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Old 03-14-2001, 06:28 PM   #1
98mpreziveRS
Scooby Specialist
 
Member#: 972
Join Date: Feb 2000
Post Boost and compression ratios

here we go again
on to pressures in the cylinders in forced inductions. When you deal with forced induction as the boost increases so does the power untill that magically cutoff point where the turbo or fuel can't keep up with the rest of the motor.

If you were to go get a 60s book on building motors it would be quite intruiging really. THey would normally suggest a compression of 7.0:1 if you run under 1 bar of pressure (2 bar for you that count 1 bar as atmosphereic pressure..although while running NA you are under vacuum most of the time ). The average car now will be running 8.5:1 due to the advancements in technology and such..BUT the big boys still tend to run lower CR's than the pocket rockets will be running. one reasib for this is the obvious intercooler. the IC actually allows a MUCH cooler charge than that of a non-intercooled setup whilst only loosing minimal pressure compared to the altertantive of playing with fire on high boost non intercooled motors . What the intercooler does is this, as the intake air passes from the air filter it will go into the turbo charger's compressor. While in the compressor it is compressed (or boosted) then pumped into the wastegate and IC (more on wastegates later). The intake charge will become hot as it is compressed. This is not from the turbo's heat..rather this occurs beacuse as you compress the air it will heat up. The intercooler uses cold air passing over it through the fins to the inside part of the IC. The air that is compressed gives off heat to the cooling air so the energy transfer happens like sodium filled valves in a way..the hot air is giving it's heat to the cooler air. The cooler the air the more dense the charge to the TB where it meets the fuel. Once the charge goes through the process of combustion it will go out the exhuast ports and into the downpipe. While the air expands if you will..it drives the turbine wheel on the turbo and this is the actual action that creates the boost (by spining the compressor wheel).
The 2 types of cores avail. are tube and fin which generally cost less and weight less..but they don't have the efficency of the bar and plate and are harder to modify the core size.
The bar and plate have a greater heat sink capacity by 35% and are easier to modify and reweld if you have to. the downside is the weight, higher cost and the size limits you have.

on to TE and forced induction still with me?
What is a common misnomer is that forced insuction affects the thermal efficency of a motor. This is NOT true. The TC or SC if you will, increases the volumetric efficency. The VE shows the engines ability to make the best of it's displacement. . When you have air going into the cyl. and is under BP, the higher quantities of air are packed into the motor than allowed when just running at vaccum or atmos. pressure

new school thoughts on forced induction are as follows... The majority of people today who suprress dett. are doing this becuase of intercooling, EFI, and some facny schmancy engine management software and retarding the timing from withing the cockpit dependant on boost levels, etc. with out this..we would still be running in the 7:1 range IMHO.
the basics right here... to think..maybe i just should type that instead of above
1. the forced induction systems out there ONLY increases volumetric efficiency not TE
2. concering boost and CR's is a fine subject here.. when you static model the boosted motors what you get is that for every 2.1 psi on average you get the CR rising by anywhere from .8 to 1.10 in points. But remember this is static using ethanol techniques so the real world IS different, based on pump gas, intercooling, turbo efficency, etc. etc. So if you have a 9.8:1 CR (the scooby EJ25 for my99) and run 1 bar of boost which is ~14.2 psi you will see your CR in the generally area of 15.48 or up to 17.61 or there abouts. depeding on Turbo IC, etc.


there ya go..LOTS more to come..my previous typed stuff had an error when i opened so i had to RETYPE it

that sums up my day pretty well

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Old 03-14-2001, 07:56 PM   #2
98mpreziveRS
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Member#: 972
Join Date: Feb 2000
Post

oh yeah..i will have more up tommorow when i retype it..i was biggie pist that i lost the file.. it was a good 5 pages long, size 10 font single spaced..now i have to retype it.. and i only have around 1.5 typed pages so far.. only 3.5 more to go
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Old 03-14-2001, 09:27 PM   #3
shadow
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"15.48 or up to 17.61"

jesus! that's freaking high as all get out!

what do you think are the limits of the engine? how much pressure do you think you could manage in that engine?
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Old 03-14-2001, 09:55 PM   #4
shadow
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Join Date: Mar 2001
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when I ask that question I mean dropping the temps, changing the heads, the cams, free flow exhaust, a good intercooler, how much boost/how much CR could I get?

another question! - using 8.5:1 CR pistons, beefed up conrods, closed deck (ala cobbtuning), the whole kitten-kaboodle etc; - how much boost / CR could I obtain?

[This message has been edited by shadow (edited March 14, 2001).]
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Old 03-29-2001, 06:58 PM   #5
98mpreziveRS
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Member#: 972
Join Date: Feb 2000
Post

sorry i never read your response..let me try to answer them as best i can

limits of the scooby motor Ej25..IMHO around 400HP before you need to close the deck and pull out all the stops. BUT i would not suggest shooting for those figures with the stock heads and pistons I would say go to an 8.5:1 or 8.7:1 cylinder if you really want to maintain good low end torque with high boost levels further up the rpm band

If you were to close the deck, go to a 7:1 piston, redo the heads, get some better valves with say a 3 angle job on them (i'll get into that later) among other things..i could see pushing 25+lbs through the motor. Mind you, this would be with EXCELLENT tuning and amateurs and those who don't want to have to run 100+ octane need not apply Or at the least 93+ octane boost and retarding the timing majorly..but that is another story

If you were to completely build the motor for boost..the only limit would be that of your tuning ability and the strength of the internals. BUT judging by cobb's work..i would say right around 20lbs with some good tuning shouldn't be that difficult. and if you plug in that boost level with that compression ratio..well..

let me do some figuring and get back to you on approx. HP
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