<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beta13 &raquo;</TD></TR><TR><TD CLASS="quote">From what I understand you always make more power with more compression, you really want to run the most compression you can taking into consideration your tuning abilities. Under low compression you have a larger room for error, while tuning while in high compression you'll make more power but there is a not as much room for error in your tune.</TD></TR></TABLE>thats it right there

 

lower compression means you can run more timing and more boost on pump gas.

we are on the way to making around 600hp on a 7.5ish:1 2liter b16. we can boost around 22psi safely on just pump gas, on full boost we are using 50/50 water/methanol injection and pump gas. the car pulls strong as hell and it spools up good as well.

 

what do you consider high 10:4:1 through 11:1?

 

exactly! here is another take on that.

the guy who wrote this was an automotive engineer at porsche, sorry for the length, but it's a real good read on some of the tradeoff's encountered when looking for real power... pay attention to the part where he talks about adding timing to counter the lower static c.r.:

-------------------------------
Compression and Boost

Well..Here we go..I could get extremely technical on this subject but, I dug up some old graphs and charts on a 2.3 liter motor we analyzed for turbocharging years ago at DME...I think you'll find it illuminating..

10.3 to 1 compression
92 octane
188 @ 6700
A/F 13.6-13.8
35 BTDC

Turbocharger installed..
10.3 to 1 C.R.
92 octane
236 @ 6500...Ping caused boost limiting to 5 p.s.i.
35 BTDC
A/F 13.6-13.8
Timing retarded to 30 BTDC
Richened mixture to 12.7-13.0
Allowed boost increase to 7 p.s.i.
279@ 6500

Further reduction in ignition timing in attempts to apply more boost became futile.
Exhaust gas temps rose dramatically with less than 2% per lb of boost increase.

Next we ran the motor on 116 octane sunoco race fuel...This is pretty amazing..

We kept feeding more and more boost and just kept making more and more power..Finally we had to install secondary injectors and change to a loose scroll huffer...We were stunned..

From 7 p.s.i and about 280 h.p we were able to pump in 23 lbs of boost and made 522 H.P.!! This was with 32 degrees of ignition..

Next we rebuilt the motor with 6.8 to 1 slugs
The curve below boost was lazy so we remapped the ignition curve to feed more timing down low maxing out at 35 degrees again...

Now we could feed 33 p.s.i. into it and H.P. shot up to 639@7700.....

Someone want to educate me as to how their high C.R. turbo motor makes more h.p.?
I'm all eyes and ears...Go for it.

Well...I'm tired folks, still jet lagged and this screen is playing with my eyes so, I'll post some more on compression ratios and other ways to extract more power from our little air cooled wonders soon...

Joe
Moore Racing/
Moore Part Source

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">Originally posted by Marty:

#1 - what type of motor was this?
#2 - with the 10 to 1 motor, what made you stop at 23 pounds of boost?
</TD></TR></TABLE>

Hi Marty...
The motor was a 2.3 litre Ford. A highly modified SVO series motor. The same basic motor Roush was modifying for use in closed course road racing. I believe his motors eventually produced 740/750 H.P. No question. Those guys are wizards.

The reason for dropping C.R. at that point Marty, was we just weren't making anymore power. We moved the ignition curves around, fattened up the mixture and still couldn't add any appreciable manifold pressure without ping. Once we had to start backing off on ignition, our EGT's started rising. We basically hit a ping wall.
At those levels of boost, r.p.m and load, motors scatter before you can audibly hear any pinging going on. Matter of fact, in the best of circumstances, harmful detonation may be occurring at a frequency which is inaudible.
We used a Siemens hi-low amplitude sensing unit. Back then these were rather esoteric devices. Todays OEM applications have a far higher resolution, and the inevitable technological advances in all areas of the automotive spectrum has resulted in the availability of simple, yet very effective knock sensors to be made available to the end user for, in some cases, less than $150 bucks..

