BluRR

iTrader: (1)

I have some uncertainties I would like to get cleared up from the more informed ones among us about boost, octane, fuel and porting and the rules of thumb which many live by here.

Rx7club.com single turbo rule of thumb #1
"Dont run more than 15 psi on your single turbo FD on 93 pump gas (without auxillary injection)"

Most cars here with single turbos are at least streetported and lets say may around 400 rwhp at this level . If I have a stockport engine does this same 15psi limitation apply to me as well? As with a stockport the car would be making saying 350rwhp. Or could the boost be turn up on the stock port on the same pump gas to like 18psi to give the same 400 rwhp and still be walking on the same margin as the streetport at 15psi?

What "imposes" this "limit" of safety?

What about fuel consumption? Does it essentially take the same amount of fuel to produce the same 400rwhp on a streetport at 15psi as it does on stockport on 18psi? So that comparing both tunning maps will essentially show similar or close enough injector opening time say 6ms +/- .3ms (assuming same tuner and same injectors).

Lets idealise everything so the external situations (i.e. outside the combustion chamber are identical, such as big turbo, such as a T-78, T-51r or gt-42, etc so that there is no appreciable excessheat generation at this boost level.

killahrx7

i know people say that the limit on pump gas 93 octane is 15 psi, but i have a friend with a fc who has been running 17 psi over a year now making 430 hp with no problems. i personally think you can go further than 15 psi what has to be done is make sure your engine and air temps are on the cooler side. i don't know if that answers your question as i'm no guru here. btw you know jamal?

BluRR

iTrader: (1)

its not really about if the actual limit is 15psi or not becuase it will vary from setup to setup and external conditions, engine health etc. I guess its really about compensation and equating for porting.

As for Jamal, no i dont know a Jamal

Chaotic_FC

I'm pretty sure Busted7 is running 20psi on pump.

manveru

busted7 IS running 20lbs on pump 93 and has been doing so for the past two years, reliably. BUT, his car is a freak and he has gone over EVERYTHING. There isnt a single part of the car that he hasnt upgraded in some way or another.

When you're a crazy old man, you can get away with that kind of ****. He's also not running AI, and you can hold your hand on the intake manifold after back to back dyno runs.

Dudemaaanownsanrx7

iTrader: (23)

Busted also has lower compression rotors. that makes a big difference. There are a couple others that have been running over 20 psi, one is even on the stock twins. But it all comes down to what you want to risk, and how solid your whole setup is. If you're running a stock intercooler even 12 psi would probably be pushing the limits. And yes the 15 psi conservative limit is regardless of porting. It's about the pressure inside the chamber not actual power or fuel consumption.

Everything plays a role. If you can guarantee that every tank you fill up is going to be true 93 octane and you will never have a bad tank of gas then you could probably run higher pressures daily. But it's all unknown, so to reduce the risk we tune conservatively. If you want more power then you start pulling the safety margin out, maybe it will last just as long, but can you afford it if not? That's what you have to ask yourself. How much is the extra power worth to you?

The thing about gasoline or really any fuel, is the higher the pressures the easier it is to auto ignite. Think about a diesel engine, it doesn't use spark plugs to ignite the mixture just combustion pressures. Raising the boost effectively raises the combustion pressures. This also generates more heat. Heat is the another thing that makes gasoline want to pre-ignite. So if you throw in a hot summer day a crappy stock mount intercooler without a duct, a stock radiator that isn't up to cooling the engine, a tiny exhaust housing that keeps the EGT's hotter, and now you decide to crank the boost up. Compare that to a properly intercooled car, a matched turbine housing, a big radiator, extra oil cooler, some heat shielding this car will have a much better chance of surviving higher boost. Add some WI to increase the knock limits of the gasoline and you're solid. The further you can reduce temperatures and extend the knock limits of a fuel, the more reliable power you can make.

Busted7

^^^^^^^^Very good points compression,heat,fuel consistensy.

The fuel i use is sold by a friend no ethanol and tested monthly, but on the highway i leave the right foot in safty mode!

Houstonderk

iTrader: (3)

THis is why I find my setup freakin weird. I have the high comp rotors from the s4 n/a with a t67 and run 91 octane, but I run around 25 psi. I was trying to figure if my exhaust downpipe was making the boost raise to much and loses power.

