I know this has to do with soooo many different things that even if one has all the information, a sure answer can't be had before the engine's built, but I'm looking for opinions, from experience, on compression ratios and pinging.

I just put a bunch of money and a bunch of time into a totally built 408 for my truck, and then it blew up, I think because of massive pinging, caused by a 10.3 compression ratio.

As I rebuild it, I want to lower the compression ratio to a much safer level, by using thicker head gaskets. I have closed chamber iron heads (Cleveland 4V), flat top thick top (meant for propane) pistons with 3cc reliefs, and .041 head gaskets now. I've done all the math and with the distance down in the hole and everything else, a .093 gasket will give me a 9.3 CR. I want something in between those.

How high do you guys think I can/should go for maximum power, and minimum pinging, even in dense winter air, and on pump gas (regular would be great, but I'm unfortunately used to buying primo)?

Give me some info to start with, and I'll take it from there, AleX.

 

What cam are you running? Your cam has a lot to do with your dynamic compression ratio.
I have 9.84:1 static CR but I have a higher duration cam 274/286 adv dur. which brings the dynamic CR back down to just under 8:1 so I have no problem with pinging.

 

As you said, there's a ton of stuff that goes into this including engine geometry, cam profile, etc. The short answer is 10:1 with iron heads and 12:1 with aluminum. The fact that your engine detonated with 10:1 leaves me to believe tuning was very wrong. You would not believe some of the things I've seen from "professionals" when asked to re-tune a car. It's amazing some of them ran at all. Get someone that really knows how to set up an engine to put the final tune on it. You shouldn't have any problems at all.

 

It's worth the money to get it dyno'd who has a good setup. For one, you'll get the most out of your engine, but you'll be able to tune it just right. It's something I didn't do and now I wish I would have because I don't think I have mine tuned to perfection by no means.

 

10.5:1 with premium fuel... 10:1 with regular.... Either way, tuning has got to be spot on.

As a rule of thumb, each point of compression is worth about 4% more HP.

 

WHAT exactly "blew up"?!?! What failed?! You need to start there,, 10:3 is not at all bad,,,, you really need to know what caused WHAT failure before you start again,,, cam timing correct,,,, pre-oiling done,,,,, tolerances checked EVERYWHERE! Engine put together with good pre-lube,,, I have NEVER had any engine take a crap after a rebuild,,, something was not right. Find your failure FIRST and then analyze why so history does not repeat itself!!

 

I personally never go over 10 to 1 with iron heads and 93 octane. I've done this with several builds on different engines and never had a problem. I'd go with 9.8 before I went with 10.2.
I remember your thread about excessive pinging and my answer was too much compression.

 

I'm guessing he broke or melted a piston and lowering the compression will prevent history from repeating itself, but we'll have to wait for his response.

 

Interesting.........I keep hearing different things. The guy who I bought my stroker kit from builds Racing/pulling engines and he told me 10.5:1 (which is mine) would be tough to run on premium (92 octane) and that's with aluminum heads. Hence, why I'm running E85. Others like you have stated I should be able to run premium. I'd like to run premium just for the fact that I don't have to drive 4 miles just to fill up and not every station has ethanol.

 

Well first off I'm still not sure exactly what failed, I'll have more info for you guys on that as soon as I tear it open, but, to give just a little bit more info: the engine didn't fail on first start up or anything like that. I had driven it a few hundred miles, taken it on the hiway, long trips, short trips, start ups, and even gunned it around a little bit, and everything was fine, but as soon as it would get close to warmed up, ping central. It sounded like I'd dropped a box of screws into a blender. Oil pressure was great though, it was staying cool, the run and idle were smooth and strong, and eventually, and it would start right up. The day it died, I was heading out to college with 2 of my friends, and all of our stuff in it, probably 800 pounds more than empty with just me, we got about 25 miles down the hiway, and all of a sudden, there is a cloud behind me. There wasn't any noise that I noticed, and the engine kept running fine, but I pulled right over, hopped out, and threw open the hood. There was coolant everywhere. It was coming from both sides of the exhaust (fully seperate duals). The engine still idled OK though, then I turned it off. I got it towed home, popped out the plugs, and most of the cylinders had some coolant in them, one in each bank was almost full.

My computer battery is dying, but that's all I know as of right now anyway.

 

High compression with a small cam and ton of ignition advance = melted pistons end of story.

For high compression the camshaft HAS to match the combination as well as getting the distributor curve dialed in for total advance.

Static compression ratio is one thing, it's been mentioned before, but read up on DYNAMIC COMPRESSION RATIO otherwise known as DCR. That ratio is far more important to streetability manners than Static Ratio. You could have a 10.75:1 engine that runs all day long on 87 octane and a 8.5:1 on 91 that pings like a coffee can because of mismatched parts and set-up.

Josh

 

And that's where I'm probably at. I didn't want a big ol cam so my truck would be doggy off the line and I wanted more torque so I went with a smaller cam (I believe 262 duration or something like that). Even though I have aluminum heads, I think I still would have trouble running 92 octane. I guess all I really could do is try a gas carb and see if it pings or not and to play with timing but I'm not sure it's even worth it. E85 isn't bad, its just tempermental when it comes to cold weather and you have to be on the money with tuning or it won't run very well.

 

Kind of off topic but do you know what your duration at .050 and lift is? Just curious. I went with 220 228 duration at .050 and .545 lift in my 460. Thats quite a bit bigger than I've used in 5.0 Mustangs and also went the the Edelbrock Performer RPM air gap intake. My dad thought I was going to kill my low end but I can still take off in 3rd gear without touching the gas and man does it scream up top.

 

I understand the concept of dynamic compression ratios, I get quite well what all you guys are saying, but I'd just be taking shots in the dark if I tried to guess at mine. Here's the new shot of info for the day though, and maybe you guys can come up with an idea of where my DCR is at, based on your experiences.

The carb was an older vac secondary 4bbl Holley 600, electric choke (which was working). The intake was a 2V (Cleveland, as opposed to 4V, not meaning to barrel) Edelbrock air gap. The intake fed into 1 inch spacer/adapter plates, which I ported to match the intake side, and left dropping/stepping sharply into the big 4V head ports on that side, to prevent reversion back into the intake. The big 4V exhaust ports hit up against 2V port sized headers, something that I was going to change for flow reasons eventually, but something that I think now could have caused alot of exhaust gas to flow/be trapped back in the cylinders, making a very high DCR. I had a 281/292 duration roller cam, with a .595/.595 lift at .050, I thought plenty of cam to let my high compression setup breathe.

What do you guys think of all of that? This thing was pinging at steady speeds, on Premium.

 

What were your base timing and total timing numbers set at?Did you indicate the camshaft at build?