Chevelle with 307 - Chevelle Tech
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post #1 of 26 (permalink) Old Nov 26th, 19, 9:49 PM Thread Starter
 
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Chevelle with 307

My first question is about figuring cylinder pressure as related to compression ratio. Is it as simple as one point of compression equals 14.7, atmospheric pressure. I've been told that 307s have really low compression but mine checks 126 to 135PSI and I forgot to open the throttle, the water temp was 160. My 68 Chevrolet chassis manual, in the tuneup section says the cranking compression should be 150PSI so it doesn't seem bad. I'm pretty sure it is a 307 but I can't see the casting number. It was a Rochester two barrel, single 2" exhaust, with the oil fill in the intake when I got it. Any other tell tales to help identify it? And please if you don't live in central Illinois and have a 350 that you want to give me don't bother with the obvious. I might consider a 396 too.

Dave
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post #2 of 26 (permalink) Old Nov 26th, 19, 10:01 PM
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Re: Chevelle with 307

No, valve events have a huge factor in cranking compression. What does your balancer look like? How wide, what diameter? That can be an indication of what engine you're working with.

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69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

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post #3 of 26 (permalink) Old Nov 27th, 19, 9:48 AM Thread Starter
 
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Re: Chevelle with 307

I'll have to check the damper specs today and let you know. The engine has heads have no accessory bolt holes and the head casting number, which I didn't write down, are correct for a 307. But the car is fifty years old and so unlikely to be completely original anyway. It does have a HEI distributor in it and I've put dual exhaust on the stock manifolds, and a four barrel carburetor and Performer intake. This is the first Chevrolet I've owned since my 68 Camaro in 1974 or thereabouts.

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post #4 of 26 (permalink) Old Nov 28th, 19, 7:51 AM
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Re: Chevelle with 307

a small block Chevy with high miles wears the timing chain and gears, the cam is retarded and this hurts low rpm performance, the cranking compression is directly related to camshaft timing position, I bought a 307 from a junk yard that wasn't in a car but I believed it was from 71 or 72
probably no help to you
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post #5 of 26 (permalink) Old Nov 28th, 19, 10:05 AM Thread Starter
 
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Re: Chevelle with 307

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Originally Posted by pbcbob View Post
a small block Chevy with high miles wears the timing chain and gears, the cam is retarded and this hurts low rpm performance, the cranking compression is directly related to camshaft timing position, I bought a 307 from a junk yard that wasn't in a car but I believed it was from 71 or 72
probably no help to you
Thanks Bob, I never even thought about the timing chain, and me a child of the plastic timing chain era. If the crank is lagging the cam because of a loose chain would that increase or decrease the cranking pressure and why? And I still don't understand the relationship between static C/R and actual cranking compression. If it was just atmospheric pressure times C/R then 10:1 would be 147lbs cylinder pressure which means according to my chassis service manual my 307 has 10:1 C/R. Not likely! I'm just curious as to whether or not it would be worth the trouble to raise my compression and how much, since this is a 307 and I'm not trying to set any records even Power Pack heads would raise my compression without costing airflow or much cash for that matter.

Dave
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post #6 of 26 (permalink) Old Nov 28th, 19, 10:57 AM
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Re: Chevelle with 307

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Originally Posted by D Berry View Post
Thanks Bob, I never even thought about the timing chain, and me a child of the plastic timing chain era. If the crank is lagging the cam because of a loose chain would that increase or decrease the cranking pressure and why? And I still don't understand the relationship between static C/R and actual cranking compression. If it was just atmospheric pressure times C/R then 10:1 would be 147lbs cylinder pressure which means according to my chassis service manual my 307 has 10:1 C/R. Not likely! I'm just curious as to whether or not it would be worth the trouble to raise my compression and how much, since this is a 307 and I'm not trying to set any records even Power Pack heads would raise my compression without costing airflow or much cash for that matter.

Dave
compression on those was only about 8.5-1 new.
look for a set of 305 heads to boost compression and performance.
a puny ''268'' compcam or generic cam is plenty for sound and fun...

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post #7 of 26 (permalink) Old Nov 28th, 19, 12:56 PM
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Re: Chevelle with 307

The calculation of atmospheric to compression ratio youre asking about assumes piston is at bottom of stroke so cylinder is full, then both valves are fully closed immediately and piston raises compressing that volume.
In reality, piston is at the bottom of its stroke and starts coming up and a valve may still be partially open, or the valve closed before piston bottom dead center, or a valve was still closed when piston was coming down from top dead center or valve started opening before tdc. All these valve timing events can bleed off some of that cylinder pressure or not allow the cylinder to draw its maximum volume potential to begin with. Then theres also intake restrictions that may not allow full efficiency, etc.
If I understand it all correctly.

