Cam choice for vacuum and small carb

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My427stang
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Cam choice for vacuum and small carb

Post by My427stang »

So this isn't a vacuum rule or restrictor plate engine by any means, but asking for opinions on a combo that could use logic from both

1 - Its a 461 inch FE. It will have decent heads, 11/32 stems, good porting, likely 280-290 cfm at .600 lift range, nice valve job, 10.7:1 compression or so, depends where I end up with cam, zero deck, metric rings, good machine work and blueprinting, etc. but it must remain a "mostly" concours look on the outside. Sound doesn't matter

2 - The car is a factory 4.30 gear, close ratio 4 speed Mustang with the only option being power brakes, we may do it as a 3.91 or 3.89 gear car with a wide ratio conversion, but either way, good deep gears with power brakes being the big limit for cam choice

3 - It's being done as a "Day 2" car, with headers (large or small tube, a few options), good exhaust, but other than the internal stuff above, all stock externally as a CJ Mustang, to include a factory 735 Holley (a bit small for the gear and cid) and a recurved Ford distributor. No add ons like vacuum pump or even a canister

So, if I wanted to treat this sort of like a "vacuum rule" build for the power brakes, and spread the centers, I have toyed with ideas of either a 292/242 @ .050 hyd flat tappet single pattern to keep overlap in check at 114 LSA, or potentially a hyd roller at 291/237 @ .050 same LSA

I could of course go much milder, but we are going to try to break the 500 hp mark and let it run a little with the steep gear, so I don't want to just drop compression and make it milder, I want to make it run well with a likely 5900-6000 rpm shift point, although a couple hundred RPM drop from that would be OK too. Easy to do with an FE stroker like this if I didn't try to keep vacuum up at idle, tougher the way I am doing it.

My first question is, assuming better than normal ring seal (metric rings and torque plate honed), a good matching timing curve with plenty of initial, and a 106 ICL for either cam, do you think I'll pull enough vacuum for power brakes? Is there a better choice that would still let it spin a bit?

Second, if dealing with a slightly "too small" carb, how would cam choice change, my hunch says high idle vacuum and pulling harder on the carb at higher RPM are at odds, each wanting a different resulting overlap, but wondering what you guys think

I know that I am asking to have my cake and eat it too, but I'd really like to try to get somewhere to use some gear for this rare factory configuration. 500+ HP is only a number, but it would be nice to hit that as a peak. The end result will be whatever it is, but interested in potentially applying some experience that other types of builders may have ran into with other types of engines

Thanks in advance!
Bullock's Power Service, LLC
Plattsmouth, NE
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 461 FE, 4 speed, port injected EFI, 3.50s
427dart
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Re: Cam choice for vacuum and small carb

Post by 427dart »

Not an FE but a 427 cube Windsor as in 4.125 bore and 4.0 stroke...I run a hyd. roller with 241/249 .600 lift on a 112 LCA with power brakes.
Of course getting the Initial Timing at 18-20 degrees helps smooth out the idle and gives a bit more vacuum.
HP figures in the 550 range based off the 480 at the wheels.

You might find it will run well with just a 3.50 gear if a lot of street driving is in the plan.
Roadknee
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Re: Cam choice for vacuum and small carb

Post by Roadknee »

This is a very interesting project. Could you provide a little more information including desired idle vacuum, desired idle rpm, intake manifold and intake valve diameter?

My initial thought is that given 5,800 rpm or less shift rpm (which would put peak HP below 5,500 rpm) you can meet your goals with less duration and a tighter LSA.
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Re: Cam choice for vacuum and small carb

Post by Geoff2 »

And this ^ is what DV says also. If you want a smoother idle [ more vacuum ] reduce duration, do not widen the LSA.
My427stang
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Re: Cam choice for vacuum and small carb

Post by My427stang »

Roadknee wrote: Wed Nov 14, 2018 11:20 pm This is a very interesting project. Could you provide a little more information including desired idle vacuum, desired idle rpm, intake manifold and intake valve diameter?

My initial thought is that given 5,800 rpm or less shift rpm (which would put peak HP below 5,500 rpm) you can meet your goals with less duration and a tighter LSA.
Thanks for responding

I would say that I am looking for 14-16 in of vacuum, idle RPM doesn't matter much, up to 950-ish, intake manifold is a factory CJ iron dual plane. Loosely equivalent to something just short of an Edelbrock RPM for flow, but plenum size is small by performance standards. Intake valve diameter is 2.15, exhaust is 1.67.

I agree that the 292/244 seems like a lot of cam, the roller example at 291/237 seems more right to me, but typically an FE with long intake runners and wedge heads needs some cam to pull the numbers.
Geoff2 wrote: Thu Nov 15, 2018 4:11 am And this ^ is what DV says also. If you want a smoother idle [ more vacuum ] reduce duration, do not widen the LSA.
I'll use his stuff and calculate a number, it'll be a fun experiment anyway, his book sits near my desk. However, keep in mind, at 237 @ .050, I am already reducing duration where most 4.30 geared big blocks would be. The potential 244 flat tappet was an attempt to drive the curve up while still keeping the overlap in check. I am open to any recommendations, certainly not new to cam choice, but I have to admit, I have been meaning to see what DV's chapter would result if in if I followed it anyway, so thanks!
Bullock's Power Service, LLC
Plattsmouth, NE
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 461 FE, 4 speed, port injected EFI, 3.50s
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CamKing
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Re: Cam choice for vacuum and small carb

Post by CamKing »

You're over-thinking it.
This hydraulic cam will make over 500hp, have over 14" of vacuum, and pull to 6,000rpm.
Cam# FE, H74333-76333-112
238/243 @.050"
.333"/.333" Lobe Lift
112 LSA
108 ICL

Lifter# J874HVM
.875" "Limited Travel" Hydr F.T. lifter set
Mike Jones
Jones Cam Designs

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68post
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Re: Cam choice for vacuum and small carb

Post by 68post »

Are you considering cutting some of the plenum divider down, and / or a small open spacer ?
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Re: Cam choice for vacuum and small carb

Post by mag2555 »

Your compression ratio effects your motors vacuum.
A motor with a total chamber volume needed to have 10.5 comp will make more vacuum then the same motor with 10 to 1 compression with all things being equal!
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
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