Cams, springs and boost

m1tch · July 28, 2017, 12:59

Hi all,

Just researching into various cam options along with the corresponding springs I might be needing, I don't plan to run higher RPM than stock on my project car with a built engine, so I won't be looking at lighter valves etc, more concerned about the boost pressure behind the valve.

I will probably fit a higher lift cam which will help the engine flow slightly better (unsure as to which one to go with), although due to the fact the engine is boosted I might even just run with a stock cam.

My main question is around the valve springs, will be aiming to run around a bar and a half of pressure (21psi) through the engine, should I look to fit stiffer springs or would the stock springs be ok, as mentioned before I won't be looking to run a higher RPM so should hopefully avoid valve float.

Suggestions and comments welcome s:)

s:)

lamcote · July 28, 2017, 13:54

I would say stock springs would be fine.

1979scotte · July 28, 2017, 15:08

21 psi is massive boost turbo dependant obviously.
If you are having forged rods and pistons which you MUST at that level I would think uprated springs would be a good idea. Engine will be in bits anyway. You'll want the head ported and polished. Crower or Piper maybe both to a specific FI cam which could be worth a look.
Assume you are thinking full stand alone ECU.

Carolyn · July 28, 2017, 15:18

I have to agree with Scotte, A fiercer cam will require the valves to shut faster, to keep up with the cam profile. All of that puts extra strain on (in my view) an already overworked, long, single row chain. So a new chain would also be in order.

ChrisGB · July 28, 2017, 17:32

Forced induction cam profiles can actually be milder than NA cams, often with reduced overlap too. I'd go mildly uprated as they will be cheap while you are in there, but you may not need to. Maybe a 20% upgrade?

shnazzle · July 28, 2017, 17:53

I thought it was more a high(er) RPM thing that required different springs to prevent float.
You'd almost think it'd be less the case with high boost.

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Carolyn · July 28, 2017, 18:59

Quote from: "shnazzle"I thought it was more a high(er) RPM thing that required different springs to prevent float.
You'd almost think it'd be less the case with high boost.

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I've only put high performance cams in normally aspirated engines. Usually one is getting higher RPM, which requires the valves to close more quickly. But, if you think about it, a steep cam profile also calls for the valve to close faster, so both tend to combine together.

The problem with those set-ups was the things idled like a pig and did not do well in street driving.

As Chris pointed out that cams for boosted engines may well be milder, then that argument dies a dead death.

shnazzle · July 28, 2017, 19:12

Ah I see. Would you want a valve that is steeper (guessing less duration) on heavily boosted?

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Carolyn · July 28, 2017, 19:17

Over to Chris for that one. My brain started aching working out the dynamics....

ChrisGB · July 28, 2017, 19:31

Quote from: "Carolyn"Over to Chris for that one. My brain started aching working out the dynamics....

I'm heading to Ronnie Scott's until 3am, remind me tomorrow (or maybe Sunday s:oops: :oops: s:oops: )

Carolyn · July 28, 2017, 19:32

Swine,

1979scotte · July 28, 2017, 20:32

Quote from: "ChrisGB"
Quote from: "Carolyn"Over to Chris for that one. My brain started aching working out the dynamics....

I'm heading to Ronnie Scott's until 3am, remind me tomorrow (or maybe Sunday s:oops: :oops: s:oops: )

Just walked out of the IVY.
Wish I was going to Ronnie Scott's.

lamcote · July 28, 2017, 22:53

My interpretation of the situation was that the final intention was to run the stock cam and the question being asked was "will the 20+ psi of boost behind the valves push the valves open on stock springs? ".

If that is the correct interpretation I think stock valves are fine.

Stage 1 cams should also be OK on stock springs because they tend to be designed to do so.

If you use cams with stock(ish) duration but EXTREMELY rapid lift, you may need stiffer springs, but they can be ground in a way that gets around this to some extent, eg polydyne cams.

ChrisGB · July 29, 2017, 22:31

So, forced induction cams / springs. Springs first. When you open the valves with the cam, there is momentum in the valve assembly. If the valve continues to lift when the cam is no longer driving it, or fails to close as quickly as the cam is moving away from the valve, this is valve float. This is bad because the unloaded valve assembly can wobble and resonate, rotate and generally fatigue leading to valve bounce, reduced performance, burnt seats, and valve failure. Increase the rate at which the valves open and close, or the height they lift, and you could need uprated valve springs to keep the valves pressed against the cam lobe. The downside is that you increase valvetrain load and friction.

