I was wondering how high I should go with compression on a new SBC. I may take some long trips and who knows what gas will be available. Can I safely go to 10:1 on 92 octane? What will the affects be from the winter oxygenating additives like methanol, ethanol, and MTBE? The engine will have full roller cam and vortec style heads.

im running a 10.5:1 388 stroker on 93oct

With what you've stated and you're concern for fuel octane availability, then why take the risk when you can shoot for perfection (.040" quench) and 9:1 and still have a very efficient engine? On a 350 HP engine dropping from 10:1 to 9:1 was only 10 HP less and would likely run on regular, a significant $$$ savings at the pump. Compression isn't the only requirement for good performance, the combination is. Check out this calculator and see what I mean.
http://www.wallaceracing.com/calc-crchg.php

just to clarify....the real question is about the additives. How do they affect an engine designed for good street performance?
Compression isn't the only requirement for good performance, the combination is. Yes I know,
I still want some performance. 350 cubic inches, shooting for over 400 hp. :D, so compression needs to be in the high 9's at least. Not worried about the price of gas, just the ability to get what I need. I realize this is kind a vague question, so here's some details about the engine.
- New 1pc rear seal block, 4 bolt mains
- heads 64cc chambers, 200cc runners, 2.02/1.60 valves,
- full hyd roller cam, dur 234/238* at .050, .539/.548 valve lift, 112 LS,
- RPM air gap, probably 800-850 cfm carb
- piston ?dish+gasket? to get me to the CR I need?
http://www.angelfire.com/fl/procrastination/motor.html
Timing: I expect to run about 12-16* initial and bring it all in pretty quick but we'll see.
I'm told this will get me where I want to be and still be street friendly. I just don't want to put the money into it and then have to de-tune it just so I can drive it.

Please read Pat Kelley's great DCR site here at least three times to get a better handle on your question.

http://cochise.uia.net/pkelley2/DynamicCR.html

EXCELLENT INFO to me...........

pdq67

11.25:1 iron head 350 on 91 octane. Call BS if you want to, lots of guys do. I had to 'handicap' the engine a bit... retard the timing some, run it cooler than I'd like to, and run it a little bit richer than I'd like to, but it works fine and still goes like a raped ape. I think the big-ish cam helps a bunch too.

(BTW - this only happened because this engine had a head swap done last spring... when I bough heads, I thought it still had the OE dished pistons in it, not flat tops)

are those iron heads? With that cam duration you will need all of 9.5:1 to get it to work decently. I hope you have a nice high stall torque converter.
The heads will make or break your power requirements, and a well chosen head will make 400 HP with a smaller cam than a poor head.
I would shoot for something closer to the low to mid 220° intake duration, another 4 to 10 degrees depending on the head for exhaust, and a 110 lobe separation to boost the midrange output. If you shoot for a 12 to 15 cc dish, zero deck the block and run a standard composite head gasket you'll be in the 9.3 to 9.6 to 1 range, as much as I like to specify for a daily driver pump gas engine. Yes you can run more, but it isn't always successful. You do not need/want more than a 750 CFM carburator on a 350.

What will the affects be from the winter oxygenating additives like methanol, ethanol, and MTBE?


Not likely enough to control detonation, as opposed to a summer blend.

i built a 400 pontiac for a guy here with a stock weight 69 firebird and 2.78:1 rear gears. it's a cc'd 9.0:1 engine. .040 quench with a 270 comp cams magnum grind. holley 600 v/s with milled choke horn. factory heads with big valves and pocket ported. 2800 converter. makes 145 lbs cranking compression and runs 87 octane every day. he's taking out the new GTO's with it on the street here. he's very happy running low grade fuel the last 2 years !!!!! very tame and streetable all year arround. we're changing to 3.73 rear gears this spring. that outta pick it up a bit.

compression isn't everything. you shouldn't HAVE to tune arround it. just complicates things more for the street.

rustbucket79 has some really good advice there.
hope it helps.

They don't have as much energy in them b/c of them containing oxygen so therefore when they are added in quantity to regular unleaded to up it's octane and as well as to create less CO that goes into smog production, your mileage suffer's..

