View Full Version : Decreasing Compression, increasing HP in NA(SC)

03-07-2003, 01:31 AM
Hey all, soooo... it looks like decreasing compression is going to be the best bet for overall HP and performance. It seems to me that it in an ideal world though, we would still want as much HP as possible out of the NA engine. So... GIVEN that the compression is being reduced (probably down to 8.0:1) what are some ways to increase the HP? A couple of the gimmies would be:

Underdrive Pulleys
Rocker Rollers

What else? Lighter rods and pistons??

I'm not super familiar with the internal engine details, I understand the whole concept of crank, bearings, rods, pistons, head valves, rockers, camshaft(?????) (unless I put the crank and the cam at the wrong ends??)

What kind of adjustments can be made to get peak horsepower out of an NA engine? I would think that the more HP the engine makes NA (keeping in mind the reduced compression) the more HP it will make when the supercharger is added???

I've also thought of things like switching to the aluminum flywheel and what not (not really an engine issue... but I guess underdrive pulleys aren't either...)

What else???

Thanks :-)

03-07-2003, 11:35 PM
bumpbumpbump... of course it may just be time for me to see if there is a "basic performance engine building theory for dummies" book out... I would think that the question of how to get increased HP while at the same time reducing compression would be a bit unusual though??

03-07-2003, 11:58 PM
8-8.5 is perfect for most cars with boost, unless you plan on running insanely high octane or different fuel

03-08-2003, 03:16 AM
I'm not sure why you want to reduce the static compression ratio. The question would be more about increasing or leaving stock.

When building a car that will be supercharged or turbo charged you need to build it much stronger than you might if it was going to be naturally aspirated. To achieve strength, you often need to use heavier components than you might use if you are trying to build a high performance engine. Much of that is because you are going to achieve much greater power than the same engine would be capable when naturally aspirated.

And there are books all over the place for building engines. Though building an optitmum V-6 is quite different from a V-8, you can learn much of the basics from a book on V-8's. (The biggest differece would be optimizing the head flow characteristics)

I'd start at the library.

03-08-2003, 03:26 AM
oh i get it now I think.

Get some better cylinder heads and cam , along with a larger MAF meter and throttle body for starters. What runs better for N/A(no blower etc) is not always better with boost or nitrous, different flowing heads and cam specs etc.

03-08-2003, 05:54 AM
The reason for reducing the static compression ratio is that it should allow for more boost without causing detonation... if I increase the static compression ratio, I will not be able to run as much boost. Therefore (seems to me) that ideally, I want to get as much HP at as little compression as possible (finding the happy medium) and then if I can run... I don't know? 16-17 pounds of boost on 91 octane (best we have at the pump here in AZ) or 92 (only on post) with no detonation, thing should fly like a bat out of hell... had problems with it when I was trying to run a 10% pulley on the engine stock (with del res and 3rd cat and bigger exhaust piping... bigger IC tubes, 3/4 spacer, 76mm C&L, bigger fresh air intake, and stuff ;-)

03-08-2003, 02:38 PM
I guess this just dosent make sense to me. If you decrease compression anytime the engine is not on boost , like just driving around town you will have a lot less power. Then If you increase boost to 19+ pounds the blower will have to work a lot harder decreasing its life. More power will be needed to turn the blower. And te most important point is now you have a big heat pump. Which is less effective. I read on one of the threads that our blowers are efficent to around 13-15 pounds. Tim

03-08-2003, 03:20 PM
The most important 3 factors in making more power in an SC is Breathing, Breathing and Density. ;)
Changing compression and blower speed without increasing input and exhaust flow will change how much detonation and where the power comes on, but not very much in overall performance.

Concentrate on low restriction, hi volume intake and exhaust, higher blower speed, which heats up the the air more, requiring better intercooler efficiency and creates greater density. Porting, polishing and bigger valves also improves flow volume.

