924Board.org

[wdb]

what is the lowest compression ratio possible without causing a hard start problem. the reason I ask is I had a thought.some of you may have read about the garret hydra charger. Its a super charger with a turbo style compressor but not run by exhaust,Its run by hydraulic fluid,that comes from a pump (similar to a power steering pump) powered by a belt off the crank.this super charger is tunable,by changing pully size,and valve settings,you can have max boost at idle,mid range,or top end,instant throttle response,and possibly adjust it on the fly.after reading several post about the limits on boost (ping),I started thinking on how to get more power out of an engine.You need a bigger air fuel charge,but how to do that with the boost limits,need more room,larger combustion chamber,lowered compression.what if you took an aluminum plate 1" or 2 cm and cut it out to look exactly like a head gasket,all the holes in the right places. place a head gasket above and below it and insert it between the head and the block,that will creat a large combustion chamber to fill. there are other issues ,thickness of the plate determines comp.ratio ,better guide pins to keep everything in line,cooling,custom head bolts,etc. whick can be dealt with. back to the original question. whats the lowest compression ratio possible,without causing hard start problems ( I dont know the details on what rpm is nessasary for the hydra charger to work,several hundred rpm's the starter can muster may not be sufficient). what would the comp ratio be with this 1 inch spacer and what would the displacement (C.I.D.)now be . can anybody figure it out.what are your thoughts

[ This Message was edited by: wdb on 2001-12-25 03:29 ]

[wdb]

I think the compression ratio with a 1" spacer would be just under 4 to 1 3.? and the C.I.D would be about 180 C.I. or 2.5L. . I think using a spacer like this will more than double the combution chambers volumn and allow a turbo to be turned up to 2 bar (assuming a turbo can go that high and the fuel cut off at 1.4 bar can be remedied ).which I believe would produce just under 400 HP . .O.K. now you can poke holes in my theory.

[ricomartinez]

The engine will still displace 2 liters (1984cc?). Displacement is the volume that the piston displaces. It is not affected by combustion chamber volume.

[wdb]

thanks for the info. Ilooked around to see if any engine's ran at low compression,it turns out that some early aircraft engines from 30's and earlier ran at 4.9 - 5.5 compression using roots type super chargers.top RPM's on these motors was only around 2500 and as a result thier HP #'s were fairly low. the idea that a large comb. chamber would produce high HP is sound but ,trying to fill that chamber in the tenth of a second that the valve is open,is the problem.so what seemed to me as an obvious way to increase performance,has its limitations.I'll need to take a closer look at turbo's and the like to see whats
possible.

[wdb]

blitz dual selenoid boost controller,capable of 2.5 bar 36.75 PSI
http://www.modacar.com/audi/electronics_audi.htm

Try, for a moment, to comprehend a four-cylinder engine capable of achieving 4,000 hp at 55 psi turbo boost on nitromethane.
http://www.turbomagazine.com/archives/tech/0202_tech01.jsp

[ This Message was edited by: wdb on 2001-12-27 14:26 ]

[larso]

The "book" displacement of an engine is not at all a true reading of actual displacement.

If you lower the comp or grind out some of your cylinder head it ups the displacement, just doesn't show in the book.

[numbers]

Sorry Larso, gotta go with Rico on this. Displacement is just that. The volume of the air displaced by the cylinders. It is determined by the bore and stroke. Changing the size of the cumbustion chamber impacts compression ratio, but not displacement.

[larso]

That's like saying changing the piston cup size doesn't decrease displacement. Think about it, your changing the amount of air that is being sucked in, if you have a really really deep cup more air will enter when the piston goes to the bottom of its stroke! Not by book value, but in true displacement.