BluRR

iTrader: (1)

Whats makes a ported car make more power than a stock port car at the same boost on the same turbo?

classicauto

iTrader: (3)

If a streetport is done right it will. But most stock ports make plenty of power, and usually in all cases more torque, then a SP. Some streetports may actually LOSE power.....its all about how the setup works as a whole.

Dudemaaanownsanrx7

iTrader: (23)

i'm not sure what you're asking or saying? It sounds like you're saying your setup is running high boost and high compression but makes less power.. ?

.



The reason porting makes more power is it allows more air into and out of the engine. Just like a larger cam on a piston engine. This raises the volumetric efficiency or VE of the engine. The higher the VE the more power the engine can make especially at higher rpms, sometimes at the cost of a bit of low end power depending on the porting. Thats where so many people get confused, they think that psi or boost pressure makes the power its not the pressure its the amount of air that makes power. Imagine a 1" water hose at 10 psi compared to a 12" water hose at 10 psi, which one will make a bigger puddle in 10 secs? Thats what porting does.

gusfd3s

at a certain engine speed on stock ports the time the ports are open will be to short for it to fill the "cylinders" effectively, VE.

by reading the torque curve you will see where the highest VE occurs (max torque).

by porting the engine you are increasing the time the ports are open (port timing), allowing them to breath more effectively at higher revs = your torque will still be climbing for a couple of hundred rpm more, here you have your power gain.

With bigger port area you will lose low-end power because of lower gas velocity - this is the downside.

Houstonderk

iTrader: (3)

Well its kind of a question as to the flow. I've been trying to figure out if that would cause that much psi without much difference to power. I was wondering if the downpipe gets changed from the 2.75 that it is now to say 4", if the boost would go down and the power would be the same or even more since the bridge flows more air.

Indian

iTrader: (1)

very informative thread.

BluRR

iTrader: (1)

Ahhh...so now Gus and Dude are on my thought wavelength. I am not a rotary genius, but my suspicions as to what happens with a ported appear to be similar to what you guys are saying. In my layman's terms this is how i saw/see it.

For identical situations, with the only difference being the port on the motor (i.e. same turbo, same boost) we dont even need fuel or tune in this idealisation i am going to present. So you have an engine attached to a turbo or a pump maintain a constant 15psi and the engine itself is being rotated by a motor thru the eccentric shaft at a constant 6000 rpm.
My assumption is that up to a certain pressure at 6000rpm the combustion chamber during a single open to close (from seal 1 to seal 2) will not actually inject enough air to stabilise the pressure between the intake line and in the chamber itself. so that if you could actually test the pressure in the chamber during that intake stroke you would get less than thats what would be measured in the intake. So with a bigger port opening it allows more into the chamber during that very minute time the port is open, simply because it has a larger opening and the same concept of the 1" hose and 12" hose as Dude said. Is that correct?

If so then it would mean for a stockport engine at 6000rpm and 15psi you would inject X lbs of air during that intake cycle, while for a streetport/bridge port, etc you would ingest say 1.4X (hypothetical) at the same rpm and boost.

So now to maintain the same AFR of 11.7 (hypothetical) for both setup (reintroducing the fuel into the equation now) you would need more fuel to with the ported engine over the non-ported engine.

Now is where i need Howard Coleman to chime in give the explanation and quantification of energy released from the combustion of X lbs of air and Y cc of fuel.

So lets say that X lbs air + Y cc of fuel in combustion gives Z horsepower.
and 1.4X + 1.4Y = 1.4Z (HYPOTHETICAL).

So that means stockport engine at 15psi is ingesting X lbs of air and Y cc of fuel to give Z hp
and streetport engine at same psi is ingesting 1.4X +1.4Y = 1.4Zhp.

The question now is..why can I turn up the boost on the stockport engine to say 18psi so that the engine now ingest 1.4X lbs of air, tune it to flow 1.4Y cc of fuel to give me somewhere near 1.4Z hp and still be safe?????????????????????

I know some1 is gonna say that the air gonna be hotter because of more compression, but one of the very first assumption from the first post is that its large turbo so the heat difference between 3psi is almost negligible. So can any1 shed some light there?

