Howdy everybody, I've always been wondering, what are the disadvantages to low compression? I am a fan of road racing, and I want my car to have plenty of low end for road courses, not so much drag. If I run low compression, is my car really going to suffer on the road course? My plan is a B18B1, and ideally I would like to run 11.0:1 with type r pistons, eagle rods, endyn block posting, and then on top of that I'd like to be able to run (at the track) about 20psi, properly tuned of course. However, I am aware that I can extract much more power by dropping my compression down to 9.2:1 or somewhere in that neighborhood. Once again, would my car really suffer on the road circuit? Thanks in advance!

~Jon

 

20 psi on high compression? sounds to rought

 

Disadvantages of low compression:

Less off-boost power
Less throttle response
Slower revving
Less power increase from cams

That's all I can think of at the moment. You can do high compression/high boost, but the tuning is gunna bleed you bad.

 

You really mean a low compression RATIO correct? There is no advantage to low compression. A low ratio will let you run more boost, get away with lower octane, run cooler, have less chance of detonation and be easier on parts in general.

 

Thats simply not true,
There is disadvantages to anything.
The disadvantages are like said above,

Slower throttle response, and
depending on how low the compression, could lead to
dramatically less power when boost is not present.

if you have a turbo that doesn't provide full boost untill about 4500-5000
like alot of guys here, that means you wouldn't have power driving around, or exiting a turn for instance untill your RPMs were in a higher range again..

A good example is the new Mitsubishi Evolution, no doubt a fast car, but lacks top end power (highway) as compared to many of turbo hondas. but mostly because of its AWD power distribution and drivetrain loss.

The engine is low compression, 8.5:1 or something like that, so we did a test,
we ran as low of boost as we could and tried to see how the car acted. It was literally SLOWER than my ford escort pony, wich only has like 96 hp to the flywheel by the way. but man it was slow. And we saw immediately that the car relies heavily on boost to make ANY, YES ANY, power at all. But the size of the turbo i think its a version of the 16g twin scroll or something like that, is very acceptabe to the car, and can provide full boost right around the 3300 rpm mark.
The car exits corners unbelievebly quick due to this, and its AWD configuration,
among other things.

any how, i did the same thing to a car that has a b18b1 in it, 98 Civic, with 95 integra LS engine, on 8 PSI with a t3/04 50 trim. 2.5 inch downpipe.

His car can actually beat the evo on the highway, and the evo was running 21 PSI at the time.
but from a stop the evo would beat him before he had a chance to catch up.

The LS = 9.2:1 compression ratio
evo = 8.5:1 compression ratio

according to the owners
the LS dyno = 245 hp to wheels @ 8 PSI
the evo dyno = 235 hp to wheels @ 21 PSI

the lower compression takes away from the total amount of HP capable of being produced by a lower boost amount.
Takes more boost to achieve the same HP goals with a low compression
with a slightly higher 9.2:1 compression, the LS made 10 more HP with less than half the amount of boost as the evo.

interesting huh,

now, the cool thing was though, when we took boost away from the civic, with LS.
It still had some *****. Not like on boost by any means, but still way faster than my escort LOL, the car acted like an LS integra, and when on boost pulled harder than the evo. the thing is, the LS engine has a 9.2:1 compression, and is very responsive to boost. but still has low end torque to make up for no boost in the lower (non full-boost) rpms

Most likely you will not be running 20 PSI on a 11. ANYthing compression ratio.
for one, The tires you are likely to use on a road course, arent anything like a drag tire. And with 20 PSI on an 11 compression LS engine (even tuned perfect) is going to lose traction every chance it gets. shoot for stock compression or stay in the high 9s in compression area, and u might be ok with 20 PSI,but build the block and valvetrain. and you'll have lots of low end power, and tremendous pull afterwards. that still might be too much for your application. personally i'd run less boost on a slightley smaller turbo for a road/circuit application.

you could always go with a small turbo, and lower compression, it will make up for a less powerful setup.

but a b18b1 is a good option for boost and low end grunt, but has its tiny disadvantages of a slightly heavier weight up front. you might even think about the b16 and running much less boost. that way, you could get a lighter weight engine in the chassis, thus turn better, and also have a decent power serge under boost.

ANYHOW, the good things about low compression is

ITS SAFE, and will keep your engine very tunable. (easy to tune)
also keeps you able to run lower octane fuels, on specified boost amounts. You'll be able to run more boost also, due to the octane/detonation benefits.
makes your engine last longer, because there isn't any power being produced while not under boost, and the detonation factor goes down with the lower compression ratio you have.
there might be more advantages,

but there ARE disadvantages, and they are

not much power when not in full boost, not to quick on throttle response, need more boost to make same power as a slightly higher compression motor, the vehicle relies HEAVILY on boost to be any fun. and you sacrifice LOTS of low end to achieve a safer motor. Unless you have a fast enough responding turbo, which i recomend.
There are probably other disadvantages,

but don't let anyone tell you there are no disadvantages.

