What's the highest CR you can safely run with pump gas?

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Captainkirk

Captainkirk

Old School Mopar Warrior
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This is a tough one.....and there are a jillion internet gurus who 'think' they know the answer....all based on modern technology and computer-controlled spark advance, variable vale timing, etc.
But for us 'old-school A-body' guys running LA engines, what do you think is the highest compression ratio you can safely run without detonation, and why?
Be sure to mention important details like spark advance, cam lobe overlap, cam timing, etc?
Also, how do additives like 104+ octane booster help in your opinion?
 
With iron heads no quench and a mild street cam, probably no more than 9.3:1.

I could get into dynamic and all that crap, but that's not in the spirit you meant your question, I don't believe.

There are exceptions, and I m sure someone will be along to argue, but that's my opinion and I am stickin with it.
 
There are too many combinations to say what an exact safe number is. Some guys have safely ran pump gas on engines with 11.5 to 1 static compression but the total combination has to be just right to do so. Aluminum heads dissipate heat quicker so they help. The larger the cam the less dynamic compression which makes using pump gas easier. Closed chambered heads with a good quench pad and tight proximity to the piston (.040" or less) for quench to be effective helps. Not running the engine too lean helps. A lean mixture runs hotter thus it will promote detonation. Polished combustion chambers help. 104 octane booster is a waste of money IMO. It only raises your octane a couple tenths, not to 104 octane like they like you to believe. Off the top of my head those are the points that come to mind. I'm sure there's more.

If your running an older LA engine with iron LA open chamber heads I'd say don't go over 10 to 1 if your running a pretty hefty cam. If your running a smallish cam don't go over 9.4~9.5 to 1... If you do your asking for tuning problems
 
I like to keep it under 9.5:1 for street engines...

Alot of it depends on the fuel that is available.

Build it to run on shitty pump gas, without additives to keep costs down...

Chicago likes to use a "special blend" in the summer that they claim will reduce smog so we don't end up like L.A.... I think it's just an excuse to charge us more money than the rest of the country...


Keep in mind that gas isn't going to get any better....
 
A few years back I bought a brand new Chevy Cavalier to drive back an forth to work, brand new off the dealer lot it got horrible gas mileage, an Eco Tech 4 cylinder and it was embarrasing. After a couple months I was reading some literature about that motor and found out it had.....................10:1 comp. ratio, I was running the cheap gas, 87 octane I think, I filled it up with 93 octane and got an instant increase on gas mileage, much more to my likeing but there was that trade off of $$$ to get the mileage, really wasnt worth it to me so that car got sold.
 
I run 200 psi cranking. And the static is 11.2 with j heads, I run a total timing of 30 and a very cold NGK 9 plug with a MSD. It is on the razor edge but has been this way for 10 years, the hi stall and somewhat lighter car help. I would recommend keeping your cranking psi under 180 to run 93.
 
I have used 104+ since 1980 .

I could run Sunoco 260 or regular gas with a can of 104+ and they performed the same in my 70 coronet R/T 440 six pack, .

I ran my 340 dart with Iron heads and 11-1's and 35degrees total timing on the stuff and it ran well with no pinging and great throttle response.

I did mix it 1 bottle to 10 gallons of gas though and still use the stuff in my current 340 that puts out 495hp and 467ft lbs of torque.

I lowered the timing to 33 degrees total and have aluminum ported heads on it now.

As far as I'm concerned the stuff works well
 
GEO said:
I have used 104+ since 1980 .

I could run Sunoco 260 or regular gas with a can of 104+ and they performed the same in my 70 coronet R/T 440 six pack, .

I ran my 340 dart with Iron heads and 11-1's and 35degrees total timing on the stuff and it ran well with no pinging and great throttle response.

I did mix it 1 bottle to 10 gallons of gas though and still use the stuff in my current 340 that puts out 495hp and 467ft lbs of torque.

I lowered the timing to 33 degrees total and have aluminum ported heads on it now.

As far as I'm concerned the stuff works well
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only thing I ever had that 104 worked on, was one cyl engines. if it works for you, you ain`t got much engine.
 
I'm not a fan of additives, and honestly the question is very broad. I can run more static compression than some because I can tune it well. Others can run more because the fuel they get out of the ground is not 10% ethanol. If it has to be "idiot proof" so to speak, then follow what RRR said. Use 9.0-9.5 as a max. Pretty much anyone should be able to tune for that, and it will run on pump 87 with ethanol.
 
It really is all about the combo,the fuel,and the tune.
Its been said and written that A good target with iron is 8.5 Dynamic/180psi.
I have run for about 10 yrs now; Aluminum heads at 9.0/200psi, on 87E10 with sub .040 quench. Ive gone 9.2Dcr/225psi on 91E10,safely.Both of these on full timing;34 to 36*, and a little fat on the fuel.
If your gonna play with cams, bear in mind that every camswap will mess with the Dcr.And so will messing with the Installed CL, and lash on a solid.So for a streeter its best not to stray too far from the original build.
I set mine up at zero deck and .050gasket.Then after each larger cam swap I reduced the gasket thickness.010.Seemed like a good idea. The .020 gaskets couldnt take it. The .039 FellPros can. I shaved the decks .005 at the last swap for .034quench. Been there for many,many years now.
If you get on the edge, you will have to have a sharp tune.
- Flat-tops and a closed chamber with sub .040 quench, seem to be the most tolerant of slight mismatches.
 