Of course, with more air, you need more fuel - bigger pump & injectors. :p

03-08-2003, 03:45 PM
Well, I guess I must be nuts. I do have all the regular stuff done. But I went the other way on compression 9.89 to 1. Slow the blower down with a late model pulley and use a "different" type cam. On paper which may or may not be accurate. 470 foot pounds of torque, 412 horse power from a very respected and reliable source. Cut the timing back and use a good tuner with a good program. Running 12.8 pounds of boost and the best gas I can get. Tim

03-08-2003, 04:38 PM
I'm not absolutely positive on this Tim, but I think Coy went the route of high boost to get his power. :rolleyes:

Without taking the same setup and changing the compression and boost, I don't think we're ever going to resolve that question. :p

BTW: Nasty #'s :D

Oh yeah, I usually get my gas from brussel sprouts. :(

03-08-2003, 07:05 PM
Physics. Improved potential for power from an internal combustion engine comes from more combustible materials in the smallest possible space. The more fuel/air mixture in a smaller place at the time the spark plug sparks, the more power that will be created.

Trying to find an optimum combination of variables to allow for maximum power under any condition is very difficult. But trading Static Compression to allow a larger change in effective compression just doesn't really work.

What Ford and Eaton engineers did with our car is try to optimize the engine configuration while utilizing the optimum power band of the Eaton supercharger. Of course they had to optimize for power and emmissions. So there has to be some room for tweaking if we are just optimizing for power. But changing the compression ratio dramatically, one way or the other, is likely to pull the engine out of the optium power band for the Eaton supercharger. Can it turn more than 15,000rpm? Yes, but is it operating at it's optimum when it is? No.

So, in my mind, looking at compression ratio, I would leave it at stock, or increase it slightly. Because then any change to the SC will be to leave it alone, or slow it down. Both of which cases involve less stress on components, rather than more stress.

03-09-2003, 06:14 AM
Alright, perhaps I should rephrase and reask??? Let's assume that we leave the static compression ratio the same... whether it is 8.2 or 8.4 or 8.5 I don't know... anyway... what can be done to the engine internals to maximize the HP without adjusting the compression ratio in anyway? It seems to me to be logical that an engine the makes 200 hp NA with a compression ratio of 8.5:1 (for the sake of argument) will make more HP with an SC at say... 12 pounds of boost (again for the sake of argument) than an engine that makes 150 hp NA with a compression ratio of 8.5:1 that has the same supercharger making 12 pounds of boost... I might be oversimplifying, but again, following my example of the switching to roller-rockers (which are supposed to be good for 10 or 15 hp) what are things along those lines can be improved to increase the HP of the NA engine so that it produces more HP as a finished project? I realize that a bigger or more efficient intercooler and better breathing, etc etc will improve horsepower, but these mods (along with exhaust, the maf the throttle body <etc>) are external to the engine... as my engine has imploded and I have the opportunity to do internal strengthening and upgrades during the rebuild, I'm just looking for things to watch for and suggestions on what might be done to get more HP.

Back on the reducing compression issue... I realize that there is a critical minimum... obviously I don't want the SC to be kicking in if I want to maintain a steady 85mph on a flat road... I know that NA our cars can currently easily maintain over 95 mph (because was able to maintain that speed without a problem with my SC belt off)...


03-09-2003, 10:11 AM
Internal parts for more horsepower also need to be strong enough to handle all the bolt on power adders. The obvious stuff better rods and pistons. For brute strength 351 rods and custom pistons at whatever compression you decide on. Crankshaft gurdle. Coy miller makes a real good one. Cam take your pick. It will depend on heads and exhaust. Better heads is a must If you want any serious horsepower. bigger valves and porting. Balanced rotating assembly. Good for more power and engine life. O-ring the block or heads. Now you have a strong base to build what ever you want. Tim

03-09-2003, 10:22 AM
and just talking short block, I've got balanced heavy duty rods with custom 30 over pistons (5-10% higher compression) = aprox. 4.0L. Going over 30 may create cooling problems, tho some have gone as high as 50. Also make sure all internal tolerances are correct including deck heighth and level. A BHJ Harmonic balancer helps also. The less the engine has to fight itself, the more final power you get. :p

Whoops, Tim got in ahead of me and included cams. The cam gets you into a lot of other issues - rockers, valve clearance and springs etc. But that goes back into breathing. :rolleyes:

03-09-2003, 01:12 PM
What he said. Tim