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  #16 (permalink)  
Old 02-11-2003, 10:10 AM
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wanna read about higher compression...here you go,
<a href="http://www.motortecmag.com/archives/2001.jun/JUN0101/JUN010101.html" target="_blank">www.motortecmag.com/archives/2001.jun/JUN0101/JUN010101.html</a> {these are zeros not o's}

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Old 02-11-2003, 11:14 AM
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"Page not found" Nickel, check the link. I'll try it later.

There are so many variables that affect detonation tendency in a motor it can be hard to pin down one trait or design feature that might be causing the problem. The problem with my 400 was that Ford put to much quench clearance into the engine and consequently you don't get that last split second "squirt" of A/F from the clearance space that causes the mixture to rotate and mix better (also helps extinguish any hot spots). That last little squeeze can be the difference between being able to run 9.5 to 1 or 8.5 to 1.

I guess what I'm trying to point out is sure you can run 10.0 to 1 on the street but be prepared for it to be picky on how well all your other components are performing, a set of bad plug wires coupled with a slightly worn distributor can cause your engine to suddenly be only able to run 6 degrees of lead instead of the usual 10 where it runs best. Long duration camshafts certainly cover up most of the clues to detonation and hence it goes completely unnoticed...unless you know what to look for.

One thing is for sure you can sure see the results on the dyno, EGT temps skyrocket all of a sudden for no reason...a sure sign of detonation in the chamber. Remember it only takes one cylinder to detonate for it to affect the entire engines power production, after all by definition detonation actually tries to turn the crank the opposite direction of rotation.

Think about that for a minute and you can see why you never want to detonate an engine for any length of time...unless your looking for mileage in which case you would just reduce the load to the point where it is on the ragged edge and run it as hot as possible.

Me I like the flexibility low compression gives me, I can run any fuel I want...even the water logged crap from Domo. I can up the timing past it's optimum without any dangerous effects and the cooling system has less to deal with so the engines generally run cooler and more consistent.

I too have built them up to 10.5 to 1 and I reverted back after seeing the difference...not worth the extra 10 cents a litre they charge up here for the extra performance that is hardly measurable. Of course Aluminum heads can handle higher compression becuase of it's heat absorption properties and typically a slightly better chamber shape because much has been learned in the last 50 years of automobile engineering.

9.0-1 is still a good ballpark figure to use for any wedge chamber disco era V8 from all makes. <img src="graemlins/sweat.gif" border="0" alt="[sweat]" />
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Old 02-11-2003, 01:48 PM
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I was looking at a balanced stroker kit made by eagle that included everything and it said that it would make 10.9:1 compression with 64 cc heads and that is what vortecs are, right? This might be the next "phase" for my 350 so would that run well on premium and what would the comp ratio actually be
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Old 02-11-2003, 01:50 PM
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I was looking at a balanced stroker kit made by eagle that included everything and it said that it would make 10.9:1 compression with 64 cc heads and that is what vortecs are, right? This might be the next "phase" for my 350 so would that run well on premium and what would the comp ratio actually be
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Old 02-11-2003, 01:56 PM
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CW that would be right on the ragged edge of being able to run on even premium pump gas with any kind of decent initial timing (10-16). You have to step up to a pentroof or hemi head to run any higher without serious detonation.
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Old 02-11-2003, 02:42 PM
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I'm going to run between 8.7 and 9.0 on my 305, I don't trust the fuel here, these days it's only 91 octane unless you buy "fake lead" fuel which is 95 or something but that's a few cents more expensive some of the more serious drag race guys fill up at the airport on 100/130 AVGAS, I had thought about running LPG only because you can buy LPG anywhere these days for half the price of petrol and I allready have a LPG tank but then I would need to change a few things to get 11:1 or more or loose power
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Old 02-11-2003, 03:05 PM
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well heres the way I figure it... I'm young... if it works out for the best having 10.5:1 compression then great... if I have to run the thing on premium with stock heads.. still okay I'll just have to save up for aftermarket ones..

as for the loss of power well i have experianced an acception to that... my old bagged out 305... I was pissing around with the timing and the thing sounded like a desile anytime I gave it some gas but it pulled LIKE CRAZY.. well maybe not but it was alot better than what it was before... I didn't like the thought of killing my engine so I put the timing back..