EPA say's E10, (plain old gasohol), will drop gas mileage about 3 to 4 percent less miles per gallon.

And E85 drops like 20 to 24 percent b/c it really needs to be burnt in an about 13.5 to 1 CR. engine to reap the full benefits of it's about 105 octane. But we can't dedicate our engines to it's use unless you can buy E85 everywhere which you can't right now..

You might find this site neat reading... "FAQ Gasoline"

http://www.cs.uu.nl/wais/html/na-dir/autos/gasoline-faq/.html

pdq67

74RAT has seen the light with Poncho power. There was a fairly well stripped Pontiac that raced at out track a few years back with an 8:1 400 Pontiac engine, home ported heads and a hyd cam, that sucker ran mid 12's all day long, suprized the hell out of me. (We did some of the short block machine work, nothing special but a set of TRW forged pistons from what I recall)

no doubt,, he was surprised when he took the GTO's with that gearing. now he says to the new-commers,, beat the GTO's,, then i'll run you. he's 19 and wanted a cheap gas/low octane use setup.
it's a very mild build. dual plane w/headers. nothing fancy. comp valvetrain.
here's a couple of pics of it:
http://f5.putfile.com/5/14713492319-thumb.jpg (http://www.putfile.com/pic.php?img=2708798)

http://f5.putfile.com/5/14713455210-thumb.jpg (http://www.putfile.com/pic.php?img=2708767)

Looks like a good combo, but if you want the higher hp out of a 350, I would look at a good head in the 190-200cc range (TFS, Canfield, AFR, Brodix). It is often said that a healthy head with a smaller cam makes for a healthy and efficient street motor with great torque.

My current setup is 11.25:1 with iron Vortec heads. Running a tight quench (.039"), Sunoco 93 and 34* timing. Cylinder pressure is right around 190 psi.

...but if you want the higher hp out of a 350, I would look at a good head in the 190-200cc range.
whoops, that should have been 200cc not 170cc. And vortec style heads.

Thanks guys. I’m still learning. Nothing like the experience from those that have been there. So even though this thread has gotten off topic (from what I intended)…let me see if I understand this correctly.

The ‘advertised’ compression ratios really aren’t true CR, they’re really just comparison numbers. They compare the computed empty cylinder volumes (piston at it’s lowest point and piston at it’s highest point). But since this is not a sealed chamber at the start of the compression stroke, the actual CR is going to be less. Is that accurate? Or oversimplification?
By reducing piston to head clearance, the tighter quench increases turbulence, therefore causing more thorough mixing resulting in more efficient combustion. So although the CR hasn’t really increased, you get a little more power.What you’re saying is that you can get the same power by increasing the piston dish cc (therefore reducing CR) and tightening quench? Would this then allow you to reduce timing and run a lower octane fuel?
I guess I’m surprised too that such a small reduction in quench area (.020 height) can have a significant effect.

11:1, .038 quench, runs on 93 octane, total adv @ 1100

Thanks guys. I’m still learning. Nothing like the experience from those that have been there. So even though this thread has gotten off topic (from what I intended)…let me see if I understand this correctly.

The ‘advertised’ compression ratios really aren’t true CR, they’re really just comparison numbers. They compare the computed empty cylinder volumes (piston at it’s lowest point and piston at it’s highest point). But since this is not a sealed chamber at the start of the compression stroke, the actual CR is going to be less. Is that accurate? Or oversimplification?
By reducing piston to head clearance, the tighter quench increases turbulence, therefore causing more thorough mixing resulting in more efficient combustion. So although the CR hasn’t really increased, you get a little more power.What you’re saying is that you can get the same power by increasing the piston dish cc (therefore reducing CR) and tightening quench? Would this then allow you to reduce timing and run a lower octane fuel?
I guess I’m surprised too that such a small reduction in quench area (.020 height) can have a significant effect.


