SRT_DSTRHOLC
Well-Known Member
Agreed. Get what you can run without worrying about it.
11 to 1 on pump gas
- Thread starter bentman
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Agreed. Get what you can run without worrying about it.
wheelsup73
Well-Known Member
I replied on Moparts to this same thread and my answer is still no you can't run 11.0 safely. And cam selection does not matter.Cranking compression don't mean squat. Here is an article to read that is very well written.
How do cams affect compression?
This is a good one with no single answer. There are several types of compression such as "static" and "effective" (aka "dynamic") compression. It all has to do with cylinder pressure. Static compresion is the actual mechanical compression ratio number that you get by combining a given piston size with a given combustion chamber size, with a given amount of stroke and displacement of the engine. So; bore, stroke, compression height, deck height, head gasket thickness and combustion chamber size, (among others), have to do with the "static compression", but they don't dictate the actual working or "effective" aka "dynamic" compression which directly affect "cylinder pressure". The cam dictates this for the engine.
Example: two identical engines both with 9:1 "static compression". The only difference is in the cam profiles. One has no overlap with a given intake valve timing to go along with the rest of the cam's profile, and the other has a lot of overlap with a completely different intake valve timing to go with that cam's overall profile. Overlap is the time in which both valves are open as the piston pushes exhaust out and starts to suck new fuel and air into the cylinder. When the exhaust valve is closing and the intake valve begins to open, there is a time (on high performance and race cams) where both valves are actually open at the same time. As both valves are open, a little "reversion" gets pushed up the intake valve which slows the overall velocity down of the new fuel & air charge coming into the cylinder, and exhaust gas trying to still get out of the cylinder at low RPMs. This overlap period in the valve timing is what causes that "rumpity rump" sound everyone likes so much. Overlap causes a "scavenging effect" as the engine RPM increases. This is why performance cams "come alive" at higher RPMs. It's not "just" the cam making the power, it's the cam's scavenging effect working as the RPM increases which increases cylindeer pressure (dynamic or effective comression). Cylinder pressure makes the power. The downside to that rumpidy rump sound in most street engines is that the overlap also causes a decrease in manifold vacuum, making power brakes a nightmare and throttle response a bit sluggish.
When you have too much overlap, you end-up with a dog for an engine at low RPM's and the car will require things to make up for it such as low rear-end gears and high stall converters to help "spool the engine up" quicker to GET IT TO those higher RPMs where it can begin to do its scavenging thing and increase cylinder pressure. When you have too much overlap in an engine without enough static compression to support it, it is what's known as being over-cammed. Lots of sound but not lots of GO, especially at low RPMs. Lots of engines out there are over cammed because guys like that rumpity rump sound, not knowing that although in many cases it may make more power at a higher RPM, it kills low-end power and torque which is what street performance engines should be concentratng on. TORQUE is what moves the vehicle below 4,400 RPM, not horsepower. Unfortunately, most guys only look at HP numbers, which is why there are a lot of turd cars out there.
So back to those two identical engines with different cams; if you did a compression test on either of these two 9:1 static compression engines, the engine with no overlap (or even negative overlap) would probably have about 140 -150psi or so in the cylinders. The engine with more overlap may only have 100 psi or so, depending on how much overlap the cam has, how narrow the lobe separation angle is, and most importantly... what the intake valve timing is.
Some racing engines with 13:1 or so compression only have 125psi - 150 psi or so of cylinder pressure when a compression test is done. That's less than what most bone stock engines have. It just means that the cam has a ton of overlap and has intake valve timing designed to work with the rest of the cam's profile to create that scavenging effect, which in turn actually increases cylinder pressure at higher RPMs. Static compression always stays the same. You can't change that unless you change the pistons or the heads, etc. This is why race engines have "power bands" and come alive at higher RPM's. It's because the cylinder pressure increases as the engine RPM comes-up. It's from the scavenging effect from the valve timing and overlap.