Power is all about tradeoffs. You cant have the BEST of everything, but you can do your BEST to keep as much of the power in low and top end, while remaining safe at the same time. the application that u need is different from what someone like me needs.
I have a b18b1 in my crx, and im going with a t3/t04 57 trim, I just want full boost at the 5000 rpm mark, and thats where i need it, I have a GSR tranny that puts me right at 5000 RPM every time i shift. I make full Torque at 5200 RPM, and my turbo is going to make power all the way to redline,
But im staying stock compression, and the car will have some sort of ***** before boost, so i don't rely to hevily in it to pass trafic.

Any thats my 2 cents.
Lates.

Modified by Talon12bp at 8:40 PM 12/6/2003


Modified by Talon12bp at 10:59 PM 12/6/2003

 

since we are on this topic....how would a car originally with 10.x:1 drive with NOW lowered 8.5:1 compression? just wondering cause we are taking my buddies Maxima to get tuned next weekend...stock CR was 10.x:1 and now it is 8.5:1, how will it react on the road(its only a 15 minute drive to the shop)will it just be kinda sluggish, or are there any other ill effects possibly?

 

when not under boost, yes if there were no other changes in the engine, the car will over all be MUCH more sluggish by a very noticeable amount.

Is the car supercharged? turbocharged? or setup for nitrous?

if hes not turbo or super, hes gonna be much slower than stock without the NOS. or while not under boost.

 

its turbocharged...yeah I figured out of boost might be slower, thanks

EDIT: would advancing the timing some in vaccum/"out of boost" help any??

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Talon12bp &raquo;</TD></TR><TR><TD CLASS="quote">when not under boost, yes if there were no other changes in the engine, the car will over all be MUCH more sluggish by a very noticeable amount.

Is the car supercharged? turbocharged? or setup for nitrous?

if hes not turbo or super, hes gonna be much slower than stock without the NOS. or while not under boost. </TD></TR></TABLE>

 

The other part to this of course is the turbo. If your looking for more of a road rally car. Get a smaller turbo which will give you better response and more low end power due to quick spool up. Basically look at how you drive and when you need the power and make the right purchases based on that. If it's high high horse power and you only quarter mile then get a big *** turbo and roll. If your more daily driver then a medium will do you fine. If your rally racing small is probably best. Hence the evo has a small turbo that's twin scroll. Quick spool but good out the gate power but not much up top.

 

https://honda-tech.com/zerothread?id=695113

read this thread. there are way too many variables to compare the evo to an Ls in terms of compression and power output.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Talon12bp &raquo;</TD></TR><TR><TD CLASS="quote">Thats simply not true,

Modified by Talon12bp at 8:40 PM 12/6/2003


Modified by Talon12bp at 8:47 PM 12/6/2003</TD></TR></TABLE>

Yeah Earl has got no grasp on whats going on, been building race cars longer than we have been alive, not to mention the low nine second CRX that him and his son built. I read your post and see your point but disagree on many things.
There car is going just as fast if not faster than the other guys, making less power. I think they have somehting on the ball.
Earl and Laskey racing


Rob

 

I never said he has no grasp on anything.

i just want to let him know that too many people simply say, one or the other is best, and it really depends, because its all about tradeoffs, and balances. You gain here, you lose here, give here, take here. When i just got into racing too many people told me there is no advantage to certain things, or that some things were the best, when actually finding out later, they were wrong most the time. And something i think i misread on his post.

he said
--1"there is no advantage to low compression."
when i thought that he said,
--2 "there is no disadvantage to low-compression."
sorry for my mix up and i apologize for not reading correctly, but now that i read it , either one of those statements would still be untrue. so im still correcting something.
Latez

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by earl &raquo;</TD></TR><TR><TD CLASS="quote"> You really mean a low compression RATIO correct? There is no advantage to low compression. A low ratio will let you run more boost, get away with lower octane, run cooler, have less chance of detonation and be easier on parts in general.</TD></TR></TABLE>

you have to read it in context... i think earl is trying to make a distinction between "low compression" and "low compression ratio". Notice how he emphasizes RATIO and then points out the some advantages of low compression ratio after he points out their are no advantages to low compression.

I would have to agree with earl... i wouldn't want an engine which produces low dynamic compression numbers whether Naturally aspirated or FI.... its never a good thing to have low compression because of rings not seating or your valves not seating properly.

 

true...