Chuck now I see what you mean about why you like to run the lower compression... I konw that higher compression is possible and I think for me at least I will go with higher compression but not too high... 10.5:1 or 11:1 should be fine for the street as long as i have everything up to snuff... I do plan on running stock heads at least until I can buy aluminum ones... but I'm young and need the power... I definatly think I'll be running a new ignition setup... stock just doesn't cut it... even for my 305 it seems to be crap (or old) either way to me anything under 10 is enough.. but I guess thats what makes everyones car different... it's all about preferance...


As for the engine that ran 16:1 compression... The thing I guess had signs of detonation in the low end but the detonation went away at the higher RPM... there probably was still some but I mean running 16:1 compression on 92 octane isn't too bad.. even if it does detonate a bit.. not something you would wanna keep as a daily driver...

Hehe my one friend wants to build his chevelle engine to run on aircraft fuel that still contains lead... I think this idea will go out the window when he realizes how impractical it is for a street car.. but for the track it would be great
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Old 02-11-2003, 05:10 PM
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the 10.9:1 was the advertised so what would this actually be? how low could I get this with a thick head gasket?
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Old 02-11-2003, 05:37 PM
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There are many vaiables to this.In the case of the 8.5:1 stock motor that pinged,you have to remember that the stock engine had a very short cam duration and also had a carb and ignition that was calibated for the leanest possible burn.On a perfromance engine,you have neither of those issues,not do you have the residual exhaust heat associated with polltion equipment.They also often have dished pistons that eliminate the quench area of the combustion chamber,which agrivates the detonation issue even further.
The first thing to look at when figuring octane tolerance,is dynamic compression ratio.That is formulated by adjusting actual compression ratio by a sliding scale based on camshaft timing figures and the calcualted number will give you a better idea of what can be run on pump gas.
I can't go into the explanation of how to calulate dynamic compression ratio,but I can give you an idea of how to use the therory to make your engne work better.
First off,you need to remember that the more compression that the camshaft can bleed off,the more static compression you can run.This is because even while an engine will make higher cylinder pressures with high compression and a big cam once the engine is into it's functioning rpm range,that range will be at a higher engine and piston speed,and since detonation is sensitive to the amount of time the piston spends at TDC as well as how fast it approaches TDC,the higher piston speed and shorter dwell time will reduce the octane requirements for a given amount of cylinder pressure.
Other factors are chamber heat uniformity,and quench area shape and distance.Using aluminum heads will allow high compression because the aluminum material moves the heat from the hottest parts of the chamber at a higher rate then iron,and will promote more uniform combustion tempatures.Thicker valve faces,particuarly exhaust valves will also help transfer more heat,as will a valve job with a wider valve seat angle.
Quench shape and distance also helps control detonation.The wider the combustion chamber and bore,the less the engine will want to detonate,but a good chamber will be wide in one direction,but narrow in the other,and will have at least 35% of the bore area exposed to the deck surface of the head.Tightening the clearance between the piston and the deck of the head will promote turbulance inside the chamber as the piston approaches TDC,which helps move the air and cool the spark plug and valve a bit to reduce detonation.
Other outside factors also contribute to the ability to run higher compression on low octane fuel,one of them is the load that is placed on the engine.A stick shift car will detonate sooner then an automatic because the with a stick trans,the engine is being spun to a high rpm and building cylinder pressure before being put under a load,and with port velocity being high and engine speed low,the engine is prone to detonation.As you may know,the engines being run in the Engine Masters Challenge were being run on a dyno where the rpm is built from low to high,so the engine can tolerate much more compression.Put some of those engines into a stick shift street car or even a car with a wide ratio automatic such as a powerglide and they might rattle a bit when the transmission shifts.
I run a 11.5:1 compression engine{true calculate compression} on 93 octane gas on the street.I run an aluminum headed bigblock chevy,with a tight .040" quech clearance when cold,and a massive cam with over 100 degrees of overlap.I also run copper head gaskets which help keep the deck of the block more uniformly cool.I run the distributor locked out with 38 degrees advance even at idle which actually helps cool the combustion process at low engine speeds,I also run a fairly fat running dominator carb,and a high stall speed convertor which doesnt load the engine up at low engine speeds.My engine makes only 135PSI of cranking compression which is lower then most 8:1 compression smog engines yet it makes power like a pure race engine.I run it in the dead of summer and it runs at 215 degrees most of the time and it still doesnt rattle.That is how some of the engine masters built 11:1 compression engines that ran on 91 octane.This is why a high perfromance engine can run with 11:1 ocatane on pump gas while a smogger rattles with 8:1 on the same gas.

