increasing compression ratio

84LX89GT · 11-11-2003, 09:11 PM

I was wondering how much power i could get from my 302 by increasing the compression ratio of my GT40 heads from 9:1 up to 10:1 or even 10.5:1?

How about a 200 straight six with 8:1 compression going up to 10:1

Thanks,

note: i haven't installed my GT40 heads yet so any suggestions as to anything else i may want to do is greatly appreciated (i'm going to gasket match, don't think i want to port/or have them ported unless it makes a huge difference)
Is increasing the compression ratio worthwhile since i'm going to be using nitrous oxide - i don't want to blow holes in stuff.

Jeff Chambers · 11-12-2003, 07:52 AM

General rule of thumb is a 3% to 5% increase in output for every full point of compression that you can raise the motor. The SBF responds extremely well to increases in compression, especially if you keep the port velocities high which you will with the small port volume of the GT-40. Go for it. I've raced my old P-headed motor on 93 octane gas at 13.0:1 compression so don't be afraid to go for as much compression as you can squeeze out of the combination.

tmoss · 11-12-2003, 08:40 AM

Jeff - I've seen you quote those numbers before. 5% of 225HP is only 11HP for one full point. I've seen other sources quote that 10HP for .5 point of compression. Who's right??? You did say that that is a "rule of thumb" but how much does it vary?

Jeff Chambers · 11-12-2003, 02:29 PM

It is only a rule of thumb, and a rule that only considers the change in compression....nothing else. I've often thought of it as being way to conservative myself, but who am I to argue? I think it seems pretty reasonable if you're considering a stock motor going from 9.0 to 10.0, but it would really seem to fall apart if you think of a Hot Street motor that is pushing 17.0:1. You'd never get to 700 hp just by taking a stock motor from 9.0 to that 17.0. More often then not, you're doing other things at the same time that you're raising the compression to better optimize the combination so the gain due to the change in compression gets washed out. Oh, but I digress. At any rate, I've got several engine building programs that seem to concurr with the 3 to 5% rule. I just ran the numbers for a stock 225hp motor going from 9.0 to 10.0 and it give a new peak hp rating of a little more than 232hp. That's only 7hp or a 3% change.

84LX89GT · 11-12-2003, 03:23 PM

thanks for the replies, those numbers sound reasonable.

If i get the heads milled i have to mill the intake 1/2 of what the heads are right?
How well do the heads/intake line up to the intake gaskets after milling, would i have to elontage the gasket holes?

tmoss · 11-12-2003, 05:25 PM

Usually, milling the heads up to .030"-.040" on the outside does not require intake milling. Just have to put the intake on the shaved heads and check runner alignment.

302 LX Eric · 11-14-2003, 10:40 AM

Just an FYI...my machine shop (Fowler's Engines of Columbus, OH) milled my TFS TW's .045" and milled the intake side of the heads as well in order to compensate for the head surface milling. The intake ports line up perfectly and I had just enough space at the bottom of the intake to the 'wavy' part of the block to seal it up with black RTV. This milling took my combustion chambers from 66cc to 58cc and resulted in overall compression change from 9.2:1 to 10.0:1.

Make sure you do all the calculations so that you end up with the proper compression that you're looking for based on chamber cc, etc.

Good luck,
E

11-11-2003, 09:11 PM	#1
84LX89GT Mustangs Join Date: Jul 2000 Posts: 1,938	increasing compression ratio I was wondering how much power i could get from my 302 by increasing the compression ratio of my GT40 heads from 9:1 up to 10:1 or even 10.5:1? How about a 200 straight six with 8:1 compression going up to 10:1 Thanks, note: i haven't installed my GT40 heads yet so any suggestions as to anything else i may want to do is greatly appreciated (i'm going to gasket match, don't think i want to port/or have them ported unless it makes a huge difference) Is increasing the compression ratio worthwhile since i'm going to be using nitrous oxide - i don't want to blow holes in stuff. __________________ 2005 Suzuki Hayabusa GSX1300-R 1980 Ford Thunderbird - 255 V8 ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain hi-po Mack Truck hood emblem 1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc. Sold 02/06/04 1989 Mustang GT ET: 13.304@102.29 mph (5-24-03) Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock) Last edited by 84LX89GT; 11-11-2003 at 09:38 PM..

11-12-2003, 07:52 AM	#2
Jeff Chambers Moderator Join Date: Aug 2000 Location: Milan, OH Posts: 2,699	General rule of thumb is a 3% to 5% increase in output for every full point of compression that you can raise the motor. The SBF responds extremely well to increases in compression, especially if you keep the port velocities high which you will with the small port volume of the GT-40. Go for it. I've raced my old P-headed motor on 93 octane gas at 13.0:1 compression so don't be afraid to go for as much compression as you can squeeze out of the combination. __________________ Jeff Chambers 1990 Mustang GT 10.032 Seconds / 137.5 MPH 14-time Street Warrior World Record Setter CRT Performance 2001 Tropic Green Mustang GT - 12.181 / 113.2 MPH 2002 Ford F-250 Crew Cab 7.3l Power Stroke - 17.41@77.2 "There's nothing boring about a small block automatic shifting gears at 9400 rpm!"

