Dynamic Compression Ratio for Pump Gas

[wrobinson]

Greetings guys,

I need to know what is a safe dynamic compression ratio for 4V quench heads. The build is for my son's first car a 1973 Q code Mach 1. Currently, I have calculated the static compression at 10.29:1 and dynamic compression at 8.29:1. All calculations were done using spreadsheets provided by one of the 7173mustang forum post by DonC. I do not mind using 91-93 octane if the engine needs it. The 91 octane is fairly easy to find but 93 is getting harder in our area. I have run 9.7-10:1 static compression ratios on pump gas motors before but 10.3:1 seems extreme but this is our first Cleveland. I appreciate your input in advance.

357CI

piston .018 in the hole

.040 Cometic head gasket

Sealed Power flat top piston

top ring .375 from crown

D0AE-N heads 63CC

 

[73pony]

You will be fine on 91 Octane pump gas preferably ethanol free is available in your area. Just make sure you watch for pinging when setting the timing.

 

[Carolina_Mountain_Mustangs]

Use the Lucas gas treatment also will help lube the valves replaces the lost lead. Makes huge temp difference also. I use with 90 91 ethanol free. I do not have the CC heads but is .050" over bore, flat top pistons never checked compression. Never have a heat issue and no spark knock.

Was at car show on Sunday and guy I had not seen came in with a Chevy truck with a heavy thump. Went over and had if I remember right a 540 BB ?? with blower. He had 7.5 compression static and had 10 lbs. of blower boost was on 105 racing fuel and is over 1,000 HP at the wheels. He had I think a 4" aluminum drive shaft and titanium U-Joints. He gets 3 MPG cost him $80 to get to the show and home, lol. Not a cruiser for sure and not going to race it either.

He has another truck that he had put a blower on and had left stock drive shaft in the truck. When on the dyno it twisted the stock shaft up like a screw shattered it and no driveshaft loops and it came flying out nearly hitting several people and just missed a very expensive Porsche. With part of the shaft still on the rear it beat the truck up severely the dyno cannot stop rolling to quick. 

Be sure to add drive shaft loop to work list can save your life. 

You can get fuel for about any compression you want to run just very expensive. I have friend with GTO that runs 18.3 static. Not a street car but could drive on street just could never afford. Burns 5 gallons of special racing fuel in one 1/4 mile pass. Two 1.000 CFM Dominators on it 5/8" fuel line.

 

[Stanglover]

Jason is correct imo. 91 octane will be fine as long as you concentrate on timing.

As an example, my engine is a 71 351C 4v CC heads (GA's though) with 13cc KB dish top pistons @ zero deck giving me approx. 9.8:1 static ratio. I had pinging like you would not believe with the reman. factory distributor set at 14 deg initial advance. This subject has been hashed out a million time here, but at the end of the day, no matter what distributor you chose (and please not an HEI thing), your max mechanical advance should not exceed 36 degrees total and all in between 2500-3000 rpm. Also buy/use an adjustable vacuum advance canister. On mine I set it at 4-6 deg. on top of 34 deg, mechanical. It comes down to fine tuning once you have it running. Listen to your engine, play with the initial timing to find the sweet spot. It might take some time, but worth it.

IF you chose to use a factory Autolilte/Motorcraft dist. I recommend the following from my own trial and error. I can expand on this if you request it.

1/ the slot plate needs to be welded and recut to give .410" (L10) wide slot. Or if it already has an L13 slot, sleeve the post to limit travel. L15 is too wide and equal to 30 degs. on the crank. You need only 20 degs. on the crank, L10 with 14 deg. initial.

2/ I use 1 heavy and 1 light spring. 

3/ I chose to use a Pertronix Ignitor II electronic module WITH matching coil. Having great success with that. 8mm copper wound wire set and Autolite 25 plugs set at .045" gap.

4/ Alternatively buy a DuraSpark from Performance Distributors set up for your engine, not cheap but you'll have a no worries distributor from what I've been told.

5/ Find and rebuild a DuraSpark yourself.

To me, bottom line is you need to be in control of the advance curve and not trust some other companies "one-fits-all" approach. 

 One other note on distributors and that is gear choice. You need the correct gear material for the cam you chose. I have no input on that, do your research.

Geoff.

 

[Don C]

The spreadsheet in this thread will give you more information and help

https://www.7173mustangs.com/thread-engine-and-compression-calculations?pid=199290#pid199290

 

[wrobinson]

The spreadsheet in this thread will give you more information and help

https://www.7173mustangs.com/thread-engine-and-compression-calculations?pid=199290#pid199290

Yep, that is the spreadsheets I used to come up with the compression numbers.

 

[Don C]

I guess I should have read your entire post. Sorry. Yes, 8.29 will be fine on pump gas.

 

[wrobinson]

In case anyone is wondering about camshaft specs: Competition Cams Hydraulic Roller duration @.050 224/230, ADV duration 275/281, lobe separation 110, intake center 106, lift 569/578 w/1.73 rockers (IVO 31.5, IVC 63.5, EVO 74.5, EVC 26.5, OVERLAP 58).

 

[boilermaster]

In case anyone is wondering about camshaft specs: Competition Cams Hydraulic Roller duration @.050 224/230, ADV duration 275/281, lobe separation 110, intake center 106, lift 569/578 w/1.73 rockers (IVO 31.5, IVC 63.5, EVO 74.5, EVC 26.5, OVERLAP 58).

Wrobinson,

Is this a special order cam ?

 

[wrobinson]

In case anyone is wondering about camshaft specs: Competition Cams Hydraulic Roller duration @.050 224/230, ADV duration 275/281, lobe separation 110, intake center 106, lift 569/578 w/1.73 rockers (IVO 31.5, IVC 63.5, EVO 74.5, EVC 26.5, OVERLAP 58).

Wrobinson,

Is this a special order cam ?

Boilermaster,

Yes, it is a custom grind from Comp Cams. I really wanted to order a custom from Blykins but already had this one available. Blykins seems to understand the specific needs of the 4V.



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