Need some help

[blkZooM]

I've been doing some research that past couple of weeks on an engine build and I've hit a road block. my knowledge on engine is poor at best, but this is why I'm reading, researching and asking questions.

I'm trying understand why its better to have lower compression when F/I (if it is better at all that is) in the picture, is higher compression really bad if one wants to run higher boost.

Some members have the 03-04 cobra S/B which I know have lower C/R and other seem to be fine with stock marauder C/R, I guess what I really want to ask is why do some members rather the higher compression over lower and vise versa.

what are the pros and cons of higher C/R vs lower C/R when one wants to run increase boost

Sorry for the noob question just really trying to understand before moving forward with the engine build.. Anyone can pay for a car to be built to go fast but I want to understand whats really going on in the heart of the car also.

[SC Cheesehead]

The primary reason guys have gone with the Cobra short block is stronger lower end components. Along with that you get the lower compression pistons, plus you gain about 89 lbs. The guys running these typcially run higher boost to offset the lower compression loss, but there are diminishing returns to generating higher boost in a M112 (you start exceeding design parameters). Porting an M112 will help with some of this, but only up to a point.

The OEM aluminum 4.6L short block will easily handle up to 460- 475 hp and boost up to around 14 - 15 lbs (I think Zack is pushing 480+ with his Eaton swap). A Teskid block or aftermarket aluminum motor (i.e. Aluminator) will take way more than that.

Unless you're planning on making mega horsepower, should be no reason to go to the CI Cobra block unless you can pick one up for cheap.

[LANDY]

I run a forged 10.2.1 CR and 521rwhp 12psi at 13*timing. I'm limited to boost and timing because of pump gas.

[RacerX]

You can also use lower octane fuel with a lower compression. You can advance timing more with lower compression also.

I'm having a 9:1cr forged, balanced aluminum sb being built right now. It'll have 17+psi and a small shot of N2O for the track.

[blkZooM]

The primary reason guys have gone with the Cobra short block is stronger lower end components. Along with that you get the lower compression pistons, plus you gain about 89 lbs. The guys running these typcially run higher boost to offset the lower compression loss, but there are diminishing returns to generating higher boost in a M112 (you start exceeding design parameters). Porting an M112 will help with some of this, but only up to a point.

The OEM aluminum 4.6L short block will easily handle up to 460- 475 hp and boost up to around 14 - 15 lbs (I think Zack is pushing 480+ with his Eaton swap). A Teskid block or aftermarket aluminum motor (i.e. Aluminator) will take way more than that.

Unless you're planning on making mega horsepower, should be no reason to go to the CI Cobra block unless you can pick one up for cheap.

I'm not planning on using a Cobra short block, I have a teskid block and wanted a 5.0 stroker kit, I had read that lower compression is ideal for boost and wanted to know why? from what I was reading it sounded like higher compression with boost was a bad thing

[blkZooM]

You can also use lower octane fuel with a lower compression. You can advance timing more with lower compression also.

I'm having a 9:1cr forged, balanced aluminum sb being built right now. It'll have 17+psi and a small shot of N2O for the track.

hm, okay being that someone can do that with a lower compression what happens if you stoke it out to 302ci, do we have to start using higher gas and do we have to decrease timing? I'm not looking for high hp numbers now

if I was to bore it out to a 302ci would it be bad later one when I plan on adding a Procharger.. Don't want to mess with too much its my DD...

Wanted to try something different, but seems very complicated when you start messing with displacement..

and Landy number are good too me I would rather be in the 400 - 420 rwhp I don't no how much boost would be needed net those numbers. Would stay there until I got use too the added power

[LANDY]

This is a heavy car and 400rwhp will leave you wanting more by the third time you floor it.
The CR racerx is using is ideal for 20 psi and decent timing.
The fact that your going up cubic inches won't hurt your CR, its all on the pistons, head and head gasket.
I also use a 125shot that is ready most of the time just in case the blower ain't enough.

[justbob]

hm, okay being that someone can do that with a lower compression what happens if you stoke it out to 302ci, do we have to start using higher gas and do we have to decrease timing? I'm not looking for high hp numbers now

if I was to bore it out to a 302ci would it be bad later one when I plan on adding a Procharger.. Don't want to mess with too much its my DD...

Wanted to try something different, but seems very complicated when you start messing with displacement..

and Landy number are good too me I would rather be in the 400 - 420 rwhp I don't no how much boost would be needed net those numbers. Would stay there until I got use too the added power

If that's all you want, then why mess with the engine at all? A stocker can EASILLY handle that for years of trouble free miles. Just curious.

[SC Cheesehead]

If that's all you want, then why mess with the engine at all? A stocker can EASILLY handle that for years of trouble free miles. Just curious.

Agreed.

I'm at 430 hp with 11 lbs of boost on my Eaton swap and the car runs great on the stock block, 72K on the clock.

[slickster]

One day I'll be coyoted eatoned;)

[blkZooM]

If that's all you want, then why mess with the engine at all? A stocker can EASILLY handle that for years of trouble free miles. Just curious.

Well I have a knocking sound in it seems as if I'm going have to take it apart to be fixed.. More power is something I want but not at the moment I've never driven a 400rwhp car so would like to get the feel of it and like Landy said I will want more later on.

My thought is well if it has to come apart might as well make it stronger now when its apart, and when I'm ready for more power which I will be just a matter of when I feel ready I can add a lot more power worry free. that's all..

[blkZooM]

Agreed.

I'm at 430 hp with 11 lbs of boost on my Eaton swap and the car runs great on the stock block, 72K on the clock.

is that rwhp or flywheel?

[blkZooM]

One day I'll be coyoted eatoned;)

Was going to go the route of the 5.0 boss block but it wasn't a plug and play thing

[99SVT]

If you plan on going with a Procharger in the future, stick with the high compression.
Centri blowers can get away with having much higher comp ratios on pump gas than turbos and PB blowers because the boost builds with RPM.
Even PB blowers like the Trilogy likes having high compression up to the octane threshold.
If I were to be building a motor with the expectation of putting on a Procharger in the future, I'd put in some forged flat top pistons and some forged rods with ARP fasteners, you'll want to be able spin the motor well past the 6k factory redline.

[justbob]

Well I have a knocking sound in it seems as if I'm going have to take it apart to be fixed.. More power is something I want but not at the moment I've never driven a 400rwhp car so would like to get the feel of it and like Landy said I will want more later on.

My thought is well if it has to come apart might as well make it stronger now when its apart, and when I'm ready for more power which I will be just a matter of when I feel ready I can add a lot more power worry free. that's all..

I understand complety friend. Free advice, plan out every avenue for your long term goals. Have a goal and don't look back.

[blkZooM]

If you plan on going with a Procharger in the future, stick with the high compression.
Centri blowers can get away with having much higher comp ratios on pump gas than turbos and PB blowers because the boost builds with RPM.
Even PB blowers like the Trilogy likes having high compression up to the octane threshold.
If I were to be building a motor with the expectation of putting on a Procharger in the future, I'd put in some forged flat top pistons and some forged rods with ARP fasteners, you'll want to be able spin the motor well past the 6k factory redline.

What about the crank?

[blkZooM]

I understand complety friend. Free advice, plan out every avenue for your long term goals. Have a goal and don't look back.

thanks for the kind advice that is what I am doing now with the questions and you guys are being great with the answers.. Step one of my goal is happening today will be taking the block to a local machine shop to be looked at..

[SC Cheesehead]

is that rwhp or flywheel?

rwhp

With drivetrain loss, prolly just over 500 hp at the flywheel.

[LANDY]

You want a Kellog forged crank and manley rods