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Old 12-10-2007, 01:24 PM
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can 10.4:1cr run on pump gas?

im deciding on 2 engine kits one comes with a 9.5:1 and the either comes with 10.4:1 the vehicle is a cruiser morely,ive heard some people say that high compression is better mileage and power over lower and ive heard it the other way around too who is right?!this motor is a sb and roller cam

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Old 12-10-2007, 01:37 PM
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My T/A has 10.2:1 and thats pushing it on pump gas. My cam has alot of overlap so I can just get by on 92 oct. If I were you I'd go with the 9.5:1.My 2 cents.


Mark.
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Old 12-10-2007, 01:44 PM
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Are we talking aluminum or iron heads? Iron I'd say you're pushing it depending on whether you can get 94 regularly, the gearing, cam and weight of your car. Aluminum I'd say you're fine, and depending on the application you might be able to bump it up just a touch (.2 or .3).

K
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Old 12-10-2007, 03:10 PM
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afr aluminum heads 190cc 2.02/1.60 and 1.6 roller rockers I was going to choose a cam after I picked the cr but its going to be a roller because this block came with a roller cam
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Old 12-10-2007, 03:11 PM
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I had 11.0:1 Motor in my roadster.I ran it on regular gas on the street .Had no problems.Put better gas when i wanted to have a little fun.
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Old 12-10-2007, 03:22 PM
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your good. i had 10.6 and iron heads, cool plugs and 160 degree thermostat. ran fine until i put alot of timing in the motor.
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Old 12-10-2007, 03:28 PM
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Quote:
Originally Posted by killerformula
Are we talking aluminum or iron heads? Iron I'd say you're pushing it depending on whether you can get 94 regularly, the gearing, cam and weight of your car. Aluminum I'd say you're fine, and depending on the application you might be able to bump it up just a touch (.2 or .3).

K
They did a comparision of iron and aluminum heads in some mag I read.
They had a set of dart (IIRC) heads one iron one aluminum that flowed almost identical. There was no difference in horsepower between the two. They did dyno runs at different temps to show the power loss with heat and they were the same.
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Old 12-10-2007, 03:37 PM
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this is in a 74 c10 by the way not for towing purposes
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Old 12-10-2007, 03:54 PM
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What`s the quench distance set at? with a tight quench around .040, with aluminum heads, a cool thermostat, and a mixture slightly on the rich side with careful attention to the spark timing, it`ll scrape by on pump gas with the higher ratio.
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Old 12-10-2007, 04:32 PM
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It depends on a lot of factors.

weight
gear
transmission
cylinder head design
and cam timing to name the biggest factors

cam timing has the biggest effect. The intake closing point determines how much air is trapped in the cylinder. Long duration cams have later intake closing points so less air is trapped in the cylinder. Overlap has nothing to do with making cylinder pressure since the engine is not making cylinder pressure at that point in the combustion process. However, more overlap does aid in cylinder filling which can raise cylinder pressure at certain rpms.

What gear and transmission are you going to use?


A truck is going to be a heavy load. And if you are interested in mpg then you will need to use a short duration cam. I would use 9.5:1 max with a iron head with a short duration cam in a truck. 10:1 cr with aluminum heads. I would use new modern cylinder heads with a "fast burn" design because they require less ignition timing. Less ignition timing allows high cr to run on the same octane fuel.

Not a huge difference in power between 9.5 and 10.5:1 cr especially when using a mild cam. However, 10.5:1 can make a lot of trouble with your tune.

I use 11:1cr 350 with 93 octane but use a compcam 292H (244/244), 3.90 gear, 5 speed, and a 2800 pound with me in it. So, no load, big cam, lots of gear, no problems.

