Upping compression and pounds of boost ???

[rudycobra]

Ok so I think my new motor build ended up around 9.4:1 which is higher than I wanted but too late now . Last time stock motor 2 lb / 3.5 and made 22 lbs . I was gonna drop to 3.75 to get to 20 for street tune . Anyway my tuner said on one gt 500 they did wen upped compression 1 point it added 4 lbs boost . Just wondering if anyone else had this issue or if its true . I made need stock lower metro pulley which I just sold yesterday ! Any help ??

 

[Stanger00]

It really depends on how your cams are setup to keep dynamic compression safe for pump gas. If the cam is setup the same as your last combination then you would probably make more boost. I would to see what dynamic compression would be first before you fire this thing up with the online calculators.

How did you do the math to figure your static compression ratio?

I seen in an old post that you are running 53cc heads but didnt mention how much material was removed when they were decked.

Are you running a thicker gasket or just the factory .04" thickness?

I also seen that you are running a 9cc dished piston with a .020 overbore. That is all the information you need to figure what static compression is. I used Wallace Racing to figure mine out.

Sorry if I didnt answer your question. But I really think cam selection will play a bigger part in how boost is affected with a motor and more compression. I thought 4 valve cars can change up LSA and gain more overlap? Am I wrong?

Edit: If you used a .06" gasket you would be at 9:1 compression. A .05" gasket would put you at 9.2:1 compression ratio. But we also dont know what changed with the chambers after the heads were decked.

 

[rudycobra]

I am using stock cams not degreed . Before we're stock heads . I had lil removed but would have to ask shop to b exact . Shop figured my compression goin by all the numbers . I also used stock head gaskets .

 

[Stanger00]

Ok I am figuring this out as I go. I did the calculator for you engine setup and it is at 9.4:1 but thats not considering what was milled from the heads.

If .005" were removed than that would put you around 9.5:1
If .012" were removed than that would put you around 9.7:1

I went through this too because my engine was built by a shop and then it blew. I bought an aluminator short block that is standard bore, standard stroke for an 03-04 cobra. That short block is good for 8.5:1 with 50cc heads. My heads were 44cc's. I was told everything would be fine...Well I did the math on everything and was disturbed to find out that I am running 9.5:1 with close to 8.4 dynamic compression with the cams I have in my 2 valve. My heads were milled .010" so that raised my static compression from low 9.3 to 9.5:1. Its scary because that sort of dynamic compression is borderline with 93 octane and I am running 91 octane. No pings or any signs of detonation on my plugs when I checked them. I also had terrible blow by with my PCV system.

I got lucky with the cam profile and the motor benefited from these new found compression ratios.

Edit: some of my numbers were off. I had to look emails between Greg at TFS and me about my motor. He confirmed my numbers.

 

[rudycobra]

I was hoping to get to 20 psi and gonna run 1 can torco to 10 gallons pump gas

 

[Stanger00]

I was hoping to get to 20 psi and gonna run 1 can torco to 10 gallons pump gas

All this work and they are not degreed?

There is tons of info on this board to help you out with your combination. I would be concerened with where the centerlines are set at because that will tell you if your car will be safe running that much boost with that type of fuel.

Chances are that it will be fine but will not make its potential power.

 

[rudycobra]

Yes I'd loved to get them degreed but my shop can not do it and was out of funds to get cams or degree stock ones .

 

[Stanger00]

I know how that goes. I had to liquidate to fund my 2nd motor and transmission.

Why so much boost with pump gas? You always going to run torch?

 

[rudycobra]

Well first time around I was at 22lbs and tuner said I would b good with torco . But I rathered b safer and was gonna drop to 20 . Yes I will always run torco if needed and prolly race fuel tune for track days

 

[fbody83]

From my experience one can of torco with 10 gallons should be used as more of a safety measure if you're running just a tad more timing or boost than you normally would on 93. If you're going to run a couple more PSI then I'd recommend two cans for 10 gallons, or one can for 5.

 

[Stanger00]

If you could swing it find a larger upper and then go from there. After all you been thru and all with the last motor and getting this one built.

 

[rudycobra]

I have a 3.75 that I'm gonna use which should drop me 2 lbs from where I was but now wondering if I shoulda kept the stock size lower that I just sold too . I was at 22 and 3.75 would put me at 20 but if my compression raises boost any then ill b back above 20 .

 

[rudycobra]

Now it got me debating if I should get a smaller than stock lower pulley . I have plenty diff uppers to change around

 

[rudycobra]

Anyone else had this issue ?

 

[rudycobra]

Guess no one runs more compression these days

 

[fbody83]

I've never heard of compression raising boost. I have a 9.2:1 motor, long tubes and cams and I make 4 psi LESS than people running the same pulley combo that don't have long tubes and cams

 

[rudycobra]

Wonder if that's because of your cams ? I'd expect long tubes to lower boost .

 

[fbody83]

Wonder if that's because of your cams ? I'd expect long tubes to lower boost .

Yes, both my long tubes and the cams cause less boost because they increase the air flow and lessen the back pressure.

 

[rudycobra]

I'm still with stock cams , just trying to get an idea . Want to b prepared wen go get tuned . Just sux cause won't know till on the dyno and between pulleys and belts man it's never ending .