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post #8 of 26 (permalink) Old Nov 28th, 19, 1:00 PM
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Re: Chevelle with 307

Here is one I had back in the early 80's, it was a stock 307 in a Pontiac Ventura. The car ran great until I decided to run the rpm's up to 6800 then bent a few exhaust valves, pop pop pop out the exhaust. I needed a set of heads and at the time the only thing I could find was a old set of 520 casting 283 heads. Well that little 307 was a completely different engine and screamed, that's why I think the new 305 Vortec's would really run good on any 307. Basically a 307 is just a stroked 283 and will rev better than a 305 with the longer 350 stroke.
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post #9 of 26 (permalink) Old Nov 28th, 19, 2:11 PM
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Re: Chevelle with 307

I have never thought about atmospheric pressure and cranking compression, I have thought about whether a supercharger should double NA horsepower when it achieves double atmosphere boost. I think static and dynamic compression ratio are poor descriptions for what happens in the engine. What is called dynamic compression ratio should be called actual compression ratio, it's the volume in the cylinder when the intake valve closes to what volume is left when compressing is done. You can't compress when the valve is open so the total swept volume is irrelevant. Back to my original diagnosis, retarding the intake valve closing lowers the so called dynamic compression ratio and the cranking compression, advancing intake closing either by advancing the cam you have or by using another cam with earlier intake closing will increase cranking compression
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post #10 of 26 (permalink) Old Dec 1st, 19, 11:03 PM Thread Starter
 
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Re: Chevelle with 307

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Originally Posted by 68Chevele View Post
Here is one I had back in the early 80's, it was a stock 307 in a Pontiac Ventura. The car ran great until I decided to run the rpm's up to 6800 then bent a few exhaust valves, pop pop pop out the exhaust. I needed a set of heads and at the time the only thing I could find was a old set of 520 casting 283 heads. Well that little 307 was a completely different engine and screamed, that's why I think the new 305 Vortec's would really run good on any 307. Basically a 307 is just a stroked 283 and will rev better than a 305 with the longer 350 stroke.
Man you hit the nail on the head with your experience with your 307. I don't know what today's gas can support compression wise so I'm not sure how small of a chamber volume I could get away with. And from the standpoint of availability and cost I've been thinking Power Pack heads, I don't think that I need intake valves bigger than 1.74 at this displacement. I'm not looking for all out power but would like better compression.

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post #11 of 26 (permalink) Old Dec 2nd, 19, 3:26 AM
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Re: Chevelle with 307

Cranking compression is not calculated by using 14.7 atmospheric pressure at all.What the compression ratio is a calculation of an engines completely full cylinder's volume versus the piston coming up to top dead center and its completely compressed volume.
A large volume of an air fuel mixture is compressed into a much smaller volume of air/fuel that is now ready for the spark to light that mixture and push the piston back down.
C&P'ed :
By definition, the compression ratio is the total swept volume of the cylinder with the piston at bottom dead center (BDC), divided by the total compressed volume with the piston at top dead center (TDC)

What would wake up your 307 would be a set of 416 or 601 305 cylinder heads.I believe the 14014416 heads come off the 305 HO from the 80-86 time period.
14014416 off of 267/305/350 SBC's from 80-86 w/1.84/1.50 valves 64cc chamber's that are crack prone.
Or these 14022601 from 79-86 SBC's 267/305's w/ 1.72/1.50 1.84/1.50 valves 58cc chambers may be better.
It seems the 307 is making a comeback as a lot of TC er's are trying to modify them lately.Whatever you do make sure to replace the timing chain and camshaft/lifters as they're junk from the factory and just doing that will wake the motor up some.
Are you sure you just don't want to find a 350 ? with the LS's popularity the small blocks have come down in price somewhat and are taking longer to sell.
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post #12 of 26 (permalink) Old Dec 2nd, 19, 7:59 AM
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Re: Chevelle with 307

I say skip the 350 and go straight to the 454. Way more potential for power and more common than the 396. They can be found in big cars, pickups, motor homes, etc.
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post #13 of 26 (permalink) Old Dec 3rd, 19, 9:08 AM
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Re: Chevelle with 307

either improve your 307 or as said grab a truck 2 bolt 454 get a cast rotator, the right head and even a 502 take out cam. It will feel like a completely different car and not cost a ton.
the torque is incredible. I dont thin its worth the $ vs/expectations to do a 350.
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post #14 of 26 (permalink) Old Dec 3rd, 19, 9:40 AM
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Re: Chevelle with 307

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either improve your 307 or as said grab a truck 2 bolt 454 get a cast rotator, the right head and even a 502 take out cam. It will feel like a completely different car and not cost a ton.
the torque is incredible. I dont thin its worth the $ vs/expectations to do a 350.
Wow, that sounds pretty freaking awesome!

Where is a good place to look for 454's?

I might pick up a block and set it to the side for later. I am into the 350 build now, but that is not to say, that once I have it built and tuned that it cannot be sold or traded

I have been a sucker for big blocks all my life. My first car was a 69 caddy 472 - had buick with a 455, newport with a 440, Lincolns with 460s....and the list goes on.
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post #15 of 26 (permalink) Old Dec 3rd, 19, 9:49 AM
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Re: Chevelle with 307

RV 454's new enough to have roller cams are $500-$600 running in my area, you can always add a roller cam to a MKIV also or SFT, HFT
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