Hi. Am seriously interested in knowing how high these little four litre inline sixes we know and love can rev safely and still produce power. I admit right now that I know nothing about the real outer limits of operating this engines (I know very little about operating them within normal limits..) and would like to hear others views. Even if you've not seen it yourself, maybe your mates mate claims to have an engine doing some crazy RPM etc etc?

I have it from pretty reliable sources that 7,500 rpm is do-able and reliable enough to run 30 laps full tit from an AU bottom end with mild head work, cam (and presumably valves & springs) and running a carburetor setup.

One of the stockcar (dirt oval track, you know) classes in NZ uses a whole heap of these engines as they seem pretty well suited to the task at hand. I think they're run with a 40mm restriction in the intake after throttle (carby) by the insertion of a plate with 40mm hole.

Even with this restriction there are engines happily producing 160RWKW apparently. Then again, it could be guys general big noting or big mouthing.

The AU would be more suited to this sort of thing maybe... Doesn't it have shorter conrods but deeper pistons and a lighter crank, rod and piston combination?

Some stock car guys are spending thousands on getting Tickford EF heads done up and are getting good power, even with the carby put on and intake restrictions...


Very interested to know of actual facts and figures about this side of things. I know some people on this site claimed more like 8500rpm, but they also claimed they could completely rebuild a car in 10 days and they also happened to have a crank worth $10,000 (although all the photos provided looked like a standard item... Yawn)

LOL, wonder where that guy has got too

I wouldnt know how well the bottom end would cope at an rpm over 8k, would be pretty cool to here though before it goes bang, i have not heard an i6 rev past 6k in person yet

Id say those stockcar guys would definately be running high rpms around 7500rpm like you said. Some of the cam profiles ive seen from some cam companies who do stockcar cams are f**k monsters and would probably be making power from about 3500-4000rpm onwards

With revs that high you would want to be making a figure closer to 200kw though

when i first had my ecu fitted there was no rev limiter activated and the engine did rev out to 6800rpm..on the dyno it sounded like it was close to its limmits..lol
but did rev without valve bounce or breaking down..
up untill last week when i added more boost the limiter was previously set to 6000rpm and i was hitting it a fair bit lol...
ive now got it set at 5800rpm..
without the luxury of the supercharger or decent head work/cam i think 5700-5800 would be about the most you would really want to rev it to while keeping it streatable and reliable

Thanks guys, useful stuff. Was hoping some other people would happen past with their thoughts too.

The stock valve springs are pretty good huh? Have heard someone say that the original springs work at 7000 or more with standard XR6 cam, but higher lifts might start to cause trouble?!

6800rpm and it was on its limits Troy? And that is standard valves, springs and what cam did you have in there at the time? AU VCT standard issue?


Obviously there is a lot of scope for head work in these cars (along with cam choice) and any little bit of improvement made there would pay off greatly once boost is added.

It probably is quite realistic to be sitting on 7500 RPM with cam, head, valves and springs being worked then?

Question is then more about the bottom end keeping up. Easy first step would be to throw the EF out and put the AU under it?.........

well i say it was close to its limits as it was starting to not want to rev anymore..
i assume the valve train was at its peak??
thats on a std vct head,valves and valve springs..
but it had the non vct xr6 cam fitted i had removed the vct cam and gear..

My motor is still on the running in tune, so it is limited to 4150 atm. I can tell you it gets there damn quickly. We all know the cam has an effect on how much it will rev, but clearances have to be perfect, and rotating assembly balanced as well. I doubt i will need to rev mine past 5500. The engine wont last as long if you ring the tits off it most of the time. Power and especially torque drop off toward the upper end of the rev limit, so why keep revving???

Cheers

Daz

Mine spun to 6500, kept making power due to the cam. Was an EL 'hybrid' motor however (AU head - lighter valve train).

mine made power after the standard limiter as well. I have mine set at 6250rpm now it made max power at 5900~ though.

All good stuff... Getting there. Still no one who has actually made useful power at 7500 though!

