BigBadWhitey

iTrader: (3)

I have a build question regarding heads and compression ratio:
Im going to be building a 355 for boost this summer, as my car is running a stock internals lt1 and id like to turn up the wick and not worry about its insides turning to mush, so i figured before that happens id get another mill underway.

My issue is this: Current plans are to use stock heads cleaned up and maybe ported a bit (this is not a max effort build, rather a safe 8-10 psi build) and with the 54cc chambers and use of a 22cc dish piston i figure I can get a compression ratio of around 9.4 (assuming cometic head gaskets at 0.040, and a deck height of 0.010). However my brother (max effort build person) just let me know in the next while he MIGHT be swapping out his afr 195's with a 65cc chamber for the new 227s and would let me take them off his hands. This would drop me down nearly a whole compression point to about 8.45.

My question is: Is there a happy medium I can find, using a good 2618 piston to get me in a compression ratio that will work well if i end up using the stock head, or get to swap to the AFR's in the future? Using a 14cc dish would get me about 10.3 with stock heads, and 9.1 with the AFRs, for that i assume i would need to get a custom piston made from diamond or someone of the like, has anyone done anything similar and can provide some insight?

Thanks guys

F0x Slaughter

You do realize people will run 8psi on a completely stock engine?

If you don't plan on running large amount of boost, have a good tune, I wouldn't be afraid to run stock compression on 8psi.

8.45:1 would be a dog off the boost.

Diamond is literally down the street from me, if you are going the power adder route some custom pistons are definitely nice.

duh

iTrader: (16)

With the LTx engines and forged internals, I wouldn't be afraid to run 10:1 on 92/93, whichever you have in your area. As long as the tune is right, you would most likely get away with up to 10psi. I wouldn't go much higher, but in my experience you'll be fine. Fwiw my brother did a turbo cavalier (I know, assinine) that was 10.5:1, ran on pump 93, and had ignition issues, but still was a mid 12 sec. car. Not too bad when the best 60' he ever had was a 2.2x

BigBadWhitey

iTrader: (3)

I do, its just I DD the car and an internal failure would rather hinder that. Also im a working student so the longevity of the car right now is very important, as well no one locally tunes lt1s unless you have bigstuff3 or something of similar caliber.

With that in mind, could I run say a 12cc dish, and that way be in ok shape either way on the heads? (10.5 on stock heads, 9.3 on the AFRs)

I guess, what is the lowest static CR id go without it being a dog out of boost?
Is dynamic compression a large thing that comes into play here? I hear people saying you want to shoot for around 8:1 dynamic compression ratio, based on cam selection as well. If this is a bigger factor than static, that gives me more room to adjust as i could delay the selection, but still purchase pistons and rotating assembly and get that together while waiting on the decision of the head. (IE, i could run 10.5:1 static compression on a 12cc piston, 10psi and run a cam with an ABDC of about 60 degrees and have a static compression around 8.5:1. If i get the large heads, I would be running a 9.3:1 compression 10psi and get a cam with an ABDC of closer to 50 if thats possible, and have a dynamic of 8:1) ?

Shownomercy

iTrader: (1)

If you are dropping the money on good forged pistons, getting em custom to what you want CR wise is nothing. JE did mine for basically what the normal set would have cost.

BigBadWhitey

iTrader: (3)

Im for sure ok with ordering a set of custom pistons. I guess my main question is: Is dynamic, or static CR more important to design around?
If static is very important- is 9 on the low end, 10 on the high end going to be ok to run?

nitrous2fast

iTrader: (2)

If it were my build i would have a 10:1 CR and run 8-10lb on the street with 93. When at the track run some sort of high octane 112 and turn it up to 15-18, but hey thats just me. i would have a good set of forged rods in it though; nothing crazy expensive.

BigBadWhitey

iTrader: (3)

So with that you figure stock heads 10:1 i should be ok? Thatd be running a 15cc dish, meaning if i used the AFRs id be around 9:1

Can anyone confirm for me the deck height on a lt1 block? Ive been using 0.010 just for calculation sake.

It would be getting good rods and pistons, think stock crank would be ok with those kind of numbers?

F0x Slaughter

Stock crank is stronger than cast aftermarket cranks.

Stock crank would easily hold 500hp.
Just use ARP fasteners.

nitrous2fast

iTrader: (2)

Stock crank would be fine.

With boost head choices get funny. The great thing about boost is it pushes air into the chambers. Air doesn't flow the exact same as when being pulled. AMAZINGLY ported heads don't make as much difference now. Good, quality ported stockers will flow plenty of air to make pretty big numbers.

