Modular piston help!

Marvin97 · Jan 29, 2009

Im building another motor to go in my car, but I need a little more information before I can purchase the last few pieces. 97 block, cobra crank, forged steel rods(stock length), ported piheads with maybe a set of cams if a cheap set comes by before its assembled. I need help with the piston selection though. I had planned on using the stock pistons with new rings and such since the car will only make around 400 to the wheels(conservative tune) when its finished. It will also have either a KB or a turbo setup from HP perfomance. A local guy here who is a modular guru apparently is worried that with the compression bump, I will only be able to run like 5psi, or have to run like 103 octane to keep it from pinging with anymore boost. So what size piston do i need to go with to lower my compression ratio? What is the stock compression ratio on these cars, what does it change to with the pi swap? What is optimal compression for boost? Thanks for the help, sorry it so long!

reivaxtorres · Jan 29, 2009

stock is like 9.5:1. with the pi swap, you are up to about 10.5:1. with ported PI heads, you are obviously going to be lower than that, but what it comes out to will depend on the end size of the combustion chamber on the head.

yes, with the PI heads and a blower, you will need to run lower boost. but take this as an example...

lets say at 9.5:1, you are making 100hp without the blower.
at 10.5:1, you will be making lets say 110hp without the blower.

lets say that at 9.5:1 with the blower @ 9lbs, you are making 160hp
now with the blower at 10.5:1 and at 6lbs, you are making 160hp.

the higher compression, in either n/a or f/i setups, will always make more power than a similar setup with lower compression.

you want my senario? i went from 218rwhp and 284rwtq with the NPI setup, and changing nothing but the PI heads, cams, and intake, i went to 265rwhp and 310rwtq, after a tune i went to 270rwhp and 313rwtq.

MustangChris · Jan 29, 2009

reivaxtorres said:
stock is like 9.5:1. with the pi swap, you are up to about 10.5:1. with ported PI heads, you are obviously going to be lower than that, but what it comes out to will depend on the end size of the combustion chamber on the head.

yes, with the PI heads and a blower, you will need to run lower boost. but take this as an example...

lets say at 9.5:1, you are making 100hp without the blower.
at 10.5:1, you will be making lets say 110hp without the blower.

lets say that at 9.5:1 with the blower @ 9lbs, you are making 160hp
now with the blower at 10.5:1 and at 6lbs, you are making 160hp.

the higher compression, in either n/a or f/i setups, will always make more power than a similar setup with lower compression.

you want my senario? i went from 218rwhp and 284rwtq with the NPI setup, and changing nothing but the PI heads, cams, and intake, i went to 265rwhp and 310rwtq, after a tune i went to 270rwhp and 313rwtq.

stock is 9.0:1 and 10.0:1 after te swap

99-04 are all 9.5:1 stock comp.

correct me if im wrong... i dont wanna be wrong... lol.

Marvin97 · Jan 29, 2009

the intake and exhaust runners are ported, the combustion chambers haven't been touched.

reivaxtorres said:
stock is like 9.5:1. with the pi swap, you are up to about 10.5:1. with ported PI heads, you are obviously going to be lower than that, but what it comes out to will depend on the end size of the combustion chamber on the head.

yes, with the PI heads and a blower, you will need to run lower boost. but take this as an example...

lets say at 9.5:1, you are making 100hp without the blower.
at 10.5:1, you will be making lets say 110hp without the blower.

lets say that at 9.5:1 with the blower @ 9lbs, you are making 160hp
now with the blower at 10.5:1 and at 6lbs, you are making 160hp.

the higher compression, in either n/a or f/i setups, will always make more power than a similar setup with lower compression.

i understand that i can achieve the same hp with less boost with higher compression, but i don't want to have to worry about pinging if i want to up the boost later on.

MustangChris · Jan 29, 2009

97whitevert said:
the intake and exhaust runners are ported, the combustion chambers haven't been touched.

reivaxtorres said:

stock is like 9.5:1. with the pi swap, you are up to about 10.5:1. with ported PI heads, you are obviously going to be lower than that, but what it comes out to will depend on the end size of the combustion chamber on the head.

yes, with the PI heads and a blower, you will need to run lower boost. but take this as an example...

lets say at 9.5:1, you are making 100hp without the blower.
at 10.5:1, you will be making lets say 110hp without the blower.

lets say that at 9.5:1 with the blower @ 9lbs, you are making 160hp
now with the blower at 10.5:1 and at 6lbs, you are making 160hp.

the higher compression, in either n/a or f/i setups, will always make more power than a similar setup with lower compression.

Click to expand...

i understand that i can achieve the same hp with less boost with higher compression, but i don't want to have to worry about pinging if i want to up the boost later on.

jsut get custom diamond pistons... id suggest 8:1 comp if you want boost... its gunna be a slow car N/A, but it will be ready for a blower.... also, if you are feeling fancy, take your heads to a head shop (which you should do anyways with a motor rebuild) and see if they offer combustion chamber boring... they bore out the cumbustion chamber on your head a little bit to lower compression and to give you a larger combustion chamber...

