270 magnum flat tappet idle speed and vacuum thoughts?

[F-BIRD'88]

It will run very nicely with the big hi perf 2-1/2" out Corvette style Ram Horns. It will make noticeable more power and torque with 1-5/8" long tube headers VS the Ram Horns.
About 12℅ more.
The hi rise RPM manifold is more powerful than the performer. If using a carb adapter be sure it is a divided style on the dual plane manifold.
The cs262H-10 Extreme XE262H-10 cam is not a bad cam for this too. 218-224 110 .464 .470

The idle manifold vacuum will be fine with either for power brakes. Both benefit from a custom diz recurve.

[77cruiser]

I had a 383 with that cam,but only was 9.2 cr AFR 190 heads ran good, idled with 14" @ 700.

[Geoff2]

I would use the Performer manifold, not the RPM with this cam. The cam peaks about 5500 rpm, same as the Performer manifold. Don't know why you use a 5500 rpm cam with a 6500 rpm intake manifold.

[F-BIRD'88]

It don't work quite like that.
The rpm hi rise is more powerfull .
There is a version of the RPM with a spreadbore flange for the Qjet. It is just right.
More power and perf than the low rise performer.
Its a better manifold.

[Geoff2]

The RPM may make a few more HP at the cam's peak, but is very likely to give up some serious low/midrange torque. The std Performer would give the best overall performance with the 270 cam. That is why Edel makes two manifolds, not one, to cater for lower rpm engines & higher rpm engines.

[F-BIRD'88]

The rpm man with the full divided plenum will make more power every where. Right off idle and up. Its is just a better maniold.
The hi rise has a lot to do with it.

[travis]

The rpm man with the full divided plenum will make more power every where. Right off idle and up. Its is just a better maniold.
The hi rise has a lot to do with it.

Not always

[abi]

I have everything except the cam. Just to confirm I have 2 different manifolds to choose from - Performer RPM Qjet, this looks like the one I will use as it has nice looking runners and plenum. The other manifold I can use is an iron z28 GM manifold with both square and spread bolt patterns,I think this manifold is used in marine applications. It looks very similar to the RPM Qjet,actually slightly more volume in the plenum but wow, the weight, to think its only job is to move air and fuel from one place to another, it could also be used as a jacking point!

[F-BIRD'88]

Hard to go wrong with either of those intakes.
If vacuum is scary and you want more of a driver than racer
And or the vehicle is heavy and or under geared.,
Consider this milder comp cam
CS265DEH-10 12-208-3
211-221 .442-.465 110. 106/114
Now you can play with rockers.

Use the biggy 2-1/2" corvette ram horns or shorty headers or long tubes. Headers is better...
Strong midrange torque, good power right where you do most of your street driving.

[F-BIRD'88]

Here are 2 other cams for this motor with corvette ram horns and hi rise Rpm intake and qjets.
Lunati. UDharold design
301A3LUN. 266-272. 211-217 110/104 .441-.454

301A4LUN 266-276. 211-221 112-106. .441-.454

With either/both these cams you can fine tune with rocker ratio especially on the intakes
And by playing with the camshaft installed phasing
To optimize the torque curve for the car.

[abi]

thanks for the recommendations F-bird

[jeff swisher]

My first 350 got the 268H cam and heads were 624 casting milled to 72cc after porting etc.
Log style exhaust manifolds that I ported to be larger than the exhaust ports to give me some anti reversion.

It ran well pulled my boats and trailers well it was in my 78 nova and got 20.5 MPG if I took it easy 18 MPG with spirited driving.

I got beat a lot at the drags as it ran 14.50's

Now onto what you are dealing with ..I decided I needed more power so I got the 270 magnum and some 186 casting heads old double humps from 1969.
I left the 1.94-1.50 valves and ported them and hooked the intake side chamber and then milled them to 54cc.
Lot of time spent porting the heads for best shape.. air flow was so so.
220 at .500 on intake side and 178 at .500" on exhaust I think it was.
I raised exhaust port and intake port and pushed the pushrod pinch open quite a bit.

Block was decked for .013" piston down the hole and I ran .015" steel head gasket.
Flat tops with 5.5 cc reliefs.

Yes compression was up there but i was challenged to run 13's with Q jet iron intake and iron exhaust manifolds.

