\'t that many constants, values you attain at 100mph will be different from those at 200mph.

x = power at y
If y = 4500rpm, the engine will produce x, the car will travel at 100mph
If y = 9000rpm, the engine produces 2x, the car will not travel at 200mph
If y = 9000rpm, and the engine produces 4x, the car will travel at 200mph, I believe.

If you make 100mph of 200bhp @ 4500rpm, you need 800bhp @ 9000rpm to make 200mph, just to overcome drag.

That\'s just a quick example of the logic. You can find the more complicated formula online and read about it,

Pd = Fd x v = 1/2pv^3 x ACd

Of course I could be wrong, tell me what you make of it though.

yeh im talking about gearing though mate, id be concerned if it took 200bhp to do 100mph!!

from what car manufactures have done to quote big top speeds in supercars is use big ovals, or that long straight for the germans.

This takes out the big thing of distance and from what ive seen, 500bhp will achieve 200mph, now thats with a very long distance and a quite aerodynamic car, ie ferrari etc.

the reason i need 630bhp peak power at 200mph is because i only have about 1.6-1.7miles of full throttle, about 31secs worth.

This is why gearing is so massively important to me.

So say i run a 4.1 diff, if i made say 700bhp at 8.5k but 630bhp at 9k and 9k was 200mph ill be ok, any less than 630bhp at 9k and im not gonna do it, ie if the power drops off to quick.

its alot harder to make 700bhp than 630bhp so i need to get my gearing right

I was gonna mention that but I didn\'t wanna seem too negative on the dude\'s thread.... I\'m pretty sure Bruntingthorpe have rather nazi noise restrictions, and have had for a long time. Bike magazines have been testing there forever, and the number of tests that read "we never got to do a top speed run because they threw us out for being too noisy as soon as the bike fired up" are countless.....

Already spoke to stav about this, we are sorting something out to cover us

[S3TH]

That\'s power though dude, not gearing. The OP was just talking about gearing when he was referencing that.

Yeah, just re-read, you\'re right. Just looked up the FD\'s coefficient anyway, min bhp required is 400 to overcome drag @ 200mph... so with 650 drag shouldn\'t effect the output. This looks interesting, though, someone needs to give him a cash injection or a loan... I want pics :Giggle

[lee mills]

Will your porting support that sort of power?
  I dont know enough about porting to answer but  from my own experience when my car was streetported it ran out of puff @1.4ish bar on reworx dyno and there was no more gains to be had.
 I didnt see those ports to be able to compare against yours.

[BlitzBoy]

As been said before, you can get huge power even from stock ports if you put huge amounts of boost in as shown in the US, Porting will help power be made with less boost(therefore less heat),help spool, and change where peak power is made.

[lee mills]

but this is the bit i dont understand hence my question.
If there were no more gains to be had past 1.4bar on a streetported motor then surely it should be worse on a standard motor?.
 I know you can up the boost to whatever you want(within reason) but whats the point if your not making any gains?

[BlitzBoy]

your gains will start to decrease after a set point depending on turbo size efficenency due to the compressable heat, you can help to expand the effeciency of the turbo with the fuel you use and if you run WI.
There are a couple of guys with stock ported engines in the US well over 600hp, one is running E85 (therefore combustable temps are lower) at 30psi, he cracked a 9 second run in his FD

[turbotoaster]

but this is the bit i dont understand hence my question.
If there were no more gains to be had past 1.4bar on a streetported motor then surely it should be worse on a standard motor?.
 I know you can up the boost to whatever you want(within reason) but whats the point if your not making any gains?

ports will help with spool, let you use a lower boost to achieve the same power and where you make most of your power.

The guy in the US was using an unopened stock port mazda engine running 32psi of boost on the bw s475 and he made 709 at the wheels, thats roughly about 700bhp fly for us. now he made that power at 8200rpm, im guessing that ill make it a little higher, maybe 8500rpm because im ported, on that turbo with a bridgeport engine on guy with an rx3 made 765whp at 9.5k so it does push the power band higher.

