BannedCR500Riders.com

OK, this is not related to 500cc 2-stroke motorcycles...though it is engine related.

To reduce pre-ignition issues....why do you lower the compression ratio when turbo charging an engine? Wouldn't running a lower boost level acomplish the same thing?

If say a motor with a stock compression ratio and 8 psi of boost has a combustion pressure of "X" and a motor with a lower compression ratio but running higher boost pressure....though still having the same combustion pressure of "X". I've been told the stock compression ratio motor will tend to be more succeptable of pre-ignition. Pressure is pressure right? And if the combustion pressures are the same between the two motors wouldn't the HP and torque peaks be the same??

Any Geniuses out there who can help me ??

MAHALO!!

"Skiddy"

first off, are we talking 4 stroke?

Theres alot of factors in place here

Are you comparing stock engines?

The camshafts made for boost applications that have a tighter overlap, hold more compression in so it would make more power at 6psi, than an engine with a camshaft meant for naturally aspirated

whatever you are working on, keep the compression, buy a water or water/methanol injector system for it which allows you to boost even more than an inter or after cooler and is the mose efficient thing you can do to draw power from boost.
FJO Racing has the best one, but AEM also released one lately, both are stand alone systems and digital. A place like theblowershop will have mechanical water injection also.

if you have an engine making 650hp at the max of its fuel load, but the engine can take more, a water injector will "cushion" the burn allowing you to add more timing, or crank up the boost and can achieve another 150-200hp

More boost = more oxygen = more power.

High compression + boost = BOOM!
(actually sounded more like a can of fairing bolts fell down the intake.)
This is what happens when you think you have 10:1 and you actually have 13:1 and add 10lbs of boost.
This motor survived a few dyno runs, and then made it about 10 miles. 190 whp.

thats decent! I've seen guys make pistons to vapor, there wasnt a sign of them left in the engine, just the pins in the rods, and a coating of aluminum thru the header.

back when I worked on honda rice we moved away from lowering compression and eventually started raising it on turbo motors. We had the best luck with around 11to1 with moderate boost maybe 8-12.

I'm still not seeing a good explanation as to why people go through the expense of lowering compression ratios instead of simply lowering boost pressures. Equal cylinder pressures should equal same HP numbers.

"Skiddy"

Skidmark wrote:I'm still not seeing a good explanation as to why people go through the expense of lowering compression ratios instead of simply lowering boost pressures. Equal cylinder pressures should equal same HP numbers.

"Skiddy"

You cant burn more fuel with out more air!

Less boost = Less air = Less fuel burned = LESS POWER

My truck threw two rods out the side of the block due to high boost and to much timing, you gotta love know it all tuners

Shitty rods + Shitty mail order tune + shitty brain dead dyno guy = boom.

Nodge wrote:

Skidmark wrote:I'm still not seeing a good explanation as to why people go through the expense of lowering compression ratios instead of simply lowering boost pressures. Equal cylinder pressures should equal same HP numbers.

"Skiddy"

You cant burn more fuel with out more air!

Less boost = Less air = Less fuel burned = LESS POWER

Obviously, but you missed the "combustion pressures being equal" part of the equation.

"Skiddy"

you gotta love SHITTY FORD POWDERED RODS[/quote wrote:

Hey dont be tricked now on the powdered metal stuff. with the excusion of some diesel engines Every GM, ford, and chrysler product 1996+ has them, many import brands before that, Johndeere, Case, Cat and other brands get into it aswell.
I have stock car classes where we have to use a "stock" rod, the GM PM rods are good, they dont need bushings to float the pin, they take alot of shock, and alot of my guys are turning these engines 7400+ for entire 25-30 lap races. I ran a set for years in a 500hp smallblock that i also threw 175hp of nitrous at.
Sorry ford guys this is where the tears start falling, the 5.4 series engine and others that share that design have crap design for the rods, and basiacally the rest of the engine, seems like ford had a little relapse into their flathead days and made everything dainty for economy, but the whole engine is shit. The rod is probaly less than 1/2 the size of a gm 5.3 rod. The camshafts on stock engines with high mileage and frequent oil changes still wear into the cap that holds them on the head and loose oil pressure. There is no bearing, just the cam (a tube about 1"dia with lobes swedged into place) running on the head casting.
Carol Shelby must be slipping a bit to put his name on anything with such a junker engine.

Back on the boost deal, I've got a guy running 12:1 with 18lbs boost on pump fuel and water injction on the street. The 614 dodge i did for a "ice cream trip" 69 charger has 10.5:1 and 16lbs boost and is expected to dyno 1200hp on the street tune with water.
I dont have a logical explination of why guys build less compression today, other than possibly thats the only performance piston on the shelf?. Intercooled and aftercooled you can run the big squeeze, but most guys just throw boost at it and expect it to live. Water injection doesnt need either rediculously inefficient cooler, saves a pile of room and recently has become less $ than the other systems.

100hp honda wrote:

I lol'd at my engine too.......those are not turbo pistons.