My guess is that this belongs in the Race section.

For those that have a BIG inch street engine, at least 500 inch if not 572, with high compression,
say above 11-11.5 to 1, if not above 13:1.

Under WOT, these engines will typically make so much power that, when driving on public streets,
you really only very rarely actually use WOT. So if you are mostly putting around under part throttle high
manifold vacuum conditions, isn't this what actually allows the engine to not detonate on pump gas ?
Not so much a big cam bleeding off cylinder pressure, but high vacuum conditions ?

At some point (manifold vacuum), a 14:1 engine at part throttle will make less cylinder pressure
than an 8:1 engine at WOT, and therefore will not require race gas octane.

But if the engine is big enough, even at high manifold vacuum it will make "enough" power for
spirited street driving. A "restricted" engine.

Isn't it true that even a long duration cam will still show high manifold vacuum with the throttle mostly closed ? Above idle that is.

By the same token, if you actually are at WOT for some time interval, maybe 5 seconds, the engine will indeed
build enough heat to then require much higher octane at that point.

Somewhere in here I have a question, I don't think I'm putting it into words very well.

I don't think I have the appropriate info for you but I can give you a reference point. My 11:1 572 Hemi with a solid roller with 260+ duration on a 110 LSA gives me about 9 inHG at idle. Seems to run OK on pump gas.

AG.

Turbobitt, do you ever floor the throttle for more than 5 seconds ? No audible detonation, probably impossible to hear though.
Must be a nice Challenger you have !

Um, maybe. Talk to some of the high power Drag Week guys and see what they do. They have high compression race engines and they drive from race to race. I think they use a few tricks such as changing the timing and/or the carb when they are on the street. I guess it all depends on what you're trying to do. My Duster has a 600 hp pump gas engine and it doesn't need anymore power. I don't even go WOT with it on the street. I took a buddy for a drive the other day and rolled into the throttle a bit. Looked at the data logger when I got back to the shop and at 75% throttle we went zero to 80 in four seconds. No real need for me to have anymore power than that!

My 11.6:1 big inch 605 Hemi has been fine on 93. Its a cast iron block, alum heads...I suspect I could go 12:1 no problem...And WOT on the street is not happening....roads around here have a crown on them for snow, and they are not friendly to big tire cars....

For the most part yes, I understand what you're saying and I'm sure you could run big compression at low/no load no problem.

For me, I don't want the feeling that if I want to load it up on the throttle I'm knocking the botttom end out of it because there's not enough octane to do so.

Could you build it to drive on eggshells? Yes I think so, but I'll pass on that.

Not sure where the limit is on pump gas cruising, but i would think 13:1 is gonna want more than 93. Depends alot on the cam. I have a 12.5:1 511 and just keep good gas in it at all time because i'm in the throttle all the time on the street. I do all my wide open testing on the freeway now because the car will run 60 to 110 in 4.5 seconds. I only use a 100 octane mix of 1 5 gallon can of Dragon 110 to 10 gallon of sunoco pump 93. I use to run it on 93 pump when it was 11.25:1 compression no problem. My cam is only 270@50.

Dave, yours is probably right at the limit. Other than testing I do not know how you figure it out. The wedges only seem to tolerate about 11:1 on pump at WOT with full timing...

When i was on pump 93 with 11.25:1 i could only run 1/8 mile wot runs on the back roads, so short passes and no sign of detonation on plugs. Now at 12.5:1 and 100 octane, no sign of detonation of plugs yet. This Dragon fuel(Terragon 110) is really 113. Don't understand their rating. I don't hear and or see it on plugs.

Me thinks your playing with a bag of deadly snakes messing with high compression and todays pump gas
Theory and talk is one thing not directly tied to the real world and real world results
My current S/P car runs on E85 with 15 to 1 compression and that would be the only fuel besides race gas I would build a motor with more than 11.0 to 1 compression to run on the street today regardless if hemi or wedge, Mopar or any other brand motor with aluminum heads
I am going to build a pump gas 572 C.I. wedge motor with no more than 11.00 to 1 compression with a set of B1 heads on a new KB aluminum block, I'm hoping to exceed 800 HP on today pump gas safely
My last serious pump gas car made 612 HP with 9.25 to 1 compression with iron heads on a stroker 400 block with 511 C.I. using CA pump swill and a low deck six pack set up, that combination ran 10.69 at 124.5 MPH through the 3.0inch complete exhaust system with the six pack air cleaner on weighing 3450 LBs with me in the car
I swap heads twice and ended up with a set of Indy SR M.W ports and a single 1050 CFM Dominator on the motor, I had swap the crankshaft from the 4.25 stroke to 4.300 to help gain some compression which ended up at 10.78 to 1 That combination ran 9.993 ET at 134.7 MPH cork up with the air cleaner on
I ended up swapping that car off I learned a lot with that car and motor and some times wish I had it back
There are many ways to make power safely, there are as many ways or more to screw up good parts up trying to learn how to do it safely and properly
Can you afford to hurt parts searching for more power a new way? If not, don't try

