compression ratio questions

[TexasNomad]

Hiya every one. I was talking with a friend today and got on the subject of comp ratios I know how they work and how its measured but what ratio is good for what. I think my 460 with D3ve heads is 7.9.1 and that can run lower grade pump gas but how high can you get untill you have to run racing fuels? what comp ratio is good for just a good strong daily driver what is good for a tough weekend street fighter, do you really need 12.1 can you bore a engine 40 over and still run low grade gas and still get good power?.

 

[SuperCab]

Well, I'm no expert but here is my humble opinion.

7.9:1 is a pretty sorry c/r. I think 8.9:1 or 9:1 maybe 9.1:1 will run on good old 87 grade juice and would increase you power, probably a quite noticeable difference. I would say that 9:1 would be good for your "good strong daily driver" OR "a tough weekend street fighter". If you go higher than 9:1 you'll have to run premium gas and that's no fun.

 

[TexasNomad]

Ya I think your right 7.1 is pretty sorry, i've been looking at a 306 stroker kit and you can get it from 8.1 all the way to 15.0! my god might as well plug up the spark plug holes and run Diesel in it. Thank you super cab for your opinion.

 

[SuperCab]

Yeah, if it was me, i'd go for 8.9:1 or 9:1. According to the book, that should still run on 87 octane pump gas. And yeah, 15:1... You could almost run diesel in that. IIRC diesels run about 20:1. IDK where you could find gas to run in a 15:1 gasser. It'd prolly ping awful.

 

[TexasNomad]

Now if there was any engine builder any where near here that were not crooks to the tenth degree I'd be in good shape, called one place and said I had a ford small and BB that I want to rebuild, he just said I don't like fords and then the conversation just stopped then he said 5G a piece with out me even saying what I wanted... Gotta love this town

 

[DOHCmarauder]

Great question that has more to it than just CR.

Chamber/piston shape has as much, if not more, to do with it.


For instance, the Tundra 5.7 is extremely happy with 87 octane with a CR of 10.2 to 1.


The old Ford 351/400M open chamber heads would ping themselves to death on premium with 7 to 1.

I'm not familiar with your heads/chamber but I would start there.

Don't know how accessible Jon Kasse is, but he is the undisputed Ford big block guru.


Maybe shop your area for a reputable head porter. I'm certain with proper bowl work and the correct valve un-shrouding you can up your compression safely to make some decent power.


But like you already figured out; stay with someone who likes or builds your brand.

 

[TexasNomad]

For instance, the Tundra 5.7 is extremely happy with 87 octane with a CR of 10.2 to 1.

87 on 10.2.1... Wow I didn't know you could do that I thought you had to run perm.. But the Tundra is a computer controlled engine that can change engine parameters to what it needs to run different grades..
And who is Jon Kasse?

 

[DOHCmarauder]

87 on 10.2.1... Wow I didn't know you could do that I thought you had to run perm.. But the Tundra is a computer controlled engine that can change engine parameters to what it needs to run different grades..?

You are correct, it has knock sensors that will pull timing if needed.

But it makes nearly 400HP on 87!!! Supposedly there are tuners in the works that will be able to utilize higher octane.....as of now, 91 make 0 difference.

The 4 valve pentroof chamber is the key.

And who is Jon Kasse?

http://www.jonkaaseracingengines.com/

 

[TexasNomad]

Wow, I think his engines are umm slightly outta my league....
Hmm sadly my 79 is lacking knock sensors, maybe i'll just stick with a small low comp stroker =)

 

[SuperCab]

For instance, the Tundra 5.7 is extremely happy with 87 octane with a CR of 10.2 to 1.

The old Ford 351/400M open chamber heads would ping themselves to death on premium with 7 to 1.

How does that work? Can someone please edumacate me more about this?