Are you wanting this car to be street driven quick machine or drag beast?

BigBadWhitey

iTrader: (3)

Its going to remain street driven, but id like it to be quick when it is tracked. The thing is the AFR heads have already been ported and put to boost use on a 383, so the ported AFRs should outflow ported stock heads right?

My problem is trying to find a piston that i can use to get myself a good middle compression ratio using either head, which is proving to be very hard.

With ported AFRs, 10psi and a suited cam, am i going to be pushing the safety of the stock crank? I assume thatd get me close to if not over about 500?

nitrous2fast

iTrader: (2)

There really isn't a safety limit on the stock crank. IT IS A BEAST! to say the least... rpms probably are going to be the only concern pushing a lot of power though it, But you probably won't be getting close to those rpms... 10psi is relative to the setup it is ran through. The actual amount of air required to run 10psi with the stock heads is going to be quite a bit different than the amount for the ported afrs (assuming vs stock non-ported heads). As well as power achieved by both. A very fun street car that will get into max boost fast as well as not rip the tires off the rims will be to run a higher CR and less boost I.E. Stock heads on 10-12lb with 10.5CR.

BigBadWhitey

iTrader: (3)

Thats what i mean about the heads. The AFRs at 10psi is going to be much more air than the stock heads at 10psi no?

Also i will be using the SC wastegate boost controller method to be able to adjust boost more readily than swapping pullies. Meaning ill pulley for say 15 but have the WG set for 10, that way itll come into the 10psi sooner.

That aside, i guess ill mostly need to wait to see what happens with the heads as to what piston i pick. Best case senario would be the AFRs with a flattop forged piston to get about 9.8:1 compression

Edit: next question would be, if using stock crank, what rod should i go with? 6 inch rod or 5.7?

F0x Slaughter

Are you planning turbo or Supercharger?
I assumed you were talking about a turbo at first.

A 355 is only stock 5.7" rod stock 3.48" crank and the bores bored 30 over. (4.030) that makes a 355.

Yes psi is a measurement of restriction. 10psi in a 1/2in pipe is the same pressure but less volume than 10psi in a 1in pipe.

~9.5:1 I wouldn't be afraid to run 20+ psi on.
So 9.8:1 would give you decent power off the boost, and give you room to run more psi. I'd probably wouldn't run more than like 18psi on that compression ratio.

As Nitrous says if you run the smaller heads requiring less volume to get the same psi it will get to full boost quicker. You start opening things up and you need more volume which means more rpms to get to the boost. Yes it will make more power, but you are talking about a street machine. You don't cruise the street at 3500rpm. The track you shouldn't see below 4000rpm.

Street, quick spooling is more fun.

BigBadWhitey

iTrader: (3)

Its a supercharger: Procharger D1 (not the d1sc, this has a higher max impeller speed as well as a larger step up ratio)

Can you not get a 355 out of a 6inch rod, 3.48 crank and 30 over bore as well? I didnt figure rod length effected CI, however the benefits of dwell time and decreased side load i thought may be beneficial in a boosted motor.

The SC will be pullied for a higher RPM than the boost it will be making, if that makes sense, using the wastegate boost control method. So mid 9:1 will be ok to shoot for in a street car.

nitrous2fast

iTrader: (2)

with the smaller stroke of the 3.48 crank, the side loading and dwell time has little difference between the 5.7" and 6" rods.

myltwon

iTrader: (1)

Make sure if you run a SC, to key the crank and run a decent hub/balancer.

BigBadWhitey

iTrader: (3)

The crank is keyed and will be in the new engine as well. Im debating going to one of the ATI dampers but im not sure if thats really needed? If i could get a good deal on one id grab it i suppose.

These are the parts im thinking of going for bottom end, and input is greatly appreciated!
Crank: Stock
Rods: Scat 5.7 (or 6? may need to get more input) H beam rods PN 2-250-5700-2100A
Pistons: Diamond 22cc dish PN 11420
Piston Rings: Total seal Maxseal gapless top PN MS3690-35
Main bearings: Clevite H series (are these good?)
Rod bearings: Clevite H series (again, ok?)
Cam Bearings: Clevite 77 cam bearings PN SH-290S (i have no idea on these)
Main bolts: ARP main bolts PN 134-5002 (do i need the small journal or large journal ones?)

anything else im forgetting as far as major assembly pieces?

F0x Slaughter

I wouldn't run gapless rings.

It has been proven by changing the gap a stock SBC 350 picked up 33hp. Just from reducing the gap. Ring gap is very important.

Z28tek

Have you completely ruled out a 383 rotating assembly and heads/cam package?