5.0 compression dessisions

mikemurf202 · Oct 4, 2007

From what i've heard, 11 to 1 compression will run on premium but will i be able to still use stock head on it. And what cc are stock 5.0 heads any way im working on my second 5.0 engine and i've havn't done anything with high compression yet. I would really appriciate as much help as i can get. thanks.

Daggar · Oct 4, 2007

I believe that the stock E7s are 58cc, E6s 60cc, and have no idea what the chamber size is for the GT-40.

Why would you want to run 11:1 compression with stock heads anyway? It's not like they flow enough to support the HP that compression that high can produce. You'll still have to deal with the heads being a bottle neck.

It would be like putting a restrictor plate on.

5spd GT · Oct 4, 2007

You can run more compression with an aluminum head over an iron head. It disipates heat better...

So it is going to be tricky with an off-the-shelf camshaft and it's valve timing.

I have read and heard where Ed Curtis recommends not using a domed piston on a daily driver. Something to consider...

Gt-40 heads are 60cc if I recall.

vristang · Oct 4, 2007

I have my concerns about the opinion of Mr. Ed, (personal issue)
but a dish is better.......

There are other ways to bump the compression... shaving the heads...

If you're going high compression, junk pile the stock heads.
Get some Al. heads that will flow what you need.

jason

Fast63 · Oct 5, 2007

True, dish is better than dome, but flat is better than both. It helps the exhaust gas escape better and makes the combustion better.

Stock E7 heads have 61cc chambers roughly
GT-40's have 64cc
GT-40P's have 60cc roughly
I'm not sure what E6's are but I believe it's more like 69cc
E5's are 69cc

I would not recommend shaving heads or doing any weird tricks like that to bump compression, they often can make matters much worse. The best way is to select a piston with the compression you want. Or get new heads with smaller chambers. The best combo for high compression is a small cc chamber head with a flat top piston that has 2 valve reliefs. You also want the piston's deck height to be right up to the top of the bore, or even slightly protruding so that there is minimal distance between the piston and the head (the least clearance necessary for preventing piston to head contact under any condition, which I believe is about .030). So if you have a head gasket that is .030 thick, you want the piston right at the top of the bore when it is TDC. .040 is still good, but from the factory, it is like .060 or more total, and that allows too much more room for the gases to combust outside of the combustion chamber area in the head.

vristang · Oct 5, 2007

Fast63 said:
True, dish is better than dome, but flat is better than both. It helps the exhaust gas escape better and makes the combustion better.
...
I would not recommend shaving heads or doing any weird tricks like that to bump compression, they often can make matters much worse.

I completely disagree with the BOLD comments
Dish is better. I have never seen a dish piston for these motors that didn't have a squish area... (assuming that is what you are getting at...)

shaving heads is done everyday, by MANY professionals.
Please explain why you recommend against it...

jason

Fast63 · Oct 5, 2007

Ok, well dish pistons have squish areas in the main part of the cylinder, but the combustion chamber is not D shaped like a dished piston, so there are some poor looking areas of combustion. Well, stock heads often have a D shaped chamber, but most aftermarket heads do not, and even if they did, I would still use a flat piston because you can maximize the squish areas. But, to each his own.

The reason I don't like heads being shaved is because if you shave enough off, you'll have to shave your intake to compensate for the difference. Who needs that cost and hassle? Especially to make an intake useless for any other head but the one you are currently using. I have also seen a few flow bench data sets in magazines that showed a loss of flow volume from the heads being shaved, though not much, it just doesn't make a whole lot of sense to me to change something that the manufacturer of the head designed for a reason when you can easily change the pistons to whatever compression you want. Pushrod length may need to be changed if too much is shaved off as well, though highly unlikely.

Shaving heads is great to get your head surface true, but I just think it unwise to do it merely for a compression boost.

samickguy15 · Oct 5, 2007

I have to agree with Fast63 as far as head shaving. Also, with the variety of different piston combos, why not just go that route. If you bump your comp. by shaving the heads and then decide, well maybe now I want a blower, then you are probably going to be buying another set of heads. On the other hand, if you bump comp. with pistons, all you have to do is change pistons and you're done.

5spd GT · Oct 5, 2007

Yeah, that is the only thing I have against shaving heads

Once it is removed, you can not add it back

Milling can throw off pushrod length needed, intake milling.

Resale value, if needed, goes down usually.

I think picking a piston is a good idea, and get as close as possible to what your goals are. Then if you really want that extra .1-.2 compression, then maybe a little shaving and hopefully not enough to throw-off the geometry.

vristang · Oct 5, 2007

DON'T put any stock in what the Magazines tell you...
Especially flow data, which can easily be skewed.
Remember that the mags also suggested setting the TPS to .998v ...

If your choices are running less than ideal CR, or shaving the heads
Then shave the heads.

I will be doing this with my 410w rebuild.
Will I have to spend some time dorking around with the lower intake?
Probably,
but it will be worth it to KNOW that the motor went together RIGHT.

