If I can get my hands on this supercharger I am trying to get, I want to run the 12psi pulley its coming with. What is a safe compression ratio to build my engine for so I can still run crappy 91 octane.

 

i wouldnt.

id start lower and work my way up with that...

on some other motors ive worked with, you can have 10.2-10.5 compression and still run almost 10lbs of intercooled boost... putting you in the 550rwhp range.. but anything past that either uses alot of meth, or they've broken stuff.

id start out with 7lbs or so, and thats still ALOT for a 10.7 motor.... id run less if it isnt intercooled...


its cheaper to dump a hundred or two into pulleys then it is to build another shortblock.... and possibly the long block... and rebuild the supercharger......


besides, whats your fuel system consist of anyway?

 

I'm planning on 9-12 with the 8:4:1 355 motor we got

 

Sorry for any confusion, Ignore my sig, I tore that motor down and it is getting new heads and pistons, so I can tinker with the compression ratio. That's what I ment.
What should I build my new engine for, to accomodate the 12psi.

 

9.5ish range for alum heads...
8.7-9.0 for iron....
assuming a modern fastburn style chamber in either one.

thats what id do atleast. i have my own theories.. im sure someone will disagree that its too high... others will say you can go higher.

for a street car with something like a D1SC, thats where id stay....

 

Going to run an intercooler? very big one?
meth injection?
which blower?

 

8:1-8:5:1 if you wanna really use anything bigger than a sh*tty P600 or so...for a D1-SC thats the CR you need to really utilize that blower. Heck, my D1-SC blown 383 had 7:8:1 with 16psi and 30* total timing! and that car was RAW brutal. The key is to run a low enough compression so you don't have completely kill your timing to add boost...thats where the real power is at.

 

He's lookin' at the older P600B/2-core intercooler I believe.
With his plans, I don't think he'll be using the FMU, would have to be big 42lb'rs + lots of tuning.

My CR is ~9.7:1 w/ alum heads and I was going to run 6-7psi Travis.
I'd think you'd have to be quite a bit lower to run 12psi.
I doubt the P600b will really net you 12psi with that pulley on your planned setup though, Jeremy would know for sure I guess.

 

Thanks guys, got the info I needed, I will shoot for 8.5:1 I think.

 

unless you're planning on it being a drag car that sees limited street useage, and you're running a large blower (or near maxing a D1SC)...... that motor is going to SUCK.

 

why do you say that? too low?

 

What makes you think that? He never once gave any info on a cam so who knows what the real compression is. You are just guessing.

If you post more info you will get more realistic responses.

 

I have had plans to pick up the new xfi cam, 242/248 580ish lift 113 lsa

 

I think it will be like MrDude_1 said with that cam. It is big for the street and big for low compression. It will not have much low end but will scream up top. The 19* of overlap will bleed off compression down low. Put your numbers in the KB-silvolite.com dynamic compression calculator.

 

What should I aim for in dynamic compression? I am not sure what number to punch in for "Intake Closing Point (degrees)
ABDC @ 0.050 lift plus 15 degrees"

Also, I wasnt sure for deck clearance, I put a 0 because my block was zerodecked originally so the pistons came perfectly to the top.

If anyone wants to help me out, my specs are 75cc head chambers, +16cc dished pistons, 4.030 bore, 3.75" stroke, 6" rods, headgasket is a .041 compressed thickness, 4.166 bore, (I might choose a different brand head gasket which will slightly effect numbers) This should get me about 8.8:1 CR for static CR. I just don't understand the dynamic

United Engine and Machine Co.

 

Look up your cam card. It should have that closing point listed on the sheet.
I just looked for it, and couldn't find it...?
Call up comp and ask 'em for the cam card I guess.
12-468-8 Comp 292XFI HR13

yes, that's right for deck clearance, 0.

That's a big cam, it'll drop your dynamic a fair bit i'm guessing. It'll be pretty lazy at low RPMS before you get up on the cam (and blower). After 2500 or 3000RPM it'll just launch though.

 

Hmm, don't have the cam yet, so the card I obviously don't have. I have a 3000stall so I don't think I would care much for lack of power under 3k. I bet even with the torque and slippage, it would launch really fine still.

 

I've been following this thread because I was interested in hearing more about dynamic compression ratio theory with boost.

