Cranking pressure on competition 4v engines

[Pumaracing]

It's always nice to see a decent cranking pressure if you've got the CR nicely matched to the cam duration but what's a decent pressure? I have my own rules of thumb but would appreciate hearing the wisdom of the collective.

I've just helped a friend to rebuild a 2 litre 16v Peugeot Mi16 engine. Alloy block 1905cc to start with but with a long stroke 91mm crank and 83.5mm bores for 1993cc. Unfortunately due to what had been done by the previous builder we could only get the CR up to 11.7 on cams of 262 degrees duration at 1mm lift. Pretty full race in anyone's book. I would have liked 12.5:1 but such is life. I was worried the cranking pressure would be a little low and the tractability and peak power maybe not quite what it could be.

Anyone care to guess the cranking pressure double checked on two gauges?

I'm sure DVA has considerable data from K series engines which he might share.

Edit. Just spoken to the owner and it's actually 268 degrees at 1mm not 262. Struth.

Edited by Pumaracing on Monday 15th September 11:38

Edited by Pumaracing on Monday 15th September 13:41

[Pumaracing]

Fit ye wittering awa aboot ye daft loon? The cranking pressure, compression ratio and cam duration are all related. To say the cranking pressure is unimportant is daft. When the cam duration and compression ratio are matched the cranking pressure should stay something like constant. If you go up on one of them you need to go up on the other. Stick to yer electronics and leave the engine theory to those of us that ken it fine well.

[Pumaracing]

Cams are the Catcam 4901553. Nominal seat durations 307 degrees inlet, 298 degrees exhaust. I set them up at 105 degrees LCA, 3 degrees advanced i.e. 102 ATDC inlet, 108 BTDC exhaust. Inlet closing event is therefore 75.5 degrees ABDC.

[Pumaracing]

Anyway, long story short. The main reason for the rebuild was a bit of smoke and oil consumption caused by the liners that the original builder, or whoever he had subbed the job to, had bored (badly) from 83.0mm to 83.5mm and there was bugger all in the way of cross hatch honing pattern and even traces of boring tool marks visible. Bored too close to finished size and no proper honing. It went well enough though and cranking pressure was 200 psi.

Rebuilt with properly honed liners and new rings and a small increase in CR from 11.4 to 11.7 we were pleasantly surprised to get 220 psi fully run in on the same gauge and the same numbers on someone elses. More frankly than I'd been hoping for on very big cams and lowish CR. My own cranking pressure charts had only predicted about 205 psi so I might have to rethink those.

[Pumaracing]

stevesingo said:

I use this...

https://www.rbracing-rsr.com/comprAdvHD.htm

I used a 143.5 standard rod length.

The calculator asks for inlet close @0.050", so I used the inlet close in relation to the CatCam spec of 268@1mm+cl.

209psi

Very close to the most I'd been hoping for myself. Well chuffed to achieve better. We did do an inordinate amount of prep work on rings, gaps, bores and everything else I thought we might squeeze any tiny bit of advantage out of though.

[Pumaracing]

DVandrews said:

That is a shed load of duration for a 4 valve..,

Sure is. They wouldn't have been my first choice but they'd already been bought and I had to use the bits I was presented with. To try and compensate for the low CR I dialled them in a bit more advanced than Catcams suggest. Their suggestion is 106 degrees straight up but I usually prefer a bit of advance on the straight up timing.

So far it's just had a running in session and a rough fueling set up on the rollers and needs to go for a proper dial in somewhere else but surprisingly it's pulling from about 4700 rpm which is a good bit lower than it used to and rather more than I'd been hoping for with cams like that.

[Pumaracing]

226bhp said:

Were your cranking pressures cold? Presumably they were.
I know you know this, but thought it was worth chucking in - from what I'm learning two cams which have the same spec on paper can be very different in real life. I know Cat do some with very steep ramp angles, on holding one up against another you can see that one has a pointy nose compared to one with a very rounded one, the latter giving much more duration at high lifts, but presumably giving the valvetrain a good beating. I'm expecting the cam you have is similar to one of those which is why you're getting good results.
I think Cat are very strong on Peugeot cams as they've done a lot of development work and seen real life results.

No, cranking pressures are always taken hot if possible. I did insist he took a baseline reading before the engine had even been started up and those were cold of course and showed 180 psi so we've pulled another 40 psi with it fully run in. That's more than I usually expect but they are very thin section low pressure race piston rings and need some gas pressure behind them and hard running to really show their best. 1.0mm, 1.2mm and 2.8mm oil ring from memory.

With stock 1.6mm - 2.0mm road engine rings I usually reckon to see within 10 psi of the final run in pressures even before the engine has started if I've got the bore finish right.

Seat duration is pretty meaningless on cams but if you go by the 1mm lift duration there's actually not a lot that can be done to change the overall profile.

[Pumaracing]

226bhp said:

Were you originally a Northerner btw? You talk like a Yorkshireman.

Laird preserve us. A Yorkshireman!!? We'll nae hae that kind of talk about the god forsaken English up here in Aberdeenshire. Muckle wee numpties the lot o' them.

[Pumaracing]

GavinPearson said:

If there is one thing I love about PH, it's the willingness of the inexperienced to argue with established professionals talking about the subject they make their living from........

Ay ah ken fit yer saying. Would yer credit the mad loon Max Torque arguing wi the likes o' me eh? Still, he'll learn as he gets older and gains more experience.

[Pumaracing]

Max_Torque said:

I think Pumas been having a wee dram a little earlier than usual today....... ;-)

There's nae sich thing as too early fae a dram. It starts the day aff just fine and fortifies the spirit.