Zwolaver21 · Jan 29, 2009

Definitely get low compression forged pistons. There is no reason not to especially if you are getting a high dollar poweradder.

MustangChris · Jan 29, 2009

Zwolaver21 said:
Definitely get low compression forged pistons. There is no reason not to especially if you are getting a high dollar poweradder.

just live with a slow 8:1 for a while till you run 12 PSI :-D lol

Marvin97 · Jan 29, 2009

so what size piston dish would I need to keep stock compression? or go lower? there's not a good modular machine shop around here to have the heads worked on

MustangChris · Jan 29, 2009

id honestly call daimond pistons and ask to talk to a sales rep. tell him your keeping stock stroke and bore and ask what piston they have to lower the compression to 8:1...

Zwolaver21 · Jan 30, 2009

Call modular powerhouse 770-591-0000 They will tell you what pistons to use

Jrgunn5150 · Jan 30, 2009

You're local modular guru is a sissy, you'll be fine with that compression ratio. I swear to god I'm going to make that statement a flipping macro I've typed it so many times.

Marvin97 · Jan 30, 2009

So your saying at 10.5:1 compression and 12lbs of boost I wont have any pinging issues?

MustangChris · Jan 30, 2009

97whitevert said:
So your saying at 10.5:1 compression and 12lbs of boost I wont have any pinging issues?

.... lol... im going to say that you are... but JR has much more expeirence than i.... im just going off what i read.

and what exactly are you doing to get 10.5:1?

Marvin97 · Jan 30, 2009

its hard to say what its gonna be, your saying stock is 9.0:1 while another is saying 9.5:1, your saying after the swap 10:1, he's saying 10.5:1, im so confused! I think i'll just give them a call monday and see what they can tell me.

Jrgunn5150 · Jan 30, 2009

97whitevert said:
So your saying at 10.5:1 compression and 12lbs of boost I wont have any pinging issues?

Ugh, here we go again...

1. PI swap is 10.3:1, not 10, not 10.5, right up the middle, not that it makes a hell of a bit of difference.

2. If the car is tuned like it has 9.5:1 compression, of course it'll ping. Most people don't know this, but SCT give's everyone that buy's their software a huge collection of "base" tunes, as a starting point for tuning a car. A TON of "shops" load a base tune, dink with the shifting and fuel a little, and kick you in the ass and send you out the door. You need a real tune, by someone really competent, who will actually tune the thing. If they say that's to much compression, walk right out the door.

A car with more compression will always be snappier, better throttle response, make more power at the same amount of boost, and get better mileage. Sure sounds like something I don't want.

I ran 10.3:1 for years with a blower, it was fine. I also didn't have an intercooler, it was fine. I also lived in California, it was fine. I drove it everyday, everywhere I went, it was fine.

MustangChris · Jan 30, 2009

J.R. said:
97whitevert said:

So your saying at 10.5:1 compression and 12lbs of boost I wont have any pinging issues?

Click to expand...

Ugh, here we go again...

1. PI swap is 10.3:1, not 10, not 10.5, right up the middle, not that it makes a hell of a bit of difference.

2. If the car is tuned like it has 9.5:1 compression, of course it'll ping. Most people don't know this, but SCT give's everyone that buy's their software a huge collection of "base" tunes, as a starting point for tuning a car. A TON of "shops" load a base tune, dink with the shifting and fuel a little, and kick you in the ass and send you out the door. You need a real tune, by someone really competent, who will actually tune the thing. If they say that's to much compression, walk right out the door.

A car with more compression will always be snappier, better throttle response, make more power at the same amount of boost, and get better mileage. Sure sounds like something I don't want.

I ran 10.3:1 for years with a blower, it was fine. I also didn't have an intercooler, it was fine. I also lived in California, it was fine. I drove it everyday, everywhere I went, it was fine.

dang... what happened to this magic carpet? (jsut teasing you, lol... i really am curious about the car and your set up)

Jrgunn5150 · Jan 30, 2009

There's pics floating around of it and old threads on the boards here. It met the side of a Corrolla and that was that.

MustangChris · Jan 30, 2009

:-O :-( im sorry to hear that man... i woulda loved to have seen it! :-(

what kinda set up did you run? 10.3:1 with 15 non-intercooled PSI? geez... what kinda blower?

Jrgunn5150 · Jan 31, 2009

I never hooked a boost gauge to it, except on the dyno. How much boost I make is of little concern to me, but it was right at 10 psi, a Paxton Novi2200 on a LIvernois 5.0 stroker with VT heads and custom cams.

Marvin97 · Jan 31, 2009

Thanks for all your help J.R. I hate that you had to go over this one more time when it seems like you've had to tell others this before, but I hope you could understand the importance of it. It could easily have effected they way I built the motor, and effected the way the car performed. Thanks again for repeating yourself =)