civicqm

I have been doing a lot of reading and talking to people about FI setups. I am in the process of building a new one and have reached a rather tough spot in the road. I was wondering what would be better; a b16 with 10.5:1 compression and about 10-12 lbs of boost or a motor with 9.5:1 compression and about 16-18lbs of boost? I have heard that a higher compression motor will act more like and NA car until the turbo reaches full spool and is much more pleasant to drive on the street because you are not frying the tires off when you reach boost. I know that boost controllers can be used to designate boost in each gear, but I am curious as to if anyone has a setup like this and knows the pros and cons of it? I notice that many b series motors are equipped with a 9:1 or a 9.5:1 compression ratio and was wondering if this is just cheaper, easier, or is it the most practical? So I guess my main question boils down to....What makes more useable power? A higher compression ratio or a higher boost level?

drvfast

The thing with the high compression motor is that with less boost you will acheive higher hp ratings. then the lower compression motor will need more boost to do so. yes u are right with the higer compression motor its like na without the turbo you don't get the bog type feel. the thing is the only time you will get a slugish feeling if the cr ratio is 8.8:1. 9.5:1 is perfect for turbo imo. i tend to like the higer compression motors better with boost. if i were do 10.5:1 but you have to make sure u have very good fuel managment and perfect timming because with the higher cr its easier to detonation.

Bspec18

I myself had the same decision to make. I didn't want to rely on just the turbo and I didn't want to be N/A at the light. So i went with 9:6:1 just to be in the middle i guess you could say, but theres still the option of bumping the compression a lil with a head gasket. Also you might wanna consider wat turbo your going to use so you know where your power band is gonna be. I'm using a garett T3 and i spool at 2200 and hit full boost at 3000

civicqm

Thanks for both of the replys. This is what I was looking for, I did not think about the idea of detonation with the higher compression. Point well taken. Also another thing I wondered is at what point do the pistons go from dished to a raised piston? An off the wall thing but do they make a dished 10:1 and above compression piston??

-iLLuZioN-B18C1

This is what i know.

if you decide to go with high CR, yes you will have better spool and everything that you mentioned, but at the same time you are making the engine do the work (and IMO causing more wear on the motor), and you will have to have top notch tuning with little room for error.. one tank of bad gas and your toast (well, maybe not with 10.5 but something like 11.1ish)


if you decide to go something like 9.1, you will make your turbo do the work, making it more efficient. yes you will have more lag, but its a lot healthier IMO for your motor, even with forged internals and all that jazz..all in all, lower compressino with higher boost is more practical.

this is what i would recommend you to do. i was thinking about doing a high CR turbo motor back in the days, but then changed my mind. let your turbo do the work!

either way, make sure you dont skimp on the fuel!! (injectors, pump, hondata or whatever you like)most important!

im not sure if this helps.

civicqm

Any more thoughts?? Race teams, engine builders???

st00pid

Boost = Torque.. No way to argue about that.. you can almost tell what kind of boost a person is running by their torque numbers.

its all pretty much personal preference however it seems that with the higher compression you lose ultimate hp on pump gas. My customers have been VERY pleased with the high compression/ turbo setups. I personally would go with 10.5:1 good low end, crisp throttle response.

thats my thoughts on it.

jason

Dee

High cr/low boost produces a flatter torque curve for better response while low cr/high boost you will get a peaky monster with more topend.

DeepSi

Damn this is a nice post I prefer the higher compression levels with less turbo. But after reading this i might like a slightly lower compression but still like the 10:1-10.5:1 compression area for better response and low end.

lazerus

i think a real good medium for high HP on pump gas is 9:1 C/r

if you want a peppy car with lower power ratings, go high c/r. but be prepared to do more tuning, and live more on the edge.

there is definately much less safety cushion the higher static c/r you go.

But they can be remedied with water/alky injection and or propane injection, but this takes extra effort to function properly.

I would say, if you want a really reliable, FWD street car with 325-400 HP on pump gas, i'd go with 9:1 or 8.5:1.

If you want a reliable, FWD street car with 200-300 HP, i'd go with a bit higher compression, and (of course) a properly matched turbo.

what would be optimum is to find friends who have done both, and ride (or drive if possible) in both of them, see what you like as far as lag power etc..

Traction issues can be solved with correct suspension setup, and a good set of tires, but of course the more power you make the harder it gets to put it toe the ground. Also a limited slip will help here, too.

Of course if you like best of both worlds, you can go with 9:1 C/r and go with a dual ball bearing turbo, and make some serious power, and still have decent turbocharger spool.

Brad

Dee

The days of sub 9:1 compression are over. Sub 9:1 was used as a saftety valve for poor tuning or if you were running 30+psi. With all of the user-friendly ECU's available today 9.5:1 and higher produces much more reliable power.

civicqm

I am thinking from what I have read so far the 9.5:1 cr looks like what I will be needing. I think I am going to get a precision sc61 turbo and run around 15lbs. However, I would like to hear more inputs if anybody has them. Thanks for all your help on this so far..

Muckman

iTrader: (2)

Ive always been very intrigued by this topic as well. Never really agreed that low boost is always "safer". If you read Maximum Boost, theres a good section in there about "effictive compression" which is basically what the actual real world cylindar pressure is, composed from piston compression and boost.