Many exotic modern cars these days have compression ratios MUCH higher than your average cars have. BMW M3's and M5's, Porsche 911's (non turbo), Ferrari's, Lamborghini's, etc., commonly have static compression ratois in the 11:1 - 12:1 area. In a regular car that is REALLY pushing it on 92 octane pump gas, but with EFI systems and knock sensors that adjust timing before detonation happens, they can get away with it. You ain't going to get away with that (reliably) with a carbureted engine, or an EFI engine without a knock sensor. As a rule of thumb (and this has a LOT of grey area), you can run upwards of 9.0 - 9.5:1 compression on pump gas with cast iron heads, and "about" one full point higher with aluminum heads because of how quick the combustion chambers cool. Can you run 11:1 static on pump gas? Sure. There are still LOTS of older muscle cars running that have comprssion ratios that high that run on pump gas. You just have to back the timing off a little. Timing is VERY important when dealing with compression ratios, cams and fuel octanes.
Like I said in the beginning, there is no one answer to this whole ball of wax question. There are simply WAY too many variables involved. Let's just put it this way; we COMMONLY run compression up in the 10:1 area on our street engines (with aluminum heads) on 92 octane pump gas and they run BAD ***, BUT we also commonly build them with between 9:1 - 9.5:1. It just depends on the circumstances and what our goal was with the engine.
Steve
95teetee
Well-Known Member
the guy who beat me is a professional (well, semi-pro I guess) engine builder and drag racer. And I beat more than enough other Chevies (small and big block) to make up for itYou got beat by a chevy, shame on you, tell you what, i'll run you with my 78 cutty, its a 406 sbc & doesn't have nitrous
.If I still had that 340 you'd be on. If a '68 Cutlass with a built 455 couldn't run with me without his funny button...
JoeDust451
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[quote
If I still had that 340 you'd be on. If a '68 Cutlass with a built 455 couldn't run with me without his funny button...[/quote]
Can't compare a 68 to a 78, there 2 different animals, I've beat up on chevys to, but now that i'm running one, i have to talk the talk.:glasses7:
If I still had that 340 you'd be on. If a '68 Cutlass with a built 455 couldn't run with me without his funny button...[/quote]
Can't compare a 68 to a 78, there 2 different animals, I've beat up on chevys to, but now that i'm running one, i have to talk the talk.:glasses7:
I still stand by my statement on that car. You put Buick 430 power in that car bud, and you'll throw that chebbie s%$t in the ditch. My boss's partner wasn't a believer until he did JUST that. When an engine has 500 plus LB FT or torque STOCK, it's got somethin goin on. But you can't tell people anything. They have to experience it.
JoeDust451
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What car :burnout:
The Cutlass.
Isaac
Active Member
No.Not with an honest 11:1 with cast iron heads. See wheelsup73's post.
Hay dude.....the bottom line is it's YOUR car. Now that you've asked, you know the risks involved. If you wanna risk runnin race gas or octane booster, it's all good. It's your decision. I will say this.....it'll make more power for sure runnin on race gas all things equal with a lower compression pump gas motor.
turbodart68
Well-Known Member
Yep, 5 years of hard abuse and when I tore it down no signs of detonation.
Thank you.
True about hearing it. By the time you are hearing it, its very severe. My car is really quiet and I read my plugs to look for detonation. Also if it detonates you can see the BSFC go up on the dyno.
You are right atmospheric conditions affect how much you can run, but you have much better gas than Ca does.
BTW I am in SoCal not Oregon.
My car has been in 100 degree heat too and runs on 91 no problem. I even raced it at Vegas at the MATS with a 150 shot on 91 octane and no detonation.
JoeDust451
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Remember, i'm not running a 355, its a 406 & it has plenty of torque trust me.
I replied on Moparts to this same thread and my answer is still no you can't run 11.0 safely. And cam selection does not matter.Cranking compression don't mean squat. Here is an article to read that is very well written.
How do cams affect compression?
This is a good one with no single answer. There are several types of compression such as "static" and "effective" (aka "dynamic") compression. It all has to do with cylinder pressure. Static compresion is the actual mechanical compression ratio number that you get by combining a given piston size with a given combustion chamber size, with a given amount of stroke and displacement of the engine. So; bore, stroke, compression height, deck height, head gasket thickness and combustion chamber size, (among others), have to do with the "static compression", but they don't dictate the actual working or "effective" aka "dynamic" compression which directly affect "cylinder pressure". The cam dictates this for the engine.
