Compression and engine displacement, relating to HP? and speed factors

[232FIed]

hey, i'm a student and i have a few questions to anyone who can answer them or at least give me a few examples of why or why not these concepts are not an issue.

first off: does a higher compression ratio automaticly mean that the engine will produce more horsepower? say you take two identical motors. put a thinner gasket in one and test them. how will one change performance wise from the other?

with what dimensions of designing a motor, bore stoke compression layout (flat,inline,v) weight material, ect. ect. can you calculate how much horsepower it will produce before building it? is there a formula?

if you were building your own sports car, what would you prefer, midengine or front? would you rather frontwheel, rearwheel or All-wheel drive? Pushrod, SOHC, DOHC. 2valves 4valves 5valves per cylinder?

why do turboed sportcoupes (evo, sti, gsx, ect..) have low compression ratios in the mid to low 8.0:1 area. i know safety becomes an issue which is why lower is better when adding boost but should you then lower your compression if your car has a 9.6:1 CR if your planning on turboing your motor?

i have other questions but i'll wait and see what you guys think before i go on. just keep the posts coming -

[TheSilentChamber]

All of these have been discussed in detail before, the last three very recently. Simple anser form:

1. Higher compression to a point when tuned for it will make more power than an identical motor with lower compression.

2. I'm not sure how you calculate the power before you build it, but yes everything can be a factory, differnt motors have differnt power band tendencies, differnt materials will offer differnt properties, rod/stroke ratio comes into play- numerus varables.

3. design of the car comes into play, most of your high end sports cars will have mid engine design with mutiple cams and high number of valves.

4. With boosted applications lower compression you run less risk of detionation.

[curtis73]

1- a little bit, yes. The main reason is that higher compression makes more efficient use of the BTUs in the fuel. More BTUs released, more pressure on the piston, more power. All things equal, a one-point increase in compression gives you about a 5% increase in power. An 8:1 engine making 100 hp would make 105 hp at 9:1. In my opinion its not worth the ignition tuning headaches

2- do an internet search for dyno2000. Its an inexpensive volmetric efficiency model that is relatively accurate. It uses a windows-type menu selection in which you dictate engine size, head flow, cam specs, etc and it spits out a dyno chart. Intensely fun and if you truly read the owners manual, its darn accurate. Short of that, visit this site
http://www.bgsoflex.com/flowcalc1.html
It lets you plug in a cylinder head flow, engine displacement, and number of cylinders. What you get is its peak HP potential and at what RPMs it occurs. Of course it means very little because you can't plug in cam specs or intake configuration, but it just relates airflow to power potential. There are subtle differences in layouts, but for the most part you can't predict things like that. All things equal, a V8 of 4" bore and 3" stroke has the same potential as that of an inline 8 with a 4" bore and 3" stroke.

3- I personally prefer front engine rear wheel drive. They are the most simple layout and properly setup will outdrive my abilities at this point. Working on them is simple, parts are everywhere for whatever you wish to build, weight distribution is still quite good, and its what I'm most used to. In general, a midengine AWD would probably give the most performance in the twisties, but a front-engine rear-drive is still the choice for drag racing. Pushrod engines are my choice, but only because to get DOHC V8s like I prefer, you have to put up with a ton of electronics. You can make or buy a carbed intake for a ford 4.6, but how do you drive the ignition? smog equipment? Not for me. I can get the same power out of a pushrod engine with a good old fashioned Edelbrock and an old-school distributor. But again, that's personal taste. If I needed the extra breathing provided by the extra valves or OHC and I had the in-depth knowledge required to hack the EFI computer I might consider the switch, but not now.

4- Lowered compression is almost a requirement on pump gas. As I discussed in #1, lowering your compression a full point will only cause you to lose about 5%, but the resulting ability to throw 10 psi into the mix will increase output by about 70%. Compression ratios often need to be dropped to allow boost. An engine that starts with 9.6:1 could be boosted, but only about 4 psi on pump gas. Lowering compression is not the terrible thing everyone thinks it is. I have an 8.6:1 chevy 454 making a very happy 420 hp on cheap gas. I could have picked up another whopping 20 horses by going to 9.6:1, but then I would need at least 93 octane (not available in LA) and some careful tuning of the spark curve. As it is I just load up on cheap gas and put my foot down.

