What Limits RPMs...?

[AutoXRacer]

So what limits RPMs in our motors?

Obviously, the first thing that comes to mind is valve springs.

So, how do you know how high you can rev relating to upgraded springs?

Does fuel type have a limiting factor; race/E85 vs pump?

My stock ROUSH used to rev up to 6400 (fuel cutoff).
My new JDM motor used to rev up to 7000 (fuel cutoff), although it was capable of more.

But what factor determined the limit?

What are your setup revving to?

 

[dysan]

This is a whole bag of worms...Design of the engine, weight of the rotating assembly, stroke, valvetrain weight & harmonics, etc, etc....stuff that is wayyyyy over my head. As far as the fuel cutoff you are talking about that is in the tune plain and simple. I don't like the fuel cutoff as my rev limiter so I use my N2MB Wotbox rev limiter and set the one in the PCM 3-400 rpms above that this way my limiter is spark cut but still have a backup just in case from the pcm.

Highest my last engine was revved to was 6800 during the dyno competition but I didn't go above 6600 since I don't make any power up there so it is just a waste.

With good cams, a well balanced rotating assembly and a blower that made power up top I may go to 7k but that's it.

 

[AutoXRacer]

Interesting video showing valve float.

www.youtube.com/embed/_REQ1PUM0rY

(I have no idea how to embed youtube... sigh

This one shows combustion in the actual cylinder (just thought it was interesting enough to share):
http://www.youtube.com/watch?v=cy_yaAOKjA8


So basically, the RPMs are determined by the camshaft specs?
Depending on the operating range of the camshafts, then you set your RPM range accordingly.

So if your cams are speced between 1,500 to 6,900, how far past 6,900 do you go?
Is this figured out on the dyno and looking at the horsepower curve and seeing where it starts dropping off?

But then the question remains, how do you know how much your engine can handle before valve float happens?

 

[JeremyH]

The rev limiter.... lol

Like dysan said, pretty much every part of the engine effects the rev limit. More importantly you don't really need to rev past your peak power or when tq and power start to nose dive due to limits of the intake, exhaust heads etc.

I rev'd to 6800 for 3 years on the stock valve train because I made power all the way there.

I've been revving to 7k rpms for 4 years now with a built motor cams valve springs etc. Stock crank as well. Allows me to finish the 1/4 mile at the top of 3rd gear.

That being said I don't see any reason to every need to spin a 3v past 7k rpms regardless of setup.

 

[AutoXRacer]

Interesting??

Man, with my current setup, 7K RPM comes pretty quickly; almost instantly.
My horsepower curve is still making power, although my torque is decreasing.

With bigger heads and more aggressive cams, I can see my power curve extending passed my current setup. Thus wondering if 7,500 - 7,700 was feasible.

 

[Captainstr8edge]

yeah, its much more than just cams and springs. its the size of the engine too. I believe a shorter stoke can get faster RPM's. Look at a sport bike. 7k is cruising on a lot of them.

 

[JeremyH]

I don't think you will gain any power past 7k rpms without extensive engine work. Most guys never go past 6200-6500.

 

[dysan]

I don't remember what it is but there is a hard limit in our pcm's for the rev limit so you obviously can't go beyond that. Unless you are going to put jesel stuff in the heads I wouldn't want to take it crazy high.

 

[drive_55_not]

I don't remember what it is but there is a hard limit in our pcm's for the rev limit so you obviously can't go beyond that. Unless you are going to put jesel stuff in the heads I wouldn't want to take it crazy high.

We tried setting mine to 7800 in the tune but it still hit the PCM limiter at 7640 IIRC, but like you said, absolutely no reason to spin it that far unless you've built for it.

My cams peak at 6500 and I found that setting the shift light at 6850 has me shifting at 7100 and that is where my car runs quickest.

I slowed down when shifting at 7400.

Gearing and power (drag) will set your absolute limit in high gear. I had 4.56 gears, a .58 5th and 26.5" tires, Those 4.56 were short and my car ran out of gear and topped out at 190 @6450rpm. With 4.30's I would've hit 200.



.

 

[fdjizm]

I found shifting my car at 6800 rpm netted the best ET's.
I think that's about the limit for me.

 

[lito]

common sense

 

[05stroker]

I spun mine to 7550 the most part of last year. I had 4:10s in the rear and a 4r70w trans with non lock up. Its all in the design of your build. If the right parts are there is no reason besides the tune and PCM limitations to go higher. You have to have a very good valve train and a well balanced motor. By the end of the year I plan to be back in the 7200 rpm range at the traps with my 3:73s. I prefer to shift at 7200 rpms so the car stays in peek power. My rpms never fall below my peek torque. My setup never had a power drop at high rpms though. It was still making power at 7000 rpm with the 127550 cams.

I am a fan of rpm! It sounds so much different after 7k.

 

[lito]

^^^ he doesn't have it, common sense I mean, lol.

 

[05stroker]

common sense

I am not average...

 

[05stroker]

^^^ he doesn't have it, common sense I mean, lol.

I wish I could have seen the look on you face when you read my email last year where I asked you to raise my rev limit to 7800 rpms so I would never hit it again! LOL

 

[RLF9409]

Funny I just asked Lito this question yesterday...
I think that is the best way to see where to shift is to getting on a Dyno. You will be able to see where the car makes power to and where it starts to drop off.

 

[05stroker]

We tried setting mine to 7800 in the tune but it still hit the PCM limiter at 7640 IIRC

Is 7640 really the PCM max? If so, can it be reset somehow?

 

[lito]

I wish I could have seen the look on you face when you read my email last year where I asked you to raise my rev limit to 7800 rpms so I would never hit it again! LOL

I just said to myself, what the f! well, is his car, always give the customer what he wants, I am over 2500 miles away anyway, so I am safe.

 

[05moneypit]

There are guys spinning 4V mod motors to 10K. I race with a guy that runs a procharged 4V 281cu motor with a Liberty trans. He launches at 7800 and shifts at 9400....car runs 5.30's in the 1/8.

I bet the 3V can be spun close to that with some of the voodoo they do to the valve train on the 4V, of course you would need an engine controller like a BS3.

 

[lito]

There are guys spinning 4V mod motors to 10K. I race with a guy that runs a procharged 4V 281cu motor with a Liberty trans. He launches at 7800 and shifts at 9400....car runs 5.30's in the 1/8.

I bet the 3V can be spun close to that with some of the voodoo they do to the valve train on the 4V, of course you would need an engine controller like a BS3.

How much money those these guys have in those engines?!??