What do my heads need to flow to match my cam?
Thread Starter
Registered User
Joined: Jul 2001
Posts: 146
From: Wichita Falls, TX
Re: What do my heads need to flow to match my cam?
Originally Posted by Zero_to_69
I meant the valve timing. IVO, IVC, EVO, EVC (or even the intake centerline),
not the spark timing. There are a few guys here that will be able to calculate
the cylinder pressure based on the valve timing.
The 4.10's are good fit for heavy cars. Good multiplier happening there.
With such a stout rear assembly, I'm assuming you are using a slick?
not the spark timing. There are a few guys here that will be able to calculate
the cylinder pressure based on the valve timing.
The 4.10's are good fit for heavy cars. Good multiplier happening there.
With such a stout rear assembly, I'm assuming you are using a slick?
Oh no
, I'm on my 2nd set of gears. 
I could have almost purchased a ford 9" for that. The BFG's are working OK for now. As soon as they are gone, I will get some ET streets. 
Shortly after, I will have to get a 9" or a GM 12 bolt.How do you determine valve timing?
Mark
Registered User
Joined: Sep 2002
Posts: 655
Re: What do my heads need to flow to match my cam?
The valve timing is on the cam card. It will detail the valve events @ 0.050" lift.
These opening and closing points are related to piston position which relates
to the amount of charge drawn in, retained, and released during the combusition
cycle.
You'll need these figures to dial in the cam (phasing) to get the MFG's suggested
performance from the camshaft (either that, or the intake centerline value).
With those figures, there are programs/calculations made to determine how
efficiently charge is ingested and exhausted, pulse tuning, cylinder pressure,
etc.
These opening and closing points are related to piston position which relates
to the amount of charge drawn in, retained, and released during the combusition
cycle.
You'll need these figures to dial in the cam (phasing) to get the MFG's suggested
performance from the camshaft (either that, or the intake centerline value).
With those figures, there are programs/calculations made to determine how
efficiently charge is ingested and exhausted, pulse tuning, cylinder pressure,
etc.
Last edited by Zero_to_69; Aug 13, 2004 at 07:53 PM.
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Re: What do my heads need to flow to match my cam?
There is a problem in running cams that are too large. A lot of guys fall prey to this approach.
BUT, I don't really get too hung up on the thing. I look at the goal and a way to achieve that. For any given displacement of engine, that engine needs to turn X number of rpm to make power, depending on the cylinder head. If you're class racing, you have to know how much HP it's gonna take to be competitive. Then you look at the weight breaks, decide on a displacement and gear the car up to turn the revs. Guys, this pretty much locks you in on the cam selection because it generally takes such and such duration to turn XX rpm in a given displacement.
It's not so much different for a street build. So, if someone tells me they want to build a 383 stroker that is going to see very limited street time, alot of track time, I just ask them what kind of rpm range they want the car to work in. Everyone wants idle to 7k rpm but we know it doesn't work that way. "No free lunch" always applies to this stuff. If you're working with an LT1, where you're staying with the factory computer... you're limited to 7k. Max effort in that case is something that peaks right at 7000. Then you have to set the car up to work in that rpm range.... converter, gears, etc.. Will the guy be happy with that on the street? I dunno but he'll soon find out.
I think one of the biggest problems with gearheads who haven't really gotten the taste of a more-radical street build is in the fact that they don't truely know what they want. Other than, "my cake and eat it too".
-Mindgame
BUT, I don't really get too hung up on the thing. I look at the goal and a way to achieve that. For any given displacement of engine, that engine needs to turn X number of rpm to make power, depending on the cylinder head. If you're class racing, you have to know how much HP it's gonna take to be competitive. Then you look at the weight breaks, decide on a displacement and gear the car up to turn the revs. Guys, this pretty much locks you in on the cam selection because it generally takes such and such duration to turn XX rpm in a given displacement.
It's not so much different for a street build. So, if someone tells me they want to build a 383 stroker that is going to see very limited street time, alot of track time, I just ask them what kind of rpm range they want the car to work in. Everyone wants idle to 7k rpm but we know it doesn't work that way. "No free lunch" always applies to this stuff. If you're working with an LT1, where you're staying with the factory computer... you're limited to 7k. Max effort in that case is something that peaks right at 7000. Then you have to set the car up to work in that rpm range.... converter, gears, etc.. Will the guy be happy with that on the street? I dunno but he'll soon find out.