11-12-2003, 08:40 AM	#3
tmoss Registered Member Join Date: Mar 2003 Location: Saint Louis, MO Posts: 634	Jeff - I've seen you quote those numbers before. 5% of 225HP is only 11HP for one full point. I've seen other sources quote that 10HP for .5 point of compression. Who's right??? You did say that that is a "rule of thumb" but how much does it vary? __________________ Tom (Torque) Moss 88Gt 5spd Vert, FLowmaster Catbacks, stock cam advanced 4° @ 108.5° ICL, NMRA prepped GT40P heads 1.85/1.55 valves and 1.7 rockers, MAC P headers Jet-Hot coated, 97 Exlporer intake (ported lower), TB and injectors. 277RWHP/330RWTQ (SAE). http://www.fastlanecars.com/

11-12-2003, 02:29 PM	#4
Jeff Chambers Moderator Join Date: Aug 2000 Location: Milan, OH Posts: 2,699	It is only a rule of thumb, and a rule that only considers the change in compression....nothing else. I've often thought of it as being way to conservative myself, but who am I to argue? I think it seems pretty reasonable if you're considering a stock motor going from 9.0 to 10.0, but it would really seem to fall apart if you think of a Hot Street motor that is pushing 17.0:1. You'd never get to 700 hp just by taking a stock motor from 9.0 to that 17.0. More often then not, you're doing other things at the same time that you're raising the compression to better optimize the combination so the gain due to the change in compression gets washed out. Oh, but I digress. At any rate, I've got several engine building programs that seem to concurr with the 3 to 5% rule. I just ran the numbers for a stock 225hp motor going from 9.0 to 10.0 and it give a new peak hp rating of a little more than 232hp. That's only 7hp or a 3% change. __________________ Jeff Chambers 1990 Mustang GT 10.032 Seconds / 137.5 MPH 14-time Street Warrior World Record Setter CRT Performance 2001 Tropic Green Mustang GT - 12.181 / 113.2 MPH 2002 Ford F-250 Crew Cab 7.3l Power Stroke - 17.41@77.2 "There's nothing boring about a small block automatic shifting gears at 9400 rpm!"

11-12-2003, 03:23 PM	#5
84LX89GT Mustangs Join Date: Jul 2000 Posts: 1,938	thanks for the replies, those numbers sound reasonable. If i get the heads milled i have to mill the intake 1/2 of what the heads are right? How well do the heads/intake line up to the intake gaskets after milling, would i have to elontage the gasket holes? __________________ 2005 Suzuki Hayabusa GSX1300-R 1980 Ford Thunderbird - 255 V8 ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain hi-po Mack Truck hood emblem 1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc. Sold 02/06/04 1989 Mustang GT ET: 13.304@102.29 mph (5-24-03) Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)

11-12-2003, 05:25 PM	#6
tmoss Registered Member Join Date: Mar 2003 Location: Saint Louis, MO Posts: 634	Usually, milling the heads up to .030"-.040" on the outside does not require intake milling. Just have to put the intake on the shaved heads and check runner alignment. __________________ Tom (Torque) Moss 88Gt 5spd Vert, FLowmaster Catbacks, stock cam advanced 4° @ 108.5° ICL, NMRA prepped GT40P heads 1.85/1.55 valves and 1.7 rockers, MAC P headers Jet-Hot coated, 97 Exlporer intake (ported lower), TB and injectors. 277RWHP/330RWTQ (SAE). http://www.fastlanecars.com/

11-14-2003, 10:40 AM	#7
302 LX Eric or '331 LX Eric' Join Date: Jan 1999 Location: Columbus, Ohio Posts: 2,142	Just an FYI...my machine shop (Fowler's Engines of Columbus, OH) milled my TFS TW's .045" and milled the intake side of the heads as well in order to compensate for the head surface milling. The intake ports line up perfectly and I had just enough space at the bottom of the intake to the 'wavy' part of the block to seal it up with black RTV. This milling took my combustion chambers from 66cc to 58cc and resulted in overall compression change from 9.2:1 to 10.0:1. Make sure you do all the calculations so that you end up with the proper compression that you're looking for based on chamber cc, etc. Good luck, E __________________ 1991 5.0 LX Coupe - 40,750 miles 331 cu. in. / Tremec 3550 / BFG Drag Radials 12.22 @ 114.31 mph - w/1.89 60'

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