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Old 02-10-2003, 05:27 PM
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Post streetable compression

what is the highest compression you can run on unleaded premium? also can you lower compression ratio by installing thick head gaskets?

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Old 02-10-2003, 05:36 PM
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In california we have the privelege of being limited to 91 octane so the magazines say stick with 9.5 with iron heads and 10 to 10.5 with aluminum heads.

You can lower compression by installing a thicker head gasket
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Old 02-10-2003, 05:37 PM
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depends on if you are running aluminum heads or not... I have heard all sorts of people say 10:1 is the limit.. yet I know a guy who runs 12:1 compression on regular pump gas with no sign of detonation.. Ignition timing, cam lift, valve size, even piston dome all have an effect on this.. remember anything that is think and sticks out will become a hotspot and cause detonation..

For the most part I wouldn't reccomend going over 12:1 running on premium pump gas.. Sure it can be done but 12:1 compression is plenty for the street and you won't need to worry if you have the thing setup properly.

You can decrease compression slightly by putting in a thicker gasket... not much but some guys do it.
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Old 02-10-2003, 05:38 PM
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you can lower compression slightly with a thick gasket but you sometimes run into other problems when you increase the quench area.
compression ratio max depends on a lot of variables, piston and chamber design, head material (aluminum or iron), ignition timing curve, fuel air mixture, cam timing, and exhaust system.
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Old 02-10-2003, 07:31 PM
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It really depends on how big of a cam you run, Mike Morgan {been in every magazine} has a Nova with a 9 second street 400 smallblock chevy and hes at 14.5:1, yes i said 14.5:1 Now hes also running in the 280's for .050 duration. How much compression you should run is directly proportional to your camshafts Advertised duration and your Advertised intake closing angle. Remember you dont want to double your head gaskets because that will probably put your quench way out of the desired .035-.040 range. hope this helps a little...good luck and have fun
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Old 02-10-2003, 07:43 PM
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12 to 1 on street gas Dave? Ya right. Either that guy is feeding you a line or he doesn't know himself.

Sure you can run 10-1 on street premium but guess what you usually make less power because you can't run optimum spark timing. Most small blocks need anywhere from 12-16 degrees of lead to run at their best, this is a function of head design. Run 10-1 in one and see if you can even get 8 initial before it pings. Don't forget it's the high rpm detonation you don't hear that kills an engine, examine your plugs with a magnifying glass at least 10X power and look for little shiny balls. Those are balls of aluminum that have melted off the piston caused by detonation. Most people don't even know they are detonating at high rpm unless they looked for those little shiny balls.

I like 8.5 to 9.0-1 (calculated) for the street, much more requires premium and gets you maybe 5-10 HP...whoopdeedooo. I'd rather pay less for my fuel thank you very much. Big blocks should stay near the low side unless the cam is huge like .600" lift and 300 degrees duration. Quench in a wedge chamber is all important to running even these low compressions. Remember large cams only bleed off compression at low rpm, once they are functioning at intended rpm the long duration starts to pack the cylinder even more efficiently and compression is right back to where you started.

Hmmmm...shiny aluminum balls...aluminum tinfoil helmet...maybe there is a connection <img src="graemlins/mwink.gif" border="0" alt="[mwink]" /> ....Hmmmmm... <img src="graemlins/drunk.gif" border="0" alt="[drunk]" />
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Old 02-10-2003, 07:57 PM
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I don't think he is feeding me a line... and he knows what he is talking about... The engine is an engine that ford sold only to racing teams.. It has giant valves and a giant cam in it.. he has had the engine since he was in highschool (bought it for quite a bit back then after working construction, it was toast so he rebuilt it)
It may have some detonation that you can't tell at higher RPM but the thing still runs so it can't be doing very much damage if it is..

We'll just have to see though cuz I am planning on running 10.5 or 11 compression whenever the hell I have money to build the engine... may not be for a year.. I plan on running regular gas and if that don't work I'll be forced to use premium... There are quite a few guys running higher compression engines around here... maybe it's the alberta gas or something but people have done it around here.

Also gotta think about it like this... my 85 camaro came from the factory with 9.5:1 compresion (from any specs I can find) now really you can't tell me that they were gunna run the limit of compression in the 80s.. especially with the gas shortage scare and whatnot..

about it only giveing 10-15 Hp... I know that you are more exprianced than I am but why would people be running higher compression then 8 then... I mean this would only give them 20 hp.. I dunno it just seems to defy anything I have been taught..

