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Old 05-16-2008, 12:42 PM   #1
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Thinking out loud: 3.6L V6DI

So, I'm bored at work today - and I'm goin' through GM's powertrain site, reading up on the Direct Injection V6 we're likely to get. Mainly trying to figure out if it's possible to Turbo it.

Here's what I found:

The engine has a forged crank and connecting rods.
It has a fuel shut-off at 7000rpms.
CR is an astounding 11.3 : 1 (why is that astounding?)
Because it only requires regular unleaded - they don't even "reccomend" premium!!!

I'm no powertrain engineer...but does this engine seem to be built for Forced Induction, or at least "willing" to accept it? The innards should be able to handle it; and by simply using premium fuel; you should easily be able to push 5-7 lbs of boost into it.

Thoughts?
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Old 05-16-2008, 01:01 PM   #2
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A CR of 11.3:1 doesn't seem like its ready for FI to me.
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Old 05-16-2008, 01:18 PM   #3
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A CR of 11.3:1 doesn't seem like its ready for FI to me.
Agreed,

That is extremely high CR for blowers or turbos. You might get away with 5 pounds of boost, but I wouldn't go past that. For the money you'll spend on FI, I don't think you'll be happy with the end result. That high of a compression ratio doesn't really give you a lot of room to grow.

Now, don't get me wrong that's a great little motor.
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Old 05-16-2008, 02:20 PM   #4
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Can someone catch me up on what CR is, and what the numbers mean?

Thanks
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Old 05-16-2008, 02:35 PM   #5
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Quote:
Originally Posted by AirGoya View Post
Can someone catch me up on what CR is, and what the numbers mean?

Thanks
Compression Ratio
It's the ratio the air is squeezed when the piston compresses during it's upstroke. So, 11.3 : 1 means the air taken into the cylinder is compressed to 1/11.3th of the cylinder's volume. For Reference, that's higher than the LS7's compression ratio. An "average", or "normal" CR is anywhere from high 9
s to mid-low tens. 11+ is crazy-compression!

Which brings me back to my question. You guys makes sense, but doesn't Direct Injection shift our conventional understanding of CR's a little? That's where I was coming from: I mean, 11.7 with Regular Fuel!!! I thought that could mean with a tune, and premium fuel - one could get an extra 100 horses out of the little 6.
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Old 05-16-2008, 02:48 PM   #6
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Can someone catch me up on what CR is, and what the numbers mean?

Thanks
CR=Compression Ratio. A Compression Ratio of 11.7:1 means that the volume in the cylinder at the time when the piston goes from the bottom of it's cycle to the top gets compressed 11.7 times it's normal pressure while being ignited by a spark. A higher compression ratio means that more fuel/air are being compressed in a given volume and thus the resultant "bang" is bigger and more power is generated.



At least I'm pretty sure that's a simplified version of how to explain it. If anyone knows better, please educate me.

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Old 05-16-2008, 02:56 PM   #7
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Quote:
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Which brings me back to my question. You guys makes sense, but doesn't Direct Injection shift our conventional understanding of CR's a little? That's where I was coming from: I mean, 11.7 with regular Fuel!!! I thought that could mean with a tune, and premium fuel - one could get an extra 100 horses out of the little 6.
I think that the difference between the DI and other fuel injection methods is that DI is much more precise in the metering of fuel and the control of fuel and spark mapping throughout the RPM range of the motor. That's why, with the aid of better internals, stratospheric CRs can now be achieved. Mostly I think it's attributable to the precision of the system altogether. It's badass isn't it?!!!
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Old 05-16-2008, 03:07 PM   #8
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O ok, thanks both of you.
Dragon, you could be right about the forced induction idea. In the other engine thread about Gen V engines, asrapid mentioned rumors of GM making a turbo V6 with over 400hp. EXCITING!
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Old 05-16-2008, 03:12 PM   #9
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you do not want to run that high CR with boost. most guys i know who build FI motors swap stuff around and build about 7.5-10 CR (without the blower)to run anywhere from 22lbs to 6lbs (respectively)


running at 11.3:1 (which is higher than the 4th gen v-8 camaros stock) allows you to get more power from your gas.

in a way its like when the LT1 motors were introduced, the vette motors were around 10.5:1 and the maro motors were around 10.3:1 CR. they just about demanded premium (mid-grade was recommended) due to the high CR.

heres an interesting writeup about CR ratios and boost and octane
http://www.sdsefi.com/techocta.htm
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Old 05-16-2008, 03:33 PM   #10
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All LT1s in Y and F-bodies were the same CR to my recollection (10.5:1?.) The LT4 was higher (10.8:1?,) but LT1s were the same. The biggest differences were Y-bodies had four-bolt mains, and B-bodies had iron heads and cams I think.