Dudemaaanownsanrx7

iTrader: (23)

The pressure inside the combustion chamber is greater with higher psi regardless of how much air is actually there. The lower the octane of the fuel being used the more prone to preignition it is at higher pressures. The higher pressures will create more heat inside the combustion chamber regardless of the original intake temperatures. This is the second area where the fuel starts reaching its limits. The hotter it is the more it wants to ignite when its not supposed to. Its the pressures not the total amount of air and or fuel. Though extra fuel is used to help cool things down, which is why as boost pressure goes up afr's are richened up. Its also why you don't tune for different afrs between a ported engine and non ported

BluRR

iTrader: (1)

So...I was under the impression that once that apex seal closed the chamber at the end of the intake stroke, the pressure outside the chamber in the intake no longer has any effect or relevance on the air inside the chamber, as now its a fixed mass of air in a closed system thats gonna be compressed. So regardless of what pressure it came in at, the mass of air and the temp is what counts, especially seeing that during the rotation of the engine the pressure it going to increase significantly during the compression stroke. So based on that line of though..if you can get in 1.4X lbs of air at 25*C in the chamber regardless of what pressure the turbo maintaining in the inlet tract..and provide the correct amount of fuel, I dont see why you would have increased risk of detonation

Dudemaaanownsanrx7

iTrader: (23)

Ok i see what you're saying now. I'm starting to get confused myself now. Idk the answer i'll have to think some more on this. Maybe some others can chime in.

arghx

iTrader: (3)

porting a rotary is like putting heads and a cam on an LS1: it increases flow through the air passages while changing the VE curve of the engine.

BluRR

iTrader: (1)

Do most ppl here really know what that means, or do we just regurgitate it when some1 asks? We know it increases flow and changes the VE curves. Thats taken from experimental data. Now lets move beyond that and start to analyze the data so we can know why it happens, how it happens, what is the likely effect of making a change to the system and at the end of the day, how to improve it all.

Dudemaaanownsanrx7

iTrader: (23)

Well its pretty straightforward how and why the porting works. It opens the intake sooner and holds it open later. Same with exhaust. Of course it depends which direction the porting goes. I think this thread was long past how and why porting works. But i'm just a bit hung up on whether a ported engines safe boost level on pump fuel is actually the same as a non ported engine. If air temperatures were the same and a ported engine moved exactly as much air at 10 psi as a non ported engine moved at 15 psi would it actually be safer to run... And if we now increase the boost to 15 psi and air temp still remains constant will this be more apt to detonate compared to the non ported engine because of the increased pressures from extra air being compressed? I feel like if the engine is breathing easier due to porting that it somehow reduces the pressure on the engine and this makes up for the increased air going into and being compressed. This is just a feeling though and i have no way to explain it with anything technical to back it up. The increased overlap would make the pressure in the chamber release sooner so maybe this is why the general 15 psi rule would apply to both. I'll continue thinking now.

gxl90rx7

iTrader: (1)

well we all know compression causes heat, too much heat causes detonation. all other factors constant, a ported engine will have a higher chance of detonation due to the larger amount of air being compressed in the rotor chamber. the more air your engine flows, larger exhaust, porting, the more susceptible to detonation it is

the limit is not 15 psi. I believe the limit is more dependent on how much air your engine flows, which is directly related to how much power it is making

this all assumes air temp, afr, and timing is the same between setups

killahrx7

i'm not sure if it's already been said but basically the limit depends on how cool you can keep air and engine temps; cause at 15 psi, the air is ok, at 18 psi it will be hotter. so if you can get the air to the temp it was at 15 psi then it would be also safe at 18 psi or whatever psi you want to run.

silverfdturbo6port

iTrader: (26)

i ran a turbo motor years ago with 9:4:1 compression boosted to 16 psi on 93 pump gas and i drove it so hard and no problems for months that was on my old fc.

Dudemaaanownsanrx7

iTrader: (23)

Air temperatures and engine temperatures are always a factor in the safely of any engine, but its been proven in piston engines for a long time that higher compression engines are at a higher risk of detonation. This is one reason why factory engines had to lower their compression when the fuel octane levels available started dropping after the muscle car era. It wasn't for fuel economy because a higher compression engine is actually more fuel efficient.
.
With this knowledge, compression and boost pressures still play a major role in an engine's resistance to detonation regardless of engine and intake temperatures. I have never heard of a larger cam being a factor in making an engine more prone to detonation, which is essentially what we do when we port our engines. Although that doesn't mean it isn't a factor, because i never really researched the effects of a larger cam aside from the hp gains. I will do more research and see what i find. - posted from mobile phone