The Stovebolt Site Forums The Shop Area The HiPo Shop Hot rod engine builders- - - -help!

Can some of you who build high performance engines help me out a little? I'm getting some figures I think are correct, but if they are, the results are pretty predictable. Let me submit the following facts and figures, and let's see if you come to the same conclusions I have?

Engine cylinder volume: 771 CC
Cylinder head volume: 66.5 CC
Head gasket thickness: .045"
Flat top pistons
Piston deck clearance: .005"

Let's see if you get somewhere close to the compression ratio I've calculated. Then, please tell me what is likely to happen if a supercharger is added to the mix.

Yes, I have a compelling reason for asking- - - -it's not just a pipe dream scenario!

Thanks!
Jerry

Jerry,
I THINK I'm right here. Without knowing the bore diameter, one can't calculate the volumn for the head gasket.
v=pi x r squared x H

Also the 0.005 " for the deck.

The bore is approximately 4 1/8". Chevy 6.2 L V8.
Jerry

Hold on...numbers are crunching.

Correction- - - -4.06" bore, about 4 1/16". I'm getting close to 12.5:1.
Jerry

OK...recalculating.

OK, here's the situation- - - -I'm due to observe a forensic disassembly of a 6.2 liter V8 2011 Camaro engine. It's in a limited production "Indianapolis Pace Car" clone with 21K miles on it. Prior to its initial sale, it was retro-fitted with a supercharger by the selling dealer, and carried a GM factory warranty.

Fast-forward 21K miles- - - -during a warranty repair of some minor oil leaks at another dealer, the original owner was sold on installing a set of GM high compression aluminum heads, with 66.5 CC combustion chambers. It made 20-something miles at normal highway speed (not being hot-rodded, according to the owner) and the engine hand-grenaded- - - -broken connecting rods, BIG holes in both sides of the engine, and fire damage from spilled hot oil to the underside of the car. If my number-crunching is anywhere near correct, the engine was built up with over 12:1 compression, plus being supercharged. That combination looks like they built a bomb! The bottom end is totally stock- - - -sintered metal connecting rods and all!
Jerry

Sorry, my spouse called me to dinner. Am trying o crunch the numbers. Somebody jump in here please.

I can't comment on the science part of this thread, but I bet that was the funnest 20 miles that guy drove the car!

Also, high compression and forced induction is never a good idea....

I doubt it- - - -that car would have been knocking, detonating, and generally running like crap! Too much of a good thing, like compression, is a nightmare- - - -not a pipe dream!

I miscalculated piston dome height on a round track engine once. It made it half a lap before it blew the head gasket out on all 8 cylinders. A 16:1 compression 327 is just about a Diesel!
Jerry

Here's the way I figure compression ratio- - - -

Add cylinder volume + combustion chamber volume + deck clearance volume + head gasket volume = total volume

Divide total by: chamber + deck + head gasket. That's coming out somewhat over 12:1 on this engine.
Jerry

O.K, my SWAG is 12.3099 to 1. Don't know about supercharging.

Just passing by. I'm showing my work!

d=4.06, r=2.03,h1=.045,h2=.005
pi*r^2*h1=.582ci=9.54cc
pi*r^2*h2=.065ci=1.07cc

9.54+1.07=10.63cc

@BDC=771+66.5+10.6=848.1
@TDC=66.5+10.6=77.1

848.1/77.1=11 (11:1)

OK, it looks like I was a bit high, but not out of the ballpark. Add a little blower boost, and we're into at least racing gas compression range, if not the low side of alcohol-burner territory. A supercharger forces more mixture into the cylinders than atmospheric pressure can, effectively raising the compression ratio. It looks like somebody at the dealer didn't do his homework, plus there was no recalibration of the electronics or chassis dyno testing done before releasing the car to the customer. Thanks!
Jerry

At times, I've run the compression a bit high in my Morgan but backed the timing off for street to bleed off the effects of the higher compression. Being a 2011, not sure what the effects of the computer and knock sensors would do. Apparently not enough.