I think they can get away with higher CRs on the DIs because it atomizes the fuel better which makes it more efficient by enabling it to burn more completely. I don't think it's FI-friendly, especially since the pistons don't look like they're forged. Maybe some spray, but SC and turbo seem to be out of the question.
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Old 05-16-2008, 04:11 PM   #11
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All LT1s in Y and F-bodies were the same CR to my recollection (10.5:1?.) The LT4 was higher (10.8:1?,) but LT1s were the same. The biggest differences were Y-bodies had four-bolt mains, and B-bodies had iron heads and cams I think.

I think they can get away with higher CRs on the DIs because it atomizes the fuel better which makes it more efficient by enabling it to burn more completely. I don't think it's FI-friendly, especially since the pistons don't look like they're forged. Maybe some spray, but SC and turbo seem to be out of the question.
yeah, the LT1s were 10.4:1 and the LT4s were 10.8:1 some f-bods ended up with 4-bolt mains left over from the vette production lines. and b-bodies did have the iron heads, but the reverse flow cooling was what really allowed the higher compression to be run. cooling the heads first then the block allowed for more controlled and constant temps which helps to prevent detonation which can be a biiiiiiiig problem when you start bumping up CR
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Old 05-16-2008, 05:59 PM   #12
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well there was already concept of v6 3.6 l TT in torana (it wasn't DI ) with around 380 hp. Now rumors on other forums are that present 3.6 l v6 can go up to 4.0 l and there is v6 TT in development which should come instead of new ultra v8 (for cadillac engine)with more then 400 hp in DTS/STS replacement. I don't know how much of this is true but i wouldn't be suprise if GM start developnig more and more smaller displacement engine with turbochargers. Even 2.8 l V6 TT (on e85) in saab aero x was developing around 400 hp.
I hope some of the rumors are true but we know there was also rumors about new ultra v8 engine and v12..and we know both of this engine was cancelled later on. So until we see it
And also when you have direct injection you are not limited (like in SFI) to inject fuel when(before) intake valves are open..you can injected much later (actually computer can determine when fuel is injected and it is not limited by time when intake valves are open) so you can avoid detonation which can occurre when fuel is injected at the start of compression(at higher CR).But offcours when you have 11 something compression ratio and you put turbo on that there are more strees to other internal engine parts..so i assume GM would lower CR before putting turbochargers.

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Old 05-16-2008, 11:30 PM   #13
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i'm hearing about how all this injection correlates to the compression ratios being so high, but the 1970's SS engine the L-78 and it had a carb on it... this engine achieved a compression ratio of 11.0:1 so what does the better atomization of the fuel have to do with the compression ratio?
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Old 05-17-2008, 02:47 AM   #14
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i'm hearing about how all this injection correlates to the compression ratios being so high, but the 1970's SS engine the L-78 and it had a carb on it... this engine achieved a compression ratio of 11.0:1 so what does the better atomization of the fuel have to do with the compression ratio?
I am no expert, but I believe the advantage of the DI is the placement of the precise quantity of fuel 'directly' into the cylinders by the injector, more so than the improved atomization. I am not sure how that will allow the old standards of lower CR for turbo to change....
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Old 05-17-2008, 02:53 AM   #15
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O ok, thanks both of you.
Dragon, you could be right about the forced induction idea. In the other engine thread about Gen V engines, asrapid mentioned rumors of GM making a turbo V6 with over 400hp. EXCITING!
As long as my turbo 6 has pull all through the power band, I'll take it any day. No lag....no time to spool up...I'll take it. 400hp?? Hell yeah...from a 6??
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Old 05-17-2008, 02:59 PM   #16
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As long as my turbo 6 has pull all through the power band, I'll take it any day. No lag....no time to spool up...I'll take it. 400hp?? Hell yeah...from a 6??
Imagine what you could do with some major modding... Remember the Supra? Can you say 900+ HP out of a low displacement V6?!?!?!?!?!?!
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Old 05-17-2008, 04:10 PM   #17
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Thanks for all the input guys.

