Questions about CR and heads...
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 7,163
Likes: 61
From: NH
St. Jude Donor '14
Questions about CR and heads...
I've got a set of 113castings that have been machined with 2" intake, 1.6 exhaust. They've been cut with the fastburn style combustion chambers making them around a 62cc, which from what I understand is 4cc up from the stock 58cc.
I'm bolting this up to a SR manifold/plenum and 219 cam, 1.6rrs...
What kind of compression ratio should this be? Is there anything I should keep in mind with this set of heads?
I'm also debating on a set of pistons, but haven't made a decision on them yet - Mostly because I'm not sure what would work best for my application. Any advice? I'm shooting for around a 10 - 10.5 CR
And slightly off topic, with those heads and those modifications, would it warrant 1 3/4s LTs rather then 1 5/8ths? I know theres a whole can of worms opened there, but my thought is that the car is already really torquey, I don't see the lowend gaining anything because the car spends most of its time off the strip, and I won't gain as much on the high end with the 1 5/8ths as I will with 1 3/4s. Plus, exhaust from the heads is around 202CFM, and I'm not sure which header would be more appropriate for amount of flow.
-Joe
I'm bolting this up to a SR manifold/plenum and 219 cam, 1.6rrs...
What kind of compression ratio should this be? Is there anything I should keep in mind with this set of heads?
I'm also debating on a set of pistons, but haven't made a decision on them yet - Mostly because I'm not sure what would work best for my application. Any advice? I'm shooting for around a 10 - 10.5 CR
And slightly off topic, with those heads and those modifications, would it warrant 1 3/4s LTs rather then 1 5/8ths? I know theres a whole can of worms opened there, but my thought is that the car is already really torquey, I don't see the lowend gaining anything because the car spends most of its time off the strip, and I won't gain as much on the high end with the 1 5/8ths as I will with 1 3/4s. Plus, exhaust from the heads is around 202CFM, and I'm not sure which header would be more appropriate for amount of flow.
-Joe
Le Mans Master
Joined: Mar 2002
Posts: 5,889
Likes: 2
From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Static CR is derived from the bore, stroke, combustion chamber, quench, head gasket bore, piston reliefs/dish/dome, and where it is in the hole at TDC. We need more info 
Here's a great SCR/DCR calculator:http://cochise.uia.net/pkelley2/
Here's a great SCR/DCR calculator:http://cochise.uia.net/pkelley2/
Racer
Joined: May 2001
Posts: 472
Likes: 0
From: Tampa Bay Area
Originally Posted by Draven
I've got a set of 113castings that have been machined with 2" intake, 1.6 exhaust. They've been cut with the fastburn style combustion chambers making them around a 62cc, which from what I understand is 4cc up from the stock 58cc...
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
If you want to ASSUME a compression ratio with the stock 58 cc chambers, it is simple arithmetic to check the cr with 62 ccs or any other size chamber. There was a post like this recently.
The 1 5/8ths" tube headers won't limit your high rpm performance one bit. They WILL boost your low speed response. Afraid if too much torque???
RACE ON!!!
The 1 5/8ths" tube headers won't limit your high rpm performance one bit. They WILL boost your low speed response. Afraid if too much torque???
RACE ON!!!
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
A rule of thumb is the OD of your header tubes should be 10% larger than the diameter of the exhaust valve.
Between the two, I'd opt for 1-5/8ths though, especially on a 350 size street engine (350, 355, 377 and even 383).
Jake
Between the two, I'd opt for 1-5/8ths though, especially on a 350 size street engine (350, 355, 377 and even 383).
Jake
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
If you know the stock CR you can easily calc the potential new value.
What year L98 is it?
Did you machine the block, i.e. deck it?
Did you (will you) change pistons?
What was stock CR?
As mentioned, the question on LT header size is do you want to gain low end or high end with them. LPE shows the recommended crossover between 1 5/8" to 1 3/4" at around 350-400chp. They also mention that you can increase low end torque with an 8-10" collector extension.
What year L98 is it?
Did you machine the block, i.e. deck it?
Did you (will you) change pistons?
What was stock CR?
As mentioned, the question on LT header size is do you want to gain low end or high end with them. LPE shows the recommended crossover between 1 5/8" to 1 3/4" at around 350-400chp. They also mention that you can increase low end torque with an 8-10" collector extension.
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 7,163
Likes: 61
From: NH
St. Jude Donor '14
Originally Posted by 65Z01
If you know the stock CR you can easily calc the potential new value.
What year L98 is it?
Did you machine the block, i.e. deck it?
Did you (will you) change pistons?
What was stock CR?
As mentioned, the question on LT header size is do you want to gain low end or high end with them. LPE shows the recommended crossover between 1 5/8" to 1 3/4" at around 350-400chp. They also mention that you can increase low end torque with an 8-10" collector extension.
What year L98 is it?
Did you machine the block, i.e. deck it?
Did you (will you) change pistons?
What was stock CR?
As mentioned, the question on LT header size is do you want to gain low end or high end with them. LPE shows the recommended crossover between 1 5/8" to 1 3/4" at around 350-400chp. They also mention that you can increase low end torque with an 8-10" collector extension.
Block Decked? No.
Have I changed pistons? No. Am I considering it to opt for a better CR? Yes.
Stock CR is stock for a '91 as I haven't touched the motor yet, personally.
Thanks for the help everyone. I had my heart set on the 1 5/8ths EMs, I just needed to find a matching Y-pipe, I'm going to stick with that plan. : )
-Joe
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by Draven
Year is '91
Block Decked? No.
Have I changed pistons? No. Am I considering it to opt for a better CR? Yes.
Stock CR is stock for a '91 as I haven't touched the motor yet, personally.
Block Decked? No.
Have I changed pistons? No. Am I considering it to opt for a better CR? Yes.
Stock CR is stock for a '91 as I haven't touched the motor yet, personally.
Good choice on the headers.
RACE ON!!!
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