black_89_vette

Right now, I'm at a really low CR (9.3 ish on paper). If I went up to say 11:1 what kind of octane requirements would I need?

On 91 pump gas, what kind of safe compression could I run with a base timing of 6?

Is there a good balance I can acheive between bumping base timing and adding compression?

speedjohnson

It really depends on your engine management system and your camshaft.

Fuel injection and knock sensor will help alot.

A small duration cam will develope more cyld pressure and will tollerate less static compression ratio. A large cam will leak down alot of the cyld pressure during overlap and will tollerate a higher static c/r.

I'd say: If you're using a cam of 224 degrees @ .050 or less, then you'd better stay with 10 to 10.5:1 c/r in your fuel injected vette.

If you go above this c/r then you'd better have a big cam. I sucessfully ran 11.3:1 c/r on pump gas with a street roller cam in the 244 degree range. sj

Trog

My brother has a '67 elcamino w/ 11.5 327 & a mild cam. Will not run on 91 without octane booster. Well it will run but it knocks & pings like a total b*tch... :chevy

black_89_vette

my cam is 222/226 split duration @ 0.050".

I know my compression is kinda low, but what kind of results could be made from 9.3 to 10.3:1?

Is there some kind of "rule of thumb" between hp and CR? Like adding 1 full CR point will yield x hp?

speedjohnson

Yes. there is a rule of thumb, I believe that you increase your torque 10% for every 1 point jump in static compression...however, at the point you get detonation, your torque and horsepower will fall through the floor as your engine management system starts to pull timing out,

Mid 10's sound just about right. Do you have extremely hot summers? How's your cooling system? Keeping the intake air and motor cool might help to fend off detonation. sj

Oh, I see you're from Ontario! Forget the whole overheating thing...heck in subzero weather you just might pull off 11+:1 c/r :D


[Modified by speedjohnson, 12:28 AM 1/10/2004]

CFI-EFI

One of the magazines (Popular Hot Rodding?) did a test 6-12 months ago. They established that one full point of compression was worth about 4% in horsepower. It is worth trying to establish an optimum when you are ordering parts for a new engine combination, but *I* wouldn't spend a lot of money, fine tuning an exisiting package. Good luck, and...

RACE ON!!!

black_89_vette

Heh, actually both times I took my car to the track, the temps were around 90 the first day and about 80-85 the second time. I got a sunburn the first time I went.

I just recently attained a laptop with an ALDL diagnostic cable and have started using DataMaster. With a slipping clutch and the winter here, I basically only did an idle data log to get used to things.

The summers here are hot and I did have a failing cooling system until I found that one of my fans wasn't working. Now, I run 175-180 on highway, and usually around 180-190 in town.

At the track, I can't seem to wait to let the motor cool down, I just wanna keep running and get as many runs in as I can and get my money's worth.

I do believe that for some reason I am getting knock counts though. Not being able to scan it at the moment I'm not sure how many, but with a 3/4 throttle quick rev while in neutral with no load on the engine (basically useless but I was having fun with new software) I did get a lot of knock counts and a knock retard of 7.0.

I'll have to wait until I can get it on the road and see what it really does under load with the laptop.

I do remember it getting knock counts with my mechanic and his snap on scaner around 4000-4500 RPMs.

With such a low CR (9.3) on 91 pump gas, I'm still trying to find out why it would detonate.

CFI-EFI

As I imagine you are aware, the ECM doesn't go into the PE mode under your test conditions. Also, as you probably know, detonation isn't the only thing that will produce knock counts. Remember the test for a functioning knock sensor? Good luck, and...

RACE ON!!!


[Modified by CFI-EFI, 6:02 PM 1/9/2004]

neverendingproject

read this, it'll answer a lot of your questions.
http://members.uia.net/pkelley2/DynamicCR.html

ZD1

altitude will help as well--lower density will act to lower effective pressure before ignition. Also temperature will play here too. Colder temperatues will keep the knocks down too (below 65 def F)

JAKE

This topic was kicked around not too long ago, but it's important. Mag articles have been written on the subject and basically what you need to do is to look at two difference CRs.

Static Compression Ratio and Dynamic Compression Ratio.

Static compression ratio is simply the ratio of swept volume to compressed volume; this is the number that most recognize and use.

More important though is dynamic compression ratio, what the engine actually sees as its running.

An example, you could build an engine for the street with 12.5:1 static compression ratio and install very long duration camshaft and use pump gas and the engine wouldn't detonate at the RPMs you'd normally see on the street. Some say the engine would still detonate up top, but you'd never get up there anyway. The engine, however, would be a REAL DOG, but it wouldn't detonate.

Why, the long duration would have so much overlap that a lot of the calculated static compression would simply go out the exhaust and never actually get "compressed".

One of the tricks that many use - when they mistakenly build an engine with too much static compression ratio - is to put in a longer duration camshaft. The static compression ratio remains the same - since swept volume and compressed volume hasn't changed - BUT dynamic CR would.

So, without getting much more wordy, your camshaft and your CR have to be considered together.

Static CR has to be increased to cover up the loss of bottom end when a longish duration camshaft is installed. You'll notice that many of the camshaft offerings tell what the CR should be; the longer the duration of the cam the more static CR is called for.

Even some of the high leak down hydraulic lifters call for a certain maximum CR. Their leakdown rate makes the engine seem to have a shorter duration camshaft installed and can put the engine into detonation if the static CR is too high.

With a FI engine and aluminum heads, 9.5/10:1 static is a good safe CR to shoot for. 10.5:1 with the PROM properly tuned; higher than that will call for some really sharp tuning and monitoring.

Some stock engines run CRs that were unheard of a decade ago because the engineers put in overtime addressing all the factors that cause detonation. I'm sure they lost their share of engines in the process too. Funny we never read about those.

CR is another one of the things that is "better to have not quite enough than to have a little too much".

And, yea, 4% or so per point in static CR. is what I understand too.

Just my understanding of it.

Jake

LD85

Pay close attention to your quench dimension,, down load this program
http://www.empirenet.com/pkelley2/DynamicCR.html