OK - so I'm now running a stock 346CI shortblock w/ ported LS6 heads, LS6 intakes, cam, etc.

The ported LS6 heads were an early version (circa 2002) from SDPC where they shaved the heads 60 thousands! I'm assuming the compression now is north if 10.5 to 1. I'm not sure but I think the compustion chambers were polished a tad but otherwise were unchanged in size.

If I use these same 60K shaved LS6 heads and bolt them to a shortblock with a 4.1 stroke and a 4.005 bore with -8cc pistons, will I get a pump gas friendly street engine?

 

Well.... seems like it will be a bit high with that small of a combustion chamber.

with a 64cc heads your kind of pushing it, those are milled 60 thousanths (are you sure)? what cc are they supposed to be?

 

If they've been milled .060 they are around a 55.858571428571428571428571428571 chamber.

With a -8cc piston, a .040 compressed thickness gasket, and a zero deck you end up with 12.701:1 compression. That's a close to ideal setup considering quench distance. You could run that much if you had a really large cam to bring the DCR down. Considering you currently have a 220/220 I'd look for a larger dished piston.

 

See this is where we differ... I would never run that high of compression where I live. The gas wont support it and honestly most builders will tell you 11.2 or 3:1 is good. I also come up with a higher number than you but what ever.. thats negligible if your going to run 12.8:1 on pump gas.

 

You can get away with 12+:1 static compression in the Texas heat as long as there's some camshaft in there to bring down the dynamic compression. That is what is going to dictate if it's safe for pump gas really.

 

Why take the chance? Running higher compression is harder on the engine to begin with. This guys is New York anyway so who knows what the gas quality is. You Texas guys have the benefit of good gas, around here we shoot for 11.2:1 and hope it was a good batch.

 

Higher compression, once the engine is running, pushes the rotating assembly down harder and it burns cleaner (which is why late model engines run higher comrpession, for EPA satisfaction). Take a look at diesels...these turbo diesels are running close to 17:1, with the NA diesels running 20+!!

The extra compression isn't harder on parts perse....the combo of compression and poor tuning is (due to detonation). If you know what you're doing on the design of the engine (camshaft, STR, DCR, quench, etc) extra compression is easy to get away with....even with the cheap 91 octane we have here (no, we don't have great gas just because we're in the greatest state in the country).

 

Diesel engines use compression ignition, a process by which fuel is injected after the air is compressed in the combustion chamber causing the fuel to self ignite. By contrast, a gasoline engine utilizes the Otto cycle, in which fuel and air are mixed before ignition is initiated by a spark plug.

SO its apples to oranges


I like to play it safe. You never know what your going to get for a batch of gas, unless your using an additive or race gas. I dont see a need to push it. Its cool, we'll agree to disagree on it.

 

If you really milled .060 you are probably around 54-56cc depending on anything else you did and valves etc.

This means around 12.5 to one compression with a 418 and 8cc pistons depending on what gasket etc.

I have not seen a good running engine be able to run that much compression on even slightly crappy pump gas and be able to tune them right.

To put it in perspective, in the Engine Masters contest, that is run by Popular Hot Rodding and has had over at times 100+K prize money and many NHRA and IHRA builders including Jon Kaase, no one has successfully run that much compression and made it work very well when they were really trying to make power on pump gas. In fact over the half the 12+ to one engines blew up in only a few dyno pulls since the gas in NY was a little worse than in other areas of the country.

Most winners were in the 11 to one range and even some of them had detonation damage after the tests which were only a few dyno pulls. Now the rules are in the 10 to one range on compression and people are still making 700hp in 400 inch small block at under 6500 rpm but almost no one blows up anymore from detonation.

Keep in mind that these engines blew up with perfect cooling and on an engine dyno and not in a real car on a hot summer day with the AC on and with out half the stuff going wrong that happens on 90 percent of the street engines I see. Several of these engines had NHRA and IHRA Pro-Stock tuners working with them and several melted pistons in three pulls or less! MANY were pissed that running a half point too much compression on pump gas had cost them a hundred grand after the pistons fragged.

Granted that the average engine on LS1tech is not getting the VE numbers that these EMC engines are making but they are close so I would not run that much compression on anything I built for regular pump gas myself. Also the FAST 90 type deals are VERY good manifold for the rpm range these engines are working in and are actually banned for EMC competition and it's not because they suck!

 

Matt, I am not saying it isn't possible because like you say it is and people do it. I would just rather a camshaft add VE than subtract it. Anotherwords why put a cam in to reduce VE since you have too much compression for pump gas?

On the other hand if you have poor VE like with a really big cam or too big a heads or overly short runners and too much cross section and no runner length you CAN run more compressin for sure and it will also usually help you.

I would rather choose stuff that promotes good VE across the range you want to make your power in and then add the most compression you can reliably get by with on pump gas and so far that's been around the 11-11.5 to one range for us.

I have guys that tell me thats crazy too and if you are running over 10.5 to one on pump gas your engine muxt be a hoopty! Oh well I guess we haven't quite acheived the EMC level of TQ and power yet on pump gas but it's fun trying.

 

So to run 11-1 compression with these 60K shaved heads heads, I would need -12cc pistons?

 

You would need to CC a chamber to see what you really have first as you may have a lot more or less compression than you think. With 55cc heads and a 413 CID engine you would still be a little over 12 to 1 with 12cc dishes. You HAVE to find out what your true chamber CCs are first.

 

Ok thanks.

Is this something SDPC could tell me based on what their specifications were for these heads back in 2002 or does this mean physically measuring the combustion chambers which requires a tear down first?

Does a large dish piston (say -18cc) adversly impact the burn characteristics of the head? I've also read that thicker gaskets result in a crappy burn too. Can you increase the compustion chambers without compremising the integraty of the head?

I'm sure these pups were milled 60 thou since I had to order special length push rods for the valve train.

 

Most of the big companies try and keep records as best as they can so they might be able to help you but to be safe the best way is to have them CC'd so you know for certain what chambers you have. If you have to take the heads off anyway you can do it then. Maybe they can tell you exactly what you have but they have had a ton of stuff go through the doors. Give Brian Gruben a call as they will certainly help you out of they can and they are good guys there at SDPC.