I think one of the biggest problems with gearheads who haven't really gotten the taste of a more-radical street build is in the fact that they don't truely know what they want. Other than, "my cake and eat it too".
-Mindgame
Thread Starter
Registered User
Joined: Jul 2001
Posts: 146
From: Wichita Falls, TX
Re: What do my heads need to flow to match my cam?
Originally Posted by Zero_to_69
Do you have the valve event timing available?
I think those specs would help determine if the cam is retaining cylinder pressure
to produce good torque early in the RPM to launch that "fat and heavy" car.
If the cam is designed to make power later, what are your transmission details
and differential setup?
Just a guess, but I'm thinking that cam is fairly wild and will need some gearing
and a t-con (if auto) to get the motor into the power range.
I think those specs would help determine if the cam is retaining cylinder pressure
to produce good torque early in the RPM to launch that "fat and heavy" car.
If the cam is designed to make power later, what are your transmission details
and differential setup?
Just a guess, but I'm thinking that cam is fairly wild and will need some gearing
and a t-con (if auto) to get the motor into the power range.
valve timing open close
at .050 int. 5 btdc 47 aboc
exh. 50 bbdc 8 atdc
Ok, what does this mean?
Thanks,
Mark
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Re: What do my heads need to flow to match my cam?
Ryan,
In a 383 with an adequate 23º head, it's gonna take ~260º to make peak power at 7k.
Oh... I know, I know...
everyone just exercised their sphincter muscles but I'm shooting it to you straight. That seems like a lot and it is for a "pure-street" engine, but that's about what it takes to get there with standard stuff which brings me to my next point...
Consider the valvetrain... namely the valvetrain mass. One reason these LS1's are turning the revs they do is because of the valvetrain (VT). Less spring and retainer mass, lighter valves and such all contribute to the ability of the engine to turn more rpm. A heavier VT engine may peak at 7k, the same engine with lighter VT (let's say titanium valves) won't make any more power at 7k but it may very well go 4-500 more rpm before it stops pulling! It's also going to be easier on valvesprings.
On the cylinder heads... they're a big part of the equation. I remember a build up from a few years ago... engine was a 434 small block. They used a Cam Motion grind ~255º @ .050, 110 lsa, then swapped out between a set of Bow-tie (392) heads with a peak flow of ~260cfm and a set of 18º heads flowing ~380cfm. Of course they needed to change the pistons too but maintained the same static CR for the two comparisons. The 23º setup peaked at 5500rpm while the 18º pulled to 7000rpm. Granted, the 23º head in this comparison is way too small to be "competition" and the test wasn't meant to be that. Just shows the trend in the effect of cylinder head port volume on the powerband.
Back to the 383 and 7k... Go with the new Ferrea hollow stem intake valves, sodium exhaust valves, less spring mass and a milder camshaft grind could get you there.
-Mindgame
In a 383 with an adequate 23º head, it's gonna take ~260º to make peak power at 7k.
Oh... I know, I know...
everyone just exercised their sphincter muscles but I'm shooting it to you straight. That seems like a lot and it is for a "pure-street" engine, but that's about what it takes to get there with standard stuff which brings me to my next point... Consider the valvetrain... namely the valvetrain mass. One reason these LS1's are turning the revs they do is because of the valvetrain (VT). Less spring and retainer mass, lighter valves and such all contribute to the ability of the engine to turn more rpm. A heavier VT engine may peak at 7k, the same engine with lighter VT (let's say titanium valves) won't make any more power at 7k but it may very well go 4-500 more rpm before it stops pulling! It's also going to be easier on valvesprings.
On the cylinder heads... they're a big part of the equation. I remember a build up from a few years ago... engine was a 434 small block. They used a Cam Motion grind ~255º @ .050, 110 lsa, then swapped out between a set of Bow-tie (392) heads with a peak flow of ~260cfm and a set of 18º heads flowing ~380cfm. Of course they needed to change the pistons too but maintained the same static CR for the two comparisons. The 23º setup peaked at 5500rpm while the 18º pulled to 7000rpm. Granted, the 23º head in this comparison is way too small to be "competition" and the test wasn't meant to be that. Just shows the trend in the effect of cylinder head port volume on the powerband.
Back to the 383 and 7k... Go with the new Ferrea hollow stem intake valves, sodium exhaust valves, less spring mass
and a milder camshaft grind could get you there.-Mindgame
Last edited by Mindgame; Aug 14, 2004 at 01:07 PM.
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