Alex Posted November 19, 2002 Share Posted November 19, 2002 Whats the maximum lift you can use on the stock valve springs? ie if you get 9mm lift cams "should" you replace the springs with tougher alternatives? or do you not need to worry till you hit 10mm's? Or 11?? Cheers Quote Link to comment Share on other sites More sharing options...
Adam W Posted November 19, 2002 Share Posted November 19, 2002 Stronger valvesprings are normally needed to stop the valves bouncing off their seats at high revs. As you're pushing the valve further into the head you need to be sure that the valve springs won't reach full compression (coil bind) before the peak lift is reached, you'll have a pretty chewed up cam lobe if that happens. Disclaimer: I've been reading up on pushrod engines lately but I think the above is still true! Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted November 19, 2002 Share Posted November 19, 2002 Originally posted by Alex Holdroyd Whats the maximum lift you can use on the stock valve springs? ie if you get 9mm lift cams "should" you replace the springs with tougher alternatives? or do you not need to worry till you hit 10mm's? Or 11?? Cheers The cam maker should be able to tell you if stock springs and retainers are OK, or if stronger springs, different springs, different retainers and / or machining is required. Even then you should check for coil bind when assembling. Quote Link to comment Share on other sites More sharing options...
Alex Posted November 19, 2002 Author Share Posted November 19, 2002 Thanks so far.... What RPM would you guess was the most stock springs can run at? With stock valve train. I would assume 7k was a "safe" reliable figure??? Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted November 19, 2002 Share Posted November 19, 2002 Originally posted by Alex Holdroyd Thanks so far.... What RPM would you guess was the most stock springs can run at? With stock valve train. I would assume 7k was a "safe" reliable figure??? What a lot of amateur (no disrespect to the remarkable things i see "amateurs" achieve every year) builders forget on turbo cars is this. As boost is increases so effective valve spring tension reduces. A valve has a finite head area, say 1 square inch, to introduce a round figure. Under a boost a boost figure of say 20 PSI this acts as a force of 20 pounds AGAINST the valve spring seat pressure, so your 100 pond valve spring is only an effective 80 pound spring. You need to equate the boost pressure against spring seat pressure, but at stock boost I'd say the stock springs and stock cam profiles will control a valve train at a crankshaft speed of up to 7000 RPM. Raise boost, increase RPM, change cam profiles and seat pressure will almost certainly need to rise. Complex stuff, the cam designers should have figures for this, but change ANY prameter from the norm and the seat pressure figure may be well out. basically it's a complex equation that needs professional advice given the whole set up looked at as a whole. Quote Link to comment Share on other sites More sharing options...
Alex Posted November 20, 2002 Author Share Posted November 20, 2002 Thanks Chris....I'll pass this info onto a friend that is running higher boost and wants to change his cams... TTFN Quote Link to comment Share on other sites More sharing options...
Ian C Posted November 20, 2002 Share Posted November 20, 2002 Originally posted by Chris Wilson Under a boost a boost figure of say 20 PSI this acts as a force of 20 pounds AGAINST the valve spring seat pressure, so your 100 pond valve spring is only an effective 80 pound spring. I don't understand this bit. The valve spring holds the valve shut, yes? So it's in effect trying to push the valve out of the top of the cylinder head, away from the piston. If the spring is 100lb and there is 20lb of boost *also* trying to push the valve out of the head, away from the piston, surely the spring is now effectively 120lb, not 80? -Ian Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted November 20, 2002 Share Posted November 20, 2002 Originally posted by Ian C I don't understand this bit. The valve spring holds the valve shut, yes? So it's in effect trying to push the valve out of the top of the cylinder head, away from the piston. If the spring is 100lb and there is 20lb of boost *also* trying to push the valve out of the head, away from the piston, surely the spring is now effectively 120lb, not 80? -Ian No, you have the valve spring pulling the valve against its seat at say 100 pounds pressure. Under boost the back of the valve (inlet manifold side) has boost pressure trying to open it, during the period no combustion pressures exist in the chamber, hence the effective valve spring tension is reduced. At say 4 bar boost on 1.25 diameter valves this makes a BIG difference. It makes some difference at any boost, and percentage wise a bigger difference on soft (read stock) valve spring rates. Quote Link to comment Share on other sites More sharing options...
Digsy Posted November 20, 2002 Share Posted November 20, 2002 Interesting one that. I had to read it a couple of times before it sank in, then its obvious! Must be a pretty short period of time where the intake valve is at risk of "blowing open", though, since as soon as it shuts you will begin to a pressure rise acting in the opposite direction on the compression stroke. Also, since there will always be one pair of intake valves open at any point in one crank revolutionm won't the air always tend to take this easier route out as opposed to pushing a valve open? I guess manifold pressure at any point is a function of the air flow through the manifold and the exit resistance, since (for the reason above) its not like you are blowing into a closed volume. If another valve popped open, then it would vent more air and the pressure would drop again instantly so the valve should close itself. I don't know if this would lead to the valve hammering against its seat, or whether it would effectively stop the effect from happening to a noticable degree at the boost levels you turbo'ed guys talk about. At mega boost, though - I can see the logic. Very interesting. Quote Link to comment Share on other sites More sharing options...