[ February 10, 2003: Message edited by: stonedchihuahua ]</p>
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Old 02-10-2003, 08:34 PM
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alright I just tested what you said on the desktop dyno chuck.. I know it isn't the most accurate way... but I was seeing consistantly that no matter where you started an increase of 2 in compression lead to a 50 hp increase.. at least on my test engine
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Old 02-10-2003, 08:54 PM
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That sound right, if you have the octane numbers to match the compression. Otherwise, like he said about the whole ignition timing thing being the limiting factor.
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Old 02-10-2003, 08:59 PM
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well yeah but my whole reasoning is even if I have to sacrifice ignition timing to keep it from detonating I will have the option of using better gas if I ever wanna take the thing out to the track and i can just change the timing
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Old 02-10-2003, 09:04 PM
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That's what I do (11:1 streeter).
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Old 02-10-2003, 10:07 PM
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ok
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Old 02-11-2003, 05:40 AM
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I think it has a lot to do with your combustion chamber too. A chamber with hot spots will pre-combust much more readily than one without. Theoretically, a hemispherical combustion chamber should be the most octane tollerant of all. Sharp edges, carbon buildup, rough casting all tend to be hot spots and cause pre-ignition requiring higher octane.
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Old 02-11-2003, 07:44 AM
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Don't forget advertised and calculated compression are two completely different things, take a burrette to the combustion chambers and actually measure it...you might be surprised. I don't know how many guys who come to me and say they have 12.5 to 1 with popups and when I cc the chamber we have 10.2 or so including the dome.

For example the 400 Ford in my profile cam factory with 8.5 to 1 compression and it detonated right from the factory with only 6 degrees initial timing, that was in 78, premium didn't help. When I asked why it pinged right from new, Ford said it was considered acceptable and not detrimental to the engine...right... When ever I examined the plugs they where decorated with little shiny Christmas "ornaments" and it drove that way for 200 000 KMs before it spun a rod bearing.

Hmmmm...I wonder if the detonation had anything to do with the #7 rod bearing being pounded out...Hmmmm...but Ford said it was OK...Hmmmm...

You will never hear the light detonation caused by too high compression at speed, you can only see the effects. As for desktop dyno's...don't get me started, there is no substitute for experience and actually doing things in the real world.

You can't expect a program to compensate for local conditions and engine temps not to mention crappy fuel and imperfect carburation. Computer programs live in a fantasy land of perfect carburation and non-scattered timing and cool, straightened intake air flowing through no restriction and no exhaust backpressure. Not exactly real world conditions if you ask me. You can add fudge factors to improve the accuracy artificially but in reality it is all just a game, reality is far more complex than a simple desktop program can model especially when you consider all the variables.

Does the desktop dyno ask for the quench clearance in the engine? Does it account for the slight rod stretch that occurs at rpm which decreases this clearance? Does the program give you EGT's? How about reversion? How about worn plugs? Ignition wires with less than perfect continuity changing timing slightly from cylinder to cylinder? All these are factors that materially affect an engines performance and change it's octane requirements.

How about this Dave, you run my engine on your desktop dyno and tell me what you get using stock exhaust manifolds unported heads and single exhaust, here's the specs;

400 Ford, 74 heads, open plane intake-heated, Cam Dynamics 272H cam, Holley 650 spreadbore vacuum secondary and 8.5 to 1 compression with a dish. I know the top speed of the vehicle it was originally installed in so I can calculate a road horsepower figure from that so we can compare.

Have you looked at your plugs lately!
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Old 02-11-2003, 07:52 AM
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I'll try that one at home.. now the thing is that I know desktop dynos aren't realistic... still they give some idea of what can happen..

now then if compression doesn't matter then why does anybody bother to raise their compression... from what I have learned it is best to have the highest compression possible witout having detonation... I dunno... it's not that I am saying you are wrong because I know you have way more experiance then me but if something checks out on a compression tester at 11:1 doesn't it have at least 11:1 compression?? I mean there are even guys who have calculated it all out and can run higher compression on the street by just adjusting their timing... now I know you are sacrificing timing but you can change that for the strip to have more power and still have a streetable car


Also in a magazine it says that someone ran 16:1 compression on 92 octane in a competition or something... This was with aluminum heads but how is it possible if you say that 10:1 will detonate... I dunno to me it seems like there is no rhyme or reason to why some engines detonate and others don't

[ February 11, 2003: Message edited by: stonedchihuahua ]</p>
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