Boost\compression ratio?

1989GT · 02-08-2001, 01:09 AM

Does anyone know the formula so you can to your compression ratio add the boost and come out with the total compression? And how much compression is too much. I read in a post the other day that the "new" thing is too run higher compression and then boost on top of that. So that got me to thinking.

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89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on!
Throttle Junkie Powered by Ford!
12.824 @ 107.05 MPH
PVT Blankenship leaving for BCT 14MAR01
MOS 67 Romeo

1989GT · 02-08-2001, 03:33 PM

Anyone?

------------------
89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on!
Throttle Junkie Powered by Ford!
12.824 @ 107.05 MPH
PVT Blankenship leaving for BCT 14MAR01
MOS 67 Romeo

02-08-2001, 04:54 PM

Sorry, but I'm not aware of any formula to calculate what your "equivalent" compression ratio would be. I'd like to know that myself.

It probably wouldn't incorporate boost in the equation, but rather the volume of air that the blower/turbo puts out(ie cfm). Boost pressure can vary depending on the amount of restriction of the intake,heads, etc.

Also, it would have to be a pretty complex equation. The volume of air that the blower puts out rises with RPM, so, in effect, your "compression" ratio would be variable.

------------------
91 coupe
3.73's,long tubes,2 1/2" exh,ported stock t-body,ProM-77mm
cobra intake,edelbrock heads,B303,1.72 rr's
36lb inj's,tremec, & a Paxton NOVI 2000!!!
11.020 @128.95 on BFG Drag Radials (275/60/15)

1989GT · 02-08-2001, 11:10 PM

Yeah I know...I just remember someone posting it(it's been a while, atleast a year or so) then all the blower guys started calculate the compression they were running. Some were like 14:1.

------------------
89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on!
Throttle Junkie Powered by Ford!
12.824 @ 107.05 MPH
PVT Blankenship leaving for BCT 14MAR01
MOS 67 Romeo

Brcuda · 02-11-2001, 08:55 PM

(boostpsi/14.7)+1)xmotor compression=effective compression
example: (18psi divided by14.7)+1)x9.1=20.242

------------------
Kenne Bell supercharged stang 93LX (the perfect sleeper) Named Wheezer 11.901@117.40 on BFGs/w 1.71 60's

Rev · 02-11-2001, 09:25 PM

One also might remember that dynamic CR is not the same as static CR. As RPM increases, dynamic CR would be better at increased RPM because there is not any reversion into the intake. CR inceases with (boost+14.7)/14.7 X CR = actual CR. eg. (8 lbs.boost+14.7)/14.7 X 10 = 15.44 CR. This does apply to dynamiic CR which changes over the RPM range. Does this make sense?

Rev

------------------
'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.

Unit 5302 · 02-12-2001, 02:15 AM

Hmmmm...

I can't resist, but Rev, you could always simplify your formula, right?

(b+14.7)x.68 = CR

Where b = Boost pressure, and .68 is 10/14.7.

Of course 10/14.7 is technically .680272108844, but I'm just getting rid of some numbers and being my usual late night smartass!

1989GT · 02-12-2001, 11:28 AM

Thanks! I knew that someone knew the formula. So what's the difference b\n dynamic and static CR? And what kinda compression ratio is too much? Thanks!

------------------
89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on!
Throttle Junkie Powered by Ford!
12.824 @ 107.05 MPH
PVT Blankenship leaving for BCT 14MAR01
MOS 67 Romeo

02-08-2001, 01:09 AM	#1
1989GT Registered Member Join Date: Apr 1999 Location: Cincinnati,OH Posts: 1,511	Boost\compression ratio? Does anyone know the formula so you can to your compression ratio add the boost and come out with the total compression? And how much compression is too much. I read in a post the other day that the "new" thing is too run higher compression and then boost on top of that. So that got me to thinking. ------------------ 89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on! Throttle Junkie Powered by Ford! 12.824 @ 107.05 MPH PVT Blankenship leaving for BCT 14MAR01 MOS 67 Romeo

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02-08-2001, 03:33 PM	#2
1989GT Registered Member Join Date: Apr 1999 Location: Cincinnati,OH Posts: 1,511	Anyone? ------------------ 89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on! Throttle Junkie Powered by Ford! 12.824 @ 107.05 MPH PVT Blankenship leaving for BCT 14MAR01 MOS 67 Romeo

02-08-2001, 04:54 PM	#3
SisuStang Guest Posts: n/a	Sorry, but I'm not aware of any formula to calculate what your "equivalent" compression ratio would be. I'd like to know that myself. It probably wouldn't incorporate boost in the equation, but rather the volume of air that the blower/turbo puts out(ie cfm). Boost pressure can vary depending on the amount of restriction of the intake,heads, etc. Also, it would have to be a pretty complex equation. The volume of air that the blower puts out rises with RPM, so, in effect, your "compression" ratio would be variable. ------------------ 91 coupe 3.73's,long tubes,2 1/2" exh,ported stock t-body,ProM-77mm cobra intake,edelbrock heads,B303,1.72 rr's 36lb inj's,tremec, & a Paxton NOVI 2000!!! 11.020 @128.95 on BFG Drag Radials (275/60/15)

02-08-2001, 11:10 PM	#4
1989GT Registered Member Join Date: Apr 1999 Location: Cincinnati,OH Posts: 1,511	Yeah I know...I just remember someone posting it(it's been a while, atleast a year or so) then all the blower guys started calculate the compression they were running. Some were like 14:1. ------------------ 89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on! Throttle Junkie Powered by Ford! 12.824 @ 107.05 MPH PVT Blankenship leaving for BCT 14MAR01 MOS 67 Romeo

02-11-2001, 08:55 PM	#5
Brcuda Registered Member Join Date: Feb 1999 Location: St. Louis,Mo. USA Posts: 37	(boostpsi/14.7)+1)xmotor compression=effective compression example: (18psi divided by14.7)+1)x9.1=20.242 ------------------ Kenne Bell supercharged stang 93LX (the perfect sleeper) Named Wheezer 11.901@117.40 on BFGs/w 1.71 60's

02-11-2001, 09:25 PM	#6
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	One also might remember that dynamic CR is not the same as static CR. As RPM increases, dynamic CR would be better at increased RPM because there is not any reversion into the intake. CR inceases with (boost+14.7)/14.7 X CR = actual CR. eg. (8 lbs.boost+14.7)/14.7 X 10 = 15.44 CR. This does apply to dynamiic CR which changes over the RPM range. Does this make sense? Rev ------------------ '66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph 1/4 mi.

02-12-2001, 02:15 AM	#7
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	Hmmmm... I can't resist, but Rev, you could always simplify your formula, right? (b+14.7)x.68 = CR Where b = Boost pressure, and .68 is 10/14.7. Of course 10/14.7 is technically .680272108844, but I'm just getting rid of some numbers and being my usual late night smartass!

02-12-2001, 11:28 AM	#8
1989GT Registered Member Join Date: Apr 1999 Location: Cincinnati,OH Posts: 1,511	Thanks! I knew that someone knew the formula. So what's the difference b\n dynamic and static CR? And what kinda compression ratio is too much? Thanks! ------------------ 89GT,T-5Z, D.S.S. 306 shortblock w\main support, B-303 cam, Cobra intake, GT40P heads, 1.72rr, MAC "P" headers, MAC off-road H-Pipe, MAC true-fit w\tips, 373's...and the list goes on! Throttle Junkie Powered by Ford! 12.824 @ 107.05 MPH PVT Blankenship leaving for BCT 14MAR01 MOS 67 Romeo