You will be suprised what you can squeeze into stock ports buddy, im guessing the 1.4 was on pump gas? because we are talking race gas or e85 here

As been said before, you can get huge power even from stock ports if you put huge amounts of boost in as shown in the US, Porting will help power be made with less boost(therefore less heat),help spool, and change where peak power is made.

spot on:Santa and beat me to it ya bugger!

[BlitzBoy]

Here is the famous stock ported FD , power dyno thread
http://www.rx7club.com/showthread.php?t=833684

And here is the tread when the same car did the 9 second run, awesome achievement, stock ports and gearbox!

http://www.rx7club.com/showthread.php?t=865403

[fil_auto]

totally different angle but thought it maybe of interest??!! or just the fact that it\'s an interesting vid with one achievement mind!

http://www.youtube.com/watch?v=p1cLfiVSyrw

[DRAGON]

Cool vid, thanks.
Remonds me the bonnet on the drag car lifted when we ran some big terminals, prob worth running 4 pins.

[turbotoaster]

thats a pretty cool video actually, shows how power can overcome aero, but thats a massive amount of power, he mentioned 1000bhp

for comparison scoot ran 204.6mph with 680bhp running a bonnet with holes in, a wing on the back and a rear diffuser, so overall more drag than a stock car, might be more stable but doing over 200mph down some back B road needs a bit of stability haha

[matt p]

Here is the famous stock ported FD , power dyno thread
http://www.rx7club.com/showthread.php?t=833684

And here is the tread when the same car did the 9 second run, awesome achievement, stock ports and gearbox!

http://www.rx7club.com/showthread.php?t=865403

Bloody hell that second link!

Those runs are somthing else

Good luck turbotoaster dude!, if you have rod on board thats a bonus too even though the rex isn\'t quite aerodynamically challenged as saph lol!!:spank

You have save big balls my friend maximum Kudos to you if you pull it off as i think it may spur more people into modifying there 7\'s and not blowing big cash monies in the process.

Matt

[Ed_RX7]

yes mate all true......what i should have said was restriction....exhaust side...housing and exhaust.

Not strictly true there Ed, the only thing that will help spool is changing the actual A/R (eg decrease in Size) not decreasing the size of diameter of the exhaust. Decreasing back pressure on a turbo engine will increase top end power. To explain what will increase spool I shall quote a response from a geek I found, which explains it quite well.

"the restriction must be the turbo itself. The turbo utilizes the pressure differential between the manifold and the downpipe for its energy source. putting a restrictor infront of the turbo will reduce the pressure differential of the flow after that restrictor and the downpipe. thus actually hurting performance.

this is why turbo\'s use different turbocharger housing a/r ratios. The smaller the ratio the greater the restriction at the turbine nozzle and thus greater force applied to the turbine wheel.

If you want an every day phenomenon to impress your brain, go get your garden hose and turn it on without a nozzle.... spray a wheel of a bicycle that is upside downwheel, and see the top rpm you make. then put a nozzle on the hose and look at your new top rpm. try altering the nozzles spray pattern and thus pressure and see what changes it makes.

if you want your turbo to spool sooner you need to meet any or all of these criteria.

1. produce MORE total volume of gasses infront of the turbo, thus building higher pressures because the nozzle will only flow so much for said pressure.

2. mechanically lighten the turbo so less force is needed to move it.

3. alter the nozzle characteristics so that for the same flow a greater pressure is created and thus a greater force impacts the turbine. ***


The problem with altering the A/R to a smaller number is the restriction in the turbo gets greater. Thus you may spool the turbo faster but your total flow is more limited and thus hurting horsepower."

[turbotoaster]

this i found online and i think its a really good explanation

Here we go:

This will be a long one, so get something to drink, something to eat, and settle down for some science.

Lets get some vocabulary out of the way or you won\'t be able to follow anything I say....errr...write. Do you know the difference between an inducer and an exducer? Well, take a look at the compressor and turbine on a real turbo. Funny looking fins aren\'t they? Not the same size all the way down the blade from tip to base. There is a reason for this. Each stage of the blade (small section vs big section) has a special purpose.

