This may seem completely obvious to some of you, and maybe thats why I couldn't find an answer in any of the threads. But what are the weak points in the motor that prevent it from revving to 7000rpm. Just for a little clarification of my goals; I'm building a 355, eagle forged crank and H-beams, SRP pistons -24cc, I have ported LT1 heads (do you need flow #'s?), and a ported LT1 intake, cam i am still unsure of, but the heads have your run of the mill ferrea valves with comp 987's and steel retainers. I think i already know some of the answers but if anyone has some proffesional/experienced insight it would be much appreciated.

 

There is a bit of a cool factor in reving to 7000 but you may not need to.

I am not an engine builder and I am restating what I have read on this site over the past 4 years:

I would say that a 355 with the previously mentioned internals could be spun to 7000 rpm....assuming that the builder is aware of your desire and take care to ensure that proper tolerances are met.

However,

A quality valvetrain becomes more and more important as you climb the RPM ladder so keep in mind that you may end up spending more money in your valve train setup to acheive your goal. So far what you've shown looks good but just keep the valvetrain in mind.

and

Since you haven't picked a cam yet I assume that you picked 7000 rpm out of the air for some reason...maybe it came to you in a dream ...but when you speak with a quality builder most will want to match the cam to the heads. Since you said it is a ported set up stock LT1 heads I assume that yours will flow well enough to meet the RPM goal but maybe not.

It probably safe to say that some of our resident Cam guru's can get you to a comparable power level and keep the Rs down all at the same time.

There are quite a few much more knowledgeable people on here that can give you more info than I can but I thought I would help to get your wheels turning.

 

Big cam to get the hp peak up produces little vacuum. This causes issues with power brakes. Radical idle is not pleasant sitting still at a light. Stalling and surging or really high idle speed. No real point on the street. Too many issues to address spending dollars for no gain below 6000 and losses below 4000.

 

The heads and intake tract are the major "hidden" impediments to high revs. Everyone knows you need a big cam (and the parts to go with it). Look at it this way.

CFM needed for a 355 with a 95% VE based on CID*VE*RPM/3464

5,500rpm - 535CFM
6,000rpm - 584CFM
6,500rpm - 633CFM
7,000rpm - 682CFM

It takes 30% better head and intake flow to turn 7K vs. 5.5K. If your heads and intake won't flow these numbers, manifold vacuum will go up and power will be down from where it should be.

Rich

 

dont forget the stock lt1 computer wont work past 7k rpm either...

with my old setup, 355, ported lt1 heads, and comp xe236 cam, i took it to 6800 without problems, but no real power advatage that high, but i could see a bigger cam onteh street, i never had brake problems and had a decent idle at 850 rpm, drove it about 50 miles daily ...key to streetable big cam is also in the tune

 

A strong bottom end--like ya have.
Valve train that will take it-- better to over spring
Heads that flow 275+-- to make power up there
BIG cam--250+@50/108+4 mech roller but hyd will do it too
Intake tract to support the heads-- ya loose 15% with a stock one
Computer tune-- been told no fuel tables above 7200,could get a Fast System.
A gear-- 4.56/4.71/488 so it will pull right out the back door
4500+ stall converter-- to make it leave right
A DCR of 9.0-- to support the cam,could be 12.5-1 static
And NO talk of "it don't have no bottom end torque".

 

What does it take to rev high? The answer to that is very simple. MONEY!

And 7000 RPM is not high RPM unless you're trying to do it with a diesel.

 

Let me just clarify a few things:

1) I appreciate everyones opinion, i really do
2) 7K is just a number i chose because i know it is the limit of my stock computer, i have no real desire to turn the motor that high.
3) As you might guess from point 2, this is all hypothetical. I have no cam yet and havent even talked to an engine builder yet, so the only thing set in stone at this point is 355ci of displacement.

Thanks again for the input, i really would like to hear from those that have done it with a "similar" set up. But from what i gathered the requirements are airflow, cam size, and valvetrain stability?

 

Nothing wrong with turning a SBC to 7K. It certainly can be done, but 6,500 is more practical given the available head and intake choices, at least if you are keeping it practical. A HR with ~235-245 degrees intake duration @ 0.050ought to peak around 6,500rpm. It will make good power on a 110 degree LSA, but it is not gonna idle great or have much idle vacuum on a 355. You will need ~125-130lbs on the seat and a 350-400lb/in spring rate. You will need a good spring to get decent spring life, which will be much better if you keep the lift at 0.600" or less. Bret Bauer, who knows this stuff is pretty high on the "beehive" type spring. You may want to talk to him about a valvetrain package. Titanium reainers aren't a bad idea. With a set of good rockers and pushrods, you will be good to go. Skilled programming will be needed to make drivability tolerable.

Rich

 

Thanks, thats good info. So maybe something a little bigger than GM's 847 cam? But with no more lift, or atleast not with 1.6 rockers.

The driveability issue is something i've always wondered about. When you say poor driveability (not that you did) do you mean no low end torque? Or just poor running conditions outside of its "sweet spot"?

 

I take my BBC to 7000 but I also have a lot of exotic stuff inside the engine. The cam specs say it makes power to 7800 but I've never taken it that high.

The camshaft determines how high and engine can go. Everything else determines if it can actually get that high without failures.

I've seen a 377 SBC easily go to 8000. It also uses 5.13 gears in the diff.

 

FWIW, a SBC will apparently tolerate about 14,000 rpm before the cylinder walls collapse.

This was learned during the 80's when high revving 287's were popular.
(I think that was the displacement...)Ultralight components, gas-ported pistons and shaft rockers, highly oversquare bore/stroke ratio...

It's also kind of silly.

The original DZ302 could be spun to 8000rpm, and I've seen it done many times; but that wasn't really a "street" engine in any PRACTICAL sense. It was a great toy though, and a lot of fun with a manual tranny.

In terms of revving to 7000 or 7500, it takes very little in the way of upgrades, and that's more in the valvetrain area than the block. Even a stock block with decent heads (to support the revs) can rev that high, although you would be wise to go with at least lighter weight Hypereutectic pistons.

With todays technology & the ability to build large displacement smallblocks, the 7000rpm (7200?) limit of the stock computer is plenty.


The preceding has been nothing more than my opinion, and while I feel it's perfectly valid, there are other viewpoints that are equally valid. Make of this what you will.

 

The lift depends on the valve sizes, but you should have lift if you are going to spin the motor. As for durations I wouldn't compare it to anything in terms of size, the lobe area and valve events are what matter.

Driveability is going to suffer due to the overlap that you need to do this and the low speed TQ is going to suffer due to the durations to get the motor to peak at 7000rpm.

Now thats all if you want it to peak there.... If you want to shift there which is what you are actually talking about. This is basically a LE3 setup on a 355 with the right parts supporting the cam and springs and a decently built bottom end.

Bret

 

Help me out here, what exactly are the "right parts," good roller lifters, light-weight valves, Ti retainers?

 

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