Example: two identical engines both with 9:1 "static compression". The only difference is in the cam profiles. One has no overlap with a given intake valve timing to go along with the rest of the cam's profile, and the other has a lot of overlap with a completely different intake valve timing to go with that cam's overall profile. Overlap is the time in which both valves are open as the piston pushes exhaust out and starts to suck new fuel and air into the cylinder. When the exhaust valve is closing and the intake valve begins to open, there is a time (on high performance and race cams) where both valves are actually open at the same time. As both valves are open, a little "reversion" gets pushed up the intake valve which slows the overall velocity down of the new fuel & air charge coming into the cylinder, and exhaust gas trying to still get out of the cylinder at low RPMs. This overlap period in the valve timing is what causes that "rumpity rump" sound everyone likes so much. Overlap causes a "scavenging effect" as the engine RPM increases. This is why performance cams "come alive" at higher RPMs. It's not "just" the cam making the power, it's the cam's scavenging effect working as the RPM increases which increases cylindeer pressure (dynamic or effective comression). Cylinder pressure makes the power. The downside to that rumpidy rump sound in most street engines is that the overlap also causes a decrease in manifold vacuum, making power brakes a nightmare and throttle response a bit sluggish.
When you have too much overlap, you end-up with a dog for an engine at low RPM's and the car will require things to make up for it such as low rear-end gears and high stall converters to help "spool the engine up" quicker to GET IT TO those higher RPMs where it can begin to do its scavenging thing and increase cylinder pressure. When you have too much overlap in an engine without enough static compression to support it, it is what's known as being over-cammed. Lots of sound but not lots of GO, especially at low RPMs. Lots of engines out there are over cammed because guys like that rumpity rump sound, not knowing that although in many cases it may make more power at a higher RPM, it kills low-end power and torque which is what street performance engines should be concentratng on. TORQUE is what moves the vehicle below 4,400 RPM, not horsepower. Unfortunately, most guys only look at HP numbers, which is why there are a lot of turd cars out there.
So back to those two identical engines with different cams; if you did a compression test on either of these two 9:1 static compression engines, the engine with no overlap (or even negative overlap) would probably have about 140 -150psi or so in the cylinders. The engine with more overlap may only have 100 psi or so, depending on how much overlap the cam has, how narrow the lobe separation angle is, and most importantly... what the intake valve timing is.
Some racing engines with 13:1 or so compression only have 125psi - 150 psi or so of cylinder pressure when a compression test is done. That's less than what most bone stock engines have. It just means that the cam has a ton of overlap and has intake valve timing designed to work with the rest of the cam's profile to create that scavenging effect, which in turn actually increases cylinder pressure at higher RPMs. Static compression always stays the same. You can't change that unless you change the pistons or the heads, etc. This is why race engines have "power bands" and come alive at higher RPM's. It's because the cylinder pressure increases as the engine RPM comes-up. It's from the scavenging effect from the valve timing and overlap.
Many exotic modern cars these days have compression ratios MUCH higher than your average cars have. BMW M3's and M5's, Porsche 911's (non turbo), Ferrari's, Lamborghini's, etc., commonly have static compression ratois in the 11:1 - 12:1 area. In a regular car that is REALLY pushing it on 92 octane pump gas, but with EFI systems and knock sensors that adjust timing before detonation happens, they can get away with it. You ain't going to get away with that (reliably) with a carbureted engine, or an EFI engine without a knock sensor. As a rule of thumb (and this has a LOT of grey area), you can run upwards of 9.0 - 9.5:1 compression on pump gas with cast iron heads, and "about" one full point higher with aluminum heads because of how quick the combustion chambers cool. Can you run 11:1 static on pump gas? Sure. There are still LOTS of older muscle cars running that have comprssion ratios that high that run on pump gas. You just have to back the timing off a little. Timing is VERY important when dealing with compression ratios, cams and fuel octanes.
Like I said in the beginning, there is no one answer to this whole ball of wax question. There are simply WAY too many variables involved. Let's just put it this way; we COMMONLY run compression up in the 10:1 area on our street engines (with aluminum heads) on 92 octane pump gas and they run BAD ***, BUT we also commonly build them with between 9:1 - 9.5:1. It just depends on the circumstances and what our goal was with the engine.