GeZza200 - Rev limiter set a 6250rpm, most power a few hundred RPM below that...
Do you consider harder springs, a cam change and some additional head work would be fruitful in the power department?

DA22LE - You see no need to rev past 5,500. Fair enough on a street car with some weight in it. However, am thinking that if you changed the shape of the head a touch, changed ports, air intake, extractors and what have you --- Well.. I dare say that the power would keep going well past 6,000 if that's what you wanted it to do and tuned it to match. Spark advance and mix and quality of fuel would start to come into play... If you have it tuned to suit various parameters at high RPM I'm guessing that you'd get MORE power at 6,500 than you would at 5,500? Maybe it's not reliable for towing a load of tools to the building site, but would be great for burning around an oval track sideways....!

DATAMINE - Is that the same car that blew its tail off its auto box on a dyno at some later date?!!!!!!!!!! Interesting there was power at 6,500 with an AU head and cam. I take it most other things were stock (otherwise you'd have said....)?




Have a look at this thread by WAGON DAD.

Heads can flow very well without having to go through a high lift cam. He seems to have as good flow through .5" valve lift with his home head job as you would through a .7" on standard head.

This could be used very well with a moderate cam such as Wade 1673. Again, I'm only guessing, but a high lift cam at high revs could lead to high rate of failure?

Yes it was.

It was a stock bottom end and head. But custom cam, intake/exhaust mods etc.

The stock cams, peak well below the stock limiter, and thus there's no point revving a stock one harder.

there is no doubt that you can get the engine to rev 7000+rpm..
the problem is what you gain up top you will loose down low..

If you're going around a slippery dirt track sideways in a light little roll cage with wheels you may not really care much about "down low" I guess.

Obviously, our theoretic high revving engine has had its standard cam thrown out long ago - along with exhaust manifold and intake snorkel! In fact, the intake snorkel was handed over to be blown up with a large fire cracker by some Holden fans.

Maybe this topic should have been started with some more thought. Maybe...
"We are building a miracle 8,000rpm inline six with single over head cam 1990's as its base. Forget money. Forget driving it on the street. Forget what your Grandma once said about hoons. Forget what the Government says about emissions. NOW - What parts should we look at putting into this engine?"

camtech have a cam design for 4000rpm - 7000rpm, it looks like the more you want it to rev the less bottom end you will get. Which isn't good for a street car. Which is what Troyman said

They do spin up to 8000rpm but it is not reccomended. A mate of mine was racing the 4.0L engines in his stock car in NZ and he was spinning them to 8000rpm and makeing big power. I won't say a power figure because it is alot more than the usual powere you see and it will just start a arguement on the possability of that amount of power. There are a few mods done to achieve this though. Oversized springs, 100mm throttle body, wolf3d, mass head work, 4" pipe dumping out of front guard, they changed from a pressed gudgen to a floating, bog stock rods and crank, custom cam and alot more tricky stuff they ran avgas too.

The thing you need to look at is the piston speed and the load it will put onto the rod etc. The motor we are working on for my car has a fuly forged bottom end, with tool steel pins and is ALOT lighter than the stock gear. The engine builder has run the numbers through his computer, he has a program to work out big end and and pin loads in the motor at different rpm. The program came back with 7500 to be a safe rev limit for a race engine. The builder thinks we could push it higher but we are going to leave that till it is on the dyno to decide.

To spin the engine that high you do need a cam that can cope with it too. The highest one I have seen goes to 6800rpm. Therefore if you want to make good use of the extra rpm a custom cam is a must. The rocker arms tend to let go at high rpm too. There are rockers that handle it better than others but the best way to reduce brkage is to have a properly set up valve train. Valve float is the reason they break.

8000rpm you say. That's gonna take some miracles.

The I6 has a long stroke, big bores, that means heavy rotating mass. You're gonna need some exotic materials (titanium rods, aluminium pistons, all weighted balanced etc., some fancy material crank and bearings.) and designs to make a bottom end reliable at 8000rpm.

Now that it won't break apart at those revs, you'll need to make it breath at 8000rpm, otherwise there'll be no power made. porting, bigger valves, higher lift cam, intake/exhaust work.