Definitely do a search. This is all so incredibly overdiscussed, some of it just yesterday. Once you've searched if you still have questions please feel free to post them; some of us like to hear ourselves type

[MattSF20]

So if I lowered the compression in my car I could increase boost pressure and get higher output overall?

[Reed]

Almost if not all of this has been disgussed, but as Curtis said some of like to hear ourselves type so i will respond to the points that really interest me which is stuff about sportscars.

If I were building a sportscar (and i use that term loosely) it would be either mid-engine rear wheel drive (like my spyder yay) or it would be rear engine all wheel drive. it is best for a sports car to have a mid or rear engine so it is heavier in the rear. this is important beacuse the car will rotate around the polar moment of inertia which is complicated but means that the car will start to lose grip at a higher speed so that a car can carry a higher speed through a corner. also with a more rearward weight balance the car behaves better under breaking.

rear or all wheel drive is a no brainer cause they allow for better acceleration out of corners.

as far as cylinder configuration you have to consider the weight of the engine and also space available. an inline engine is lighter than a V engine which is lighter than a flat engine. now if we are talking about an 8 cilinder then an inline engine is too long so a V is much better even though it would weight more cause there is just more metal used for two banks of cylinders and two heads. now a flat engine may be better than a V engine because it has a lower center of gravity than the V8 but it also could weigh more so it may not be better. the flat engine is a bad choice in a car where the engine is a stressed member of the chassis because the flat engine provides less rigidity than a V. I think it was last year that the F1 renalt had a wider angle between the cylinder bank to give it a lower center of gravity but also used carbon fiber supports to reclaim the lost rigidity.

there is sooooo much other stuff to consider but i have already typed too much

[beef_bourito]

I'm going to expand a bit on curtis' comment (not something that happens often) but the reason you can run more boost on a turbo car is because higher compression creates more heat (if you took grade 11 chemistry or physics you would know that a change in pressure is directly proportional to a change in temperature). if this heat gets high enough to burn the gasoline without a spark, the fuel ignites. this is bad because if the fuel ignited too fast, the engine is pushing up and the expansion of the fuel is pushing down so it's like taking a sledge hammer to your engine (actually, probably worse), this is known as detonation, knocking, or pinging.

also lowering the compression for a turbocharger will provide longer lag because the exhaust isn't as hot untill the turbo spools, this is pretty small but it's the reason you don't see stuff like 2:1 compression with like 150psi, the engine would stop making power before the turbo even spooled. and just changine a head gasket won't provide any power worth talking about, you MIGHT get 1% more but i doubt it. you'd probably have to change pistons or heads.

If i were to build a sportscar, it would be either mid rwd (rear wheel drive) or mid awd (all wheel drive) because of the center of gravity stuff that was metioned before. i'd probably go rwd because it's the better setup for a dry tarmack and you'll lose some of the off-the-line acceleration (only 2 wheels trying to push instead of 4) but you'll go faster out of the turns. this might not be 100% correct, im just using current sportscars, supercars, and racecars as an example (ferrari enzo, all f1 cars, carrera gt, viper, all other ferraris, pretty much anything except the murcielago and maybe the gallardo). if it was for rallying, offroading, or a wet track, i'd go awd because there's more traction when you slide.

different materials, setups, dimentions do affect an engine. lighter materials for the block will reduce weight but might not be as strong, lighter pistons, rods, etc, will allow the parts to accelerate faster and allow for faster and higher revs but won't be strong enough for really high compression or boost. there's no way to calculate exactly how much because there're too many variables (intake/exhaust flow, heat transfer, compression, heat of the pistons/internals, valves, etc, etc, etc). you can get a ballpark with programs like what curtis mentioned but you won't ever see something that calculates power really precisely other than a dyno.

[Reed]

sorry to hijack but i think this is relevent...

How is a Mid engine AWD drivetrain configured?
how can a car be front mid engine like an S2000?

[beef_bourito]

there are various configurations, i can't remember them off the top of my head but if you do a search for mid engine awd you should find a thread containing it. it was started not too long back.

In terms of the s2000 thing i've never heard of mid-front. they might be saying that the engine is toward the midle of the car but in front of the driver, or they could be saying it has a 50:50 weight distribution with a front engine configuration.