Big Mike

iTrader: (4)

*And still run on pump gas? and *What's the highest compression ratio currently being run (race gas)?

VINCE

iTrader: (1)

To tell you the truth Mike. I do not think anybody knows really.. I asked that question on ls1.com and all I rec'd was we think. We are going to need a vendor who has experimented with this to tell us..

Nine Ball

iTrader: (38)

It will depend more on dynamic compression ratio, not static compression ratio. Dynamic involves the cam timing events and can make a motor run much differently than "planned" when just calculating by the static method.

I'm running 11.1 CR right now, and only use 93 Octane. when I have the catalytics on the car and its very hot outside, it will sometimes ping under heavy load (5th gear, freeway, at lower rpm). With the cats off, no pinging ever.

I'd stick with less than 11.5 for pump gas when using a "streetable" cam that might keep dynamic factors less dramatic.

Tony

VINCE

iTrader: (1)

Tony do you know of anyone that is running 11.5:1 on a 346cu. I know of guys with strokers running higher compression..

Pro Stock John

iTrader: (34)

Strokers eat up compression.

I have heard a few 346ci cars ping in the middle of the summer. One was a Westech heads/cam M6 that I got to drive one day. I put in 5 gallons of 100 octane and it went away... Came back after I burned the tank off...

I'd say 11.25:1 is the max for a street car. Better to run on pump gas and not have detonation issues.

2quick4u

Vince and Big Mike, notice that Tony is running 11.1:1 CR. That should give you some ideas. Hint, hint. <img src="images/icons/smile.gif" border="0">

[ November 14, 2001: Message edited by: 2quick4u ]</p>

Big Mike

iTrader: (4)

I'm running 11:1 also. Back when I ordered my H/C from MTI I requested they shave off the .30 for 11:1 compression. I think this came back to haunt me because I had the Knock Retard removed. I obviously had issues somewhere along the way because now I have piston ring problems. Anyway, I getting ready to plan my next setup and I'm looking to go with the same 11:1 or just a tad higher like PSJ mentioned 11:25:1, just this time add back in the Knock Retard and make sure the car is tuned perfectly!

[ November 14, 2001: Message edited by: Big Mike ]</p>

VINCE

iTrader: (1)

Teasing me again 2quick.. I get the point, but Jason has no issues with his 11.3:1 compression. There is a local guy running 11.3:1 compression down here in florida on pump gas and they are not having any issues.. I am going to try the 11.3:1 compression and if does not work I will get the money from 2quick to get a stroker to eat up the compression.. <img src="images/icons/smile.gif" border="0">

MIGHTYMOUSE

iTrader: (59)

yea gtp heads can be 11.3, are is 10.8 i think

Colonel

Not having cats (actually, a free flowing exhaust in general), cool plugs, cool heads (cool coolant), cool air, and high density altitude, all contribute towards being able to run high C/Rs on pump gas. Like Tony said, it's really the "dynamic" or "effective" C/R that matters, not the static C/R. Of course, static plays a major role in determining what the dynamic C/R will be too.

ChrisB

Actually not having cats will probably lower the compression ration you can run safely (assuming you have a decent set of headers) - if you have cats you have more back pressure, causing more burned/inert exhaust gasses to stay in the chamber - this acts like an EGR and dilutes the a/f charge, reducing the thermal delta and thus your chances of pinging (and power output).

They key points have really been hit though - cam timing has a huge amount to do with what you can get away with. The same compression level that you can achieve with a 226/234 114 cam will cause massive problems with a stock cam.

Generally more intake curation will cause the intake valve to open sooner = more overlap and a higher compression tolerance. The intake valve will also close later which = more compression loss.

More exhaust duration = earlier exhaust opening = less cylinder pressure = more octane tolerence and a later exhaust closing = more overlap/charge contamination and more octane tolerance.

A narrower LSA will increase overlap which will have a slight impact on octane tolerance, but will kick your dynamic compression wy up with a later exhaust opening and earlier intake closing - so the net effect will be less octance tolerance. A wider LSA will have the converse effect.