This is tough question to answer. Mine is 10.5:1 static and makes 160CPSI. Runs on pump gas and has aluminum heads. I've asked my machinist this question and he says max is 10.50:1 with iron, 11.5L1 with aluminum. There are too many factors here to consider to come up with one definitive concrete answer. Cam, chamber, piston, plugs, timing, stroke, bore, operating temp, head design, head material. etc...
 
roccodart440 said:
This is tough question to answer. Mine is 10.5:1 static and makes 160CPSI. Runs on pump gas and has aluminum heads. I've asked my machinist this question and he says max is 10.50:1 with iron, 11.5L1 with aluminum. There are too many factors here to consider to come up with one definitive concrete answer. Cam, chamber, piston, plugs, timing, stroke, bore, operating temp, head design, head material. etc...
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Agreed! My 273 is 9.6-1 with Egge domed pistons, polished 63-4 cc chambers, 12° initial timing with 35° total, and a mild Isky e-4 cam. I have ran 87 regular with no ping but I usually run 93 non ethanol. I need to do a compression test. I don't know what my cranking compression is. I would agree with the others here. 10-1 with cast heads unless everything is right but no more than 10.5. tmm
 
My 360 is 10.5 to 1 my 416 is 11.9 to 1 both run just fine on 93 pump gas..
 
I can't tell you for sure, but I can share this. I have a 440 RB with 11.5 to 1 and aluminum heads. The cranking pressure is 210 psi. I run 93 octane with Klotz KL-628 octane booster. I ran it with out the booster and got some rattle at throttle tip in and occasionally it would diesel when I shut it off. I used the 104 octane booster at first and it worked but it was pricey to run because I had to run a high concentration of it to get results. After researching octane boosters I found out that 104 and the other off the shelf boosters are too weak as sold. They only raise the octane by points and in .5 so your 93 become 93.5. The real racing boosters like Klotz raise it by whole numbers so it superior to the Walmart/parts store crap. I order the Klotz by the gallon off the web. Ts about $55 a gallon and I mix it 1 ounce to a gallon of gas so it adds about 40 cents to the cost of a gallon off fuel. It worked perfectly too, never pinged or dieseled at all.
 
stroked340 said:
My 360 is 10.5 to 1 my 416 is 11.9 to 1 both run just fine on 93 pump gas..
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Aluminum or iron headed?

Otherwise, RRR short answer and Fishy's long answer are dead on IMO. As an example of cam size effecting the engine, I ran a (zero deck height) 9.8-1, 360 with J heads (@72cc's) on 93 octane with MoPars 292/.509 Purple cam.
The same engine later got Edelbrock heads which are 63 cc's and it speced out to 11-1. This also ran fine on 93 octane.

The cams bleed of compression. A smaller cam in the engine like a XE268 would have issues. The Dynamic ratio would be to high.

104? Walk away from it unless you get bad gas and then have like 4 or more bottles on hand.

Here's the thing Capt. If you want to build a rock solid engine with as much ratio as possible for use with most any cam, keep it 9-1, 9.3-1 is pushing it, 9.5-1 the ceiling.

You will get more power from better flowing heads. A GREAT example is/was the NASCAR Truck series. Over 700 HP on a 9-1 engine.
A lack of compression make the cam and engine perform a bit inefficient compared to a high ratio engine and it's better matched parts & combonation
 
Sorry - It was not intended to read that way. I meant it's conservative and it will work for basically anyone, anywhere.
 
The way I worded it, is set in stone. You cannot run much over that with a mild cam and no quench and iron heads. Idiot proof or not.

Once again, this will be 30 pages of totally useless information.
 
Yeah this an open ended, multi-answer question. There are guidelines you can follow, whereas some don't follow those guidlelines and push the limits further and have no issues while others go the other way. Fishy68 has a pretty good answer for you. I've read 8.0:1 dynamic is about the limit for iron and 8.5:1 dynamic for aluminum (as a guideline). I have also seen guys on here who run higher than that and are fine. It's all about the combo. I've been running 91 in my 11:1 iron headed SB (8.0 dynamic) and it runs 91 no issues. I always purchase from Cenex where 91 is no ethanal here. The heads and cam combination make it work. I don't think it would have worked with the smaller cam I had before.
 
I didn't think his question was open ended at all. I read "'old-school A-body' guys running LA engines" to mean no modern quench technology and iron heads on the cheap.

Maybe I was mistaken.

Yall go ahead and make another bullshit argument thread that yet again does no one a lick of good. I am out.
 
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