vortec heads with flattops,compression problems?

[inglewatt102]

I went ahead and got some vortec heads and had them cleaned up,valve job and all but I have flattop pistons in my 350 with a 480lift Lunati cam,Edelbrock RPM intake and Holley 650 carb.Can I still run pump gas with this setup or will my compression be way too high(its a daily driver)?Do I have to get dished pistons?What about bigger gaskets?Anybody have this setup?How did it run?THanx for any answers

[novaboy22]

i really depends on your cranking compression, put the heads on then do the test, if its around 200 or less you should be ok, and why would you change pistons if your concerned about compression just install a bigger cam with more overlap to bleed off cylinder pressure. i have a similar setup and i run fine on 91 octane. thats cali gas btw so its crap. just make sure your timing is good and you shouldnt have any problems. a thick gasket might be enough though. what was the combustion camber volume in cc's on your old heads?

[inglewatt102]

The old heads were camelhumps I think like 76cc?its bored .20 over also.The machinist that worked on my vortecs was concerned about the amount of compression with putting the new heads on and suggested putting in dished pistons in place of the flattops that are in there.I really dont want to have to replace perfectly good pistons but dont want a ride that runs like crap on pump gas ya know.Just really curious exactly how much compression it will actually have once the heads are put on.

[MrPbody]

A "standard bore" 350 w/64 CC heads will yield a static compression ratio of about 9.7:1, which IS a bit too high for 93 octane combined with an iron head. It's "close", though, as we use a "line" of 9.5:1 for "pump gas".

If you were to make sure your mixture is a tad "fat", and timing curve isn't TOO aggressive, you can make it "work". You WILL "give up" a bit of power to keep the timing "down". It will be more forgiving and easier to tune if you reduce it to under 9.5:1. A small "dish" in your existing pistons would do the trick. 3" in diameter, .060" deep. A good machine shop can do the job for you.

327 heads (refered to as "camel humps" here) were ALL 64 CC chambers. Some of the "Power Pack" heads had 72 or 76 CCs. Those have a sharp "peak" on the head where the "humps" are on the large valve heads.

Jim

[novaboy22]

i suppose its a matter of opinion, in that case but i know i have almost 10 to 1 and i run FINE on 91 octane. 35 degrees total timing. if you ping under load, then just get a bigger cam, its cheaper then changing pistons and you might be happier with performance. o and btw vortec heads have an excellent camber design that will support more compression then older style iron. but hey what do i know im only 22 years old.

[MrPbody]

novaboy,

It's true, the Vortec head uses a little "less" timing than older designs, due to the efficiency of the chamber design. However, the "laws of physics" won't be denied. Even though you can't HEAR it, it IS detonating, at least a bit.

Using a cam with more overlap to reduce detonation is okay ("bleeds off" pressure at low engine speed) for low-RPM applications, though it kinda defeats the purpose. But in higher revving engines, detonation is not only present, it is increased in the upper ranges.

Your "age" is of no concern here. The "net" is full of "old guys" that don't have a clue, and youngsters that do enough "real" research, they're very knowledgeable. The key is to know WHO to listen to and who NOT to... Pay close attention to what professional engine guys (or gals, I guess) have to say. Still pay attention, but take with a grain of salt, what experienced ammetuers have to say. Many of them are VERY good sources of information. Gotta watch it when it comes to wannabes and "arm chair" quarterbacks. Many will tell you their ideas are the best, more to make themselves feel better about THEIR choices, than to give you solid advice. The above issue regarding higher overlap and detonation is one such example. Mythology is that you can retard the timing and reduce detonation. Not so. It WILL reduce "spark knock", but it can't make the fuel burn slower. The quicker "burn rate" of lower octane fuel is the reason and there's no getting around it. By using more overlap to bleed off the pressure at lower engine speeds, you "fool" the engine iinto thinking it's got less compression (dynamic). Once the cam gets into it's "operating range", all that goes out the window. The ONLY thing working "for" you is the higher RPM leaves less time (on a clock) for the mixture to burn at peak cylinder pressure. Aside from that, the bigger cam will actually "pack" the cylinder better at higher revs, compounding the problem. MANY a piston has been removed here, and the rings "fall off" due to detonation, and the customer says "It sounded fine."

The bottom line regarding octane, compression and detonation is "There's no free lunch". Get something somewhere? Give something up somewhere else...

Just to clear things up...(:-

Jim

[toddman67]

In order to satisfy your determination to find out what compression you will end up with on your engine You will need to read the following link and do the math as explained in the article. There may be some complex measurements that need to be obtained, but it is the only methold that will get you the ball park figures.
I will send you another web site with further information soon, but this will get you started.
www.chevyhiperformance.com/techarticles/46778

[toddman67]

Here is another article that explains the calculation process as well.
www.carcraft.com/techarticles/calculate_compression_ratio

Assuming that the heads were not shaved during the machining process, they should be approx 64cc (vortec 906) correct?

Felpro 1003 head gasket 8.44cc calculated - 9.1cc advertised volume.

Pistons? Look at advertised specifications for vavle relief cc.

Block: is it O- decked or stock. Straght edge tool with dial indicator a TDC to determine this volume. Dial caliper would be sufficient unless you want the exact measurements.

You know that the block was bored out so the piston diameter for calculation purposes will be 4.020"

There is no quick answer here, however if you are persistanat and want to know the real deal. Do The Math.

My Gut feeling on this, as previously posted, is that if you put the Felpro 1003 head gaskets on this engine w/flat tops and vortec 906 heads, you will be fine on pump gas. I have run this combo 64cc heads w/flat tops and less than .002 deck height with no problems, however they were not vortec heads, but same combustion chamber volume.

Get a good calculator, pull out the old math skills from the past and get to work.

Good luck.

[toddman67]

Here is a calculator for static ratio that will help.

www.rbracing-rsr.com/compstaticcalc.