High Compression and FI

blk98gt · June 24th, 2007, 01:53 AM

Im new at all this stuff and I have seen people say that the PI heads are bad for any FI becuase the heads raise the compression. Why is a Higher Compression bad for FI?

DrHolliday · June 24th, 2007, 02:54 AM

FI raises the compression on an engine by cramming more air into it. If your compression is already high then you can't run as much boost without upping the octane in your fuel.

MustangMatt96GT · June 26th, 2007, 05:54 PM

Also with less compression and high octane fuel you can pull more timing and create more power! The 99-04 motors have a 9.6:1 CR and our NPI 96-98 motors have a 9.3:1 CR. When the PI (99-04) heads are put on our NPI motors the compression bumps up to 10.6:1. The reason being is the different size combustion chambers and different dished pistons. PI motors have a 17 cc dished piston with a 42cc combustion chamber where as the NPI motors have a 11cc dished piston and 51cc combustion chambers. So if you put PI heads (42cc chamber) with NPI pistons (11cc dish) it creates the higher CR. Now if you run low boost with your PI heads with NPI motor it wont cause a problem but you would be better off to take your NPI heads and get them Ported and polished and keep the low compression ratio to make more power safely!

Boomer · June 26th, 2007, 06:21 PM

Removing some of the combustion chamber will help lower the compression ratio, you'll end up with around 46-48 cc chamber. If you take to much out you will end up hurting the performance you are trying to gain. Average combustion chamber on a P.I heads is actually 44cc's they rate them at 42 as the smallest of the average, I have actually seen some measure out at 46 untouched. If you have your heads ported by a real head porter all this information should be sent to you with the heads.

Back to the topic, you can run higher compression but you will need a higher octane fuel to support it. One way around this is to adjust the timing trim around to support the added compression. Built modulars running forced induction love compression, unless you plan on digging into the lower end I would look into other options.......

MustangMatt96GT · June 26th, 2007, 06:26 PM

You will also be digging into your wallet alot with the high octane gas! lol but then again Horsepower sure aint cheap!

DrHolliday · June 26th, 2007, 09:12 PM

Originally Posted by MustangMatt96GT

You will also be digging into your wallet alot with the high octane gas! lol but then again Horsepower sure aint cheap!

Why'd you have o bring up that evil 3 letter word? Ever since the KB my best tank has been about 13 mpg, lmao

Darrell · July 3rd, 2007, 11:27 AM

Originally Posted by blk98gt

Why is a Higher Compression bad for FI?

Its not. Some one fed you some crap.

June 24th, 2007, 01:53 AM	#1
blk98gt Regular Join Date: Jun 2007 Posts: 61	High Compression and FI Im new at all this stuff and I have seen people say that the PI heads are bad for any FI becuase the heads raise the compression. Why is a Higher Compression bad for FI?

June 24th, 2007, 02:54 AM	#2
DrHolliday Enthusiast Join Date: Mar 2007 Location: New Port, NC Posts: 717	FI raises the compression on an engine by cramming more air into it. If your compression is already high then you can't run as much boost without upping the octane in your fuel. __________________ Mustangless atm 2006 2500 HD Diesel. Built allison trans and other goodies

June 26th, 2007, 05:54 PM	#3
MustangMatt96GT All Throttle with a Bottle 1996 Mustang GT 12.91@105.29 @ 5800 FT Join Date: Dec 2006 Location: Lakewood, CO Posts: 7,609	Also with less compression and high octane fuel you can pull more timing and create more power! The 99-04 motors have a 9.6:1 CR and our NPI 96-98 motors have a 9.3:1 CR. When the PI (99-04) heads are put on our NPI motors the compression bumps up to 10.6:1. The reason being is the different size combustion chambers and different dished pistons. PI motors have a 17 cc dished piston with a 42cc combustion chamber where as the NPI motors have a 11cc dished piston and 51cc combustion chambers. So if you put PI heads (42cc chamber) with NPI pistons (11cc dish) it creates the higher CR. Now if you run low boost with your PI heads with NPI motor it wont cause a problem but you would be better off to take your NPI heads and get them Ported and polished and keep the low compression ratio to make more power safely! __________________ We expect shit to break, look foward to realizing exactly when it is happening, and than obsess about dropping plenty of cash to rebuild stronger, so we can try to run it hard as hell and break again. We are sick individuals. I support the MM Staff

June 26th, 2007, 06:21 PM	#4
Boomer Enthusiast Join Date: Oct 2006 Location: warren, Mi Posts: 880	Removing some of the combustion chamber will help lower the compression ratio, you'll end up with around 46-48 cc chamber. If you take to much out you will end up hurting the performance you are trying to gain. Average combustion chamber on a P.I heads is actually 44cc's they rate them at 42 as the smallest of the average, I have actually seen some measure out at 46 untouched. If you have your heads ported by a real head porter all this information should be sent to you with the heads. Back to the topic, you can run higher compression but you will need a higher octane fuel to support it. One way around this is to adjust the timing trim around to support the added compression. Built modulars running forced induction love compression, unless you plan on digging into the lower end I would look into other options....... __________________ Just another 10 second Mustang, what did you expect ? http://youtube.com/watch?v=BgykaUrmfvM

June 26th, 2007, 06:26 PM	#5
MustangMatt96GT All Throttle with a Bottle 1996 Mustang GT 12.91@105.29 @ 5800 FT Join Date: Dec 2006 Location: Lakewood, CO Posts: 7,609	You will also be digging into your wallet alot with the high octane gas! lol but then again Horsepower sure aint cheap! __________________ We expect shit to break, look foward to realizing exactly when it is happening, and than obsess about dropping plenty of cash to rebuild stronger, so we can try to run it hard as hell and break again. We are sick individuals. I support the MM Staff

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High Compression and FI

High Compression and FI

Re: High Compression and FI