Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - brainstorming an engine build
View Single Post
  #20 (permalink)  
Old 12-09-2012, 07:54 PM
cobalt327's Avatar
cobalt327 cobalt327 is offline
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Posts: 5,031
Wiki Edits: 1616

Thanks: 128
Thanked 616 Times in 565 Posts
Originally Posted by trillobite View Post
I found this to be an excellent read during my research: Racing Engine, How to tips

cobalt brought up some good points, if it is a truck, then it should have the ability to pull something, even though I VERY highly doubt I will EVER pull anything above 500 pounds, you never know.

For a truck engine as a general rule, is it 9-9.5:1 for iron heads, and 10-10.5:1 for aluminum heads? I read that running aluminum heads allows you to increase compression by one point. I'm thinking that it would also be a good added insurance to run .035 quench since I do not expect to ever rev above 6000.

thanks for the good advice cobalt
I would call your estimate of "9-9.5:1 for iron heads, and 10-10.5:1 for aluminum heads", safe. I would not, however, want you to go any closer than 0.040" quench. 0.035" leaves NO room for error on the part of the machinist or the dimensions of the parts or even a missed shift or broken driveline part, there's nothing of any real substance to be gained from the extra 0.005", and the downside can be expensive.

While it's true you can usually get away w/more compression using aluminum heads, this is only because aluminum sheds heat easier into the cooling system. There are no free lunches- if you build w/alum. heads, the added compression is needed just to stay even w/an iron head- not that aluminum magically allows you to run more compression. I don't much care for aluminum heads on a SBC daily driven vehicle myself, I like good ol' cast iron. That's not to say the good aluminum heads aren't good- they are- but you do pay for the privilege.

In the same vein, I also never build a street engine right on the edge, compression-wise, either. The general rule of thumb on compression is one point- like going from 9:1 to 10:1- will net you about 4% more power, all else being equal. Now, that IS 16 hp on a 400 hp engine, so isn't exactly insignificant, but the down side to the added compression can be a huge loss of power if you misjudge things and you end up needing to retard the total timing from what gives you the best power- just so the engine doesn't detonate. And it has other downsides as well like less tolerance to a bad tank of fuel, or something that causes the coolant temp to rise past normal. At least in the last two cases you can stop, get out, and turn the distributor to lower the timing to limp home w/o ruining the engine- as long as you caught it soon enough.

My bottom line is: There have been a LOT of VERY strong running SBC 350/360s built using 9-9.5:1 CR. A properly spec'ed and assembled engine w/that CR can expect to make more power than you can safely use on the street. A racing engine is another animal altogether and the two (racing engine on the street) seldom meet happily in the middle, in my experience. Again, that's not to say there aren't 10:1-plus iron head or 11:1-plus aluminum head engines being driven on the street every day, w/o issue. But it IS to say that if you choose to run on the edge, it can bite you.

So I cannot in good conscience advise anyone to build a 11:1 aluminum or 10:1 iron head engine for street duty. When the parts on hand absolutely HAVE to be used, sometimes this means compromises must be made- I understand this. But if at all possible, hedge your bet to the safer side rather than the "faster" side.

Good luck.
Reply With Quote
The Following User Says Thank You to cobalt327 For This Useful Post:
trillobite (12-10-2012)