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Old 06-03-2009, 06:15 PM
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Couple of questions.....

Hey guys got a few questions so I figured might as well put them all in one post. My first question is about compression ratio. I am wondering what the MAXIMUM SAFE compression ratio for a sbc 383 with forged pistons to run on PUMP GAS would be? I always here about this magical number of 9.8:1 compression ratio but couldn't you go higher than that safely without detonation? It's pretty strange how a sport bike, take a 2004 suzuki gxr600 for example, has a compression ratio of 13.5:1 and runs on pump gas yet you here guys tell you not to go over 10:1 or you'll be detonating. Next question is how much of a difference in POWER does a higher compression ratio equate to? I can logically see an engine with 11:1 CR opposed to an engine with 9:1 CR making more power from a chemistry point of view, but how much of a difference does it really make? One last question that relates to CR I have is a simple one but one that I need some clearing up with. Can you please define what "quench" is relating to an engine.

Thanks,
Keith

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Old 06-03-2009, 06:57 PM
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really it all depends on well the set us done and how well the motor is tuned, ive heard as high as 11:1 on high test, but i would be a little leary of that, but you can get away with around 10.5:1 agian with a good tuning job and from time to time you're likely going to have to keep tunning it, safely without having to tune it everyother week i would probably say around 10.0-10.2:1, this is liable to start a debate on here cause everyone has their own opinion, quench is the clearance between the piston @ TDC and the head, example:
piston is .005 below deck and you use a .040 gasket, your quench is .045 - what ever the compressed thickness of the head gasket it once tq'd down (.041ish), or your .025 below deck and you use a .015 gasket = .036ish, you want to keep you quench in the area of .035-.044, otherwise you run the possibility of the head smacking the valves or the head at higher RPMs
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Old 06-03-2009, 09:03 PM
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A tight quench of around .040 with the chamber and pistons sharp edges smoothed over I wouldn`t go over 9.8:1 on 93 octane. There are many other factors to take into consideration between a V8 and a motorcycle engine.
Compression with aluminum heads can be higher since the head cools faster than does cast iron (I`m sure you knew that). Also if the cam has alot of overlap it can be higher however this is somewhat misleading. At lower RPM the overlap causes the compression to bleed out somewhat due to the valves still being partially open, but at upper RPM there is not the time to do so, so full compression is reached, detonation occurs and you can`t hear it due to the exhaust rap. Besides what`s been mentioned there are other ways to help the cause when the compression ratio is higher than it should be on pump gas and they are:
Run a colder spark plug.
Keep the engine cool as possible, run a 160 degree T stat.
Slightly richen the mixture, richer mixtures burn cooler than lean ones.
Carefully set the spark timing and the advance curve. A tight quench with a efficient chamber like a vortec chamber requires less spark advance to make the same power.
Cold air induction. Cold fresh air plumbed in from a outside source is better than hot stale underhood air. The mixture is more dense when it`s kept cooler. Blocking off the exhaust cross over under the intake helps.
Since you`ll be starting your combo over, go with a D dish piston and a tight quench, keep the ratio in the 9.5 area and you won`t have to worry about this happening again unless you get crazy on the spark timing or run it excessively lean.
Quench is the distance between the flat portion of the piston and the flat portion of the combustion chamber, when these 2 surfaces are close, say .040, it`s so tight, that when the piston is coming up on it`s compression stroke, the mixture that is inbetween the surfaces becomes more compressed and mixes into a "soup" making the mixture far more combustible. The shock wave from these 2 surfaces forcing out the mixture forces it into the open part of the combustion chamber where it`s ignited by the plug. If you need a better idea of how Quench works, place a tomato on a table and hit it with a rubber mallet. The sweeping action of the quench helps to cool the chamber as well as reduces the octane requirement and needs less spark advance to create the same power. Quench is pretty much non existant past .060.
Quench distance in the .040 range is safe with the proper piston to bore clearence since piston rock has to be taken into consideration.
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Old 06-03-2009, 10:18 PM
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Agreed... too many factors to say one number. For instance, if its an EFI with knock sensors, coated pistons, efficient chambers, and the right timing, 11.5:1 would be a snap. Old-school chambers, poor ring seal, and a carb with lots of ignition lead might make 9:1 a stretch.

