SBC 408 head which of the 2 do I choose

[racin69z]

I am building a 4.165x3.75 400 based engine for my sons 71 blazer. 5000# 4x4 SM465 35" tires 3.08 gears(but with the SM465 low first it is just right almost like having an OD in 4th). Right now it has a standard bore 9.2:1 350 with vortec heads and a 216 226 on a 108 lobe sep hyd roller cam. The truck runs good, but we want a little more. It has plenty of low end grunt, but seems to peter out early (not much cam, so I can see that). He would like it to rev a little better.

I came across a nice 400 block, so we have been working on getting it ready together. I bought a comp hr282 cam 230 236 510 520 110. I have 2 options for heads.

1. Reuse the vortecs, which I know will make killer torque, but will it peter out real low? I will have to do some work to these for the over .500 lift.

2. I have a set of old Pre RHS Pro action 220 iron heads. These are over .500 ready obviously. My thought is if I build it at 10:1, since it has 58 more cubic inches even with the larger heads and cam I think it will have as much low end torque as the 350 does now, but will have stronger mid to upper end power.

I have read every thread I can find on the large vs small head debate, and can't say I feel any more decisive. I am leaning towards the larger head, and maybe I need to learn a lesson to get this thought out of my head because I have wanted to try it for years.

Any input is appreciated.

Thanks
Lynn

[rustbucket79]

Vortecs with the intention to upgrade the top end down the road. You might want more rpm, but a large runner head on a daily driver with 3.08 gears and 35’s!! Is going to feel soggy in high range on the normal drives.

[Baprace]

mag2555 head advice will make more power, no question , BUT rustbucket79 advice will drive much better because of no gear and the big tires. Both guy's are correct it just matters on what you have to compliment the engine and what amount of driveability you want.

[bobmc]

use the bigger head and higher ratio rockers for more lift, 1.65 or 1.7 rockers will help

[lefty o]

i would be a lot less concerned about what head to use than i would be concerned over having 35" tires with 3.08 gears. really bad combo imo

[dfarr67]

For the money and application, do some work to the vortecs- they will surprise.

[racin69z]

I knew I shouldn't have mentioned the rear gear. Many of you may know, but for those that don't, the sm465 has super low gears

1st 6.55. 2nd 3.58 3rd 1.7. 4th 1.

So doing the math, in high range first gear, with 3.08 gears the ratio is the same as a th350 with 8:1 rear gears. I know that since the torque converter will multiply torque the ratios are not a straight conversion, but it really does not drive like it has 3.08 gears. The gearing is just right for daily driving. I've been arguing that with my son since he thinks 3.08s are too high. I have some 3.73s, but I don't think it needs it.

I was considering trying to get the lift up to the .550 area with some 1.6 rockers.

[jeff swisher]

I have ran the SM465 with a 3/4 ton chevy and a camper it had 2.75 rear gears and a bone stock 350"
Surprisingly quick in L and 1 and tires were 32" tall.

Now the deal I do not get is the lack of RPM with the current 350" with Vortec heads.

I have ran way smaller cams in the 350 and they would rev well the little 268H will rev to 5500 for best shift point with stock 993 heads.
That is a flat tappet 218@ .050 single pattern.
I have ran that cam many times and 110 lsa 106 lsa and on a 112 LSA they would all rev 6000+ RPM with bowl ported old iron heads.
Valve spring pressure on that cam would work all the way down to 80 psi on the seat.

I have Vortec 906 heads but I ported them and installed PAC Beehive springs with 130 seat pressure and 313 Rate.

Now if you are running the stock valve springs that may be your issue with lack of revs.
The cam I had made was off the old GM Hot cam that was 218-228 @ .080 and .492" lift. 112 LSA

I have seen that cam run in a 355" with non ported heads and it will go 6500 lickity split.

I had my HOT cam ground as a single pattern 228 on both sides with a 110LSA.
Ported vortecs and those PAC springs it would go 7200 RPM really quick.

Plenty of seat pressure and with that kind of lazy lobe it did not need a lot of open pressure with the link bar lifters.

1.5 Rockers were used.

I have ran the same grind on an LS which is a 1.7 rocker and lifter in bucket.
That needed more open pressure as it would still rev and buddy went 7500 RPM in his 6.0 with the same PAC springs..TOO much RPM that really needed more like 145 seat and 375 rate spring if he was going to rev like that.

