Ok, so I'm in the process of getting a parts truck with a 240 in it. It's not stuck, but I have no clue of the condition otherwise. I would like to convert it to a 300 and build it up for the rat rod, that is assuming that it's worth rebuilding. I know it's kind of putting the horse before the cart, but we're car guys here, thats just what we do. Anyways, I'm thinking leave the 240 head with flat top pistons, a very very hot cam, Big Block Chevy 1:7 rockers, and I'll build a custom header, and a custom intake that uses either two Autolite 2100 2bbl carbs, or 3 Carter YF carbs. What do y'all think, does it sound like a solid build? All of this among all the other "little things" that go with building a performance engine. Correct me if I'm wrong, but all the 240 needs to be converted to a 300 is a crankshaft. Correct? Would I be able to run premium with this set up? I really dont want to have to go to the airport every time I fill up...

 

Your compression will likely be too high for pump gas.

 

What combination would be best for running premium? Stock pistons and 240 head?

 

For premium gas in a 300 six the Dynamic Compression Ratio (DCR) should be limited to 7.5
Example:
If you had a cam with a 292* advertised duration, 110* LSA installed straight up, you would need a 10.4 static compression ratio to give you a 7.5 DCR.
If the 240 head has a finished 68cc combustion chamber the piston would need a 12cc dish.

A 240 would need a 300 crank and custom pistons with a 1.200" CH to become a 300.

Important question: What year 240 is it?
There are some differences between years.

 

It's a '67 assuming the engine is original to the truck. So that still doesn't quite answer my question since i'm not terribly great with the technical number stuff. So, what would i do to achieve the optimal compression ratio with this set up?

 

To answer your question, the camshaft specifications determines the compression ratio needed for the gas octane you want to run.
The intake valve closing point determines the Dynamic Compression Ratio (DCR)

So here is the proper order.
The cylinder head comes first because the completed head determines the rest of the build. You will need to know the finished combustion chamber volume
Then the camshaft is chosen to match the port work on the head for the desired power band.
Once you have those two things figured out then you can calculate the piston dish volume based on the combustion chamber volume to get the needed Static Compression Ratio that will yield a 7.5 DCR to run premium gas.

If the 240 is a 1967 then is has the good rods worth reconditioning for high performance work.

So lets go through an actual build:
The 240 heads gets 1.94" intake valves and 1.60" exhaust valves and is fully ported with an intake port flow over 200 cfm.
After the combustion chamber is unshrouded for the larger valves and the head has a cleanup cut taken off the deck, lets say the combustion chamber volume is 70cc.
The stock 3/8" rocker studs are replaced with 7/16" screw in studs for roller rockers.
You can also get one of the new casting SR heads when they become available later this year, with intake port flows in the 250 cfm range.

Chose a cam with a 236* 050" duration with at least 292* of advertised duration on a 110* LSA.
This cam profile will give you a lot of power with a lopey low rpm response and still be very street worthy.
The intake valve will close about 76* after bottom dead center.
If you enter that value into an online DCR calculator and adjust the Static Compression Ratio (SCR) until the DCR is 7.5 you get a SCR of 10.4
https://uempistons.com/p-27-compress...alculator.html
So now you know the engine's compression ratio needs to be limited to 10.4 to limit the DCR to 7.5 to run on premium pump gas without detonation.

Have the 240 rods polished and shot peened and resized with ARP bolts.
AutoTec can make 4032 aluminum alloy forged pistons with a 10cc dish.
The 10cc piston dish along with the 70cc combustion chamber volume will give you a 10.4 compression ratio with the piston set at zero piston to block deck clearance.
This is what the exact rod and piston combination looks like.



Did this help answer your question?

 

I think so lol.

 

Please ask more questions on anything you need clarification on other questions for your engine build.

 

I'm sure i will have more as i go. I'll let y'all know if i do end up with this parts truck.

 

...and 300 rods, and a 300 piston, and a 300 damper.

 

You can use the 240 rods with a 300 crank. But it requires some custom pistons that have a much shorter compression height. Like Pmuller mentioned earlier.