Thats the way we went with our street/strip brkt car, 589ci, only 10:1, small solid roller, 260/270@.050", tiny 950HP carb,(15" vacuum@idle) Indy 440-2, Indy heads, 33 deg total, street 4200 converter, max torque@4400, max rpm/hp 6200@traps, 4.10's with a low first in 727, fill up in the morning on the way with Shell V-power pump, drive to the track on full exhausts, race/drive home, no stress, easy on parts, heavy 3800lb car low 10's.

I only buy Sunoco now and only at the supplier, so i think it's the best i can get in the area. I do not run on straight pump now that the compression is at 12.5. I tried it will 3 gals of Dragon 110 to full tank of 93 Sunonco and could hear detonation while street driving. From stop to moderate acceleration. Full 5 gal pail and no detonation that i see yet. From what i remember back in the day when all the cars had high compression 100-102 octane is all they had at the pump if i remember the story correctly.

W/r/t "big cam bleeding off cylinder pressure"
It does reduce CCP... all the way up to 3,000 RPM (more or less, depending on too many variables to list, but stops well before the torque peak).
After that, it increases cylinder pressure - that what it's for.
It doesn't always need 5 seconds to "build heat", the piston damage can occur in 1 second.

And just because you don't hear detonation doesn't mean it isn't happening.

Kevin

I really like this conservative combo alot, and put together something pretty close, except mine is 572ci, 10.2:1 and has hemi heads.

And just because you don't hear detonation doesn't mean it isn't happening.

Kevin [/quote]
Detonation versus pinging are NOT the same thing
Pinging can hurt parts, detonation kills parts quickly

I think we can agree that a conservative initial spark lead and a longer, slower advance curve will help here, but how much? Probably related to stall and slippage?
Add 15-20° of vacuum spark to get mileage back and keep temperature under control.

I think you can run as high of a compression as you want with pump gas if you weren't ever going to get on it. You can't make manifold pressure with the throttle closed/barely open. Powerful engines can do 60 mph with the throttle barely open. But if you floor it, it will detonate and that's bad. And it doesn't take the 5 seconds to build heat that you are talking about.

Modern high performance engines have anti-knock sensors to protect them if you put lower octane gas in them than they require. You can drive coast-to-coast with 87 octane gas in your supercharged Demon or your twin turbo Mercedes and it will not hurt a thing, but if you floor it, you can't make max manifold pressure without detonation, so the computer cuts the timing back to protect the engine.

We used pump gas to warm the engine in Big Daddy's Top Fuel dragster. I doubt you're driving a more high performance car than that.

What was the mechanical compression ratio on his Top fuel motors back then and how much boost would it make idling when warming it up,? Also didn't they run it really rich when warming it up?
I thought all the top fuelers warm them up on alcohol and then switch them to nitro for several minutes to make heat

I would guess the compression was low and I doubt it made any boost to speak of at idle but we didn't have a manifold pressure gauge on the intake manifold.

I don't recall them changing the mixture to warm up.

Gasoline builds heat faster than alcohol. Then switch to nitro (but not for several minutes) to read the plugs.

Look into piston coating,cylinder head chamber and valve coatings, wide lsa cams to reduce cylinder pressure. Fuel injection setups that retard the timing if need be when it starts to detonate. $$$$$$ is it going to be worth it on a street car probably not.

Ive ran 12:1 on pump gas without any issues.... But pump gas varies from state to state, your camshaft will play a large roll in bleeding off some of the cylinder pressure... I usually use pumping compression for a good indication for pump gas. Ive seen multiple engines make a 1000 hp on pump gas naturally aspirated. Both were close to 600 inches.

For what it's worth, my advice.....keep the compression on the low side. You will never hear the detonation before it breaks stuff. You also may not be able to always access the fuel your motor needs to stay happy. On a big stroker it will make WAY more power than you can safely use on the street, you'll never notice the difference in a point or two in compression.
My old truck tries to kill me on a regular basis just gently rolling into the throttle.

my engine responded well with a raise in compression from 11.25 up to 12.5:1. almost 4 tenths and 4 mph. I did also have a electric water pump and new shocks on the front. I'm guessing the compression increase alone was worth 2 tenths. Compression makes your heads flow/work better. old slip was 10.40@130 and new slip 10.06@134. Compression does make a difference, you just need the fuel to match.

I have a customer with a Pettis 572 w/11.7 compression races on 94 pump and makes close to 1000hp based on it's 8.84 et at 151 mph at 3450 lbs with a pretty big 800 lift cam. so the RIGHT combo of parts can obviously work and his plugs look great so far......