When you have an NA engine, opening the valves very quickly produces a strong pulse in inlet and exhaust systems and resonant tuning of these pulses allows for enhanced cylinder filling. It is possible to use these pulses to fill the cylinders with up to 25 - 30% more fuel / air mixture than atmospheric pressure alone would permit in the best N/A designs. Generally, the stronger the pulses produced, the better the effect and the higher the RPM you can produce the optimum cylinder filling, the higher the BHP. So for a highly tuned N/A engine. you will have aggressive rates of lift, high levels of lift, long duration of lift and large valve overlap (the time during which inlet and exhaust valves are open at the same time). Unfortunately, this also means that at low revs everything is well below optimum and the thing runs like a pig. Also, it means that for aftermarket N/A cams, with aggressive lift and profile, you will be looking at uprated valve springs. You can do things to offset the compromises. variable valve timing can be used to fill in what would be a weak midrange (1zz) variable valve timing and lift using two sets of cams means you can optimise for both low and high speed running (2zz) etc.

A turbo changes things. Firstly, you may want to reduce the strength of the exhaust pulses relative to an N/A engine especially in applications with short and narrow runners to the turbo. You want the gas delivered more progressively where it wont backlog in the runners forming a plug if the turbo is temporarily overwhelmed. You may want early exhaust opening, more lift, and porting work, as the boosted inlet charge is more dense than an N/A inlet charge and anything to help it flow along is good. You may want improved cylinder filling at low revs for reducing lag. You may want less valve overlap, particularly at high boost. There comes a point where the boost could be pushing mixture down the exhaust port (I wonder if this is what limits 2zz turbo applications) which is bad for EGT and could cause serious boost control issues.

So regarding valve springs, I wouldn't expect 21psi to push the valves open, but some forced induction cams may lift more, necessitating springs to cope. Crower stage 2 FI cams are significantly higher lift than stock and Crower recommend uprated springs for example.

Hope this helps.

The gig was brilliant by the way. Jean-Luc Ponty, Bireli Lagrene, Kyle Eastwood. Stunning!

1979scotte · July 30, 2017, 09:28

Quote from: "ChrisGB"The gig was brilliant by the way. Jean-Luc Ponty, Bireli Lagrene, Kyle Eastwood. Stunning!

I hate you and your car ;p

ChrisGB · July 30, 2017, 12:05

Quote from: "1979scotte"
Quote from: "ChrisGB"The gig was brilliant by the way. Jean-Luc Ponty, Bireli Lagrene, Kyle Eastwood. Stunning!

I hate you and your car ;p

I have days when I do too s:lol: :lol: s:lol:

m1tch · July 30, 2017, 18:59

Thanks for the help guys, I can see that the stage 2 FI cams do have longer duration with higher lift so I will get uprated springs if I do decide to go with a hotter cam, I will have a think about it but it makes sense if the engine is open and the cams will be out anyway.

m1tch · July 31, 2017, 17:05

Another point I wanted to ask, would I be able to bump the rev limiter by say 500rpm as well as I will be running upgraded rods and pistons and looking to run higher spring rate - I think the limiter is at 6,800?

1979scotte · July 31, 2017, 17:48

Quote from: "m1tch"Another point I wanted to ask, would I be able to bump the rev limiter by say 500rpm as well as I will be running upgraded rods and pistons and looking to run higher spring rate - I think the limiter is at 6,800?

No expert but with upgraded rods and springs I can't see why not.
Standalone ECU required I would think.

m1tch · August 1, 2017, 07:41

Quote from: "1979scotte"
Quote from: "m1tch"Another point I wanted to ask, would I be able to bump the rev limiter by say 500rpm as well as I will be running upgraded rods and pistons and looking to run higher spring rate - I think the limiter is at 6,800?

No expert but with upgraded rods and springs I can't see why not.
Standalone ECU required I would think.

Indeed, will be running a standalone ECU by the time the engine is swapped in so can bump the rev limiter up a bit, might keep the stock limiter but will check things like the oil pump to see if they are ok at slightly higher RPMs.