Inliners International Forums General Discussion Hi Performance 250 RPM capabilities

What kind of RPM capabilites does a 250 series inline 6 have from the factory? I don't care about the cam or the flow of the head or the flow of the intake or carb. Assuming you've for the airflow, what gives first? I have a HEI ignition and I understand that with a few upgrades (coil, wires, plugs) they can be good to 7000 or so (have I heard right?). With a stock cylinder head at what point do the valves begin to float? I've always assumed it was around 5000 or so cause the inline 6 is a low RPM beast, but I don't actually know. If I wanted to run the engine occasionally to 6000 RPM with total reliability and good life what would I be looking at? I've heard of this with a stock bottom end.

That's the important part, now there's me thinking out loud (you can judge if it's as significant).

So I was thinking on the bus home tonight about turbocharging. And I was thinking about how turbocharging can be used to allow you to use RPM you otherwise can't. Basically the idea works like this. For every so many RPM (say 1000) you add another lb of boost or so. The intake ports and cam setup and whatnot begin to be restrictive at higher RPM but the increasing boost (which encourages flow) basically cancels them out. So what you have is a power out put that's dependent on RPM basically. I know it's more complicated than that but that's the most important part. So I decided to make a spreadsheet.

I assumed that torque is 200 lb*ft completely independent of RPM. I picked that cause my engine is rated at 200 lb*ft @ 1600. So we get power output that's dependent on RPM.

RPM Torque HorsePower
1600 200 61
2000 200 77
2250 200 86
2500 200 96
2750 200 105
3000 200 115
3250 200 124
3500 200 134
3750 200 143
4000 200 153
4250 200 162
4500 200 172
4750 200 181
5000 200 191
5250 200 200
5500 200 210
5750 200 219
6000 200 229
6250 200 239
6500 200 248
6750 200 258
7000 200 267

Now that in itself would be a feat for power. You'd be controlling a turbo pretty tight to keep at 200 lb*ft exactly. It'd be interesting. But say if I just wanted the top part of the graph - the part from 4K to 6K. That wouldn't be hard at all the do with a turbo, only a few lbs of boost. But of course this got me thinking. I have an engine with 8:1 compression. With premium fuel I could put a few lbs of boost on there to increase power output and still be able to compensate for the poor flow. So I made another graph. Same idea. The assumption though, is that torque is now 300 lb*ft. That represenst about 6 - 7 lbs of boost. That's totally reasonable. Now you assume another lb/1000 RPM to compensate for flow reduction at higher RPM. Even still. You're talking about 10 psi max at 6K RPM. Which is reasonable. But enough talk.

RPM Torque HorsePower
1600 300 92
2000 300 115
2250 300 129
2500 300 143
2750 300 158
3000 300 172
3250 300 186
3500 300 200
3750 300 215
4000 300 229
4250 300 243
4500 300 258
4750 300 272
5000 300 286
5250 300 300
5500 300 315
5750 300 329
6000 300 343
6250 300 358
6500 300 372
6750 300 386
7000 300 400

There's definetely a lot to consider with what I've said. It's not as simple as more boost to combat decreasing flow as there's more back pressure. But it's mostly true. God I love spreadsheets. I think I was meant to be an engineering (hey! that's what I'm going to school for.......). Anyways, I'd better go to bed

Greg, the limiting factor for the most part is the valve train. Stock valve springs apply pretty low pressure on the valves for durability of the entire valve train. At high rpm harmonics becomes a factor and the valve springs actually bounce because they don't have enough pressure to hold the valve on its seat at that point. The valving in the hydraulic lifters also can't support this punishment. End results are valve float and lifter collapse. On an engine with a lot of miles it gets worse as the valve springs have lost some strength and the lifters might be slowly leaking by.

To get a better idea of how far any valve train can run before the valves float, check a cam mfrs
spec book, like Crane, Comp, Isky, etc. Dig far enough into it and you get the cam rpm range and the spring float info as well.