First lets look at the compressor wheel and figure out how it works.  

The inducer part of of turbine is at the end of the shaft and can be seen by looking into the intake of the turbo compressor housing (looks like a fan). The blades that you see there extend into a larger diameter at the other end of the turbine. This is the exducer stage. The inducer on the compressor turbine is responsible for generating the vacuum at the compressor housing inlet that pulls air into the compressor. The air then "rides" the fins towards the exducer stage, which is a larger diameter, and gets sling-shot towards the outside of the compressor housing. The housing collects this moving air an expels it through the housing\'s outlet.

The size of the compressor turbine determines the maximum amount of boost that the turbo charger can produce. It also effects the spool-up time of the turbo. The type of compressor wheel is usually designated as its "trim", which is a value that describes the inducer and exducer sizes. Typically, the exducer is significantly larger than the inducer on a compressor turbine.


Next lets look at the exhaust turbine wheel and figure how the inducer and exducer work. And lets learn how the turbine gets moving in the first place. This will be important when we answer your question of making a small turbine and a big compressor.

The exhaust turbine also has an inducer and exducer, but because the exhaust turbine has the opposite function of the compressor turbine, the two are switched. The exhaust gasses are directed towards the outside of turbine through a nozzle. This is the inducer stage because it is the part of the turbine that collects the gasses. As the energy from the gasses is transferred into the turbine, the gasses slow down and exit the turbine through the exducer stage.

Most people think that the exhaust coming out of the engine smacks into the turbine and gets things moving. Like a pinwheel a kid is blowing against. The air smacks the blades and starts it spinning. Well, that is not quite the case in a turbocharger. You need to take into account the gas laws and how a gas behaves under heat, pressure, and volume. We know that a gas that is compressed heats up and a gas that is uncompressed, cools down. That takes care of heat and pressure. The volume of the gas is determined by the cylinders and other hoopla. So when the gas leaves the cylinder, it gets compressed going through the small exhaust manifold outlet. This is by design. You\'ll see in a minute. As the hot compressed gas exits the exhaust manifold, it enters the turbine inlet - a very small space. At this point, we have very high pressure and very high heat, so our gas has a very high energy level. As it passes through the diffuser and into the turbine housing, it moves from a small space into a large one. Accordingly, it expands, cools, slows down, and dumps all that energy - into the turbine that we\'ve so cleverly positioned in tho housing so that as the gas expands, it pushes against the turbine blades, causing it to rotate.

The exhaust turbine design is a balance between absorbing as much energy from the exhaust gasses as possible and allowing the gasses to flow as easily as possible. This is closely related to the size of the exhaust housing. A larger turbine can absorb more energy from the gasses and spin the shaft with more torque and speed, but too large a turbine will restrict the flow of exhaust such that engine performance is greatly reduced. Typically, the inducer is only slightly larger than the exducer on the exhaust turbine. Generally, you would want to stick with the stock turbine because it\'s size is not nearly as important as the compressor turbine\'s size. If you want to reduce restriction through a smaller housing, you can have the turbine "clipped", which reduces the size of the fins and allows more air to flow around the turbine.


Whew. Boil it down. The exhaust turbine and compressor turbine sizes are engaged in a delicate balancing act. The housing used on both sides plays an important role as well. After all, the housings are what make the compression and expansion chambers needed for manipulating the gas state. One reason for a large exhaust is by lowering the outlet pressure, you increased the pressure differential, and now the exhaust gas can expand more, and do more work. That increased work pushes harder on your turbo, and it spools up faster.

Just simply putting in a smaller exhaust turbine and a larger compressor will not reduce spool time. The amount of energy the inducer/exducer needs to get moving may be offset by the weight of the HUGE compressor wheel you are using on the cold side of the turbo.

Turbo charger work/design is a work of art.

[lee mills]

More important than all that opened the last link and was surprised to see my company name (LMC) associated with that speed run:chat:Laugh