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Old 08-09-2002, 09:20 AM
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Post compression physics question

okay, as I've said before, my mechanical knowledge is rather below average for this board. If we were a school, I'd probably get put back so many grades that my knees are sticking up out of my chair.

I know just enough about engine physics to sort of understand why you use a higher lift cam with a large carburetor. I know that you *do not* want to supercharge an 11:1 compression engine unless you want to end up like the coyote with his latest Acme roadrunner-catching scheme. I know that if you've got crappy smog heads, no amount of cam or carb is going to save you since the valves need to be able to let the carb and the cam get to know each other.

But here's what I don't understand: what is the relation between compression ratios and what kind of rpms you can run and what kind of carb and cam is too big/ not big enough?

and what do I need to do to take good care of my high compression engine? More frequent oil changes? some kind of additive to help the valves survive?

anyone feel like putting on the old mortarboard hat and giving me a lecture? I promise to take notes and to not dip Daisy Duke's pigtails in the inkwell.

hm. good view from back here.
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Old 08-09-2002, 09:38 AM
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The reason you need higher compression w/ a bigger higher reving cam is compression bleed off.A bigger cam stays open longer which bleeds off cylinder pressure.Low cylinder pressure causes a lack of low end torque.Although when selecting a cam I take into consideration cubic inch(the more inches the less responsive to a large cam)Also vehicle weigh the lighter the less torque ou would need to yank it off the light,same for nasty gears.
As far as special treatrment of high compression motors would be gas,you need higher octane.
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Old 08-09-2002, 09:47 AM
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Also w/ carburation the bigger the cam the less vacuum w/ would call for a mech sec carb.As far as carb size I can't exact cfm but close.I you under carb you'll run flat in the top end.If you over carb a little you can compensate w/ stuff like jets.If you way over carb(need 600 put two 750s)then it'll suck out of the hole.
Also you said W/ smog heads no amount of cam or carb can save you.Its all a matter of matching.To big a cam/carb will out flow your heads causeing a poor performer.But on the other hand matching a good cam to those smog heads can run pretty good.Like a 244@.050 cam will proably dog w/ smoggers but a 214int/224exh@.050 w/ a 600cfm vac sec carb will run strong.

[ August 09, 2002: Message edited by: 78 monte ]</p>
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Old 08-09-2002, 11:29 AM
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thanks Monte!
yeah, I make sure to always use 92+ gas.

Fortunately the smog heads were purely hypothetical. My Caprice may be a boat, but at least it's a '65 boat!
I have some nice old (original-to-the-car-stock, but rebuilt) double humps from february and march of '64 on my 327, at least according to the alphaumeric codes stamped on the heads.

does compression effect where I should keep the rpms?

If you're interested, the rest of the info: 3.31 12 bolt that came stock, mildly bumpy cam that Edelbrock thought was right for my dual plane manifold; I guess I trust 'em.

600 Holley now sits where that Edelbrock carb that came with the kit was. My car just didn't get along with that Edelbrock. I tried to sell it at the last yard sale the missus and I had. No one bought it.

Can't wait for the 700R4...
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Old 08-09-2002, 12:27 PM
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I assume this car (1965 Caprice?) is a street car. If it's a race car you're sick but I love it. Anyway, if you've ever played around with a dyno program on the computer you might know that a stout street engine (around 1.1hp/cube) running 9.5:1 doesn't pick up a whole lot if you go to 11:1. It's just not worth the hassle. The cam needs to be longer, the RPM's higher, the converter looser, the gears deeper, you see where I'm going. Why don't you take a look at more cubic inches and keep a more civil compression. And I'm not saying "get a big block" - but that should be an option with such a boat. Is it a 2 door? I'm a sucker for those big cars, just give me a 1979 Lincoln Continental Mark V with a 460 and a.......

The Vandal
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Old 08-09-2002, 11:50 PM
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Whew, your not asking for much info are ya? Well here goes.

Compression is good.

Todays gas is bad, even 92 is not that great.

Lots of initial timing is good...up to a point.

Too much compression + Bad gas + lots of initial timing = detonation.

Detonation kills power and destroys pistons and makes your car sound like my diesel Jetta. :p

Long overlap camshafts lower compression...at low rpm. At higher rpm when the hot cam starts to finally do its work compression bleed off is gone, if it detonates you might not hear it. If you reduce timing to compensate you might as well have put a smaller cam in it, power at high rpm needs lots of timing...how much depends on your heads, exhaust, and intake/carburation configuration.

The solution?

Select carbs, cams, heads, intakes, and headers for the power you want to produce.

When you build your engine select your compression based on the fuel you intend to run. 1 compression point is worth approx 10-20 HP, this is barely noticeable on a 300 HP engine...I doubt you could tell the difference on a 500HP engine. There is a correlation between flame front speed and compression but unless you are racing in the big leagues you can ignore it.

Taking care of a high compression engine is no different than a low compression engine.

The coyote never caught the road runner because he did not use brand name parts. <img src="graemlins/nono.gif" border="0" alt="[nono]" />

BZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ!!!!

RECESS!!!
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Old 08-10-2002, 04:20 AM
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thanks 4-Jaw!

that all makes sense.

do ya think 10.5:1 is too much compression with a cam duration of intake 204, exhaust 214 at .050?

the lift (at valve) is Intake .420 Exhaust .442

if it really is recess already I think I'm going to see what Daisy is up to...
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Old 08-10-2002, 12:36 PM
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I have been wondering about this stuff too.