I'd consider 4500rpm not a problem for a bone stock 250 and 4800 the absolute max once in a while if the engine is in good condition.

My 250 has a Comp Cams 260H with their recommended springs and retainers along with Crane Gold roller rockers. The engine is fresh and I occaisionally buzz it up to 5300 with no problems. This engine is also not done to stock specs.

Thanks Mike! I'd been supposing something like that but there's no point in guessing with so much experience on here. It makes sense given the role that the 6 was used in.

mines a ford...but who cares...its not really that different. Ive gone 5,000 RPM with mine, but its pointless unless its built up and your powerband goes that high...i peak out at about 4800 RPM with mine...slightly modified, but pretty much stock.

Yep, I agree about the powerband. There's no added power reving higher if you don't have the powerband for it. But that's where the boost would come in to provide the power band up there.

i have been thinking along the same lines. i have a chevy 235 in by '59 chevy truck and was considering turbocharging. does anybody know the point the valves float or lifters collapse. if those are the main two problems, can't you just get stiffer valve springs and better lifters?

can the stock crank, rods and pistons handle a low boost (8 psi) application, reving to 6000 rpm?

i would like to get some details ironed out, because if i cant turbo this sucker, i may have to be replaced with something with 2 more cylinders.

It seems like the issue is the valve train first (though I always like to be sure). I'm not a 235 expert but I would think you'd be able to upgrade the valvetrain for the extra RPM. And the bottom end on practically all engines is many time stronger than it needs to be. Done right I don't see a problem with 8lbs of boost.

so if i used one of the cam kits from clifford's shouldn't that help with some issues?

they come with stiffer double valve springs.

would a mechanical cam with mechanical lifters be any better than hydraulic ones?

Yes a matched cam and spring set is always the safest way to go. A solid lifter cam can rev higher as the lifters don't rely on a cushion of oil like a hydraulic cam does. If that oil leaks and the lifter collapses then at the very least the valve doesn't open its full amount and in a worse case the lifter face and cam lobe are damaged and a bent pushrod often follows.

RPM and all that goes with it are nice but you really need to decide just what the engine is going to be used for and select parts accordingly.
A street engine doesn't need to rev over 5000 so as others have said " why go there?". For an engine that makes no more power or torque after say 4500 rpm, having a valve train that doesn't float until 5500 rpm only gives you a little more room in case you miss a shift.

Otherwise its like altitude above an airplane - useless.

Mike

Normally I'd agree with what you say Mike. Normally streetability dictates that you need to make your power at a fairly low RPM. If you're making power at high RPM then you've got to have the parts matched to do so and that eliminates the low RPM you need for the street.

But I don't agree in this case.

I think I can get the best of both worlds. I think I can build an engine for low RPM power (torque as some will call it) and then use a turbo to allow that car to wind into the higher RPMs while making disgusting amounts of power. Check out my post above to get what I mean. I intend to make power at high rpms and still get great economy and torque at low speed. (and I know how difficult this will be)

Greg I think you've got the engine's rpm limit and the turbos function a little crossed up. A turbo will not let an engine rev beyond its design limits. What it will do is pack a denser charge into the engine at any given rpm and therefore make more power than if that same engine were naturally aspiriated.

So by using a cam that has mostly bottom end and running a turbo you will add to the upper end of the cams range if the turbo is set up that way. That makes your assumption of more power at top end correct but it does not override the cams rpm limits and let you rev freely to some excessive number. If the sum of the moving parts restricts the turning to 5000 rpm no turbo in the world will change that and let it run 6000 or 7000 rpm.

Mike

Maybe I didn't explain it properly (I do that a lot) cause I do understand pretty well what I'm talking about.

The sum of my engines rotating parts doesn't allow it to go above 5000 RPM or so.

However, the sum of my intake system parts doesn't allow me to go above 3800 (say 4300 for shifts). So this is the first issue.

I want to change both of these. I want to build the engine to handle upwards of 5500 or 6000 RPM. That still doesn't make it worth my time to rev that high. That's where the turbo comes in.