High compression in a big inch street motor
#2834266
10/18/20 04:05 PM
10/18/20 04:05 PM
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Joined: Jun 2016
Posts: 906 Washington
hemienvy
OP
super stock
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OP
super stock
Joined: Jun 2016
Posts: 906
Washington
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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.
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Re: High compression in a big inch street motor
[Re: hemienvy]
#2834280
10/18/20 04:45 PM
10/18/20 04:45 PM
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Joined: Aug 2005
Posts: 760 Southington Ct.
turbobitt
super stock
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super stock
Joined: Aug 2005
Posts: 760
Southington Ct.
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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.
1970 Challenger w/572 Hemi street car and my pride and joy.
1986 T-Type with 272 Stage 2 Buick V6 engine - True 8 second street car. Just updated the engine and put down 928 HP @ 35# boost to the ground on chasis dyno.
1976 Cee Bee Avenger Jet Boat - 460 Ford powered.
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Re: High compression in a big inch street motor
[Re: Dragula]
#2834319
10/18/20 07:02 PM
10/18/20 07:02 PM
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Joined: May 2019
Posts: 712 Lake Villa Il
INTMD8
super stock
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super stock
Joined: May 2019
Posts: 712
Lake Villa Il
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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.
69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700
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Re: High compression in a big inch street motor
[Re: hemienvy]
#2834417
10/19/20 12:20 AM
10/19/20 12:20 AM
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Joined: Aug 2003
Posts: 43,174 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,174
Bend,OR USA
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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
Last edited by Cab_Burge; 10/19/20 12:32 AM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: High compression in a big inch street motor
[Re: Cab_Burge]
#2834455
10/19/20 06:38 AM
10/19/20 06:38 AM
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Joined: Oct 2015
Posts: 570 UK
rb446
mopar
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mopar
Joined: Oct 2015
Posts: 570
UK
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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.
Last edited by rb446; 10/19/20 06:48 AM.
1969 'Cuda 446ci, best 9.96@133.9 in 1990 1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: High compression in a big inch street motor
[Re: hemienvy]
#2834486
10/19/20 09:00 AM
10/19/20 09:00 AM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,206
New York
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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.
Boffin Emeritus
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Re: High compression in a big inch street motor
[Re: polyspheric]
#2834516
10/19/20 09:46 AM
10/19/20 09:46 AM
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Joined: May 2003
Posts: 6,562 Downtown Roebuck Ont
Twostick
Still wishing...
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Still wishing...
Joined: May 2003
Posts: 6,562
Downtown Roebuck Ont
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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
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Re: High compression in a big inch street motor
[Re: rb446]
#2834533
10/19/20 10:26 AM
10/19/20 10:26 AM
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Joined: Jan 2003
Posts: 17,844 S.E. Michigan
ZIPPY
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 17,844
S.E. Michigan
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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 really like this conservative combo alot, and put together something pretty close, except mine is 572ci, 10.2:1 and has hemi heads.
Rich H.
Esse Quam Videri
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Re: High compression in a big inch street motor
[Re: Twostick]
#2834646
10/19/20 02:06 PM
10/19/20 02:06 PM
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Joined: Aug 2003
Posts: 43,174 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,174
Bend,OR USA
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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
Last edited by Cab_Burge; 10/19/20 02:07 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: High compression in a big inch street motor
[Re: Twostick]
#2834669
10/19/20 03:19 PM
10/19/20 03:19 PM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,206
New York
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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.
Boffin Emeritus
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Re: High compression in a big inch street motor
[Re: polyspheric]
#2834958
10/20/20 10:57 AM
10/20/20 10:57 AM
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Joined: Jan 2012
Posts: 2,645 Stuttgart, Arkansas
rickseeman
master
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master
Joined: Jan 2012
Posts: 2,645
Stuttgart, Arkansas
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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.
2011 Drag Pak Challenger
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Re: High compression in a big inch street motor
[Re: rickseeman]
#2835022
10/20/20 12:58 PM
10/20/20 12:58 PM
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Joined: Aug 2003
Posts: 43,174 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,174
Bend,OR USA
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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
Last edited by Cab_Burge; 10/20/20 12:59 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: High compression in a big inch street motor
[Re: Cab_Burge]
#2835033
10/20/20 01:19 PM
10/20/20 01:19 PM
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Joined: Jan 2012
Posts: 2,645 Stuttgart, Arkansas
rickseeman
master
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master
Joined: Jan 2012
Posts: 2,645
Stuttgart, Arkansas
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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.
2011 Drag Pak Challenger
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