Engine Speed On Modified 8V

[alexcrosse 124]

Hi,

 

Another thread has made me think a bit. I wondered what the limiting factor would be on my engine rev wise. Its nothing special, but I think it would be limited by components rather than flow.

 

1.9 8v

HFH Stage 4 head

280deg cam

jenvey 45's

4-1 manifold

 

Opinions and experience welcome, not necessarily in that order of importance...

 

Cheers,

 

Alex

[petert 588]

Mass of the internal components.

[alexcrosse 124]

Yes... as i said.. components rather than flow, but I meant what would a sensible engine speed limit be?

[TT205 42]

I'm presuming this is a follow on from your build thread where you wondered if you have an ECU fault as the engine wouldn't rev

 

- as your head has been worked surely it should rev to towards 8K whether on carbs or bodies even if the power has tailed off, are you sure the tacho you are using is accurate?

[alexcrosse 124]

No, its nothing to do with that lol. The fault was with the ecu nothing else. I just wondered what it would be mechanically safe to so I know how far I can go after the power has rolled over

 

Head will flow 8k+ im sure, but what will the bottom end let go at lol?

 

Its a standard bottom end, but if I can rev it to 7.5k rather than the usual 6850 then it would be good to know so it can be mapped up to there when I'm next at track n road. Whats the 8v mechanically safe to (given my spec)?

[pug_ham 244 3 Cars]

Have you used uprated rods bolts in your build?

 

I think for otherwise standard components I wouldn't risk prolonged use above 7.5k personally.

 

Where is your peak power & how fast does it drop off after that? Imo not much point revving for revs sake for too long after that.

 

g

[alexcrosse 124]

lol, cheers pugham thats all I wanted to know.

[welshpug 1,661]

it wont be peaking over 7.5k so why take it that far?

 

 

same rod bolts as the 16v btw, early gti6 was limited to 7450.

[alexcrosse 124]

because you always rev past your power peak for many reason like landing on a higher power in the next gear. (area under graph between current & next gear is bigger). And sometimes if you need to push for another second or 2 before braking you can, rather than having to shift up or hold revs.

[Tom Fenton 1,542]

Don't forget that the 8v crank doesn't have the extra counterweights like the 16v crank. I've revved my 8v on standard bottom end bits to 7k often enough, that is on 1600 bits with the smaller big ends.

[pug_ham 244 3 Cars]

My rebuilt 8v with a balanced crank & flywheel revved cleanly to 7k when I got it remapped so we set the limiter there but it was past peak power by then & for the sake of engine longevity didn't see the point going any further.

 

Shift points are as much about peak torque as power, ideally you want to shift when its puts you close to peak torque imo.

 

g

[Anthony 1,003]

Are you using standard valve springs? My best guess would be that they're likely to be the limiting factor, especially if they're old and tired, as they feel fairly soft and it's a fairly large/heavy valve that they're going to be losing the battle to control at high revs.

 

No direct experience or evidence to back that up though, more an educated guess.

[alexcrosse 124]

lol, not gonna get into a 'this is how to change gear' thread. Too many variables.

 

Anyway, cheers for the component limit info guys spot on.