Steve
That covers it well!
And yet there is even more variable. such as too HOT of spark plugs, and not only timing but the timing Curve.
I know. The 430 or 455 would kick its butt. Believe me. I speak from experience. Don't just take my word for it, search around on the net. Here's food for thought. Tha big block Buicks are the lightest big blocks ever made. They are about the same weight as your chebbie. When you see a full bodied 67 GS430 run low 11s on all motor, you become a believer. Think about it from a logistics standpoint. How cool would it be to open the hood with everybody expectin to see chebbie power and all of a sudden be arguin that somebody put a ford motor in it cause the distributor is in the front. lol I've witnessed that very thing. Plus, you'd be goin faster. Faster and more cool points. If that car was mine, that chebbie would be a doorstop.
JoeDust451
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People on the web will say anything to make there ride shine trust me, the chevy guys will say the 406, the buick guys will say the 455, thats nature, now, i hope you don't think i mild GS430 buick is going to run low 11s, I've been around, i'm 48 yrs. old, i was there back when these cars were new, The buicks definitly ran strong, but they were not king, i'm sure a mild 455 in my car would be a screamer, i've already got the 406, its all re-done, i'll be more then happy to drop in a 455 if you foot the bill
.
I didn't say mild. I said full bodied. As in stock trim at a 3700 pound curb weight. I'm not trying to convert you. And I'm not "anybody saying anything on a forum". I've seen the difference and it's nothing short of astounding.
dgc333
Well-Known Member
wheelsup73 said:
The issue with that article is its not overlap that effects cranking pressure it's how long the intake valve stays open past bottom dead center. Exhaust/Intake overlap occurs when the piston is transitioning from moving up the cylinder to going down.
Cranking pressure and the dynamic compression ratio is created after the intake closes on the compression stroke. Pressure does not start to be captured in the cylinder until the intake is closed (the exhaust was closed long before BDC). When you look at cam catalogs as the intake duration gets bigger the recommended static compression ratio goes up, this is to ensure that adequate pressure is captured in the cylinder for good low/mid rpm response.
Thank you. There is a great deal of false in formation in the article by "Steve" and your answer is finally a voice of reason. It is when the intake valve closes that the compression begins to build in the engine. The overlap does not bleed cylinder pressure. There is generally a loss of cylinder pressure with a cam of greater overlap because those cams likely have a longer intake duration and therefore it is of neccessity that the intake closes later causing a loss of dynamic compression.
I have personally run 11.9:1 compression on 91 octane with factory 906 iron heads, .160" quench, the distributor locked at 38 degrees and there was no detonation. You have to be smarter than your cylinder head design, compression ratio and fuel type.
I don't believe I would run the OP's engine at 11:1. If you have to ask if you can run 11:1 on pump gas you probably don't have the expertise to attempt the task. The problem with having to ask on the forum is that many answers will be given by those who don't have the knowledge to answer correctly. We read false information, assume it to be the truth and repeat it.
JoeDust451
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Pump gas street driven 446 11.3-1 comp off the juice.
100 plus pass's.
10.71 et
128 mph off the juice
Pinks 09
Are those Aluminum heads i see, he has iron, big difference.
daredevil
Well-Known Member
Question. If it does ping couldnt he have a cam ground custom with a lot of overlap and fix the detonation problem by dropping dynamic compression. Wouldnt the earlier intake charge cool it off some?Just wondering since a cam change would seem the easiest fix if there was a problem.To the original poster,Talk to Damon Kuhn at Diamondback racing engines here in Houston. He has a lot of real world experience with motors in the Houston climate.
check into e-85 converison kits for your carbs, cost u 300$ for the kit. e-85 here is 70 cents cheaper than gas & 105 octane no problems!!!!!!!!!!! i almosted passed on a 80 model warlock, 340 12.5 to 1 compression, 2x4 carbs, tubbed with 31x21 m/t on weld prostars cause i couldn't afford 7.99 a galoon for racing fuel.supper nice truck
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