Joe
Moore Racing/
Moore Part Source

 

and if you do the math on it:

10.3:1 @ 23 psi = ~26.4:1 effective c.r.
6.8:1 @ 33 psi = ~22:1 effective c.r.

remember that the thermal efficiency limit of all gasolines is ~17:1 c.r.

the 10:1 motor had over 4 points of extra *effective* compression, but that did not make any h.p. at all!! it was completely wasted... all it did was stress out the motor for nothing.

boost volume was the only thing making h.p., once those motors got over ~17:1 effective c.r.

now how relevant is all that to a 91 octane street motor, lol :-0

 

what about in the middle of those 2 CR?? 6.8:1 is so low and seems like 10,3:1 doesn't make any power compared to the 6.8:1

anyone here can explain this to me?

btw, isn't the 10,3:1 spools up the turbo faster??? the max power appear probably sooner, wich make better acceleration, wich would be the point of getting a motor built.

a dyno chart on each engine would have been appreciated, max rpm too, torque, need more infos please the next move i'll do will be getting custom piston, so what CR would be the best

 

sorry for confusing the issue... no dyno charts, unfortunately.

what is your priority for the car, both of those are kind of extreme:

best gas mileage and off-boost performance? use higher static c.r.

or really big power? use lower static c.r.

is it a daily driver? will you be taking it to the track? what octane do you have in your area?

are you going to work on it yourself? do you mind working on it a lot? etc.

 

ok, here's what I want, I'm going with AEM EMS (already bought) and I want to have like few stages

first would be a low power for daily driving/trip and less than 5psi rev limiter at 7Krpm or less
second would be higher psi about 10 higher rev limit, this one would be used when I want to have some fun in street (maybe the most used stage)
third, about 15psi but with e-cutout open and if needed propan injection or methanol
fourth, drag, all power I can get

octane, I can have access to 93 octane, or more if I go to the track wich isn't that far away from here.

i'm building the engine from a to z, swaping it with some friends, for the tuning, just make some base map by a pro, and work on it after that, and no I really don't mind putting a lot of time on it!!

 

well, it looks like fuel mileage isn't a concern :-) me either.

have you read "forced induction performance tuning" by a. graham bell? copyright 2002, it's the best turbo book on the market... i looked up his recommendations for street cars, it seems that he likes from 8.5:1 up thru the mid-9's, because the off-boost performance makes it more fun as a daily driver.

maybe the next step would be to figure out what you would like your max h.p. to be? that guy who posted about looking for 600 h.p. with his ~7.5:1 c.r. is gonna have to upgrade the drivetrain for drag strip use.

if you want that kind of power on a street car, his c.r. recommendations are pretty realistic.

 

I wonder how many people out there are running 10:1 c/r with 91 octane at 1 bar of boost. How about you(danimal)? Have you seen it?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by MIKES &raquo;</TD></TR><TR><TD CLASS="quote">To me 10 to 1 is fine provided you have some sort of boost retard device in your car. The key to success is tuning. Another thing to keep in mind when building your setup is, what is the maximum octane level at the pumps in your area. I run Sunoco Ultra 94 in my area so that helps me out alot as opposed to some areas that only yeild 91 octane at the pumps. Cause lets face it, race fuel is damn expensive and usually gets thrown in a street car once in a while.</TD></TR></TABLE>

I think that if you're even concerned with compression, then you need to have a good way to control timing. That's not even optional.

The fact of the matter is, with compression as high as 10:1, you will have to retard the timing a LOT under WOT to keep knock away. After a point, you're going to start losing power because of this, even though the compression is higher.

Tuning a motor ALWAYS is a search for the best compromise.

 

i ran 91 octane on a bar of boost, at about ~7.3:1 c.r., but it was not a water-cooled motor... that was kinda too low c.r., but air-cooled motors can't be pushed as hard.

if you look at the overall history of porsche turbo cars, including their fastest turbo'ed race cars, you'll see low static c.r.'s pretty much across the board... like, starting in the 6's and 7's(!) way back in the day.

i always look at what factory street cars are rated... a perfect example is the 2004 wrx sti, which is 8.2:1 c.r. at about a bar of boost, and it makes 300 h.p. out of 2500 cc's, 91 octane minimum:
http://www.cars101.com/subaru/....html

look at the evo8 factory spec on that page... 8.8:1 c.r. at 19 psi(!!) very radical for a factory street car... the effective c.r. is ~20:1!

no 10:1 static c.r.'s there... look at the power xtreme car that was in drag sport recently... 9:1 arias pistons, 37 psi of boost, he was lighting 'em up pretty good in phoenix last month.

the problem with boost comparisons is that boost #'s are pretty meaningless, because it's a measurement of resistance to flow... it's kind of the opposite of what you really want, which is boost volume.

