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? for the cam gurus, overlap differences based on flow

General engine tech -- Drag Racing to Circle Track

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68fastback
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? for the cam gurus, overlap differences based on flow

Post by 68fastback » Tue Jun 23, 2020 11:35 pm

Hi, I'm new to the forum. Was hoping to find an answer to a question I've been thinking about on differences in overlap required to meet a target HP with different heads and any differences in drive-ability based on that.
Here goes -
I'm planning a street strip build of my 68 Mustang fstbk. Car weighs 32-3300ish with me in it. It's a stick car, 6spd, with 4.10 gears. 8pt cage, 9in Strange with ladder bars. Goal is to comfortably run in the 10s on pump gas with as much drive-ability as I can get.

I've decided on a N/A build of a 461 SBF, 4.25 stroke, 4.155 bore. 10.5 to 1 compression.
At this point I'm trying to choose a cylinder head so I can order pistons.

My target HP is 650, about 1.4hp per cube.
I'm not looking to make the most HP each head is capable of.
(I think) All of the heads I'm currently considering are capable of getting me to my target HP.

My question is, between the heads below, which do you think would allow me to run the least amount of overlap and still make 650?
Am I right in thinking that the better the head is at low lifts, the less overlap I would need, and therefore have a little tamer engine while still making the target? I realize a lot of the drive-ability is in a good tune with a carb but all else being equal running less overlap should help right?
Are the heads different enough that it would make a noticeable difference in cam specs for overlap, the vaccuum, etc?

Happy to work with one of the cam specialists like DV or Mike Jones to get the answers on the cam specs for each head to meet the requirement for the build but I figured it's also an interesting question for everyone to weigh in on.

The heads I'm considering at this point (happy to consider others) are:

AFR 220 2.1in intake
.100 - 72/55
.200 - 135/121
.300 - 194/171
.400 - 259/215
.500 - 295/238
.600 - 315/245
.700 - 325/249
.800 - 331/250

TFS 240 High Port 2.1in intake
.100 - 72/58
.200 - 146/117
.300 - 222/162
.400 - 281/210
.500 - 310/234
.600 - 330/247
.700 - 343/257

TFS High Port Fox Lake Stage 5 244cc intake runner, 2.125in intake
.100 - 84/68
.200 - 162/144
.300 - 236/177
.400 - 291/214
.500 - 339/242
.600 - 357/256
.700 - 371/270
.800 - 373/278
.900 - 375/285

So there it is, am I crazy?
What kind of cam differences would there be between these heads to make 650, and would there be significant differences in overlap to get there?

Appreciate any and all feedback!

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Re: ? for the cam gurus, overlap differences based on flow

Post by cgarb » Wed Jun 24, 2020 5:07 am

If any of those combos won't make 650 hp you have something seriously wrong.

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Re: ? for the cam gurus, overlap differences based on flow

Post by CamKing » Wed Jun 24, 2020 10:51 am

Overlap has nothing to do with "drivability".
You go with the port size that's correct for the RPM you want to make power at, and have the cam designed for that. That will be more "drivable", then running a larger port, with less overlap.
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Re: ? for the cam gurus, overlap differences based on flow

Post by 68fastback » Wed Jun 24, 2020 11:17 am

Thanks for weighing in camking.
I should have included the rpm range in the original post. Since the car will get a decent amount of street use and for parts longevity I was looking to make peak around 6200-6500
I guess I was looking at the engine as basically a big block 460, looking at port volumes for those heads, and thinking even 244CC would not be overly large for 461 cu in.

I was thinking that overlap had a large effect on idle quality and that overlap required would be driven by low lift flow.

Happy to go with the smaller head. The AFR is the clear winner for HP per dollar if the target is 650.

I was curious how the other heads would differ

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Re: ? for the cam gurus, overlap differences based on flow

Post by CamKing » Wed Jun 24, 2020 11:30 am

The TFS heads would require less duration, and More lift. This will be harder on the valvetrain.
Even though the less duration will reduce overlap, it will be less "driveable" at lower RPM's because the port velocity will be lower.
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Re: ? for the cam gurus, overlap differences based on flow

Post by 68fastback » Wed Jun 24, 2020 11:39 am

So would that manifest itself in just less low end torque or stumbling off idle or both? Or some other less than desirable behavior?

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Re: ? for the cam gurus, overlap differences based on flow

Post by CamKing » Wed Jun 24, 2020 12:55 pm

I'll just say, it would be inefficient at the lower RPM, and that can manifest in a few different ways.
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Re: ? for the cam gurus, overlap differences based on flow

Post by wyrmrider » Wed Jun 24, 2020 2:15 pm

I would not want the Boss ford syndrome
Which TFS Mike?
the head with the greater low end flow (got .050?) would take less lift (intake opening= duration) to supply piston demand
so i'm thinking the high port head would require a smaller cam- be more driveable
would that head support more velocity?
did you mention stroke and rod ratio
I'd ignore the higher lifts listed for a street car
Mike said more maintenance with high lift and short duration
would light valves and springs help here?
I wonder what they did to boost up the low lift flow
hatever- if durability is a goal let's not eliminate valve guide support are longer valves an option so the guides do not have to be cut down?
is this for a roller cam?
I have never been happy tradig away much low lift flow
Whatever Mike recommends- pick the heads, cam, rockers, compression all at the same time

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Re: ? for the cam gurus, overlap differences based on flow

Post by Walter R. Malik » Wed Jun 24, 2020 3:26 pm

68fastback wrote:
Tue Jun 23, 2020 11:35 pm
Hi, I'm new to the forum. Was hoping to find an answer to a question I've been thinking about on differences in overlap required to meet a target HP with different heads and any differences in drive-ability based on that.
Here goes -
I'm planning a street strip build of my 68 Mustang fstbk. Car weighs 32-3300ish with me in it. It's a stick car, 6spd, with 4.10 gears. 8pt cage, 9in Strange with ladder bars. Goal is to comfortably run in the 10s on pump gas with as much drive-ability as I can get.