I am gonna be running around 11.85 compression, but my cam is 230/230 @ .050

But I dont care if I have to run racing fuel. Thats not a problem. I have 305 H.O. heads on it, but they were redone and have 1.94 and 1.50 valves, 3 angle performance valve job, and they dont have that restricting thing inside the intake runners. I sure hope they do good for me.
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Old 08-10-2002, 01:20 PM
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American fuel is much better than the fuel we have here in Canada, octane and the blends seem to be superior. I do not run more than 9:1 and sometimes run 8.5:1 on large bore engines (more prone to detonation). Our octane here is also 92 with premium fuel but for some reason it is not the same as US 92, it's measured the same but who knows. Maybe they mix water with it...Ha Ha Ha!

The last thing you want to run into is having built an engine and knowing it needs 16 degrees initial timing and 37 total at 3000 rpm and starting it for the first time and only being able to run half that. Torque production is a direct function of proper timing and torque X rpm = ?....you know. Your head design and piston shape/quench area determines your optimum timing and the engine runs best with what it needs.

Here's an example I once built two identical 351W engines back in the early eighties, I blew the first one (remember the con rods I posted?) and that engine was a 10.5:1 engine. The best timing I could get out of it was 6 degrees initial. After that one blew I built another exactly the same except I used the factory dished cast pistons and with the 69 heads it worked out to 8.3:1.

On the dyno the second engine was 50 ft pds of torque higher from idle to 4500 rpm and held a slight advantage all the way to 6800. I could run 16 degrees initial timing and dial in all the advance I wanted and it never detonated, even in 100 degree weather. Lest you think this is minor, when the engine was installed in the car it immediately dropped 1/2 a second in the quarter and from the seat of the pants it felt like I had dropped in a big block. The car was transformed and this was all done with a 300 degree duration cam with over half and inch of lift, the heads were ported within an inch of their lives too.

I learned my lesson with that car and realized high compression is only good for racing...if your rich. Personally I like to pull up to the regular pump and laugh at the guy who spends another $25 to fill the same tank as mine. The last thing you want is to run into detonation at 6800 rpm belive me, that's what killed the first engine.


Your homework assignment is to chase daisy duke in the General Lee til' she can't run no mo.

COOTER!!!Boss Hawg has got Daisy in the barn all tyed dup, can we borrow some duct tape? Man, I used to love that show. Watched it the other day and can't imagine why I did? Oh well.
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Old 08-11-2002, 02:46 PM
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Okay I did the homework, but all I got for my trouble was a slap in the face. What gives? I chased her from the front seat to the back seat. What? I got the assignment wrong? I thought you said chase Daisy around *inside* the General Lee?

One more practical on-topic question before we descend into further fiddle music and Benny Hill antics: What is the easiest way to compute your compression ratio, just to make sure you're really running what your parts numbers and math tell you that you're running? thanks again.

woo-doggie, she can really hit--that stung like hell... maybe I can get her to do it again <img src="graemlins/drool.gif" border="0" alt="[drool]" />
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Old 08-11-2002, 03:37 PM
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I still do it the old fashioned way, There are not really any foolproof shortcuts I know of. A good calculator and some knowledge of trig and you can do anything.....even take OVER THE WORLD!!! Muhhahaaaaaa.

Lodge meeting tonight everyone, don't forget your fuzzy hats. Jon is initiating me since the triple sixes were dicovered on my head.

Bring Daisy, we'll need a snack. <img src="graemlins/evil.gif" border="0" alt="[evil]" />
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Old 08-11-2002, 05:31 PM
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I still remember how cute my high school math teacher looked when she sat there smoking her cigarette at her desk and told me she didn't think I should take trig.

She was so nice, she gave me a C-- on Algebra (a passing grade that I did not deserve by any stretch of the imagination) so I could stop taking math courses.

She was right. I can add, subtract, divide & multiply... but beyond that it just ain't gonna happen for me.
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Old 08-12-2002, 01:12 PM
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4 jaw-what in the hell had ahold of your 'nads when that picture was taken! ooiiiiiieee! so. i bought this chevelle (350/auto)a few years ago that the guy drag raced (w/full interior) and swore it had flat tops, and a summit 290 duration cam. that means a compression ratio under 10:1, like 9.2:1 actually. with a performer/headers/stock gm electronic ignition/350turbo-hydro and i think 1.94's this thing ran 12's and it was a 1970 which i think is a boat. Nobody could ever figure out why it was so fast. it had to have had close to 400HP. any ideas? i just ran it on regular! and it ran fine! i don't get it either.

by the way, how do you stick a picture onto the site, and same for the little smiley faces

[ August 12, 2002: Message edited by: bullheimer ]</p>
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Old 08-12-2002, 04:05 PM
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Tuning tuning tuning, the gods live in the details and cubic inches are impossible to tell from the outside. I will not divulge any more info. <img src="graemlins/mwink.gif" border="0" alt="[mwink]" />

The instant graemlins are down and too your left when you post and you can send Jon your pic for your avatar at;

avatars@hotrodders.com

[ August 12, 2002: Message edited by: 4 Jaw Chuck ]</p>
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