 

danimal, why would I want good mileage on a high powered engine, i'll just need to go more often at the gaz station

so as I can understand, a 9:1 would be the best for me, because on drag i want to pull around 5-600hp at the wheels on a h23 fully rebuilt (+2mm overbored, valvetrain changed, pauter rods,...)

 

remember that a set of pistons is a lot cheaper than a blown motor.

since you want to go real big on h.p., it's a lot safer to start off with 8:1 or less, then up it to whatever you want after your combo is fully dialed in.

that way, you'll have a big margin of error, and maybe you won't end up like those poor saps in the mag article i listed earlier.

and if you think it can't happen to you, then look at the august issue of drag sport... they blew up a 10:1 project motor when the boost got out of control, rebuilt it to 10:1, and blew it up for the second time, because of another boost problem on pump gas.

since you don't mind working on the car, i say be safe, and sneak up on the static c.r.

 

I think that's what i'll do and if not happy with the off-boost power, go with higher cr

 

I really wish there was an easy way to measure internal cylinder pressure when the engine is running. Sort of like an ongoing compression test. That would be way to gauge real combustion chamber compression.

Internal cylinder pressure = piston compression + boost (in volume, not psi)

The realworld cylinder pressure could be the same with low compression/high boost as it would be with high compression/low boost.

 

hey,
this is my longest running thread and i just wanted to thank all you guys who have contributed. every time i read the posts i learn a little more. thanx for all you help


Modified by Slomo_civic at 9:25 PM 11/6/2003

 

Why not just keep stock compression. Keep it simple. I see no need to raise nor lower compression. Unless someone convinces me otherwise, thats what ill be doing. If you're going to do something, i'd run lower though. But not much.

 

Thanks for clarifying some things danimal, awesome input. I've always been a fan of low CR, high boost. I think the general consensus on H-T is that low CR high boost is good for more peak HP but the higher CR motors with lower boost will have more low end HP.

Danimal, would you like to explain two things to me, one, what is thermal efficency limit? You said "thermal efficiency limit of all gasolines is ~17:1 c.r." Can you elaborate on that a little bit, i'm not sure what you mean by thermal efficency limit. Are you saying the compression is so high the heat isn't pushing the pistons down?

Second, in the post from Joe from Moore Racing, it talked some about knock/ping sensors. I'm going to be tuning my car myself, obviously dont wan't it to detonate, is there a good way to tell when you've hit the maximum effective compression limit of the gasoline you are using? I wan't to make good power, but i'm not an expericenced tuner so I don't know exactly how much timing to put in and I want to keep from detonating. Thanks in advance!

 

awsome thread... keep it going!

 

I did 8psi on 12.2 compression...It feels like taking off on a bike
I was on pump gas too 93 octane...the only catch is your rings wont last on high cr pistons

 

Next goal is 11.5 cr on 19psi on 93 octane

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by TURBO4drTypeR &raquo;</TD></TR><TR><TD CLASS="quote">Next goal is 11.5 cr on 19psi on 93 octane </TD></TR></TABLE>

guess you have money to burn

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by danimal &raquo;</TD></TR><TR><TD CLASS="quote">the 10:1 motor had over 4 points of extra *effective* compression, but that did not make any h.p. at all!! it was completely wasted... all it did was stress out the motor for nothing.</TD></TR></TABLE>
Are you just going by peak power #'s, or would a severely lowered CR produce better area "under the curve" as well?

 

I just think that this is good cr because if my turbo goes out for any reason, I still have a pretty quick car. I already drove NA on 8.5 cr for a week and it sucked *****