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Old 06-10-2003, 10:02 AM
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Compression Ratio advice new vortecs

Hey all,

So I had a dead cylinder last week, thought I had burned a plug wire... pulled the valve cover off the passenger side and, low and behold the rocker is lying there in the oil passage between #6 and #8 cylinders. snapped rocker stud. So, since I'm running 882's with pressed in studs, and I'm tired of dealing with these junky heads, I decided to go with the GM Vortec upgrade.

My current compression ratio is about 9:1. That's with 72cc chambers. Now, moving down to 64cc chambers is gonna boost my compression to 10.5:1 if I stick with stock height head gaskets. (.04" height)

My question is this: I'm considering going with a .062" tall copper head gasket from SCE - this would bring my compression down to 10:1. Do you think this will be worth doing since all I have available here in california is 91 octane fuel? I'm just trying to keep my compression down as much as possible to avoid having to put octane booster in every time I fill up.

Opinions? I'm very happy with the performance of my SCE copper exhaust manifold gaskets, anyone have experience with their head gaskets?

The Blonde Weasel
San Diego, CA

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Old 06-10-2003, 01:17 PM
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I think 9.5 to 1 is about the limit before you have to start backing off timing or find better fuel.
Are you using flat top pistons and zero deck height?
If you are then the vortecs will definitely put you over the limit. If you have dished pistons and pistons that are down in hole a ways the vortec heads will work just fine.
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Old 06-10-2003, 02:36 PM
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I think you better do some math on this project. I feel that you will have a pre-detonation problem trying to do what you are proposing. First of all, about the most compression that you will want to run is about 10:1 (you should target 9.5:1) unless you want to go to race gas. If you have a cam with considerable valve timing overlap, some of the compression will be bled off and you can go a little over that, particularly at higher elevations, but this is risky and dependent upon good machine work and careful planning.

Generally, you want to have as tight a quench area as possible between the wedge section of the head and top of the piston. A quench of around .035 or less will significantly reduce detonation because the minimized quench area will not support combustion untill the gap opens up after top dead center. The tight quench also creates a violent swirl in the head as the gap opens up which increases combustion efficiency. This is the concept that the Vortec head is good at, a high swirl.

It is unlikely that you will be able to have a low enough compression and a tight enough quench using the pistons you already have and therefore, you will need to replace your pistons with a set of flat top or D cup pistons. As long as you are at it, you will probably find that to get the quench you will want you will need to "zero" the deck height of your block, so it is not going to be a bolt together project if you do it right. Or, you can avoid detonation by going to a significantly lower compression (below 9:1) and just have mediocre power.

I have built a 383 with zero deck, vortec heads and speed pro hyperutectic pistons, tight quench and am supporting 10.3:1 compression at sea level using a Comp Cams XE268H cam. It is very streetable, creates just gobs of power and even idles with just a little lumpiness.

I assume you also know that the vortec heads will not tolerate a valve lift over .488, so be careful what cam you put under the heads.

There is a very good tech artcle on quench area on the www.speedomotive.com website.

I will post a portion of the article here:



Engine Building With SPEED-O-MOTIVE

There isn't a universal set of rules that govern all engine building. The following is information that has worked successfully and should be considered when building a performance engine.

A high performance race engine, by its definition, indicates that limits are going to be pushed. The limit that is of most concern, as far as pistons are concerned, is peak operating cylinder pressure. Maximizing cylinder pressure benefits horsepower and fuel economy. Considering the potential benefit, owners of non-race engines, from motor homes to street rods, also look to increasing cylinder pressure. Increasing the compression ratio is one sure way of increasing cylinder pressure but its not the only way. Camshaft selection, carburetion, nitrous and supercharging can all alter cylinder pressures dramatically.

Excessive cylinder pressure will encourage engine destroying detonation with no piston immune to its effects. The goal of performance engine builders should be to build their products with as much detonation resistance as possible. An important first step is to set the assembled quench distance to .035". The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston height, (not dome height), is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness to the water cooled head.

If you are building an engine with steel rods, tight bearings, tight pistons, modest RPM and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Turbulence is the main means of reducing detonation. Unfortunately, the operating quench height varies in an engine as RPM and temperature change. If aluminum rods, loose pistons, (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit the benefits of the quench head design and can cause severe detonation. The suggested .035" static quench height is recommended as a good usable dimension for stock rod engines up to 6500 RPM. Above 6500 RPM rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston. Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. High performance motors will see some detonation, which leads to preignition. Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before top-dead-center. Detonation damages your engine with impact loads and excessive heat. The excessive heat part of detonation is what causes preignition. Overheated combustion chamber parts start acting as glow plugs. Preignition induces extremely rapid combustion and welding temperatures melt down is only seconds away!