And kind of intake path restrictions will allow you to run a higher static compression ration, as the restrictions reduce VE and prevent you from actually seeing the full compression (With respect to atmospheric). This is why nascar cars can get away with 15-16:1 - the tiny restrictors mean they are really seeing sane compression levels. this is actually a pretty decent analogy for a big cube LS1 motor, as the intake functions similar to a restrictor plate, and some of the same techniques can be applied.


Chris

VINCE

iTrader: (1)

What? ChrisB I hope you know what you are talking about. If not then you should be a politicial. All of that sounds good.. <img src="images/icons/smile.gif" border="0"> I was lost on the first paragraph..

Colonel

The only point I'm confused about is the back pressure. I understand what Chris is saying and it really does make sense...it's just not what I've witnessed on my two LS1 cars. Maybe there's another factor being overlooked? Also, Jayson Cohen told me that they've seen significantly less KR on cars without cats (MTI does ALOT of dyno testing while using Tech II.)

Y2K_WS6_T/A

Can you say 17:1?

http://www.motortecmag.com/archives/...JUN010101.html

<img src="graemlins/fluffy.gif" border="0" alt="[Fluffy]" />

Big Mike

iTrader: (4)

That black Camaro isn't 17:1. That's Mic5ey car from LS1.com. He just went 10.99 @ 124.xx the other night. I beleive his setup is 11:1.

[ November 15, 2001: Message edited by: Big Mike ]</p>

ChrisB

Well cats = backpressure basically - better cats not as much, but still, backpressure.

Now your overlap tunes your VE to a specific RPM level - e.g. the actual time (second) that the oerlap occurs allows for the amount of flow you need exactly at that rpm. Since the overlap is fixed in respect to degrees, but the actual duration (of those degrees) varies with RPM, you will only be optimal at/around one rpm point.

Now if we have more back pressure we reduce the rate of flow, thus lowering the peak VE point for a certain set of cam specs.

If you are running enough static compression that you are relying heavily on your cam, etc. to bleed it down to a streetable level, then doing something like this would increase your cylinder pressures at lower rpm, at which your are more susceptible to pinging.

That would only be a specific case event where you had a high compression/smaller cammed motor though.

At least that would be my best guess to explain that phenomenon <img src="images/icons/smile.gif" border="0">


Chris

Speed Demon

iTrader: (14)

Dang! ChrisB how did you get so smart?

I understand what you are saying but no way could I explain it to someone else as elaborately.

Good info!

This board rocks!

2quick4u

[quote]Originally posted by ChrisB:
<strong>Actually not having cats will probably lower the compression ration you can run safely (assuming you have a decent set of headers) - if you have cats you have more back pressure, causing more burned/inert exhaust gasses to stay in the chamber - this acts like an EGR and dilutes the a/f charge, reducing the thermal delta and thus your chances of pinging (and power output).

Chris</strong><hr></blockquote>

Chris I totally disagree with this. Like Colonel my experience with every V8 I have ever owns runs contrary to this. I believe reduced backpressure reduces knock for the following reason. The back pressure keeps heat energy in the exhaust concentrated at the exhaust manifold. This heat energy causes cylinder head temps to run higher than what they would with reduced back pressure. The less back pressure you have, the more heat energy in the exhaust that is being rejected through the exhaust pipe and out of the car rather than being radiated and conducted back into the heads from the manifolds.

ChrisB

I don't think it's really heat energy in the exhaust manifolds that's transferring throught he heads/valves, etc. and causing detonation - this can definitely make your headers glow, etc. (though we are talking more backpressure than a normal cat would induce) - the for heat energy to buildup in the manifold/header it would still have to travel through valve or head/head + coolat to make it to the chamber. I don't think you can achieve a big enough delta to make this transfer significant.

We know an EGR works (to reduce detonation) by injecting spent end gasses into the combustion chamber - these gasses are straight out of the exhaust and as hot as they get - if injecting these gasses into the combustion chamber doesn't induce, but rather reatrds pre-ignition/detonation, then I don't think heat from the same gasses building up outside the port would do it.

The only way I can see cat's actually causing pinging problems is if you have too much overlap (for the rpm band you are getting pinging in).


Chris

VINCE

iTrader: (1)

You guys go back and forth. Maybe I will learn something.. <img src="images/icons/smile.gif" border="0">