Some of the big hitters on Drag Week run 15:1 compression, but they run race gas on the street all week. Curt Johnson is a prime example of this, and his 632 CI engine with LOTS of compression, runs race gas all week. He told us if he tries to run any type of pump gas through it, it will wash the rings out right away. He changes oil every day as well.

Wow, I can't imagine what it costs him to do Drag Week! That's ridiculous.

LOTS!!! We figured it out, to around 3K for fuel alone. But he has won his class 7, or 8 times. He has the formula figured out. He one year built a Pump gas engine, to minimize maintenance. Won his class yet again. He got bored, and the next year he had a 15:1+ engine again. He told me they put a 3-4K chip on the high side of the MSD for street driving. Didn't leave any room for goofing off, and was easier on the valvetrain. If anyone ever gets a chance to talk with Curt, do it. You will not be disappointed.

x2. Modern stuff does more than just retard timing to keep the engine from pinging, most of the engines that have high cylinder pressures (either high comp NA stuff or boosted applications) will limit engine load to keep them out of detonation. I don't know of any mass produced OEM car engine that is much past 12:1 NA. On a high output motor it isn't worth the risk.

my hemi was 12.2 to 1 comp ran on pumpgas always 93 octane at 30 degrees of timming it ran 9.60s at 140+ mph i eaven sprayed it into the 8s on pumpgas and 22 degrees of timming

How True! Unless you have an unlimited supply of 'good' fuel to ALWAYS use you are playing with dynamite. You will never hear it detonate, the damage can occur in a blink of an eye. A big inch street motor with a sane compression ratio will make way more torque than you can stick to the ground. My 605 makes 750+ ftlbs at 3500 rpm.

Can Hemi's get away with more compression than a Wedge?

Compression will make a big difference in power and torque, at the track and in the street.
I live in Michigan so have only driven my cars in the summer, but, if given the chance, i much prefer running race gas. Its easy to get, consistent compared to pump swill with who knows what in it.iItsvnever hard to access
I drive a few hundreds of miles on the street every summer, typically not thousands of miles, so at 7 bucks a gallon the cost to run race gas isn't that big of deal.i figure if i cant afford to put fuel in it, i am probably involved in a hobby i cant afford.
I have had several engines where i raised compression and saw it immediately at the track.
My project over the winter here will yet again. Going from a 360 to a 418, Changing nothing other than the cubes and compression, little bit of head work. Heads will be way to small for 418 inches. But going from 10 to 1 to 12.5 to 1 compression. I am expecting big gains for primarily that reason. Figure the compression alone is probably 40-45 horsepower. Same cam, headers, carb, etc, etc

I would love to find out if I kept my high compression, would 93 octane and any of those additives work?

That's my understanding

605 Hemi, 11:1, no signs of detonation on the plugs or audible pinging on 92 octane and 32 degrees of timing.

IMG_8495 by Greg Ault, on Flickr

Yes...Typically 12:1 is no issue on pump gas....I am at 11.6:1 on my big Hemi...No issues with that.

Dead on.

I've driven high compression motors on the street using not enough octane, with the plan being to stay out of the throttle. The question is do you have enough a self-discipline when an impromptu challenge arises? I don't.
It's like being alone in a motel room with a lingerie clad super model and agreeing not to look at her.

What is your piston comp height? I have ta buy some pistons for my 572 Hemi (7.1 rod)

Well I said earlier 12:1 Hemi on pump gas is no issue, save one, ever driven a +800hp on the street? And if there was a super model in the hotel room with me, in lingerie, I was only looking to make sure it was all female..I am on a budget after spending all that money on a Hemi...

I like Mopars and I like Hemis, but I cannot see how the Hemi can "get away"
with more compression than a wedge for the same octane.
I have always read that the Hemi combustion chamber is more detonation prone
and needs more octane. Or, more cam duration to bleed off pressure under 3000 RPM.
Unless you guys are comparing aluminum Hemi heads to iron wedge.

It's the exact opposite of what you have read. The Hemi has the most efficient combustion chamber shape of the era, and a centrally located spark plug. The flame front has an equal distance to travel in all directions from the spark plug.That reduces its octane requirement.

I can see the centrally-located plug being a good thing. But there is no quench area.
Is that the reason this supposed lack of detonation resistance is based on ?

Aluminum heads help a lot also on any motor to reduce detonation on pump gas
There is quench in all Hemi combustion chambers as long as the piston has a dome that is above the block but not like wedge motors have with one side having it and the other side under the combustion chamber not having it
I use thin narrow strips of clay on the hemi motor pistons to check piston to head clearances on my high compression builds, check that out to see about quench clearances also

most wedge engines came with open chamber heads ... terrible when compared to a closed chamber like the 915 ... or the hemi.