There stands a good chance I get the V6 model over the V8. Not positive, yet - but the chance is there. So: I hope the aftermarket embraces this engine just as much as they do the V8.
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Old 05-17-2008, 04:21 PM   #18
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i'm hearing about how all this injection correlates to the compression ratios being so high, but the 1970's SS engine the L-78 and it had a carb on it... this engine achieved a compression ratio of 11.0:1 so what does the better atomization of the fuel have to do with the compression ratio?
I think it's mostly to do with the internals, but I haven't researched it much. Let me get back from Leave and finish my annual checkride studying and I'll do some research and get you a good answer. Until then, I can only give you the Joe Dirt Answer: "How's a rainbow made? How's the positrack on a Plymouth work?! IT JUST DOES!!!!" Hehe.

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Old 05-17-2008, 10:50 PM   #19
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Some of your thoughts on direct injection show good analysis- compression ratio is a volume related measurement, it is the ratio of volume remaining in the cylinder between when the piston is at the top of the stroke compared to the bottom of the stroke. In simple terms: If a cylinder contains 11 cubic inches at bottom dead center (the bottom of piston travel) and compresses it to one cubic inch at top dead center ( top of piston travel) then compression ratio is 11:1
All other things being equal then an engine with higher compression will yield more power and efficiency than one with lower compression. Several factors restrict higher compression ratios- primarily "Knocking" which can destroy an engine if not controlled. Higher octane fuel does not have more BTU energy per unit volume, but a higher quantity of antiknock additives which will control this damaging phenomenon. In the 60s some street strip engines had compression ratios up to 14:1. Ratios were lowered starting with the 71's as most of the early emission systems could not cope with the higher levels of oxides of nitrogen which tend to form when combustion temperatures are higher, as with a higher compression ratio. Modern combustion controls and catalysts have allowed compression ratios to be increased again.
Direct injection helps to control "knock" by being a finer mist which cools the intake charge significantly. Hope this helps- sorry about being so wordy.
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Old 05-18-2008, 12:36 AM   #20
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sorry about being so wordy
No! Thanks! Good information, there!
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Old 05-18-2008, 07:02 AM   #21
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just to bump on what frank said about the 60s and high CR's, dont forget leaded gasoline allowing higher CR's during that time.
but back to the main point of putting FI on a high CR motor is not safe or reliable on a regular production vehicle. now if you wanted to spend the money that goes into pro dragsters that run nitromethane and whatnot, thats a whole other story.

but for gasoline engines with a FI application, running anything over 7 psi on more than 10:1 (roughly) CR motor is asking for trouble.

however, diesel engines (semi's and large machinery) will run anywhere from 14-30:1 CR with FI with no problem, but thats also due in part to the setup of the motor itself, larger/stronger rotating assy with more cyl rings and the fact that the fuel is ignited thru self ignition due to the high CR and cyl temps. commonly referred to as "dieseling"
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Old 05-22-2008, 06:33 PM   #22
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CR=Compression Ratio. A Compression Ratio of 11.7:1 means that the volume in the cylinder at the time when the piston goes from the bottom of it's cycle to the top gets compressed 11.7 times it's normal pressure while being ignited by a spark. A higher compression ratio means that more fuel/air are being compressed in a given volume and thus the resultant "bang" is bigger and more power is generated.



At least I'm pretty sure that's a simplified version of how to explain it. If anyone knows better, please educate me.

-Tim

Dragoneye beat me to the punch... sorry for the double-tap!
Almost right, except for that line.
I know I'll sound like a nerd here, but Dragoneye is correct, it's the volume change that Compression Ratio refers to which DOES NOT equal an equivalent pressure change, temperature changes need to be taken into account as well.

Sorry about this, but thermodynamics says:
(p1)(v1)/(T1)=(p2)(v2)/(T2)
p=pressure, v=volume, t=temp, 1 is the state at bottom of piston stroke, 2 is state at top of piston stroke

therefor:
p2=p1*(v1/v2)*(t2/t1)
At an 11.3:1 compression ratio, that means p2=11.3*p1*(t2/t1)
and since t2 will be greater than t1(in compression), the final pressure multiplier will be more than just the compression ratio. I'm not too sure what kinds of temperature the air/fuel mixture would reach in the short time it's being compressed, but it will have some influence on the final pressure.

Again, sorry for doing this, but I just saw something that I figured should be explained, if for no other reason than so I can sleep better tonight

Have a good one.
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