I've learned over the last couple of years that a strong/durable motor doesn't just get assembled from a collection of parts. Each part must be custom fit to interact with the other parts.
Doing it right involves taking the time to make these kinds of modifications.

As for the dish...
I'm gonna leave that alone, as I am sure I don't have all the details for making a sound decision here.
I do know that 'Quench' isn't the only factor that should be considered...

jason

5spd GT · Oct 5, 2007

vristang said:
If your choices are running less than ideal CR, or shaving the heads
Then shave the heads.

I guess me and you differ on that, but that's cool

I would rather keep a stock mill, select a good piston, head and combustion chamber to match, and a custom cam to match the static compression and all the other variables.

I see what you are saying though.

Fast63 · Oct 5, 2007

vristang said:
DON'T put any stock in what the Magazines tell you...
Especially flow data, which can easily be skewed.
Remember that the mags also suggested setting the TPS to .998v ...

I don't put any stock in what magazines tell me. But it is arrogant to assume that everything they say is false, even if most of it is. But it is hard to argue with someone putting a set of heads on the flowbench and then shaving them to put them back on a flowbench solely for the purpose of seeing if there is any flow difference, and that happens to be the result of slightly less flow. Head manufacturers put a lot of time and thought and science into making heads, including the combustion chambers, it just seems lame to mess up all their good work by milling them and changing the design.

vristang · Oct 5, 2007

Fast63 said:
I don't put any stock in what magazines tell me. But it is arrogant to assume that everything they say is false, even if most of it is. But it is hard to argue with someone putting a set of heads on the flowbench and then shaving them to put them back on a flowbench solely for the purpose of seeing if there is any flow difference, and that happens to be the result of slightly less flow. Head manufacturers put a lot of time and thought and science into making heads, including the combustion chambers, it just seems lame to mess up all their good work by milling them and changing the design.

You are making 2 assumptions here...

1. That the flow data was not corrupted. It is extremely easy to fudge up flow data.
Were the heads placed on a 'cylinder'?
What diameter was the bore?
Radius inlets?
...

2. You are assuming that flow numbers are 'all important'
There is MUCH more to making power than flow numbers. Proper CR is one of those things. Squish is one of those things. Piston design is one of those things.

See what I'm getting at?
A motor must be looked at globally, not locally.

Flow numbers on a bench are only part of the story. Some would say they are a small part of the story.

I am not saying that EVERYTHING in the mags is false.
I am saying to look at the info, and evalutate its validity on your own.

I am not familiar with the article you are speaking about, but I have seen some pretty crappy 'Testing' done in the mags.
Step back and take a good hard look at how they came up with that data.

BTW-
Implying that I am 'arrogant' only proves that you do not know me.

5spd GT said:
I guess me and you differ on that, but that's cool

I would rather keep a stock mill, select a good piston, head and combustion chamber to match, and a custom cam to match the static compression and all the other variables.

I see what you are saying though.

Our opinions cetainly do differ

Also, I think you are focusing on a 'basic rebuild' (for lack of a better term), while I am thinking in terms of 'dream motor'

I am not about to design my short block around pushrod length, or limit my CR options in order to preserve the resale value of an intake manifold.
We are definitely discussing 2 very different scenarios

jason

Fast63 · Oct 6, 2007

The flow data was done with a cylinder included, 4 inch standard bore, measuring at increments of .100 lift. I don't know the specific article or even the mag, but I remember the results because I thought it was very interesting, and it does make sense. I know that flow data can be easily fudged, but they had no reason to. They were not supporting any certain cylinder head or anything, they just wanted to figure out if there would be any difference, so it is not really right to assume that their data was fudged.

Also, I have no idea why you are saying that I am assuming flow numbers are "all important" because I never said anything of the kind. In my original post, I stated that flow numbers were only slightly affected anyway. I don't know why you assumed that I made that assumption. So please, save the speech about global aspects of an engine, because I already know it.

vristang · Oct 6, 2007

Fast63 said:
So please, save the speech about global aspects of an engine, because I already know it.

Best of luck to you

This is headed for an e-argument now, so I'm going to check-out
before things get nasty.

Good Luck,
jason

Fast63 · Oct 6, 2007

vristang said:
Best of luck to you

This is headed for an e-argument now, so I'm going to check-out
before things get nasty.

Good Luck,
jason

Ok, well I wasn't trying to argue, but yeah there is no point in going on about it

5spd GT · Oct 7, 2007

I got to looking through Probe's piston specs and the compression it allows with certain head combustion chamber sizes.

I noticed one thing

They all have a piston that would allow from 8.4:1-11.5:1.

All by the tenth

I imagine, if you pick a piston you want to get the compression you would like, you can match your headgaskets (usually .038-.048 for us guys), and deck height, you can go without milling your heads/intake, and still keep a good quench area.

This is, if you pick the right piston from the beginning

5.0 compression dessisions

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