I've decided on a N/A build of a 461 SBF, 4.25 stroke, 4.155 bore. 10.5 to 1 compression.
At this point I'm trying to choose a cylinder head so I can order pistons.

My target HP is 650, about 1.4hp per cube.
I'm not looking to make the most HP each head is capable of.
(I think) All of the heads I'm currently considering are capable of getting me to my target HP.

My question is, between the heads below, which do you think would allow me to run the least amount of overlap and still make 650?
Am I right in thinking that the better the head is at low lifts, the less overlap I would need, and therefore have a little tamer engine while still making the target? I realize a lot of the drive-ability is in a good tune with a carb but all else being equal running less overlap should help right?
Are the heads different enough that it would make a noticeable difference in cam specs for overlap, the vaccuum, etc?

Happy to work with one of the cam specialists like DV or Mike Jones to get the answers on the cam specs for each head to meet the requirement for the build but I figured it's also an interesting question for everyone to weigh in on.

The heads I'm considering at this point (happy to consider others) are:

AFR 220 2.1in intake
.100 - 72/55
.200 - 135/121
.300 - 194/171
.400 - 259/215
.500 - 295/238
.600 - 315/245
.700 - 325/249
.800 - 331/250

TFS 240 High Port 2.1in intake
.100 - 72/58
.200 - 146/117
.300 - 222/162
.400 - 281/210
.500 - 310/234
.600 - 330/247
.700 - 343/257

TFS High Port Fox Lake Stage 5 244cc intake runner, 2.125in intake
.100 - 84/68
.200 - 162/144
.300 - 236/177
.400 - 291/214
.500 - 339/242
.600 - 357/256
.700 - 371/270
.800 - 373/278
.900 - 375/285

So there it is, am I crazy?
What kind of cam differences would there be between these heads to make 650, and would there be significant differences in overlap to get there?

Appreciate any and all feedback!
With a Ford Windsor engine that large ... I don't think you'd have much issue with drive-ability using either scenario.
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Re: ? for the cam gurus, overlap differences based on flow

Post by 68fastback » Wed Jun 24, 2020 3:38 pm

Thanks for weighing in Walter and wyrmrider.
That was the reason I wanted to go as large as possible for the car. I originally had ideas of building something you have mentioned before - a 400in 8.2 deck engine, but after talking to the guys at MPG the costs for the bottom end were pretty high.

What is Boss Ford syndrome?

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Re: ? for the cam gurus, overlap differences based on flow

Post by 68fastback » Wed Jun 24, 2020 4:57 pm

wyrmrider -

About valve length - I took this from the Fox Lake website - "Valve length of 5.300 must be used for installed height of 2.00 or more."

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Re: ? for the cam gurus, overlap differences based on flow

Post by 68fastback » Wed Jun 24, 2020 5:12 pm

also, Mike mentioned that the TFS would require more lift. I remember David Vizard saying in the post on his AFR 220 headed 408 something like "with the .700 lift that this head requires" or something to that effect. I'm guessing that there is an optimum lift number for making max HP on any combo.
Does that lift number change if the target is to make less than max HP with a nod toward reliability or does that mess up how the port acts?

I'm trying to learn as much as I can.
I see a few people running lifts in the .650 to .700 range on the street. is that a lift number that would be max for any kind of reliability? To better define the car's use and the reliability I'm thinking of - I'd think I wouldn't drive it more than about 5000mi a year with 6 or 7 trips to the dragstrip.

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Re: ? for the cam gurus, overlap differences based on flow

Post by 68fastback » Thu Jun 25, 2020 9:34 am

bump

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Re: ? for the cam gurus, overlap differences based on flow

Post by CamKing » Thu Jun 25, 2020 10:11 am

68fastback wrote:
Wed Jun 24, 2020 5:12 pm
Does that lift number change if the target is to make less than max HP with a nod toward reliability or does that mess up how the port acts?
Yes, if you're not trying to make max efficiency, at peak HP RPM, you don't need as much lift.

With the AFR heads, you will only need a valve lift around .660" to make 650hp
Mike Jones
Jones Cam Designs

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jonescams@bellsouth.net
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Jones Cam Designs' HotPass Vendors Forum: viewforum.php?f=44
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Re: ? for the cam gurus, overlap differences based on flow

Post by 68fastback » Thu Jun 25, 2020 11:18 am

Thanks Mike, I really appreciate your help. Sounds like the fox lake head would require a duration so short to make peak HP at the rpm I'm shooting for that lift would be severely limited?

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