With concerns to tuning I dont think compression really matters. You tune the overall effective cylindar pressure, not just boost, not just compression. Which has more effective compression - 8:1 CR and 20psi or 11:1 and 5psi? Of course I dont know wtf Im talking about since Im a turbo newb so I could be wrong.


Modified by Muckman at 9:01 AM 5/27/2003

shermanyang

iTrader: (1)

i'm running 9.5:1 right now in my built LS and it runs like stock from 0-3k and from there on to 7300 it pulls like crazy @ 6 psi. i hardly get any lag like my buddy's eclipse which has the same turbo as i do.

Pelican

I turned this arguement over and over in my head.
Effective CR can be equal with High/Low or Low/High combinations but think of two things; a real world app and a a metaphor

1) Top Fuel Drag Motors. refutedly they run about 5:1 CR with HUGE amount of boost. yes they run nitromethane, but even if they didn't, it would still be a powerful motor. And CFM can LOOSELY be related to hp. Says nothing about compression ratio. So its about Airflow that determines power numbers.

2) Here's something I came up with all by myself. would you rather have ten people pushing your car with one-tenth of their strength or one person exerting themslves 100%. If both ways produce the same power (your car gets pushed), which would be better.
kinda the same thing because the low compression setup has more volume at TDC

DragEG6 FI

Let me share my thoughts:
I used to have p73(itr pistons) in my b18c1 block b/c i ran it all motor for a while at first. Then I jump on a turbo kit oppertunity and went ahead and put in on with the same bottom end....itr pistons in a b18c1 block and stock b18c1 head = 10.9:1cr. Having that cr seemed to work very well with boost at first....lots of low end/mid range power without any lag ...granted i did not have any engine management....i was using an inline pump and FMU at the time. when i finnaly put my car on the dyno i found out that it seemed to loose a good bit of power from 6800-8000. This was due to it running a little lean of course. So then i melted a hole in the #2 piston So after my experience with high cr and boost i chose to rebuild with 9:1 JE.
I like having that larger reliable area when it comes to the tunning....and as many have already said....the Tunning of a high CR motor and boost is very crutial...not much room for error.

Casey

iTrader: (5)

good thread...i cant comment but am interested in others

PM-Performance

I am runnign 10.6:1 compression now and love it . but everyone is right, the margin for error is very minimal. i have a great tuner so i havent had any probs with mine , i made great numbers on lo boost and can still get thme higher with a few tweaks. we took lot of timing out to play safe.
on the rebuild im going 9.8:1 though , just for peace of mind and easier tuning methods

Muckman

iTrader: (2)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Pelican &raquo;</TD></TR><TR><TD CLASS="quote">
2) Here's something I came up with all by myself. would you rather have ten people pushing your car with one-tenth of their strength or one person exerting themslves 100%. If both ways produce the same power (your car gets pushed), which would be better.
</TD></TR></TABLE>

Id rather have ten people pushing my car using 1/10th of their strength. The 1 dude using all of his strength would probably dent my car! Better to spread the force out equally..

civicqm

Thanks for all the relpys guys, I apperciate it. Very good information...

sweet

iTrader: (5)

yes there is some really good info here with this topic.

personally i think 9:1 is too low, but 10:1 is about right. 10.5-11:1 is nice but requires the extra tuning and effort for it to last. though there is extra care needed when using 11:1 C/R and boost, i decided to go this route. the boost won't be here for some time yet so i won't have much to say about High C/R with lower boost levels until then. but i wouldn't build a turbo motor with less than 10:1 C/R. quite a few run in the mid 10:1 C/R range without trouble, with the correct tuning and lower boost of course.

you also have to take the turbo's boost efficiency range into account when planning the motor's displacement and C/R. you don't want too high of a C/R if the turbo wants to see 12-15psi and you can only run 8psi with the higher C/R. or run too low of a C/R like 8.5-9:1 with lots of boost and max out the turbo's efficiency range. matching things up within a certain range is needed, the C/R along with displacement too.

intekragsr

many people make the mistake of saying that 10:1 is high compression. IMO, 10:1 is mild compression and is perfect for turbo.

DLB1994

am a fan of 10:1-10:5.1 comp

am running 10.2:1 comp and love it and have high tq and hp numbers at low boost with a stock intake manifold ,stock cams ,stock revlimter,with just a t3/t04e 57trim .63 turbine on a nonvtec engine

you dont see many non-vtec's make 337whp with 313wtq at only 12psi it has to be the comp

but the boost is going up if the rain ever stop here

intekragsr

nice numbers! gives me some inspiration. im running same turbo and same compression, except a built head. my car should be up and running tommorow night finally after 3 months to check it out =D

EDIT:Just noticed it was an h23


Modified by intekragsr at 5:25 AM 5/28/2003

sweet

iTrader: (5)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DLB1994 &raquo;</TD></TR><TR><TD CLASS="quote">am a fan of 10:1-10:5.1 comp

but the boost is going up if the rain ever stop here </TD></TR></TABLE>

yeah same here, i like that range of C/R too.

also the rain has been here where i live for many days on end!