[belacyrf]

Ok, I'm wondering... what does increasing the compression ratio really gain you? I mean I've done some calcultions on some of the web calculators, and raising from say the stock SVT ratio of 10.2 to maybe 11.1, I think I gained 5-8 whp! I mean is that right?

It seems to me, 5-8 whp is not worth the added risk/stress to the engine. Is there more to this than I am aware. Well I'm sure the answer to that question is "yes" since I know diddly about engine internals. Can anyone enlighten me as to the wonderful benefits of a higher compression ratio?

 

[TheOtherRobert]

yeah i dont know. That doesnt sound right to me either, but then again, i dont know much about it. But im sure raising compression to 11.1 would do a lot more than 5 hp...

 

[methknome]

your gains really depend on your mods, and the type of engine. Those calculators are not very accurate. Like the 1/4 mile calculators...hmmm lets see, is that time for a front wheel drive, rear wheel, awd, fron engine, mid engine, rear engine, how much wheel spin?

You'll know what it does when you mod the hell out of your car and then up the compression. Look at a turbo, all a turbo really does is up the compression.

 

[BUR_ZX3]

all I know is you REALLY risk detonation... especially for us on the track, as soon as you start upping the compression. I can't remember what 91 octane is good up to...

 

[belacyrf]

Well I found some other materials.. and it looks to be like for each compression point your raise it, you'll get about 2% increase in power. (It's a rule of thumb anyways) but it falls in line with the calculators and so on.

Hmmm seems like it's not even worth it. Seems like it would be better to just port and polish a head, adjust some of the valve angles, and then leave it at that. The added compression doesn't seem to offer as much, for the price you have to pay for the higher octane and worst case scenario.. the increased likelyhood of deto.

 

[slidemaster]

all I know is you REALLY risk detonation... especially for us on the track, as soon as you start upping the compression. I can't remember what 91 octane is good up to...

Depends on spark and fuel mapping as well as cam timing

 

[CheaperSleeper]

Looked at from the opposite direction, if you are going to P&P head, you may as well up the compression. The cost is negligeble, you get a nice surface for the new head gasket, and a few extra horsies. Here in Texas you can purchase 93 octane anywhere, which should be good up to about 11 to 1.**

**Note: this is just a talkin out my ass figure so don't blow your [censored] up and blame me. It is about right though....

 

[Blue01Street]

In any discussion about CR the difference between static and dynamic compression needs to be accounted for. If you are installing stage 2 or larger cams you are going to LOSE dynamic compression and LOSE some bottom end torque. That's why you want to raise your static CR when going to a larger cam with more overlap. You would worry more about detonation with a milder cam and a CR above 10.5:1 on pump gas. With more overlap, longer duration, higher lift, and aluminum heads 11:1 is not too much for pump gas as long as you keep the timing advance under control. The area I live in is at 4200 ft. I can run safely on 89 octane with my 10.5:1 static CR and stage 2 cams because the altitude lowers my dynamic compression. If I go to sea level I need to bump it up to 92 octane.

 

[belacyrf]

Blue.. thanks man, that make sense, but at the same time, you'll still only see the 2% power gain. It just seems so miniscule, it's just not worth it. If you ever have to retard timing to avoid deto, then I believe it's definately not worth it, but assuming spark is unadjusted, then at least there is a little upside.

 

[doogie78]

In any discussion about CR the difference between static and dynamic compression needs to be accounted for. If you are installing stage 2 or larger cams you are going to LOSE dynamic compression and LOSE some bottom end torque. That's why you want to raise your static CR when going to a larger cam with more overlap. You would worry more about detonation with a milder cam and a CR above 10.5:1 on pump gas. With more overlap, longer duration, higher lift, and aluminum heads 11:1 is not too much for pump gas as long as you keep the timing advance under control. The area I live in is at 4200 ft. I can run safely on 89 octane with my 10.5:1 static CR and stage 2 cams because the altitude lowers my dynamic compression. If I go to sea level I need to bump it up to 92 octane.