According to that calculator, my brother's TA (in sig) has,

static compression ratio of 10.924. I already knew it was 10.9:1 before that, so the calculator seems very accurate.

dynamic compression ratio: 9.159:1

My brother has been regreting building this motor with such high compression since a few days after we got it running. He wishes he could do a rear-mount setup for no more than 6 psi of boost. Does this mean his dream could be a reality?

 

TraviZ, call comp cams and see if they recomment your XFI cam with a blower. They might be able to point you to a suitable blower cam. .580 lift is going overboard, my heathy BBC cam is not that big, and i think you can run 9.5 ish IF you plan to intercool. This will make your bottom end usable on the street (low rpm, low boost). You don't need to go an 8.5 or 8.0 motor with your chamber design. I'll look through my cam books and see what I can find.

 

John Hopkins - why does he want to do a remote mount turbo? With those HP/TQ #'s, i'd be happy as is.. Why does he regret it??? That boggles my mind... But yea, it's definately not a motor that is suited well to adding forced induction. I wouldn't even consider doing the remote mount turbo on that motor.

Yea, Traviz, that cam is GREAT for a NA motor, all the good perks, etc. But for a blown motor, you don't want the same type of cam. You don't need large duration, large lift, etc. You just want loads of exhaust lift/duration, and a wide LSA usually. Heck, i'd leave that cam in there, 230/236.

 

He built the motor to add nitrous years down the road. He regrets making the CR so high, even though it seems to be a really good match for the topend of the motor. There are no realistic possibilities for manifold mounted turbo(s). We filed the ring gaps for a 250 shot of nitrous, but turbos have become a personal preference since the development of MY car. He's probably ALL "talk" when it comes to actually doing the RMT setup, but he would appreciate his motor more if he knew there was still room for improvement, and that it wasn't 'maxed out'.

So, are you saying it would probably blow up the motor, or just that its not an ideal application?

 

Well, both. I think it's far far far from an ideal application, IMHO.

How's your TT setup working?

 

My car impresses me more everytime I drive it. I got the 4bolt block and stock rods from a (1975 van) free. I paid $81.99 for a new crank, and bought a $149 rebuilt kit w/ pistons. I don't have any measured numbers on the power output except that my 38# injectors see 98% duty cycle close to 5900rpm. That comes out to 545hp at the crank according to the holley software (.55BSFC). The motor loves to run [what I call lean] around 12.8:1 AFR and very little timing 13-20degrees depending on boost level. Since I rebuilt my tranny and put in a clutch that will hold in every gear, I suspect my car is faster than my brother's, but its not really a competition just a reference for my power level.


more on topic:
I don't even know for sure what my compression ratio is! I was stupid and 16 when I built the motor, but it has held up really well. ring gap? tolerances? I called northern autoparts, where I got the rebuild kit, a year later to ask them what the compresion ratio would be on a rebuild kit for a 70 camaro (thats what I ordered)... He said, "Probably a shade under nine and a half."

I'm peaking at 8psi@3600rpm and 14psi@5750rpm from massive wastegate creep. So, I recommend stock compression (9.3:1) for running 14psi, just make sure you can take out the timing if you have to, or run high octane fuel.

 

Well Considering what's been said that I could run a higher CR cuz the intercooler and whatnot, The best pistons I can find that match my specs, have a +5cc dish, which brings out my CR to 9.8:1
This still fine for 12ish psi? I may step up to a bigger unit in the future, but I'll never want to push more then 15psi. That's the pulley I would probably run with a d1sc style unit.
So basically, I can run pistons that will net me 8.8 CR (dished) or flattop pistons with valve reliefs that would net me a 9.8 CR. What's the choice guys?

 

I vote for the pistons that give you 8.8:1. In the end it'll be better and safer.

 

I'm probably going to be argued on this, but 8.8:1 is my thought on the matter. Your CR being high makes good power on NA. On boost, it's not nearly as important. ie, if you can drop the CR, and raise the psi boost, you can SCHWACKS AND BUCKETS, AND LOADS! more power then raising the CR. 12-15psi, is lots of pressure, I think 8.8:1 is good for that.
Remember, seriously, you don't need a big cam to make big power in this application, you also won't need to spool it up into the high RPMS. I'm thinking like 750HP or so, so why make it miserable to drive by going high on the cam? Nah...
ok, i've exhausted my limited knowledge base, so i'll step back and let the forced induction experts chime in.

 

Thanks guys, F(orced)I(nduction) is new to me as far as engine combos. I will definently heed your warnings. 8.8 sounds good to me too

 

Between the two, I vote 8.8. You can always play with the boost, but changing pistons sucks.