 

The minuses to low CR:
-If you aren't in boost and you haven't taken advantage of adding more ignition timing to pep the motor up then the motor is kind of "sluggish". The fix, run a smaller turbo to make boost down low.
-Its marginally harder to achieve good thermodynamic efficiency at low throttle angles. This reduces turbo spool capibilities.

The pluses:
-Ability to run large amounts of boost.
-Larger volume to fill with a/f mixture during cylinder filling events (reason lower CR can produce more power when done right).
-Much more tolerable motor on pump gas.
-Larger window of error when "things go wrong" with the setup.
Low compression motors are very easy to turn over and start on cold winter days.

 

Talon12bp, your arguement would be better if you compared the same engines and with the same drive trains.

Plus there are toooo many other variables that could have affected these numbers.

The LS = 9.2:1 compression ratio
evo = 8.5:1 compression ratio

according to the owners
the LS dyno = 245 hp to wheels @ 8 PSI
the evo dyno = 235 hp to wheels @ 21 PSI

 

The only advantage to low compression is more power w/ less heat. thats it. You can run more boost and make more power on pump gas.

 

wow, thank you guys so much for the very helpful, very quick responses. So far I think I may just stick with the stock pistons, keeping the compression around 9.4:1 or so, and then of course I won't run 20psi on the street, just on the track. But yes, I would like to have enough grunt on the low end to get around town real nice. Who knows, I'll check it out on the dyno without boost and see if it's enough to suit my needs.

 

I built a turbo motor (9.4-1 c/r) for a H-T member (Porsche997). He has put several thousand miles on it as his daily driver but has not installed the turbo yet. He took it to Carlsbad with stock 13" wheels and tires and ran several low 14 second passes with it. I did not think it would be that fast but I think he could pick up a second with some decent slicks with traction. Maybe they all run that good as I don't know that much about street set ups. I just thought he was making decent power for the low compression RATIO. He pumps about 170 pounds of compression using gsr cams and stock b16 head on it.

 

wow, I'm convinced!!!!!!!

what else did he have on his setups that allowed him to run 14's without boost?

 

i don't think the whole power loss, etc from low compression is as bad as most make it out to be...Look at omniman in teh allmotor forum, his motors make more power with less than 11:1 comp than alot of the other guys running 12:5:1+, personally im a fan of low compression, ie 9:1 in street cars, race cars are a differnt story.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Archidictus &raquo;</TD></TR><TR><TD CLASS="quote">Disadvantages of low compression:

Less off-boost power
Less throttle response
Slower revving
Less power increase from cams

That's all I can think of at the moment. You can do high compression/high boost, but the tuning is gunna bleed you bad.</TD></TR></TABLE>

call me dumb...but isnt the throttle response dictated more by the head and how much it can flow air, rather than the compression.

By the way, I am running 8psi on 9:1 pistons and my throttle response is tons better...

AND EVO's at 21psi will make a lot more than 235whp. I saw a stock EVO VII make 271whp on the dyno at Autobacs today. At 21psi (properly tuned) they'll do WELL over 400whp

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by WrongWD &raquo;</TD></TR><TR><TD CLASS="quote">what else did he have on his setups that allowed him to run 14's without boost?</TD></TR></TABLE>I think he uses a Hondata but I am not sure what else. You might want to IM Todd (Porsche997) directly. He has a Porsche with a rear engine Chevy V-8 in it also but he only drives his Honda. haha

 

You NEED FORGED PISTONS. DO NOT build a motor with stock pistons.

 

MiraiZ

hey,
a stock evo makes just around 271 hp to the flywheel.
you can lookl that up on mitsubishis website. but thats not to the wheels.
and the evo also runs 19 PSI in stock form, and no just bumping the boost to 21 PSI will not make anything close to 400 WHP on the stock turbo.

I've driven this car way too much to give in to that. it is fast but not 400 whp fast.

and like i said, he did what most evos are doing in stock form,
Most stock evo's make between 220 - 245 whp at best in stock form (some manufactured stronger than others)
To the wheels there hasn't been an evo yet to make more than 260 WHP in stock form.

So that 271 WHP that u seen wasnt stock. Just go tell that to anyone at
http://www.evolutionm.net
They will give you a laugh. its an evo forum, and im a regular there too. The car is easily modded, and 400 HP has been achieved on stock turbo, but thats with different intercooler, 3 inch downpipe, 25 PSI, some more fuel, and other things, but i tell you, we run 21 PSI on this car daily, with a SAFC, and he just got a 3 inch down pipe, and the thing is, its not even 300 HP to the wheels yet. he dyno'd last week and got 279 whp. which means he's probably getting around the 350 mark in flywheel hp.

Latez