Max RPM

[KamakSun]

Hey all!

 

Now that we have the head bolt discussion covered, what determines how high we can rev our motors?

[76Lil Hustler]

"Simply put, power is the biggest benefit of a high revving engine. Horsepower is a function of torque and engine RPM. Increase max RPM or torque, and power goes up. If you have a 2.0-litre engine, you’ll need large amounts of boost in order to make high power figures. On the flip side, simply designing for higher revs means you’ll increase the amount of power you make. Torque is not critically important if you make enough power, as you can gear the car appropriately to keep wheel torque competitive. Of course, you can always increase displacement to increase the power, but generally this reduces efficiency."

 

"For example, let’s say you have two 4.0-litre V8 engines that rev to 6000rpm. One has a bore/stroke ratio of 0.5 (68.3mm/136.6mm) and the other has a bore/stroke ratio of 1.5 (98.5mm/65.6mm). The undersquare engine will have an average piston speed of 27.32 m/s while the engine with the larger bore will have an average piston speed of 13.12 m/s, less than half! The short stroke engine could rev all the way to 12,500rpm before reaching the same piston speeds as the other engine."

 

hope this helped.

[Stoffregen Motorsports]

On an L motor, it's limited by valvetrain. Sure, a built L16 can rev to 10K, but it needs to be rebuilt after 3 races.

 

There are a lot more details that need addressing to get there too. You can't run stock rods or pistons, and the crank needs a lot of work to be able to rev that high too.

 

So enough with the hypothetical questions, what is it that you're trying to do? Building a custom L motor costs thousands of dollars. If you have limited budget then this hypothetical discussion is a waste of time.

[KamakSun]

Stock bottom end, the closed chamber head is at the engine rebuilder. It is being ported to the mikuni manifold, milled, new isky can that he's picking out and valve springs

[datzenmike]

The Red Line is a function of tensile strength of the parts used. Stock part strengths are fairly universal across engine makes unless replaced by forged or custom titanium or aluminum parts. The formula for finding your Red Line on a fully stock engine is ...

 

4000 X 6 divide by stroke in inches... 4000 is the max piston speed in feet per min. (about 45 MPH) The g forces generated on the rods accelerating up to a stop and down to a stop 16,500 times a minute by a crank rolling along the ground at 45MPH are incredible. More than 4000 fpm and parts begin to stretch.

 

L16 stroke is 73.7mm or 2.908".... 8,265 RPM  .

L18 is 78mm................................. 7,809 RPM

L20B is 86mm............................... 7,082 RPM

 

 

.

[banzai510(hainz)]

What can your distributor handle before it retards!!!!!!!!!!!

[datzenmike]

The Red Line is a function of tensile strength of the parts used. Stock part strengths are fairly universal across engine makes unless replaced by forged or custom titanium or aluminum parts. The formula for finding your Red Line on a fully stock engine is ...

 

4000 X 6 divide by stroke in inches... 4000 is the max piston speed in feet per min. (about 45 MPH) The g forces generated on the rods accelerating up to a stop and down to a stop 16,500 times a minute by a crank rolling along the ground at 45MPH are incredible. More than 4000 fpm and parts begin to stretch.

 

L16 stroke is 73.7mm or 2.908".... 8,265 RPM  .

L18 is 78mm................................. 7,809 RPM

L20B is 86mm............................... 7,082 RPM

 

 

.

 

The acceleration/deceleration is 2814 g. (I think) A 1.5 pound rod is pulled and compressed by 2.2 tons 275 times a second. If you have an LZ2.2 the red line is even less. 

[distributorguy]

The factory cam and the timing curve in your distributor will run out of steam by 5500, or sooner.  

[Draker]

The factory cam and the timing curve in your distributor will run out of steam by 5500, or sooner.  

 

I was coming in to state that's about where my stock L20 with closed A87 falls flat, way flat.

[scooter]

lz23, pulls good to 7000, shift 7500 on the bottle, im sure it would turn 8000 on the bottle but rod integrity becomes a concern. u60 casting l20b rods with arp bolts. cp pistons.

[G-Duax]

like Stoffregen says, valve train is you biggest concern.

The more $$ you spend, the higher you can rev.

That pertains to both build cost, and down the line parts replacement costs.

Heavy valve spring pressure is a killer on cams, followers, valves, wiper pads.

Lighter components reduces required spring pressure.

Also lighter pistons, rods, wrist pins helps with the max rev limit on the lower end.

[distributorguy]

We pushed our L20b beyond 10K rpm on nearly every run at Bonneville this August.  That required a custom designed cam, a head that flows nearly 70% more air than original, twin DCOEs, a custom header sized to the reworked head, and ALL the go-fast goodies in the block that lighten reciprocating mass, reduce heat and friction, and strengthen weak points.  The valves are custom severe duty, rocker arms are lightened and precision ground, springs are designed for the cam, retainers are stronger and lighter.  It would make one hell of a street motor, at 3x the cost of a typical 620 truck, if you want rpms.   :angel:

 

Oh yeah - forgot to mention this was done with a factory single-point distributor because the HEI matchbox version couldn't handle the rpms.  

[Stoffregen Motorsports]

There you go again with that points crap - kidding...

 

My old boss, Dave Rebello, used to joke that when they first started racing, they built L20B's for circle track racing, and they'd spin them over 10K. His joke was, they did it before they knew they couldn't rev them that high.

 

Point is, they did it even though everybody said it couldn't be done.

[racerx]

Just curious if MSD would have helped at higher rpms

[distributorguy]

It ran better when I removed the CD box.  It absorbed too much power and put less to the plug.  Dual condensers made up for the rpm.  We had no power issues at all on the course - it was all traction problems even though we were never lean enough for the altitude.