Cranking pressure on competition 4v engines

Cranking pressure on competition 4v engines

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Pumaracing

Original Poster:

2,089 posts

206 months

Monday 15th September 2014
quotequote all
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

anonymous-user

53 months

Monday 15th September 2014
quotequote all
The absolute value of Pmax(cranking) is, imo, completely un-important! As long as all your cylinders show the same, or close to the same value, you're done!


(too many factors play a part, from starter motor power, battery voltage, engine rotating inertia, bore distortion vs coolant temp, oil press in the valve train, esp with VVT etc)

anonymous-user

53 months

Monday 15th September 2014
quotequote all
Pumaracing said:
if you've got the CR nicely matched to the cam duration but what's a decent pressure?
What does ^^^ this mean? The "cam" duration at cranking (say 150rpm) is never going to be anywhere near any kind of optimum, especially for a race engine that wants to make peak BMEP above say 6krpm??


(surely, at that low speed, IVC needs to be at BDC etc as intake air charge momentum is effectively zero?)

Pumaracing

Original Poster:

2,089 posts

206 months

Monday 15th September 2014
quotequote all
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.

DVandrews

1,315 posts

282 months

Monday 15th September 2014
quotequote all
Let me translate..

The longer the cam duration , the shorter the effective compression stroke and therefore the higher the static CR needs to be. Without paying attention to this the torque will always be disappointing. Normally with longer duration cams a little torque is sacrificed at lower RPM in favour of work done (power) at the top end.

IME with long duration cams as described, the static compression pressure is up around 16-17 bar, if you tried running those pressures with shorter duration cams then it would be detonation city. This equates to a static CR. of around 12.5:1 which is what I aim for on those particular engines, with shorter durations the usual target is around 11-11.75:1 depending on specifics, this usually gives a cranking pressure of around 15-16 bar.. Normal road engines give 12.5->13 bar.

Caveat.. This is with my cheapie Gunsins gauge that I have lived with for 20+ years and gives repeatable results.

Dave


anonymous-user

53 months

Monday 15th September 2014
quotequote all
DVandrews said:
Let me translate..

The longer the cam duration , the shorter the effective compression stroke and therefore the higher the static CR needs to be.
Indeed, and rather obviously so!

However, my concern is the "cranking" bit!

I don't care what CRANKING pressure is, as long as all cylinders are the same (ish).

What i care about is burn rate, heat release and thermal cycle efficiency, all things almost entirely unconnected to the (unfired) pressure recorded during cranking. At high speed we are relying on the dynamic pressure of the incoming air charge to allow continued inflow into the cylinder, even as the piston rises and the cylinder pressure starts to climb. Luckily, because pressure is doubled when volume is halved (assuming adiabatic conditions) we have plenty of extra "time" during which we can delay IVC and hence increase volumetric efficiency above 100%. At this time, the dynamic compression ratio should be optimised to allow the engine to sit as close to the Detonation threshold in the end-gas regions as we dare, considering the chambers burn rate at that state. What the static / geometric compression ratio actually is, or what cranking pressure it develops, well, who cares. It just a "number" and is relatively meaningless except as a basic comparison between engine types?

Edited by anonymous-user on Monday 15th September 16:25

stevesingo

4,848 posts

221 months

Monday 15th September 2014
quotequote all
Pumaracing said:
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.
What is the Inlet closing event?

Based on 60deg ABDC the calculator I use makes it 202psi.


Edited by stevesingo on Monday 15th September 19:51

reggid

195 posts

135 months

Monday 15th September 2014
quotequote all
Max_Torque said:
Indeed, and rather obviously so!
However, my concern is the "cranking" bit!
I don't care what CRANKING pressure is, as long as all cylinders are the same (ish).
What i care about is burn rate, heat release and thermal cycle efficiency, all things almost entirely unconnected to the (unfired) pressure recorded during cranking. At high speed we are relying on the dynamic pressure of the incoming air charge to allow continued inflow into the cylinder, even as the piston rises and the cylinder pressure starts to climb. Luckily, because pressure is doubled when volume is halved (assuming adiabatic conditions) we have plenty of extra "time" during which we can delay IVC and hence increase volumetric efficiency above 100%. At this time, the dynamic compression ratio should be optimised to allow the engine to sit as close to the Detonation threshold in the end-gas regions as we dare, considering the chambers burn rate at that state. What the static / geometric compression ratio actually is, or what cranking pressure it develops, well, who cares. It just a "number" and is relatively meaningless except as a basic comparison between engine types?
Edited by Max_Torque on Monday 15th September 16:25
IMO The cranking pressure lets you know that the static CR and inlet close point are reasonably well matched which is basically the DCR sitting between roughly 8:1 to 9:1, it’s only a rule of thumb rather than gospel.

On 98RON 190-200psi is pretty good cranking compression. The engine won’t feel overcammed and doughy off cam if you are in this range yet wont be too prone to knock provided it’s a reasonably good chamber and piston design.

Some engines may only tolerate less, and some more so its only a rough guideline but better than a stab in the dark


Edited by reggid on Tuesday 16th September 00:14

GavinPearson

5,715 posts

250 months

Tuesday 16th September 2014
quotequote all
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........

Pumaracing

Original Poster:

2,089 posts

206 months

Tuesday 16th September 2014
quotequote all
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.

DVandrews

1,315 posts

282 months

Tuesday 16th September 2014
quotequote all
That is a shed load of duration for a 4 valve.., the most I use is around 280-285 on the inlet and 278 on exhaust. These sit at around 104/108 depending on rod ratio. I would expect your cranking pressure to be a bit lower than my results.

Dave

Edited by DVandrews on Tuesday 16th September 08:59

stevesingo

4,848 posts

221 months

Tuesday 16th September 2014
quotequote all
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

Pumaracing

Original Poster:

2,089 posts

206 months

Tuesday 16th September 2014
quotequote all
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

Original Poster:

2,089 posts

206 months

Tuesday 16th September 2014
quotequote all
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

Original Poster:

2,089 posts

206 months

Tuesday 16th September 2014
quotequote all
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.

226bhp

10,203 posts

127 months

Tuesday 16th September 2014
quotequote all
Pumaracing said:
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.
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.

Pumaracing

Original Poster:

2,089 posts

206 months

Tuesday 16th September 2014
quotequote all
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.

226bhp

10,203 posts

127 months

Tuesday 16th September 2014
quotequote all
Pumaracing said:
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.
I think that's what I was looking for - thanks.

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

Pumaracing

Original Poster:

2,089 posts

206 months

Tuesday 16th September 2014
quotequote all
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

Original Poster:

2,089 posts

206 months

Tuesday 16th September 2014
quotequote all
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.