u are probably going to mixed opinions on this but i do know that too low of compression is a bitch when it comes to tuning these 3 valves

 

Here is my 2 cents on compression and Superchargers. Keep it high (9.2:1) if you want the highest possible hp on race fuel. But you will have to back it down to 10 psi or so to run pump gas. If you drop the CR to 8.5:1 or so, you can run higher boost (15 psi or so) on pump gas, and make more hp on the street, but the power you will make on Race fuel and high boost on the track will drop quite a bit from the 9.2:1 ratio.

So basically it's where you want to be in the tradeoff. I kept my ratio at 9.25:1 in my 5.3 engine, and I still wonder if I made the right decision.

 

Here is my 2 cents on compression and Superchargers. Keep it high (9.2:1) if you want the highest possible hp on race fuel. But you will have to back it down to 10 psi or so to run pump gas. If you drop the CR to 8.5:1 or so, you can run higher boost (15 psi or so) on pump gas, and make more hp on the street, but the power you will make on Race fuel and high boost on the track will drop quite a bit from the 9.2:1 ratio.

So basically it's where you want to be in the tradeoff. I kept my ratio at 9.25:1 in my 5.3 engine, and I still wonder if I made the right decision.

So, I would probably want to run a lower compression than the 9.55:1 to get the higher boost for the street. Say a 9.0:1 I dont want to go too low and have problems with tunning. I feel The street would be more fun to play with the extra power and develop a little reputation, so everybody and there grandma in the mini van don't always want to race at every stop light.:thumb: Or maybe that will still happen, but at least I'd win most of the time. Some of these damn four cylinders are getting pretty fast from the factory. How are you guys running the tunes? in a piggyback/stand alone set up, or a hand held tunner that you have to reflash the pcm with every time you want to go racing? Anybody else?

 

9.0:1 sounds like a good compromise. I am running the standard KB tune for my KB 2.6 Stage 2 at this point. I will be going to a tuner when I get my new engine finished. Good move on the Mammoth, I will max my 2.6 out at about 17 psi with the new 5.3L I am building, where as A 2.8 would have gone up to 20 psi or so.

 

9.0:1 sounds like a good compromise. I am running the standard KB tune for my KB 2.6 Stage 2 at this point. I will be going to a tuner when I get my new engine finished. Good move on the Mammoth, I will max my 2.6 out at about 17 psi with the new 5.3L I am building, where as A 2.8 would have gone up to 20 psi or so.

If you don't mind me asking, what kit(5.3L) are going with, and who sells it. What is your stroke and rod lenght, and what brands did you choose. Light weight I beam or the H beam rods. I was planning on the 5.0L but that's not definate right now. What kind of power are you shooting for?

 

i run 11.5-12 psi on stock 3v for 6000 mile now .rev limit is at 6700 ....car makes 500 rwhp .and is 9.8 .1 comp so i just built my forge short block the same way ..9:8.1 and soon 18 psi

 

Let me just leave you this reference thread on the subject so you know the tradeoffs:
Link => Increasing static CR vs boost pressure

A common complaint of a lower CR is less responsiveness when not under boost. But with a P-D blower, I have to say that I honestly don't see why you need to run a high static CR. My '03 Cobra has a stock CR of 8.5, and has far better part-throttle response than my '01 Cobra had, which had a CR of 9.85, IIRC.

Just my thoughts.

 

Let me just leave you this reference thread on the subject so you know the tradeoffs:
Link => Increasing static CR vs boost pressure

A common complaint of a lower CR is less responsiveness when not under boost. But with a P-D blower, I have to say that I honestly don't see why you need to run a high static CR. My '03 Cobra has a stock CR of 8.5, and has far better part-throttle response than my '01 Cobra had, which had a CR of 9.85, IIRC.

Just my thoughts.

Hey black2003cobra that was a great write up. I have now switched my thoughts on this subject. I think I will try to run 8.7 to 9.2 to 1. I haven't ordered any thing yet. But this seem to be the sweet spot for booth street performance and track performance. I really don't want a doggy car for the street, but in the the other hand, I want to make killer # on the race fuel. How can you tell when your reaching your boost/detonation limits when on the dyno. What are the signs, and how do you know( if I were to keep pulleying it down and increasing the PSI) when to say thats enuff on the 93 oct and its time for the 110?:dunno2: Thanks for the info.

 

I'm running stock compression on my built motor with ported heads. I have a Mammoth and am currently producing 754rwhp / 665 lbs. on 93 octane @ 19psi.

 

Those are sick numbers !

 

I'd keep it at 9.0:1 or a little less. You should call some of the top modular engine builders and get some real tried and true professional advice.

 

Havent some of the Cobra guys been having issues with spinning the big KBs fast enough for big boost and driving them on the street that way? What is your ultimate hp and use goal?

 

I've heard of that before. Too much compression always seems to make a blown engine's top end and bottom end at odds with each other. I've never done it, but it's kind of the norm to lower the compression a bit with blown applications.

 

What's the compression ratio?

 

Stock cr on a 2005 GT is 9.8:1

 

I plan on going stock CR on my built motor as well.

 

What kind of Street boost are you shooting for and what fuel Oct.?

 

Probably around 15-17psi and 93 octane. It should make in the neighborhood of 600hp by my calculations.

 

What are your thoughts on a turbo car? I have a bullseye s366 (66mm). It has a small bit of lag, so I would like to go with a higher compression. Goal is 750 hp, but I don't want it to be a dog on the street. What is you guy's opinion ?

 

I wouldn't go any higher than stock.

 

Stock is 10:1, right?

 

Not a whole lot.