basically 3 types of compression.
1. static compression = ratio of volume above piston at bdc divided by the volume above the piston at tdc. it's actual. and does not change once assembled. physical measuring of cc's of all the parts used.
2. kenemetic compression = (some call this dynamic compression)compression ratio from the point when the intake valve closes. same as above,, but measured from intake closing point. can be changed by changing cams, changing valve lash with solid lifter cams, advancing or retarding cam. changing rocker ratios does not alter this,, it only changes the rate of valve lift,, not the seat timing points.
3. dynamic compression = differing actual cylinder pressure achieved that changes as rpm and throttle opening changes while in motion as the crank changes speeds (dynamic),, from causes brought about by engine breathing efficiency by the selected components and throttling the engine,, which effect the amount of air/fuel mix that is trapped in the cylinder. parts that effect this are anything from the air inlet point to the tailpipe tips. and many other other things.

volumetric efficiency is a combonation of all the 3 things listed above and whole lot of other things,, including air temp, elevation, barametric pressure, throttle opening, etc... the list goes on.....
it's based on actual engine size vs what amount of air/fuel mix is actually is taken through the engine and burned properly,, in a percentage of engine size..

quench distance,, is the part of a flat top piston that almost meets the flat part of a cylinder head when the piston is at tdc. it is that measured distance from the flat of the piston deck including the gasket thicknes and deck height which sets the piston to head clearance....... when that clearance is over .050,, the effects of tight quench start going away quickly.
a 9.0:1 engine can detonate easier than a 10.0:1 engine if the 9.0:1 engine has .075 quench distance and the 10.0:1 engine has a .040 quench distance,, everything else being the same on the same fuel and all.

a combustion chamber shaped dish will utilize quench. a stock 350 chevy piston with a full round dish and valve reliefs does not utilize quench because there's not a totally flat piston deck there (on the entire surrounding perimiter of the combustion chamber of the head) that brings the piston deck up to within .050 for the complete area arround the combustion chamber perimeter.

the squeezing out of the mix in those tight quench areas, from the otherwise dead area at tdc, over into the open combustion chamber area, is what makes it work.
it allows less timing to be used along with leaner jetting caused by better fuel atomization from the added turbulence,, and less octane. creates a more efficient burn in the chamber,, burning more of the mix and leaving less unburned mix.
sure hope this helps.

a 9.0:1 engine can detonate easier than a 10.0:1 engine if the 9.0:1 engine has .075 quench distance and the 10.0:1 engine has a .040 quench distance,, everything else being the same on the same fuel and all.I can attest to that. My mild 350 is 9:1, aluminum heads (Trick flow Twisted Wedge, with that fast-burn heart shaped chamber), factory LM1 pistons, and about .064-.068 quench (I used a .039" head gasket- don't ask;) ). Even so, I have to run 93 to keep it from rattling on a hot day, or under full throttle. I also need more timing (35°) than these heads ought to require, or the power really starts to drop off. I have to believe that some of this is surely due to the crappy quench. Oh, and I have 180-185psi cranking pressure.

If i chicken out on the LSx conversion, and just rebuild this motor, I intend to use flat-tops and .037-.040 quench next time. That should put me at 10.3:1 or so, and I'd still run 93, but not expect any problems.

...the squeezing out of the mix in those tight quench areas, from the otherwise dead area at tdc, over into the open combustion chamber area, is what makes it work.
So combustion chamber size and shape would be important in taking advantage of that?

So combustion chamber size and shape would be important in taking advantage of that?

yes,, generally a smaller chamber is a faster burner. but the heart shaped chambers tend to be even more efficient than say,, a bathtub shape. the vortec head is a good example of that heart shape chamber. from what's said,, those heads only require arround 32* of total spark lead max compared to about 36* for the old school 60's closed chamber small block heads.

if you're lighting the spark at 36* before tdc with bathtub closed chamber heads,, then you've got 4*of extra pressure rise working against the piston on it's way up to tdc on the compression stroke,, compared to the 32* spark lead with the vortecs.

so it free's up some negitive work that's done,, which equals slightly more power. each combo reaches that point of diminishing returns with timing lead somewhere. as an example,, that's when the car slows down at the track,, or mph doesn't gain after that next added degree of timing or 2,, the mph speed just levels off. with too much timing lead,, it starts working against itself,,,,,, next is detonation.

hope it helps.