compression vs boost
Thread Starter
Staging Lane
Joined: Jun 2007
Posts: 93
Likes: 0
From: Temple Terrace/ Thonotosassa
compression vs boost if an sts ls1 car was to change their heads out from the stock heads to 317 truck heads to lower their compression how much more boost could you run? how much more boost would you have to run in order to compensate for the drop in compression? would the end result in horsepower still be about the same before the change in heads and after considering its a stock motor and the limitations of it?
also how does lowering yuor compression affect yoru mpg while driving around not in boost?
thanks in advance.
also how does lowering yuor compression affect yoru mpg while driving around not in boost?
thanks in advance.
Well its kinda hard to say how much boost you can run since turbos flow different amounts of CFM. Since you are an STS guy, I would assume you are starting with either the 60-1 turbo or the upgraded gt67. LS1 heads(10:1 CR) with a gt67 you could saefly do 5psi. 8psi and more will probably require methanol injection if your using pump gas. With a 317 casting head, it would lower compression to around 8:1(someone correct me if I'm wrong). With that being said, using a 317 casting head will lower your lower end power due to the loss of compression but will allow you to run much higher boost, 12-15pds. You would definately see more power gains with running a 317 and more boost. Hope this helps...
When I changed from stock heads to 317s it made about the same power N/A, actually a couple more hp but that could be weather, dyno, whatever. Then I straped on a D1 and picked up 260rwhp! It also blew up a few months later lol. So don't think the little drop in compression is fail safe.
Well its kinda hard to say how much boost you can run since turbos flow different amounts of CFM. Since you are an STS guy, I would assume you are starting with either the 60-1 turbo or the upgraded gt67. LS1 heads(10:1 CR) with a gt67 you could saefly do 5psi. 8psi and more will probably require methanol injection if your using pump gas. With a 317 casting head, it would lower compression to around 8:1(someone correct me if I'm wrong). With that being said, using a 317 casting head will lower your lower end power due to the loss of compression but will allow you to run much higher boost, 12-15pds. You would definately see more power gains with running a 317 and more boost. Hope this helps...
My last shop cars 370 was 8.5:1 and made 770rwhp at 15lbs of boost. I am building my new shop cars 370 with 9.5:1 and shooting for similar numbers with less boost. I am running a bigger S480 turbo compared to the last TC76 unit. The extra compression will help with spooling and overall throttle response.
It is all in how you tune the combo.
It is all in how you tune the combo.
Joined: Jun 2004
Posts: 1,501
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From: Ft. Irwin, California (But Virginia is home)
Theoretically they'll handle 8-9PSI safely. The tune is the real factor. Fuel is 2nd. Intercooling to lower intake charge is 3rd. The whole point of the lower CR is to delay the onset of detonation so you can be greedier with the boost and get more HP. The downside is that as the boost goes up, so does the heat from compressing the intake charge, so intake charge cooling can still limit you even with lower CR heads. Great tuning + methanol injection can make up for this. But, this creates the next problem.......above the 550-600RWHP zone the stock rod bolts will not like you and will eventually say F'it and quit. This # is easy to exceed on 9PSI 
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From what I understand is that stock 317s will lower your CR from 10.1:1 to 9.7:1 on a stock LS1. The LQ4 also uses a dished piston to get 9.5:1. Ported 317s probably have unshrouded valves and other chamber work that will lower CR. Some say that 317s flow as well as the LS6 heads but I have never seen this proven (not that I have searched) although they may have similar port volume. Im gonna say that you will lose little to no power switching to as cast 317s but I dont think you are going to gain a big margin for more boost.
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Hate to be the guy to ask the stupid question but here goes.
Is it possible to have an intake manifold with a larger volume?
And if so that would lower the intake charge temps, right?
Is it possible to have an intake manifold with a larger volume?
And if so that would lower the intake charge temps, right?
