Compression ratio too high! What to do?

[PSB]

Apparently when I bought my stroker kit, I got the wrong pistons. I was shooting for a 10:1 CR, but stupid me, I didn't check specs for the part numbers I got before I had the engine put together . It turns out the pistons I have are for 12:1 CR. I've only idled the engine so far, so I don't think any damage has been done, but now I've got to figure out how to fix it, as I need to be able to run on pump gas.

Are there any other options besides changing the pistons, or should I just bite the bullet and pull the engine and replace the pistons?

Engine is a 408W with AFR 185 heads. Cams specs Here.

Building this car has been one heck of a learning experience. Since I discovered this, I've read more on compression ratios, dynamic compression ratios, quench area....then I ever wanted to know.


Thanks,
Pete

[Hotfingrs]

http://www.campbellenterprises.com/R...o%20calculator

Go to this website and make sure what CR you have....you need to know all the specs

[exrodder]

Pete,

Unfortunately, I think you have the right idea about taking pistons out and replacing them. I have 10s in my engine and I don't think I would go any higher for a street driven motor. I had to remove my motor over the winter and have the heads redone due to local machine shop not doing them right. They didn't replace worn guides and it smoked. All is right now and enjoying driving car when it is nice. Can only say good things about Randy and Sue. When you talk to Sue, tell her hi from the owner of #31

Tom

[PSB]

I've tried about six different compression ratio calculators - depending on which one I use, I get between 11:8 - 12.3:1. Piston manufacture's specs say 11.85:1.

Pete

[Tommy]

Pete -
First of all, I am an amatuer mechanic so double check anything I say with a professional before you act on it. .. By my calculations, you need an additional 0.9889 cubic inches of volume above the piston to lower your compression ratio about 2 points. Raising your head by 0.0775 inch with a thicker head gasket and/or spacer would theoretically give you that additional volume. I don't know if there are gaskets/spacers available to do that, and I don't know what other problems thick gaskets might create. But that is the only simple solution I could imagine. Good luck.

[Ron61]

Pete,

I run 11:1 in both cars and have no problem but with your 12:1 are your pistons domed? You can drop about a point with thicker head gaskets in some instances but not always. I think in your situation I would go ahead and bite the proverbial bullit and get the correct pistons.

Ron

[Excaliber]

Ron what octane gas are you using for the 11 to 1?

Is your timing reduced accordingly to avoid "ping"?

[coyled]

Pete, run a lower gear in back which will allow it to crank up quicker and you might avoid pinging. Also, you could run less total timing to avoid pinging. I personally would rather have the compression and run less total timing. That way, if you are ever at the track, you can add timing and race gas and the motor will gain a bunch of power. Scott

[brainsboy]

To lower compression with out changing pistons only leaves a few options. As one person already suggested you can use a thicker head gasket. Also find a head gasket with a larger bore size. Anouther way is by taking the heads off and porting the chambers larger.

If you goto head gasket at .050 thick with a 4.100 bore and open the heads 5cc each it will drop you down to 10.9:1 compression. If you can open the chamber a total of 8cc you will have 10.6:1 compression

hope this helps

[PSB]

I'm thinking I'll get some thicker head gaskets and maybe some larger chamber heads to get down to 11:1. Anyone have any experience with Edelbrock Vr 70cc heads?

Ron - My pistons are flat-top. That's why I didn't notice when I got them that they were the wrong pistons. Apparently, in a 408W, you have to get dished pistons to get below about 11.8:1.


Pete

[Ron61]

Ernie,

With the aluminum head stroker I can run this lousy 91 as long as I don't pull it hard but I still use the 110 Racing gas. With the big block and the steel heads it is harder to get away with this low octane stuff and I only use it when I am going far enough that I have to fill up. The aluminum lets you get away with a little more as it dissipates heat so well. So I normally run the 110 in both or mix it if I am going on a long trip. And no, I don't retard the timing unless I am out and have to use the low octane then I will retard it just enough that I can run down the freeway without pinging. I am thinking of putting wheels on my gas barell so I can tow it along behind the cars.

Ron

[Mr.Fixit]

Run racing gas. You might find out you enjoy the extra power that compression gives you.

[brainsboy]

The extra horsepower from 11:1 ro 12:1 is only 3%, hardly worth it since he's trying to stay with pumpgas and something streetable. Your cheapest way is to take the heads to a machine shop, you shouldnt have any problem getting anouther 5cc's from those heads + a .050 thick gasket should be right on the money.

[trularin]

Bite the bullet and get the 10:1 pistons. If you ping your gonna damage the pistons.

[Cracker]

I run 11:1 in my 427 with no problems so far. One thing to consider when runnig close to the edge is not advancing timing to radicaly - mine is 15 initial & 32 total. It seems like you could go with a combination of thicker head gasket and additional material off the pistons or combustion chambers and be in the mid - 10's - I think. Sorry, I haven't read all the above posts if they've already suggested this. Don't fret because prior to my rebuild last year I had miscalculated and was running 13:1 on premium for over 700 miles with no major problems. Yes, it was caught and some detonation signs were present but a total rebuild by a professional builder has been done with no cost spared and alls well. I sure miss the throttle response and sound of 13:1 though!!! Best wishes!

[PSB]

Thanks for the inputs guys.

I talked to the guy who I bought my stroker kit from, and I think I'm going to go with a different set of heads with a bigger combustion chamber. Edelbrock Victor Jrs with a 70cc chamber will bring the compression down to a more reasonable 10.8:1. Back the timing off a bit on the street and I should be fine.

Anybody want to buy some barely used AFR 185s?

Cracker - 13:1 Did you hear any detonation?


Pete

[Dennis Kelley]

I'm interested in the 185's pm me.

thanks

[Cracker]

PSB - You kiddin me! At 13:1 and the revs I was running all I was hearing (and everyone within 3 miles of me) was DETONATION! It sounded like a freakin CUP car - it was bad! No one ever asked me if I had a "427" when I pulled up - it embodied everything a BB 427 could be - LOUD! Seriously, I really didn't hear it but it was loud (and fast)! At 11:1 it's still nice but not quite what I had grown accustomed too. Good call on changing the heads.
Enjoy!

[MidOHasp]

Brainsboy...

I'm just a dumb asp but seems to me that going from 12-13:1 would net a heluva lot more than 3% more power. Figuring 500HP as a benchline, 15HP?????? Seems crazy. I always thought compression netted a LOT more difference than that....

[coyled]

The gain or loss in power from compression or loss has a lot to do with the combination. With good heads, its closer to 10% more power per point of compresion. Scott