I had a 9:5 cr 350 in my 70 c10 and had to use 93 octane with a compcam 260H (212/212) with stock 70 iron heads, 3.08 gear, auto trans, and stock converter. Needed 40 degrees of total timing to get that truck moving. Vortec heads could have used 89 octane and 35 degrees of timing due a faster burn.
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Old 12-11-2007, 10:49 AM
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these are the newer patriot heads cnc bowl blended,2.02/1.60,1.6 roller rockers(full),gear ratio right now is 4.11,and turbo 400 I was planning on swapping the gears to a 3.42 and using the th700r4 trans because this is going to be a highway cruiser I want a cam that can have good power and some mpg as long as its a roller I was thinking something like the 270HR if its not too big it is a 383 engine though or even a 262HR xe cam whatever would do a little better
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Old 12-11-2007, 01:08 PM
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Another way to estimate if it will be ok on pump gas is a dynamic compression calculator. Your motor does not start to actually build compression until the intake valve is completely closed. This does not happen until several degrees after bottom dead center.

Here is a good site that describes dynamic compression. This site recommends staying between 7.5 and 8.5:1 for maximum performance while on pump gas. They warn that at 8.5, you really have to know what you are doing. If their approximations are correct, you can assume that 8.5:1 requires aluminum heads and perfect tolerances with no buildup in the chamber or sharp spots.

Here are a couple of cars that I have had that ran on pump gas and what their DCR was.

1972 ventura 10:1 static comp
iron heads
5.7 rod
xe268h cam (intake closes 60 degrees after bottom dead center)

This motors dynamic compression was 8.27. It ran fine on 93 octane, but it gave some indication that it was on the edge of detonation. With iron heads, I would stay under 8.25:1 along with having the correct tolerances based on this.

1989 firebird 11:1 static compression
aluminum heads
5.7" rod
XR288HR cam (intake valve closes at 70 DABDC)

this was 8.47:1 static compression. Again this ran on 93 octane, but I think this was also pushing it.

Your setup, assuming a 5.7 rod, 383 comes out to the following with the two cams you are looking at and 10.4:1 compression

270HR: 61 Degrees ABDC : DCR= 8.6, I would not recommend this based on the DCR

I could not find the specs on the other cam you are looking at. To be safe, I would try not to go above ~8.25:1 with the dynamic compression. For that to be the case, you would need the intake valve to close no sooner than ~67 degrees ABDC. The smallest cam I could find that acheives this is the XR282HR (230 236 @.050 .510 .520 lift intake closes 67 degrees ABDC)

this is the calculator
http://www.wallaceracing.com/dynamic-cr.php

This is the explination
http://www.empirenet.com/pkelley2/DynamicCR.html

Adam
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Old 12-12-2007, 02:14 AM
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is dcr really important i dont really understand how that works?certain cams are better with the cr?
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Old 12-12-2007, 06:47 AM
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On the compression stroke, the intake valve can be open as the piston is rising in the bore. Once the intake valve is closed, the piston can compress the air/fuel charge. However, less charge is captured because the piston has moved some distance up the bore before the valve closes, so this lowers the cylinder pressure.

Longer duration cams by definition, close the intake valve later (unless a tighter LSA is used, but that is another story). You need to understand, you have a 4 stroke engine. It doesn't make compression or power every stroke.
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Old 12-12-2007, 01:18 PM
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454C10 is right. Think about a couple of strokes in the 4 cycle engine starting with induction.

1: fuel and air is inducted into the cylinder, the piston moves down until it reaches bottom dead center. The intake valve is still open when the piston reaches BDC.

2: The inducted fuel and air is compressed. Once the piston passes bottom dead center, it starts to move upward, but the intake valve has not yet closed. (The reason for this has to do with the momentum and pressure waves of the air and fuel creating a "ram" effect into the cylinder after the piston has reached bottom dead center.) At any rate, the piston is moving upward, but the intake valve is still open. Because of this, actual compression of the air and fuel cannot happen. Only when the intake valve closes can compression start. The fact that the piston has moved up reduces the volume of the cylinder, but has not created compression. This can kind of be seen as "lost compression". The more compression you "loose", the greater your static compression can be (or needs to be for the best performance). Imagine if the intake valve never closed. No compression would be created and all of the fuel and air would be pushed into the intake manifold.

When you look at a cam, it tells you how far after BDC the intake valve is going to be open. If it is closed by 61 degrees ABDC, it will trap more compression than if it waites unitl 67 degrees ABDC to close.

Adam
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