Maybe you just need to set those vortecs up with a better spring if you have not done so.
130 seat would be nice.

[racin69z]

The springs are some kind of aftermarket springs. Probably the old z28 sytle. I bought a truck and took the engine out, re ringed it, and put it in the blazer. Someone before me had screw in studs and guide plates installed. The springs are a single with damper, quite a bit stiffer than the soft springs vortecs have stock.

It will turn 6k, but it is wheezing. The 108 lobe sep could be hindering some of the higher rpm power If shifting by feel, you would shift at 5k-5500, Its not that we want to rev the 400 that much higher, we just want something that has a little more pep. With either head choice, 58 more cubes alone should be worth some strong torque through the mid range.

[jeff swisher]

Sounds like the springs are up to the task.
I do not think the 108 LSA is an issue with top end power, In fact I have always got just as much from the tight LSA on the top end due to more overlap.

Now if you have some of the header tube blocking the top of the exhaust port that can hurt top end.
Carb not set up correctly will hurt the top end make sure it is opening all the way.

I assume you have the Vortec heads with only 4 bolts on either side to bolt it down to the heads.
Not what were called Vortec that had 6 on each side to bolt to the heads.

The exhaust port on the Vortec heads really needs to be bowl ported.
In stock form they do not move much air.
Not any better than the old smogger heads.

If someone stuck larger valves in them with NO porting that will also be an issue.

And Timing can be a RPM deal killer.
I am sure you went through all that stuff though.

I quit using the Z springs a few years ago as I had too many of them break with mild cams 268H.
Hope your luck is better than mine if you keep those with the next build.

I did like the price of them vs the PAC beehive but not the breakage.

[racin69z]

I never had any trouble with the z28 springs. The valves are 1.94 1.50. They are 4 bolt per side vortec. I have never saw a 6 bolt per side. I know some people think the centerbolt 193 casting TBI heads are vortecs, but these are definately vortec 350 heads. I think I am going to have to try the larger heads just to get it out of my system. Not the end of the world if it doesn't work. Weve decided that it will be more fun to drive if it RPMs good.

[BTZ201]

Just a silly thought! If you by chance are running a factory type hei dist. Your 71 probably came with a resistor wire to power the ignition. If there is not a full 12v it would act lazy and not rev very well.....
Just a thought, I have seen it more than once.
I also agree the gearing is an issue, 1st gear is supposed only be used as a granny gear, not in regular street driving. 4:10 is a good compromise with a 35 inch tire and no overdrive.

[jeff swisher]

Back in the late 80's I would race people in 2 different Trucks a 3/4 ton chevy with a camper and a 1 ton dually Ford.
Both had the very low granny gear and I used it as a first gear to launch out of the hole and at 15 MPH I would grab the next gear.

Seldom would those little camaros be able to beat me.

Some say you can't shift those fast but I could shift fast enough to get a few length on the competition out of the hole and through the next gear.
If you ever went into the third gear available which was marked 2ND gear you would just hold that to the finish line or next stop light.

Never broke one of those "granny" transmissions.

[Baprace]

I never had any trouble with the z28 springs. The valves are 1.94 1.50. They are 4 bolt per side vortec. I have never saw a 6 bolt per side. I know some people think the centerbolt 193 casting TBI heads are vortecs, but these are definately vortec 350 heads. I think I am going to have to try the larger heads just to get it out of my system. Not the end of the world if it doesn't work. Weve decided that it will be more fun to drive if it RPMs good.

There is water right there , hey come on over here, where the water is , don't you see the water ??? I can't push a rope or lead a horse to drink the water. JMHO

[Baprace]

I never had any trouble with the z28 springs. The valves are 1.94 1.50. They are 4 bolt per side vortec. I have never saw a 6 bolt per side. I know some people think the centerbolt 193 casting TBI heads are vortecs, but these are definately vortec 350 heads. I think I am going to have to try the larger heads just to get it out of my system. Not the end of the world if it doesn't work. Weve decided that it will be more fun to drive if it RPMs good.

racin69z , I hope you didn't take offense of my post, I was just trying to be funny , I reread my post and thought I was being too harsh. Don't take me seriously. I understand your point on using the other heads as you have already driven one set , and if you don't try anything new you will always wonder what if .