Dropping compression: increased low end torque?

[imported_admin]

Got an "advanced" question for you mechanic types.

Somewhere I have heard that dropping an engine's compression actually improves low-end torque. Is this true?

Stock CR on the 3.8 is 9.3:1 or 9.4:1 depending on what book you're reading. My CR is 8.2:1. Granted, everything in the engine is built, but you'd still figure with a drop that severe it should run like a dog NA.

But it doesn't. Right now I'm running NA pending an intercooler repair but the thing just picks up and goes. I've blown the doors off several cars that should be "faster" than mine NA. I'm really tempted to do a dyno pull before putting the supercharger back on, just to see what kind of gains the blower is truly giving.

Am I just on crack or is my SOTP meter accurate?

[malice10985]

I am pretty sure that your SOTP meter needs an overhaul.

Higher compression should always give you more power/ torque.

I am saying this off the top of my head, I will go find some evidence to back it up.

[Tiago]

dropping compression makes engine less hp/tq

less thorttle response too.

[Magnus]

I would think that lower compression would mean less hp/tq across the board.

Perhaps it feels faster cause you're actually hooking.

[KBeezy]

Lower compression on a NATURALLY ASPIRATED motor creates less power -- lower compression on a boosted motor is actually good, safer, and depending on the amount of boost will dictate whether you pick up any power or not.

[DEE97GTP]

A mildly modded L67 dynoed about 140HP without the blower belt connected.

DEE

[Dojo2000]

<blockquote>quote:</font><hr>Originally posted by DEE97GTP:
A mildly modded L67 dynoed about 140HP without the blower belt connected.

DEE<hr></blockquote>

Don't you think thats more due to the fact that it was trying to suck air through the unconnected M90?

[ December 02, 2003: Message edited by: Dojo2000 ]</p>

[Bliggida]

Original CR = horizontal
New CR = vertical
____8:1_ 9:1_ 10:1_ 11:1 12:1 13:1 14:1 15:1
9:1_ 3.5
10:1 6.5_ 2.9
11:1 9.2_ 5.5_ 2.5
12:1 11.5 7.7_ 4.7__ 2.1
13:1 13.6 9.7_ 6.6__ 4.0_ 1.9
14:1 15.4 11.5 8.3__ 5.7_ 3.5_ 1.6
15:1 17.0 13.0 9.8__ 7.1_ 4.9_ 3.0_ 1.4
16:1 18.6 14.5 11.3_ 8.6_ 6.4_ 4.4_ 2.8_ 1.4
Difference is the percentage in power gained.

so 9:1 to 10:1 will gain you 2.9% more power. At 9:1 making 200 hp, is 205.8...206 horsepower.

Compression doesn't seem like much until it involves serious horsepower to begin with. Then it multiplies. Don't forget about the role timing can play in this as well.

So for example you have 300 horsepower on your 8:1 compression 350 Chevy. You step up to 10:1 compression. That's going to net you 319.5...320 horsepower.

This is all from http://www.popularhotrodding.com/tec...power_squeeze/

it helps me a little but wasn't quite what I was looking for. In general I've heard it said that for every point in compression your raise is another 4% more power, but I do not believe this to be entirely accurate, yet close enough for the average Joe.

200 HP @ 9.4:1 is 204.8 hp...205 at 10:1.
Where as if you are already making 250 horsepower and go from 9.4:1 to 11.5:1 is 271 hp.

[ December 03, 2003: Message edited by: Bliggida ]</p>

[3.4 slow to go]

N/A answer:
If you just drop compression, I seriously doubt you will gain low end. You will most likely loose overall power unless you match your cam to the lower cr also. This can result in more power, but usually in a limited rpm range.

Side note: How are you planning to lower the CR? thicker head gaskets, bigger chambers, dished pistons, all of the above?

I still chuckle at the ratings for both my Poncho 400 motors, 76 Bonnie had 7.6:1 CR, 180 hp & 325 tq, 77 T/A had 8.0:1 CR, 185hp & 325tq.

[Bliggida]

<blockquote>quote:</font><hr>Originally posted by 3.4 slow to go:
I still chuckle at the ratings for both my Poncho 400 motors, 76 Bonnie had 7.6:1 CR, 180 hp & 325 tq, 77 T/A had 8.0:1 CR, 185hp & 325tq.<hr></blockquote>
DANG! I thought my 8.2:1 compression was gay. Holy rusted metal Batman! 7.6:1 man, That would be a good candidate to run some high boost. Despite the Poncho being strong, I wonder if it'd hold up to enough boost to give it 12:1 effective compression ratio? Mad power though!

[3.4 slow to go]

They were smog motors alright. In 77 They bumped up compression to a "performance" 8.0-1 & gave the cam a more "agressive profile" :rolleyes: But the 4800+ lb Bonnie was able to be propelled thru the 1/4 is a staggering 17.1sec @ 78 (2.56/2.73 gears don't help) [img]tongue.gif[/img]

While these engines are reflecting net horsepower as apposed to the RA I~V engines with gross hp ratings & high compression, the torque numbers were relatively the same.