For a successful performance engine, use a compression ratio and cam combination to keep your cylinder pressure in line with the fuel you are going to use. Drop compression for continuous load operation, such as motor homes and heavy trucks, to around 8.5:1. Run a cool engine with lots of radiator capacity. Consider propylene glycol coolant and low temperature thermostats. Reduce total ignition advance 2 to 4 degrees. A setting that gives a good HP reading on a 5 second Dyno run is usually too advanced for continuous load applications. Normally aspirated Drag Race engines have been built with high RPM spark retard. The retard is used to counter the effect of increased flame travel speed with increased engine heat. "Seat of the pants" spark adjustment at low RPM will almost always cause detonation in mid to high compression engines once they are rung out and start making serious horsepower. Set spark advance to make best quarter mile speed not best ET, usually 34 degrees total advanced timing.

I hope you found this interesting and I predict, would have saved you some grief. Tom

Last edited by F-1Rodder; 06-10-2003 at 02:43 PM.
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Old 06-10-2003, 05:56 PM
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that post from the speed-o-motive site was actually taken directly from the keith black site:

http://www.kb-silvolite.com/page05.htm

can you find any other sources that more specifically mention detonation problems at greater quench heights for Vortec heads? I emailed scoggins-dickey for advice on my situation.

Once I get home I'll check my pistons, but I'm pretty sure I have double "D" valve reliefs and a flat deck on the inboard side. Once I have the heads off I can measure my piston to deck height.

My problem is I can't afford new pistons, and I already bought the Vortec heads.
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Old 06-10-2003, 08:29 PM
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okay so I have a set of 345NP SpeedPro Federal Mogul pistons. Not sure if they have valve reliefs on just one side, or both sides of the piston. Couldn't find any pictures of any online.

My cam has an advertised 292 duration, .480" lift with 1.5 rockers.

From what I've been reading, it sonds like my compression will get bled off a little due to the high duration on my cam.

Again, I'll know more once I rip off the heads, but that's a little ways down the road at this point.

Thanks for the input.

The Blonde Weasel
San Diego, CA
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Old 06-10-2003, 10:03 PM
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Okay, checking my math here... tell me if everything looks right.

chevy 350 with a bore of 4.030in and a stroke of 3.480in, the cylinder volume should be 727.41cc.

the combustion chamber volume should be 64cc (in the head) + 4cc (in the valve reliefs) + 6.58cc (SCE 0.031" copper head gasket) + 2.09cc (0.01" piston to deck clearance) =

total combustion volume is 77.13cc

727.41cc / 77.13cc = 9.43 / 1 compression ratio

that's weird. why does desktop dyno say 10.43?!?

This may explain some of the confusion on my part. Anyone else out there have desktop dyno that could try this out?

With the 0.031" gasket and the 0.01" piston to deck clearance, that puts me at 0.041" quench height, which is good. I'm actually really excited to get those new heads on now. Guess that means it's time to clean out the garage.

The Blonde Weasel
San Diego, CA
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Old 06-10-2003, 11:46 PM
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B.Weasel, you have to add the chamber volume to the cylinder swept volume and THEN divide the result by the chamber volume. You need to check your deck height, as I believe the Speed-Pro 345's have .020 less height at 1.54" than a standard 350 piston (1.56"). The H345P hypereutectics are 1.54 for sure. David Vizard & Smokey Yunick both said never increase quench to lower compression, as it results in MORE tendency to detonate and less power. KB has charts of CR's for their pistons with various chamber sizes at .040 quench. Go to http://www.kb-silvolite.com/page25.htm The KB catalog is free for the asking, & has lots of good information in it.

Last edited by jimfulco; 06-10-2003 at 11:51 PM.
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Old 06-11-2003, 02:34 PM
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I think you got this one backwards: David Vizard & Smokey Yunick both said never increase quench to lower compression, as it results in MORE tendency to detonate and less power.

I think they said never reduce quench to lower compression which is exactly the point I proposed.

But there is always one way to find out, isn't there!
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Old 06-11-2003, 06:02 PM
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I just talked with a guy named Tom from Scoggin Dickey. He says several of the guys in the garage have thrown the vortec heads on without any problems. He says, especially since my camshaft has high duration and good valve overlap, that's gonna bleed off enough of the compression so that detonation won't be a problem. He says they've run vortec heads anywhere from 0.03 to 0.05 quench height without major problems. He also said that detonation problems in these situations are almost always solved by timing and fuel adjustments. Typically, I guess many people run with their ignition too far advanced, as with vortecs you have to retard a couple of degrees.

I gave him all the numbers for my particular setup, we identified my pistons, and he feels my compression will be somewhere around 9.8:1. Not really sure how he came up with those numbers, but it sounds good to me.

Again, this all means nothing until I pull my old heads and determine definitively what my piston to deck height is.

the blonde weasel
san diego, ca
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