Cam timing also plays a huge role.

I've seen Vortec combos running 10.5:1 with a modest cam in the 220* duration range and they sip 91 octane. My 9.5:1 vortec with a tiny cam sips 87 octane.... well, its in a boat, so I shouldn't say "sip." Old school heads tend to need less compression or more octane.
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Old 06-04-2009, 02:09 AM
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Most of our 383 builds we are right around 10:5 and on some of the Vortec ones we may go a little higher as the Vortec heads only require about 30 to 32 degrees of total timing to make peak power and torque on the dyno.

And aluminum heads you may get away with more compresion and we shot for 8.3 dynamic on most of our builds as camshaft plays a big roll there.

Poor ring seal seems to require more timing.
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Old 06-04-2009, 04:59 AM
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comp

Quench is the distance (including the head gasket) from the top of the piston to the head surface. There is alot of theorys about IDEAL quench, but like everything else in the race engine world it's just not that simple. The magic number that fixes everything does not exist.

Your idea about a sport bike with 13:1 running on pump gas shows some points, they probably have a 3.0" or less diameter piston, a very efficent chamber, and weigh 500lbs. These are all factors in what makes "X" engine run on pump gas and "y" engine not.

If you take a 11.0:1 engine and install it in a 4000lb car with 3.08 gears, then take that same engine, install it in a 2500lb car with 4.56 gears it is now operating under totally different parameters. See where i am going.

If i build an engine for a customer i go way on the safe side, allowing lots of room for errors. If it's for me i will push it closer to what i can get to work.

Keith
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Old 06-04-2009, 06:12 AM
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How many more engines do you want to build and blow up trying out everyones maximum pump gas compression ratio theory?
What ever compression ratio your last motor had...WAS TOO MUCH!

Unless you got a real fat bank account you should build your next motor with less compression ratio.
There is a theroretical but not linear approx 4% gain in engine power with each whole cr increase.
If you build with too much cr and the engine is operated in detonation and the rings scuff a bit or a lot, or... you will end up down on power very soon even if the pistons don't give up. Way more than 4%.

You do not want to or need to ride the max cr edge in order to go fast on the street. Build a motor that will work reliably under ALL OPERATING CONDITIONS everyday including the possibility of getting a bad tank of gas or 2.
Concentrate your power building by improving the breathing efficientcy of your whole engine from air cleaner to the tip of the tail pipe. You'll go a lot faster for a lot longer.
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Old 06-04-2009, 09:41 AM
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Quote:
Originally Posted by sexypizzaman
I can logically see an engine with 11:1 CR opposed to an engine with 9:1 CR making more power from a chemistry point of view, but how much of a difference does it really make?
Keith
Compression alone is not responsible for big HP. Compression ratio gets tuned with the cam and head flow to produce big power. Rather, it should be said this way: If you want big power, you need a big cam. Big cams require big compression.

Compression ratio alone will only add about 3% more HP and TQ per full point of compression increase. If you have a 300 hp engine at 8.5:1 and you raise it to 9.5:1, it will now be about a 309 hp engine that probably needs premium pump gas. Not really worth it in my opinion.
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Old 06-04-2009, 05:04 PM
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Here is my 383 set up in my 3200# C3 with a 3.36 rear ratio and stock 4 speed:

Speed Pro 12cc dish pistons, '0' decked block, 64cc TF 23* heads, Performer RPM intake, stock reworked Q-Jet, coated 1 1/2 headers, chambered exhaust, CC XE274H cam, 34* timing all in at 2600 RPM, calculated static CR is 10.3, calculated dynamic CR is 8.2.

I have been running fine, for over 2 years, using BP 89 octane.
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