Doing a head swap, Compression too high?

[Jerrod4567]

Hey guys, been a while since I posted. Im getting ready to swap the heads on my bronco. Here's whats in the motor right now. 393W Comp Cams Xtreme 4x4 hydraulic flat tappet cam. Specs: .477/.496. Advertised duration: 254/262. Weiand Stealth 4bbl intake, Q-jet carb and headers. Its got late model 351W heads on it right now with 69cc chambers. The static compression ratio is right at 10:1 its always ran fine never pinged. I have a set of aluminum trick flow track heat heads that I'm wanting to use that have 61cc chambers. With a .041 head gasket the static compression ratio comes out to 10.9:1. How do you guys think this will run on 91 octane? Anyone running this kind of compression or higher? I hear you can run nearly a full point of compression higher with aluminum heads than you can with cast iron on the same gas.

Next, the pistons sit .017 in the hole. The quench space with a .041 gasket is .058. Seems kinda high but if I were to take any material off the deck, the compression would be way too high. What do you think of the .058?

Any feedback would be appreciated. Thanks a lot.

[slow poke]

Re: Doing a head swap, Compression too high?

I have a 11:1 compression 427 Dart motor in an old Mustang. Aluminum block and heads. It pings on 91 octane under heavy loads (stomping on it in 5th gear) or at low RPM (under 2500 and hard acceleration in 3rd gear). I am running octane boost in it and have it at about 100 octane which solves the problem. I buy "Boostane" by the 5 gallon can to cut down the cost....but it still makes for expensive fuel. I would try to stay around 10:1 if you drive it much.

[pippinmader]

Re: Doing a head swap, Compression too high?

It seems like you are really pushing it, aluminum heads should help some but that is a lot of CR. Is your 393 the one that uses the custom crank, stock rods, and 302 pistons? I looked into that for a while (maybe everyone has) and keeping the CR down is really hard, requires a ton of dish in the pistons.

An engine that pings kind of ruins all the fun. I have even heard that you are better off with higher compression ratio and the correct quench, I can't say if that is true but that is what "they" say. If it were up to me I wouldn't go over quench of 0.043 and a CR of 10.5, I know with a 393 a quench that small is really hard to get without using an early shorter deck block or having a newer block decked 0.020" or so. I am a big fan of the trick flow heads, I have a set of track heats on my bronco now but have you thought about getting some of the other trick flow heads, looks like you can get them up to 66cc, that would get you down to closer to 10.5. https://www.trickflow.com/parts/tfs-52516601-c01

Good luck!

[DirtDonk]

Re: Doing a head swap, Compression too high?

Cam has a huge amount to do with this too. My first thought was you'd be ok because you're not pinging now with iron heads. But we don't know just how close to the limit you really are either.
If it was mine I'd try it, but hearing what slowpoke and pippinmader had to say, I'm having second thoughts.

The new 5.2 Mustang engine is running 14:1 now! Amazing what cam timing, combusion chamber shape and electronic engine controls to fuel and spark can do!

Paul

[Jerrod4567]

Re: Doing a head swap, Compression too high?

Thanks for the replies. Ya I know its right on the border.

My 393 consists of a Scat 9000 3.85" stroke crank, stock 351 rods and a 302 piston.

I think I'm going to try it and see what happens. The reason I am going with these heads is because they are free! We just put a larger set of trickflow heads on my dads 65 Mustang (trickflow 11r 205cc runners) and these that I have now are the ones that came off of it. I'll report back how it goes. Worst case scenario is that I'll just have to put the other heads back on. We shall see! Thanks guys.

[pipelayer]

Re: Doing a head swap, Compression too high?

I have the trick flow heads on mine that have the 205cc runners at just over 10.5 compression I run 91oct in low elevation but run 89 up in the mountains also no ping after I fine tuned my timing. Also the efi helps plus i dont lug it down with such low gears much. I would go for it.