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Old 03-01-2006, 06:59 AM
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Compression Ratio

I own a 1969 c10 with a sbc 350. The engine has never been rebuilt so I assume the cylinders are nice and oval shaped. To fix this problem i'm going to bore it .030 over. If I do this I also want to make the compression ratio a little steeper. What kind of pistons, stroke, and camshaft should I run to get a 10:1 ratio? That can run on 89 octane, right? Thanks

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Old 03-01-2006, 08:03 AM
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Ok, 1969 c10 will most likely has a th350 trans and a 3.08 gear.

Here are a few things to consider:
Old style 76cc sbc heads need a lot of ignition timing advance (40 degrees total).
A 4000lb truck with a 3.08 gear and automatic is a good load on the engine.
You need low rpm torque which means a short duration cam.

So, I would build a 9:1cr 355 cid,
Use cam with about 206 degrees intake and 212 degrees exhaust
Get a set of headers and 2.25" exhaust
Edelbrock performer intake with a 650 spread bore Holley or Q-jet
Use 40 degrees total timing

If you have the money, then get a set of 64cc aluminum heads with flat top pistons (10:1 cr) with the same cam and use 35 degrees total timing. Fast burn head design needs less timing. Hp will increases by 40 or 50 over the stock 69 heads with a cam that size. Maybe a set for trick flows or 180cc afr's.

Have you considered building a 383?
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Old 03-01-2006, 01:56 PM
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it is a muncie 4 speed with a 3.73 rear end
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Old 03-01-2006, 06:26 PM
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Well... why didn't you say that sooner???

That makes a big difference in the cam you can use for your truck. Assuming that your tires are of normal height.

With a 3.73 and a 4 speed in a truck then you could run a cam with 220 to 224 degrees duration on the intake and 226 to 230 on the exhaust with 110 lobe separation. However, 10:1 cr would be a close one with those heads and the weight of the truck. Maybe 9.5 to 9.75 would be a better choice with your requirements.

Keith Black makes a piston that makes 9.7:1 cr with 76cc heads on the 350 for 220.69 (summit racing, uem-kb105030-8).

What cc heads do you have???
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Old 03-02-2006, 07:45 AM
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Quote:
Originally Posted by 454C10
What cc heads do you have???
I'm not sure man my dad bought the engine when I was 6 or 7 and he doesn't even know what year it is. But I have a book on it and just judging from the carb, which is a rochester 2gv, it is a 1974 350. I don't know if this helps you any. Sorry about not saying that sooner.
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Old 03-02-2006, 08:34 AM
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Most likely 76cc heads, but make sure to verify that before ordering pistons.

Use a Holley 3310 (750 vac sec) and edelbrock rpm intake with that 9.7:1cr engine with the bigger cam.
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Old 03-02-2006, 10:35 AM
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Quote:
Originally Posted by 454C10
Use a Holley 3310 (750 vac sec) and edelbrock rpm intake with that 9.7:1cr engine with the bigger cam.
I kinda want to run a quadrajet carb for the gas milage. I also heard that htey can support an engine of up to 600 ponies.
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Old 03-02-2006, 11:56 AM
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I do like Q-jets. But hard to find an intake with a spread bore design that will handle power over 5500 rpms. Your set up is going ot make power up to 6000 rpms. Maybe a Edelbrock RPM intake with a carb adapter with work well.

That reminds me. Install the proper valve springs for the cam. Stock springs are ok for the smaller cam but not for the bigger one that we talked about.

You will also need screw in studs and guide plates.

You may want to get a set of aftermarket heads that come with stronger springs, screw in studs, guide plates, and 64cc chambers. Will cost around 1000.00 but worth it in the end. You will have about 500.00 in the old heads anyway so just spend another 500.00 and get 50 more hp.

With 64cc heads and a 350cid, you can use flat top pistons to make 10:1cr and flat tops are best for flame travel.
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Old 03-03-2006, 07:31 AM
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Quote:
Originally Posted by 454C10
Most likely 76cc heads, but make sure to verify that before ordering pistons.
How do you check the size of the combustion chamber? Will there be a number printed on the heads?
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Old 03-03-2006, 07:42 AM
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Also I know that we are increasing the static compression, but dynamic compresion is what makes power correct? I have very limited knowledge about this, but from what I understand your dynamic compression ratio gets worse as your valves stay open into the compression stroke because some of the air/fuel mixture gets pushed back into the intake manifold, and since performance cams cams keep the valves open longer would you get better dynamic compression with a stock cam? Again this is all speculation because i know very little about it.
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Old 03-03-2006, 09:16 AM
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Pull off a valve cover and read the casting numbers. This number can be crossed referenced and should give a good indication of the combustion chamber volume. Once the head is off, I would also measure it with a cc gage or a digital scale with 1gm accuracy. FYI, 1cc of water weighs 1gm. The metric system is based on water (one cubic meter of water weighs 1000 kilos). So you fill the chamber with water and subtract the before and after weigh of the container you are pouring the water from.

Yes, you got it. A late intake valve closing point will lower the dynamic compression. That is why more static compression is used with longer duration cams. This must be kept with in a certain range to use pump gas. There are many online dynamic compression calculators on the web. The inputs will be rod length, stroke, static compression, and intake closing point. Do a search on dynamic compression valve to use. A dynamic compression of 8:1 seems to work nicely on pump gas.

A 10:1 cr engine with a short duration, high torque cam will tend to detonate a lot more as compared to the same engine with a long duration cam. It is very important to match the compression, cam, head type, load, and fuel to be used.
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