I am going to send my engine out to the builder in a couple of days, and they said that I need custom pistons to do 8.0-8.5 cr. Since I have to buy a custom set of pistons, should I just go as low as possible or is there a point of diminishing return?

 

You need to decide with your builder what is best for your application. Yes, you can have too little compression. Most likely 8.5:1 will be fine, but going anywhere from 8:1 to 9:1 will depend on your setup.

What are the specs on your setup?

 

Most people agree that the more you lower you CR the more your car relys on boost to make power. So in essense, you'll have to be into boost to have decent drivability. The lower the CR the more sluggish the car feels when not in boost. It really all depends on how your car is set up as to how bad the car will react on really low compression though.

My truck TOTALLY felt sluggish when I dropped from 9.5-8.6 CR when part throttle driving. On my new motor I am going back to 9:1. I simply don't have enough gear to help overcome the low compression i think.

 

It's kinda funny... I don't know the specs. I have a set of ported heads (278/188), a PT76 GTS turbo, and a mission . It looks like the short block will be a 383 (callies race master crank, manley rods, custom JE pistons, billet 4 bolt conversion). I was looking to make just about as much power as I can and stay safe on pump gas. But, when it comes to race fuel I want all I can get.

 

I'm not the know-all on this stuff, and I wont pretend to be. But a word of caution. Obviously you have a goal, and you obviously have enough cash to make it happen. The question is whether or not you fully understand what you're getting into. Personally I know far less then I'd like to know. But with comments like "It looks like the short block will be a 383" and "I dont know the specs" I get a little nervous.

If your heads are 278/188 at .600 lift - I'm also a bit nervous. AFR's 195cc LT4 competition package heads are set to flow 280cfm at .600 lift... but the exhaust is 208cfm, which is a tad more then yours. Not a huge difference, no. For reference, AFR's 180cc lt1 street head flows 260/194 at .600 lift... Or the LT1 competition is 268/211cfm.

Now I know well enough that heads are very different in a N/A setup as opposed to a FI setup, but I kinda get startled by your comments. Slapping parts together with the intent of them working together will not produce the best results. Your crank and rods are great, sure. Manley rods and a Callies race master. That is awesome. But why those parts? Max power is a great goal for sure - but a bit unrealistic. Till you get scared? Till you bend the rods? Till you spin the bearings? I would love to see your setup come together, but from a glance at the numbers, it looks like things have been thrown together a little bit, and more care needs to be taken about selecting parts.

Again, I'm not the know-all in this subject and dont pretend to be. But when I look at what you've said, somewhere in my brain a caution light clicks on... Maybe there's something I'm missing. If I were going for "max power" on a 383 with a T76GTS, I wouldnt be using heads that small... but thats just me. That or I'd step down to the 4340 Eagle setup and just not work the parts as hard to save some cash. Also there are other options, but again, that's another can of worms.

 

My new shortblock is 8:1, maybe 8.2:1.

I always wanted to try low compression.

The points to keep in mind are:
-you might need to make another lb of boost at say 8:1 to make the same power you make at 9:1
-8:1 348ci LS1 is not a laggy engine, it's still a 325-350rwhp n/a engine
-Most folks who put down low compression are just regurgitating what they have read
-A factory LS1 is 10.1:1, I am now 2 full points lower, so according to some sources I have reduced hp by 6%. My current motor at 10:1 would do 400-440 crank hp no problem, do the math
-I hit 21 psi on the dyno last time
-With 9:1, you will hit probably 600rwhp at the most on pump gas, maybe a bit more
-With 8:1 you might have a little more room, and you certainly have a bigger margin of error for detonation
-John Meaney's C4 is 8:1

 

I'm going off real world experiance in my truck but I'm sure timing, load and other factors contribute. That being said my 8.5:1 CR 306 setup in my old Mustang GT wasn't sluggish at all. Then again, I never had a higher compression motor in it so I couldn't compare. Just what happened in my truck.

 

AFR's flow numbers are generally with a pipe from what I've been told.
I'd still want more than 188 though.

 

The parts in the short block where selected for thier strength and quality. I didn't want to buy another set of heads, so I was going to use the ones I have. They will be sent back to AI for a little more work, I was told that I could get into the 200's on the exhaust side. This makes the project a little more affordable. Besides if I was going to go through the trouble of getting new castings they would not be LT-anything. More than likely it would be a aftermarket block, 18 degree heads, and a sheet metal intake, but that is getting away from the nature of this project (it also would come pretty close to trippling the budget). I apreciate your concern and your insiteful comments, but I have a little more time and research invested in the project than I let on (I don't want to come off as a know it all, just a regular car guy trying to have something). I am at the inital stage of the project where my main concern is block and rotating assy strength and durability. Those decisions (parts selection and clearances) where based on the reccomendation of my engine builder (Tax and all I am looking at a little less than $6000). I have not discussed the CR with them as of yet because all I did (to this point) was get a quote for parts and labor. Since the pistons are a custom deal their price will not change regardless of my decision on CR. The cam has not been selected yet because I have not consulted with anyone at this time. When I do I will give them the stats on the heads and they can make a reccomendation, at whitch point I would disclose the cam specs to this forum for review (I trust you guys). The hot side is a custom deal, I will make my own hedders (16 guage mild steel), and custom wiring (rewireing) after the hot side is compleete. I chose mild steel due to the ease of fabrication, also I will have room for expansion of the pipes. I chose the 76 GTS because of it's ability to spool up quick, and make good power. Also, it's a little smaller than the 88 (hopefully) making it easier to package. I don't know the exact specs of the intercooler (air to air), but it will be in the neighborhood of 24"L X 12"H X 4 1/2W, I will also look into making one of those rings that go arround it and spray nitrogen across it for more efficient cooling. I have pretty much written off a water to air simply because it would be more bulkey of a system, however I am open to further discussion and I would apreciate any insite into the matter. The air to air looks like it's going to be a custom core and custom side tanks (ends). I also went with a FAST for the engine management. When I say I want "max power" that means max power for the set up I have, not max power in the realm of all 383's. Personal fear and broken pars, (this might sound crazy) I guess that's just the learning curve of a project like this. If I scare myself with the power, that might be a good thing, I just might stop spending money on this stupid car .Nothing is unbreakable, the best anyone can do is investment in durable parts and good tuning. I don't have all the answers as far as tuning goes, but my dyno guy is pretty damn good. So, the best I can do for him is provide a comprehensive system for him to work with. Finaly I consulted with induction motorsports for the turbo selection and other parts (engine management, fuel system ect...). I hope I cleared some things up about my thinking/ goals of this project.