I blended the entry in the iron manifold and made the 4 holes 2 large ones and did not cut down the divider.
I ran a Q jet and single point distributor with 370 gear in a 9" in the 78 Nova.
Uniroyal tiger paws 275-60-15. TH350.

This got me a best ET of 13.028 at i think 105 MPH.
I shifted it at 6900rpm.
Log manifolds may be a bit better than your ram horns..I would port the ram horns and raise the runners as much as I could and still have them seal well.

I had 2 3/8" fence post for exhaust and glass packs.
1.8 60 foot times was best I could get with those tires and had to really drive it out of the hole.
15 MPG was the normal and could get that pulling a boat down the highway.
I swapped 2.29 gears and got 18 mpg after making timing come in early enough.
It liked 38-39 total with 91 octane depending on temps outside and 22 initial timing.
Sinclair had 93 octane back then and 41 total was the sweet spot.

Like others stated headers will help..I was dead set on nor running headers but a buddy needed 2 core SBC cams and traded me some hookers for them.
I did the headers and picked a bunch of power up from off idle to 3000 rpm.. from 3000-6900 no difference.

I then stuck on a performer RPM and it gained more power everywhere.
I then ran it at the track with headers and RPM intake and went 12.55 at 110 MPH same tires.

I ran old Z28 springs set at 110 seat pressure and stock GM rockers and pushrods from the 1969 engine.

Do not go by the RPM rating by comp cams on the high energy and magnum cams.
I think that may be with bone stock heads.

On a wheel dyno my 268H 218@ .050 flat tappet with ported 601 heads 1.84-1.50 valves makes max HP at 5700 rpm and max TQ at 4500 rpm.
This is with old school single spring with dampner set at 80 psi seat pressure.
I stuck a 280H magnum into the same engine and max HP was 5850 rpm.
That is a 230@ .050 cam and beehives at 110 seat were on those.

All those cams had plenty of vacuum and would idle down below 500 and not die..Cold -10 days I have to baby sit them for a minute as I do not have a choke.

I like the 270 and 280 for good power and decent MPG..I have went to the 292H and mpg took a huge hit but it will get you into the 11's if you are lined out.

Long winded I know ..hope some of it may have helped.

[nxtruck]

Jeff,

If I may ask, what converter did you run with the 268 and 270 cam combos? Lots of good info in your post concerning the o.p.s question.

Thanks!

[jeff swisher]

The converter was a 12" old TCI at that time that in the magazine it stated 400-600 rpm over stock stall.
It would flash to 2400 but if I launched flashing it the tires would blow off so I left at 1500 RPM and eased into the throttle.
I could not open the 4 barrel fully in first or it would blow the tires off with the 54cc chambers and 270H cam.

I had 245 psi cranking pressure on that build.
Ragged edge kind of thing for most.
One key was absolutely NO sharp edges in the combustion space that included the pistons and edges of chambers.
Intake runners from below the carb to below the valves were roughed up with a hard stone to keep fuel atomized as much as possible.

Years later I stuck 64cc 041 heads onto that engine and it would detonate out of the hole and I had to limit advance to 18 and make it come in a bit slower to get my total.

That lead me to find the SBC really can stand more compression with a smaller chamber head.

I currently run that same short block in yet another vehicle.
It has been in 3 so far.
I have a 2200 stall in it now with 370 gears but a very tight 2200 it will foot brake to 1500 rpm and start to tweak the car and try to push it forward.

Good for pulling my 20 foot pontoon. But I upped the game to the 280H cam and stock rockers RPM intake spacer and 1407 edelbrock carb.
Full headers and 3" exhaust into 2.5" straight through oval mufflers.
Ported 601 heads with 1.84-1.50 valves.
Heavy 4 door 57 chevy.

I did tweak the 57 one week for putting the hurt on my buddies 550 wheel HP mustang.
I stuck my Coan 4500 stall and tunnel-ram with 2 1405 carbs, single point distributor then 4.56 gears with a spool.
Only because I have a lot of parts laying around.

57 good friday 002.JPG

He was running 373 gears and 6 speed with the 5.0 supercharged 2015 mustang.
Fun times. I just killed him at the light. He could not make it up.

[nxtruck]

Thanks for the info and video, Jeff! Cool car, btw!!