Right now I've got a set of AFR 190's that I am a little leary of running. For some reason (many, many decisions ago...) I opted to shave them down a bit to 64cc and now I'll be running around 11.3:1 compression, which I'm a little uncomfortable with. The 190's were also purchased at an earlier "era" in this engine buildup, and I've since sold the Super Ram, and purchased a Stealth Ram. Would I see a major benefit if I were to get a different set of heads, or would I be throwing money away? Dropping compression down to 10.5 range would also calm my nerves. This car will serve street duty a couple days a week, it is not my daily driver anymore. Any ideas? Thanks!

 

Are you still shopping for short block components? It might be a good time to look at custom pistons as that compression is a bit high for 92 octane...

Those are good heads...it depends on your HP goals if you want to use them or not.

 

LOL, no way man, its WAyyyyy late in the game. The block is about ready to drop in! All the internals are there, ballancer is installed, AFR 190's are on. I just have a real problem with the compression ratio. I'm running Kieth Black #139 Hyperutectic pistons. For some reason I thuoght running a high compression was cool My builder figured the compression ratio to be at 11.3:1, not 11.4:1 as mentioned on Kieth Blacks site, but it still seems high to me, regardless of the heads being aluminum. Maybe I'm just getting some kind of last minute jitters, cause I dont want to run into any catastrophic problems and have to pull the motor out....

 

Use a FelPro 1044 head gasket, that will take a couple of tenths of a point out.

 

I'm using the head gaskets that came with my Trick Flow 400 small block gasket set. It lists the Fel-Pro # on the box, but its not listed online. I'll see if I can call up Trick Flow or Summit later on.

 

If you are stuck with 11.3 to 1...the only thing you can do is to reduce the timing in the chip to help compensate...there is another member on TGO (MikeH) who went through this same problem and ended up having to run something like 20 degrees total timing!! to solve this problem...

Good luck.

 

It's a whole lot easier to change them now, than it will be after you've been driving it for a while and can't advance the timing enough to get decent throttle response and cruise behavior without it pinging like a mother (and we all know how much those ping), and you have to take apart just that much more stuff to get to them.

Do it now.... you're not saving yourself money or trouble by putting it off.

 

I just called my builder, 165 to balance the rotating assembly, again I'm going to stop by the shop on the way home today and see what he has to say about all this. I'm thinking that a piston swap will be my best bet right now. I appreciate the help guys, this ordeal is really bothering me

 

My builder told me that I wouldnt be able to change to forged pistons, so I'd have to stay with my current pistons, or get a new set of KB Hyper's that would yield my desired compression ratio. His 2 suggestions were:

A. Get KB #147's
B. Cut my KB #139's to match the 147's

Option B will be much cheaper, and wont require a re-balance, but it sounds a little shakey to me. He is confident that he can do it, and has done it before successfully, so I'm considering it, but he still has to call KB to see if it is even possible with my pistons since the 147s and 139's may be different castings. Any thoughts on this?

 

I would change out the pistons now while the is motor able to be easily worked with!!! Playing the gasket game may or may not help you, it may actually cause additional detonation problems if you already have a big quench height. Your builder is correct, you will not be able to run any other piston besides a KB becasue he has already finished the bores in the block for proper side clearance with the KB piston. Forged slugs will require greater side clearance then the KBs. I would get with KB before cutting the pistons, you may destroy a perfectly good set of slugs and at the end of the day still end up buying new ones. Even if you do cut the pistons you will be removing a decent amount of weight, which will need a re-balancing of the assembly, i dont see how he can get around this.

Again just change the pistons now. You have the flat tops right now they are a 7cc dish. Going to the 147s will give you a 18cc dish that will drop your compression to around 10:1. It will cost you a few bucks now but at least you will be able to rest easy for as long as you own the car and motor. Heck you might even be able to sell off the pistons you have now for a few bucks to help balance out the cost of the machine work.

 

Is it not possible to re-hone the cylinder walls for the forged pistons? Not that its important I guess, I think I want to stay with hypers anyway. He told me that since he'll be removing the exact same weight from each piston (once the first one is done, the rest are done exactly the same since the settings are made), and it wont need a re-balance after that. If I were to get the 147's, I'd definately need a re-balance since they have not yet been matched to the rest of the rotating assm.

 

Right, you could rehone. Going forged will require it, thats all, more work more money! KB hypers are great pistons, I went with a set in my motor, 197s 12cc dish for 383.

 

You could beveal /round off the valve relieves (out of the engine of course) and buff the tops to reduce the chance for hot spots, run the 1044 gasket RB mentioned, and find someone in your area that will unshroud the valves to the 4.155 bore of the 406 - with the goal in mind of increasing the chamber volume to 67 - 68 cc? If you're decent with a grinder (meaning you won't walk the grinder across the head surface) and have a couple old valves (to put in place of the "real ones") you actually could do all this yourself - if you're not confident with a grinder,,, pay some one.

With the increase in gasket thickness (.039 vs .051),, if you had 11.3:1 before,,,, you'll be looking at 10.6 - 10.5:1 and should be able to give the engine a decent ignition curve running premium gas. I say should,,, because there are other variables that come into play other than your calculated compression.

Anyway,,, that at least gives you another option to consider and good luck.

 

With spot on tuning, you should be ok at that compression. I'd stick a thick gasket on it, and perhaps open up the chamber, with the hopes of getting it into the upper 10's.

With an aluminum head, 10.7:1 compression really shouldn't be that much of an issue.

 

What cam are you running? Another way to get around too much compression is to use the overlap in the cam to bleed off pressure. CC had 11.7 cr on their vortec setup and used a cam with a lot of overlap, 2xx/25x 108LSA solid lifter. You dont have to go with that much, but it is a another option for you. Also that isnt really that bad if you run enough overlap in a cam and retard the timing a little.

 

mastrdrver:
My cam is similar to the CC306 cam a lot of LT1 guys run. .510/.541 Gross Lift, 229/245 @.050 on a 112LSA. I ran the numbers through a cam-calculator, and I've come up with 12 degrees of valve overlap, which sounds kinda small, or wrong? I used the calc from here:

http://www.rbracing-rsr.com/camshaft.html

Using these #'s
BadSS:
I'm not that confident in my grinding abilities to work on a set of aluminum heads I'll let a pro do it if it comes to that.

OMINOUS_87:
Yea, more money and time, neither of which I want to spend much of

Thanks for the help!

I just used that sites "Dynamic Compression Ratio Calc.", and its telling me 10.35:1.

http://www.rbracing-rsr.com/comprAdvHD.htm

4.155 Bore
3.75 Stroke
5.7 Rod
11.3 static compression
Intake closes at 42 deg ABDC
0 PSI boost
Target Altitude: 250ft

 

12 degress of overlap isnt a lot, but it isnt a small amount, seen guys run at or less then 0 degress worth. I would think that you would be ok with that setup with you low dc. Still if you dont fell comfortable I would go have the cc enlarged to drop you compression or run a thicker head gasket.

 

Yea, I'm thinking spending the bucks on the heads would save a bunch of time (since nothing is coming out of the block), and while the chambers are being enlarged, I'll have them do a bit of porting, so hopefully everything will work out perfect

 

double post.

 

Ok after doing some research, you should aim for a dynamic at ~9:1. So I would say you are a little high.