Dennis

I put a set of Jahns 12:1 pistons in a Morgan +4 once, and the owner insisted on using his stock camshaft- - - -didn't want an idle lope. I told him it wouldn't work. We had to set the timing 10 degrees AFTER TDC to keep it from detonating on acceleration, and the thing was totally gutless! He didn't run it that way long!
Jerry

Jerry, you hit the nail on the head about the guys lack of knowledge of blown engines. You leave the static compression alone and set the cylinder pressure with the blower. We usually have initial set up of around 8:1 and the drive from 25% under to 25% over. With 1471 blowers on our tractor engines one can raise the pressure up with over driving the blowers to where the top of the block separates or the main webs are on the ground. Of course as the boost goes up so does the undesired heat. On some street applications I've read where they do run a bit higher static compression on turbochargers to get rid of some of the turbo lag but never seen any above 10:1. On a Roots type blower I would stay in the 9:1 range max on "dumb" engines. I can't comment on the "smart" engines with a computer monitoring from knock sensor to air/fuel ratio. They may be able to tolerate more static compression. If they want you to reveal the cause of the grenade event just put your finger firmly against the noggin of the higher compression head installer; which you have already done on paper.

Thanks, Evan- - - -I was pretty sure the laws of physics hadn't changed recently. Stuffing 10 pounds of "potatoes" in a 5-lb. sack means something's going to rip! With a billet crankshaft, tool steel rods, and a few thousand dollars' worth of other fancy parts, that engine might have survived, but I really doubt it! Doing race engine stuff on the street comes under the general heading of having a "stupid attack". The owner of the street performance shop that's doing the disassembly summed things up pretty well- - - - -"We cater to idiots with more money than brains!" They do build some nice rides, and they do it right. Once they show the car owner the hub dyno figures he wants to see, they detune the engine a considerable amount before it hits the street!
Jerry

Smart shop owner. One of the first lessons I learned 65 years ago was if you hot rod your drive to work car you are going to spend a lot of time being a pedestrian. One to drive, one to race, seems to be the only formula that has worked through the years.

Slightly different topic but raced motorcross as a teenager with several tweaks to get more torque. Wisest advice my Dad ever told me was " son put $100 in a jar every time you start it and you still won't break even when it explodes" .Had lotsa fun but the Jar was always empty......

The engines we built for 1/4 and 1/2 mile round tracks cycled from 3K to 7K+ RPM at least twice a lap, and the average engine lasted half a season, including practice, qualifying, and actual racing time. An engine with a thousand miles on it was an antique. A brand new one had more "wear" built in in the form of extra piston and bearing clearance than a 100K mile street engine, and if it used a quart of oil every race night, so what? We had nine quarts onboard.

Translate that kind of engine build to the street, and you've got a nightmare. With mufflers on it, one of those engines sounds like there's a hundred little guys with sledgehammers inside, trying to escape! Doing race stuff on street engines is an exercise in futility.
Jerry

Well- - - -I was wrong! (Better get out the record books- - - -never happened before, and probably won't again!) The engine destroyed itself due to some very serious assembly mistakes that resulted in a swallowed intake valve after about 60 miles of driving. If a dyno tune had been involved, it never would have made it out of the shop!

Aside from assembling the valve springs with insufficient height from the heads surface to the spring retainer which got the springs dangerously close to "stack-solid" dimensions with the valves fully open, the fatal mistake was not using hardened steel lash caps between the rocker arms and the Titanium valve stems. Every intake stem was worn to the point the rocker arms were contacting the spring retainers, and one had worn sufficiently for the split keepers to release the valve, dropping it into the #4 cylinder at 70 MPH. Titanium is light, but it sure isn't tough! All the intake rocker arm tips were blue-black from friction as the valve stem tips got rough and began eroding. No hotrodding involved- - - -just an engine that ate itself due to incorrect assembly and not paying attention to details! There's a destroyed piston, a broken connecting rod, holes in the aluminum block and the oil pan, and metal debris everywhere- - - -including the intake manifold and the blower! There's also fire damage to the bottom of the car due to the hot oil that bathed the undercarriage clear to the rear bumper and then lit off for a short time. Now the fun begins- - - -insurance adjusters and possibly lawyers will be getting into the mix very soon!
Jerry

Kids and their toys these days...

This "kid" is a 40-something year old 100% disabled veteran who bought the Camaro as an investment for its collectible potential. He really got shafted by a supposedly well-qualified dealer that didn't do their homework before recommending an "upgrade". The car's collectability value has taken a huge hit, both from the fire damage and the need to install a non-original engine now. This one's going to get messy!
Jerry

My kind of kid! Play on!