[Bob Cowan]

Now you need to calculate your effective cylinder pressure, and your dynamic compression. This is very important. Cylinder pressure is what makes power. Not enough, and it will be a dog. Too much, and it will beat itself to death. Been there, done that!

Around 8 for dynamic compression works out really well. You'll need to be carefull on your tune, and keep total timing rather conservative. People seem to want to run a lot of total timing. But dyno testing on my engine shows it makes more power with a little more timing down low, and a little less up high. I only run 33* total on race fuel, and 31* total on pump gas. I also run a light spring so the timing comes in quick.

to calculate your numbers, I'v found this one to be about the easiest to use: United Engine & Machine Co. Incorporated

BUT.... this one does not take in to account altitude. So if you live above about 1,200-1,500' ASL, use one that compensates for altitude.

Wallace Racing: Dynamic Compression Ratio Calculator

 

[2FAST4U]

Bob-
Great information. I calculated my static and dynamic compressions. They are 11.5 and 9.6. Am I going to have problems?

 

[Bob Cowan]

Yes, you are definatly going to have problems. That's way too much cylinder pressure for pump gas street driving.

The easiest change would be a bigger cam with alater closing point. Run the calculater again with in intake closing point of 75* or so, and see what you get.

The down side of that is the carb. Lots of compression and a huge cam can be difficult to drive on the street. A multi point EFI will make it easy to drive on the street, but a carb needs a good vacuum signal and a wet manifold needs good intake charge velocity to keep the fuel in suspension.

It might end up better in the long run to use a dished piston.

 

[2FAST4U]

[deleted - moved to separate thread]

 

[slaga]

I punched in my numbers and assuming my deck height is 0.035 (waiting to hear from the machine shop), I have a static compression ratio of 9.51 and a dynamic compression ratio of 8.58. That seems just out of your suggested range. Now I am just waiting on the machine shop to let me what the deck height will be.

Thanks for your help Bob.

 

[slaga]

Keep Dynamic Compression between 8.0 and 8.5 and you'll build a great engine.
United Engine & Machine Co. Incorporated

I have run my numbers and I am ending up with SCR at 10.175 and DCR 8.712. How bad is a dynamic compression ratio of 8.712?

 

[Bob Cowan]

I have run my numbers and I am ending up with SCR at 10.175 and DCR 8.712. How bad is a dynamic compression ratio of 8.712?

That has the potential of being a big problem. You'll need to run the mixture a bit rich, keep timing conservative, and don't beat on it too hard on pump gas. As the rpm's rise, so will cylinder pressures. That has a high likelyhood of beating the ring lands to death.

If you're going to run it hard - like autocross, road course, or drag strip - then use a high octane race fuel.

You could consider using a "bigger" cam, with more duration and a later intake valve closing point. That will keep the cylinder pressures lower at lower rpm's, and raise them at highr rpm's.

I plan on using Ethanol this year, mostly on the track but occasionally on the street. But I have EFI, and changing the fuel map is as easy and flipping a switch. With a carb, you'de have to rejet for ethanol.

 

[slaga]

Thanks for your help. Seeing how my cam was stolen before I even got my block back from the machine shop, using a different cam will not be a hard thing to do.

 

[Erik W. Treves]

I am leaning towards the twisted wedge heads and flat top pistons with reliefs for the twisted wedge heads. It seems the compression ratio is about 10.15:1 or 10.2:1. Will this setup be compatible with pump gas? Will I need to run 93?

I ran that setup in mine for several years and even supercharged it. Ran 91-93 in it all the time. No issue at all....that is until my FMU failed and killed a piston, block and TFS cam.

The complete TFS street heat set up with TFS 1 cam and intake was/is a nice setup and made exactly the HP/TQ it said it would. SOunded great too.

 

[slaga]

Since my cam was stolen and now I have to purchase a new cam, I went with a custom cam from Flowtech Industries. With his custom cam my new dynamic compression ratio is now 8.45... Thanks for your help.

 

[slaga]

HV/SP Oil Pump or SV/SP Oil Pump?

Like I said earlier this car will mainly be a cruiser but I do plan to autocross some and depending on how much I like it... I do have the Levy Racing fully baffled road race pan and I am looking at ordering a high volume / standard pressure oil pump. Is there any reason to not get the high volume standard pressure oil pump?

 

[Bob Cowan]

There's a lot of controversy about that. I did a lot of reading and research. Some pro builders will only use a HV pump, and some will never use one. A SBF never needs a high pressure pump.

From what I can find out, if you have external oiling components like remote filters and coolers, then use a HV pump. If not, use a standard volume/standard pressure pump.

Only use a Ford or Melling pump.

 

[slaga]

I do not revive an old thread very often but I thought this would be worth it. Going on 6 years on the road with this engine and it has been a hoot! I love my little 347. Thanks to everyone that contributed to my parts selection, especially @Bob Cowan.

 

[Bob Cowan]

You are more than welcome.