So me living in FLorida with 11:1 or maybe 11.5:1, I'll need race gas? Not really worried about it cause I don't drive that much

 

[ZXmurph]

<Shrug> just seems like the age old tuning question whether it's N/A or FI........the maximum compression possible for a given fuel quality, management and advance curve is going to be the most efficient, and make the most power.

When not into deto, compression is actually not a huge factor in engine stress compared to rpm,and absolute piston speeds due to stroke, rod length, subsequent ratio etc.

A sound example of this is the 217k I have on a 21:1 compression ratio VW diesel....low rpm, low piston speeds due to rod ratio= little or no stress in spite of the high compression needed for ignition.

 

[blueovalbaboon]

think about back in teh 60's...stock CR on any engine (then or now) tends to vary between 8.5 and 12.5. that's )12.5-8.5)*2% = 8%. What's 8 percent of 300 hp? 400 hp? 500? Yahhh nice fat gains from 24-40 hp.

Or even look at a stock engine. I think our CR is at abt 9.0 for a stock setec...make that little guy about 12 and you're getting a 6% gain...10.5 hp gain for the 50 bucks deckin a head costs you...much better than the bolt ons at price/hp.

just putting it in perspective. itty bitty percents mean a LOT at even relatively low HP levels.

 

[belacyrf]

Yea but at the same time.. lets keep in out our perspective! We have 150ish whp cars. So 2% is not much.
I'm talking about my car specifically. 10.2 stock CR, bumped to 11.1. Currently running 165whp.. so 2% is going to give me 3.2 whp! The equivalent of an underdrive pulley, virtually unnoticeable. The SVT currently requires min 91 octane.. so what does a bump of 1 compression point do to the necessary octane? I was just hoping there were other gains to be had by compression ratio increase (IE Throttle response etc...)

Murph has a point about there not being any added stress on the engine, but my worry is 110 degree weather with access to only 91 octane... how close to the knock threshold am i? If i have to worry about that, it seems the increased CR isn't exactly worth it.

 

[CheaperSleeper]

Bela, obviously from your perspective it is not worth it. End of debate.

However, folks have been running high compression on the streets since hot rodding began. The 2% figure also sounds a tad low.

Very few people are going to go through pulling the head off and putting it back on for the amount we are talking about, but if it is off for P&P it is a nice added bonus.

Since I have a regular Zetec, I did have a small amount milled to go with my P&P. But if I add a point of compression to the Zetec I am only at 10.6 to 1, the SVT engine is halfway there already so you can't do as much and still feel secure. Different engine...(shrugs)

 

[belacyrf]

Cheaper, the 2% figure is a rule of thumb that I have read. They said the gains you have decrease as you go higher into the compression ratio. A jump from 7 to 8.0 will give about 3% and a jump from 13 to 14 gives about 1 percent or so.

I was just really hoping it offered something more. Oh well

 

[OmniFocus]

See.. This is something that always confused me a bit. You see small gains on large increases in street cars, but...

We had a formula car with a 2.3L Oldsmobile Quad Four running 13:1 C/R. It dynoed to the tune of 315hp at around 8000RPM. Now, a stock Quad Four made something like 150hp... Sure, it had a super-exotic head, valves, cams, etc., but I'd always assumed that massive horsepower was due largely in part to the compression ratio... Is this not the case??

 

[CheaperSleeper]

The old hot rodders rule of thumb that I've heard many times was that milling .035 was worth about 8%......

I have no data to back it up, just throwing it in...

 

[blueovalbaboon]

omni...look at my numbers... and blue01's post. The higher the HP, the more they matter. THe greater the overlap, the more cr numbers matter again.

And it still remains that for a regular (nonSVT) zetec, deckign nets a very good price/hp. VERY good. Even on a stock engine. 10.5-21hp, depending on whose rule of thumb you use, for about 50 bucks. I liek those numbers.

 

[Solaris]

all I know is you REALLY risk detonation... especially for us on the track, as soon as you start upping the compression. I can't remember what 91 octane is good up to...

all i know is at 14 u need to start using airplane fuel

 

[OmniFocus]

all i know is at 14 u need to start using airplane fuel

Not airplane fuel specifically.. Just high octane, which is fairly easy to come by for a race car.