[ February 11, 2003: Message edited by: Super Streeter ]</p>
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Old 02-11-2003, 05:43 PM
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You have to measure your compression with the heads off and cc the chambers. "Advertised" is the figure they tell you...BS or not. Your thicker head gasket will get you less than a 1/4 point, not even worth bothering.

Dave, Imagine if you could run that old 305 at 12-16 degrees of initial without detonating under load...I think you get the picture. There are gains to be had running as much initial timing as the cylinder head design and heat load dictates. Assuming you need more and most old V8's do, ramping up the mechanical advance is beneficial also. Modern engines need significantly less initial timing due to head design changes and more efficient chamber shapes i.e. pentroof.

Super Streeter...that description deserves to be in the Knowledge Base...JON!!!

[ February 11, 2003: Message edited by: 4 Jaw Chuck ]</p>
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Old 02-11-2003, 05:51 PM
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You will make more horsepower with a little less compression and optimized {maximum} spark lead VS more compression and retarded timing. That is a fact Jack. If you push the compression envelope with crappy fuel, you are gambling with losing odds, engine damage will result. I have 400 small block in my shop that was built with just a little too much compression for pump gas. The siamesed cylinders are now nicely ventilated because the timing was just a tad too fast, and BANG!!!!!!! MELTED PISTONS---VERY BAD
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Old 02-11-2003, 06:05 PM
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Four-Jaw, my desktop dyno says about 325 horsepower at 5000 RPM, and 372 ft/lbs at 4,000 RPM.
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Old 02-11-2003, 06:54 PM
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Tryin not to start yet another arguement, but In my experiance (and I think I can speak for most of the circle track guys here in the midwest) For each full number increase in CR will contribute appox. 6% increase in horsepower up to about 14/1. After that the percentage declines with 1/2 that per full number. Cam duration can also play a huge part in what cranking compression will show. On a 300 degree cam the piston will be almost half way the way up the bore before intake valve closure. Producing a substantial decrease in working CR. This combined with polishing of the chamber and carefull attention to quench and timming control will allow a racer to exceed the 11/1, with the 10.5 being the norm on stardard pump gas 91 octane. However, this is not with out consequence, Cams this big won't run really well at street RPM's so It's probably best to shorten the cam and run a little less CR and increase the timming to the optimized 12-18.

This is my $102.42
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Old 02-11-2003, 07:09 PM
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Pretty close Nairb, now what is it with dual exhaust and some longgggg headers.

Just curious, I love racing software! :p
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Old 02-11-2003, 08:03 PM
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have any of you heard of dynamic compression? Im going to be running 11.1:1 with no problems,I can actually run up to around 11.7 but i dont want to get that close to the ragged edge, but then again im going to be running a cam with 304 actual, 264 @.050, 548 lift and a 108 l/s in my 350. This is a wicked cam, this is going to be my street driver, it will run PERFECTLY on pump gas with regular timing because my Dynamic compression is within tollerance {7.7, 8.0 is ideal} dynamic compression is the relationship between your actual compression and your intake closing angle. In other words the more duration you run the higher you can go with compression and still run pump gas but you dont want to run too much duration or your car will be a dog. Im guessing that you dont have too mighty of a cam since your running vortecs so id say 10.9 is probably out of the question for pump gas.*** NOTICE COMBAT *** if you give me the following i will tell you if you can run your pump gas without any problems or not...i need your, Advertised intake duration, advertised intake closing angle, Crank stroke, rod length, bore diameter, combustion chamber volume, your head gasket thickness, piston to deck clearence, the measure {in cc's} of the valve reliefs in your flat top pistons. And from all of that i can calculate if you will be able to run pump gas. Let me know....good luck and have fun
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