[wdb]

compression vs boost
there is more in the other sections of the site
http://www.motorsportsdigest.com/forced2.htm

[wdb]

cam timming vs compression analysis,this page goes far beyond my commitment to optimize my 924 motor,but if you have the math skills ,have at it. there is more , advanced readings at the bottom of the page in the link panics mopar tech.
http://victorylibrary.com/tech/cam-c.htm

[ This Message was edited by: wdb on 2001-12-28 17:29 ]

[TroyDest]

Rico is right everybody. The only way to increase displacement is to increase bore and/or stroke. Combustion chamber shape / piston dome shape has nothing to do with DISPLACEMENT. Look at it like this: if you had a huge pocket in your piston and it was full of air at the bottom of the stroke, that same volume of air is still there at the top of the stroke and is not evacuated. Therefore it not displacement.

As far as what overboring will do for displacement, a typical overbore of 0.5 mm (for which pistons and rings are available) will give you a displacement of 2007 cc compared to the stock 1984 cc. Not enough to brag about!

As far as installing a one inch spacer to decrease compression ratio, forget it! A one inch spacer would give you about a 3.4:1 comp. ratio. Although lower compresion engines start easier, the large combustion chambers are inefficient for flow and lower compression means lower torque.

Detonation is controlled by maximum cylinder pressure, temperature, and fuel octane. Just because you can put more boost to a lower compression motor before detonation occurs doesn't mean you will make more power. Detonation will still occur at the same max pressure which also happens to be where horsepower is made.

Sorry, just stick to fuel enrichment, intercooling, ignition retard, and water injection to prevent detonation at increased boost levels.

-Troy

[AppleBit]

very good point.

The way to make good reliable HP with the 924 is to go to at least the Euro pistons. If you want a supercharger on your car, get a bigger radiator because your engine is going to produce more heat.

The reason to go a little up with your compression from the 8.5's is because you don't want the supercharger to do all the work. If it is doing all the low-end work because your engine can't do the work itself, your take-off's will be very disappointing- just like turbo lag. You want to balance your engine with the supercharger you put on your car.

If I was to put a supercharger on my car I would be all ready for it. I have 10.0:1 pistons and I would go for something that only gives 6 to 7 psi boost. Reliability, safety and the amount of power I would get all around would be so incredible.

But I think my engine is too fast as it is... I'll skip the supercharger $$$ and just build an engine that is as fast- and more reliable

- Jon
t

_________________
Classic British Sports Car Restoration v6 + v8 Engine Conversion Swaps

Porsche 924 Wide Body LS1 Corvette 500 Horsepower Engine

[wdb]

the spacer can be any thickness,from 1/4"-3/4" (if 1" is too much) .The idea was to create a larger combution chamber,so as to allow a higher volumn of air fuel mixture into the cylinder. If 1 liter of an air fuel charge at 14.7:1 produces an explosion with the power of X than 2 liters will produce X2,twice the power. when you compress a gas,it heats up,compress it more it will get hotter,continue to compress it ,and it will reach a tempeture that ignites the fuel spontaniously,all of it at once,creating a sharp sudden explossion. that is detonation.when the spark plug ignites the fuel,the flame starts at the spark plug and propagates to the farthest part of the combustion chamber,taking more time to complete the burn, creates a slightly softer explosion. the idea is to get more air and fuel into the cylinder without reaching that critical pressure and tempeture. a 2l engine running at 4 bar of boost cramming more air and fuel into the motor will produce alot more HP than one runing at 2 bar of boost,but not for long,detonation will destroy the motor.f1 race cars run short stroke, low compression engines, and high boost,to achieve 1000 HP. Im not inventing anything here, Im trying to find a way to achieve a low compression high HP engine (cheaply) without going to the expence of buying a new crank,rods, and pistons. the question is ,what thickness of spacer, will produce what compression ratio, to allow what level of boost, and produce how much HP

[TroyDest]

Yo WDB,

I will e-mail you the formula for figuring our comp. ratios as it is kind of difficult to type up and I don't want to take up board space.

I did have a thought on your F1 comment though. I don't know what compression ratios they run, but you're right that they use a short stroke. The short stroke is the main reason they can wind them so high (15000 rpm or so). The higher rpm's also help prevent detonation.

-Troy

[wdb]

the last post in 924 racing today is about F1 racing engines,I dont remember if it mentioned compression or not.