How hard can you rev an M20B25? Opinions wanted too :) etc.

Post Tue May 09, 2006 10:27 pm

Following on from the 'It's mapped' post, the power line on the graph was pointing skywards when the injectors ran out of fuel so i think I need more revs as well as more boost!

Does anyone know what the safe rev-limit on these engines is?

Or am I gonna have to build a slightly lower compression (not much, mabey 9.2:1) engine which I can rev the bollocks off without it breaking?

Next question: What grade material are machined steel rods made from?

Part of me says that having looked at the graph, I've got what used to be max torque at just under 2500rpm now, and what used to be max power at just over 4000rpm, and then another 100bhp to go so that should be enough.

Another part says that I should build another engine to take 7500rpm and a smidge more boost (and that doesn't leak oil) before I go back, and just enjoy what I've got in the mean time.

Cheers!

Alex

Post Wed May 10, 2006 9:42 am

7K max alex, the higher comp pistons are heavier than the later counterparts due to the longer skirt design, possibly more reisitant to rocking motion though for the same reason.

I was told that the occaional forray to 7.2K was OK , but given the extra cyl temps and pressure I'd suggest the 7 K ceiling to be the absoloute limit without risking a failure

Post Wed May 10, 2006 10:03 am

What do you recon the rev limit could be on an NA M20? Assuming all the components are in top nick and it has the fuelling to cope, don't metric mechanics go t to 8G?

Post Wed May 10, 2006 11:20 am

I'd want to see lightweight pistons and uprated big end bolts for the 8K limit Widge, not saying 8K cant be done, but if you asked me to map it up that high on stock internals you'd recieve the car back with a 7 K limiter( hard cut)

I dont think anyone would warrant 8k on stock internals mate, too much weight flying about IMHO

just my 2p though, I know the Aussie's run them high 24/7, and 20 psi on stock bottom ends.

Post Wed May 10, 2006 11:56 am

Thats the opinion I thought you'd have mate, just wanted to hear it from the horses mouth, not that you are a horse or anything....................

IF I was to buil an engine, if I ever get around to it, and had to have custom pistons made, they mightaswell be light weight, and go for lighter stronger rods and bige ends, then I'll bring it to you for mapping

What is the exact differance between hard and soft limiters? Like how do they work? Cut fuel or spark etc...

Post Wed May 10, 2006 1:44 pm

Now that is interesting Ant, Dave reckoned that these engines could go to 7K without too much of a problem, was just a bit scared by the handbook saying 6400 absolute max!

Might let him go to 6600 in that case

Widge, the forces due to acceleration in the pistons and rods by far exceed the loads imposed by combustion so really and truely it shouldn't make too much difference whether it's N/A or F/I

A soft limiter cuts every other spark (or every third etc.) so that the power drops away allowing the engine to slow down a bit.

A hard cut kills everything: fuel and sparks so as to reduce engine speed.

Trouble is that if you're doing 120mph in 5th and accidentally (somehow) drop it into 2nd, no rev-limiting in the ECU will save the engine from over-reving!

Post Wed May 10, 2006 2:21 pm

But a nice soft limiter will remind you to change gear when overtaking that lorry which is coming a bit too close to you , and therefore not really concentrating on the revs!

IF you know what I mean.

Inteeresting, I wondered how they worked. Yeah your right the forces are much bigger in the riciprocating mass. so light rods and pistons, and 10k here we come!