Adam W Posted November 20, 2002 Share Posted November 20, 2002 What are the main disadvantages to running stronger valve springs? I can imagine cam lobe wear is increased . . . anything else? Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted November 20, 2002 Share Posted November 20, 2002 Boost pressure is a function of air FLOW through the intake system as a whole, and despite one or more intakes being open to charge that cylinder the other valves (exhausts included) see a positive pressure trying oppose any pressure from the chamber side. At certain momements in the combustion cycle this may become critical with regard to valve train control. Despite things happening very quickly in a high revver, the result is still an opposition of valve spring effective pressure. It can be a real problem on seriously boosted engines, particularly pushrod ones where mega seat pressures are already needed to control heavy valve train movement without the added problem of boost opposition to the spring, and pushrod engines, being usually 2 valve per cylinder, have large valve head areas making PSI figures more detrimental on the larger surface areas. Quote Link to comment Share on other sites More sharing options...
Digsy Posted November 20, 2002 Share Posted November 20, 2002 Originally posted by Adam Wootten What are the main disadvantages to running stronger valve springs? I can imagine cam lobe wear is increased . . . anything else? Increased timing drive loads / reduced timing belt durability, especially at low engine speeds. Quote Link to comment Share on other sites More sharing options...
Guest Martin F Posted November 20, 2002 Share Posted November 20, 2002 Originally posted by Chris Wilson Boost pressure is a function of air FLOW through the intake system as a whole, and despite one or more intakes being open to charge that cylinder the other valves (exhausts included) errrr how can that be ? Is it the pressure from the other exhaust gases causing pressure on the back of the exhaust valves ? Quote Link to comment Share on other sites More sharing options...
Ian C Posted November 20, 2002 Share Posted November 20, 2002 Originally posted by Chris Wilson No, you have the valve spring pulling the valve against its seat at say 100 pounds pressure. Under boost the back of the valve (inlet manifold side) has boost pressure trying to open it, during the period no combustion pressures exist in the chamber, hence the effective valve spring tension is reduced. At say 4 bar boost on 1.25 diameter valves this makes a BIG difference. It makes some difference at any boost, and percentage wise a bigger difference on soft (read stock) valve spring rates. Aaaaaaaahhhhhhh - I understand now, I didn't think about the manifold side at all. Hope you didn't mind my questioning your statement Alex is asking this stuff on my behalf, believe it or not, because I foolishly asked him to price up some cams. We are talking some sort of 256/264 mix or 264's all round. What do I get back? 11mm lift cams that will idle as well as a child on tartrazine, a whole valve train replacement, and the whole cylinder head removing to fit it all. Can anyone give me an idea of what benefits (i.e. more wellie) I'll get going from J-spec cams to any others, including the usage of adjustable sprockets, and what effort I would have to put in? -Ian Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted November 20, 2002 Share Posted November 20, 2002 Originally posted by Martin F errrr how can that be ? Is it the pressure from the other exhaust gases causing pressure on the back of the exhaust valves ? Sorry, doesn't read as I meant :-) I mean that any gas pressure in the ports or chambers, be they acting on exhaust or inlet valve, will have an effect on valve spring active rate, be it positive or negative, IYSWIM . Pos or neg depending on which side of the valve the pressure acts. Quote Link to comment Share on other sites More sharing options...
Guest Martin F Posted November 20, 2002 Share Posted November 20, 2002 Ahhh right.....I see. Quote Link to comment Share on other sites More sharing options...
Alex Posted November 20, 2002 Author Share Posted November 20, 2002 I asked the same question on the Supra Forums and got this response "I will tell you this, with 264's you are right at the acceptable limit for coil bind....I say replace them to be safe. With 272's you are a little beyond...in real world use this usually does not present a problem, but this is what we have measured. We sell springs that can be used with stock retainers for an economical upgrade, and we can get just about anything else." In my question I again failed to mention boost levels as my tiny mind hadn't thought about it... I assume the cams are HKS that this person used as an example. Hope that helps your decision, Ian, on what parts to replace. Quote Link to comment Share on other sites More sharing options...
Digsy Posted November 20, 2002 Share Posted November 20, 2002 Originally posted by Alex Holdroyd "I will tell you this, with 264's you are right at the acceptable limit for coil bind....I say replace them to be safe. With 272's you are a little beyond...in real world use this usually does not present a problem, but this is what we have measured. Odd that this guy doen't mention valve lift. I take it these longer duration cams also have increased lift over the standard ones, as increasing the duration alone won't change the compressed height of the spring. These are increased lift cams with a longer duration to make a less aggressive profile, yes? Quote Link to comment Share on other sites More sharing options...
Alex Posted November 20, 2002 Author Share Posted November 20, 2002 Though I don't know the stock profile I know the HKS has about a 9mm lift on a 264 and 9.3 on 272's I believe. I'm sure they have a higher lift than stock. Could someone please confirm? Quote Link to comment Share on other sites More sharing options...
Guest Martin F Posted November 21, 2002 Share Posted November 21, 2002 I would like to know the profiles of the stock cams as well, Jap and UK spec. AFAIK these details are not in the NCF info. Quote Link to comment Share on other sites More sharing options...
Guest Terry S Posted November 21, 2002 Share Posted November 21, 2002 Martin I think the J spec cam info is on the NZ site. I know the UK/Jap share one common cam, but I don't remember which one, think it's the exhaust. I have heard of high RPM coil bind with 264's. Quote Link to comment Share on other sites More sharing options...
Alex Posted November 21, 2002 Author Share Posted November 21, 2002 Model RZ / GZ (Twin Turbo) SZ (NA) Max Power (kw) 209 @ 5600 (280 hp) 164/168 @ 6000 (220/225 hp) Max Torque (nm) 440 @ 3600 (318 ft/lb) 290 @ 4800 (210 ft/lb) Compression ratio 8.5:1 10 Induction twin turbo n/a Transmission 6 spd man/4 spd auto 5/6 spd man/4 spd auto Bore * Stroke(mm) 86.0 * 86.0 86.0 * 86.0 Capacity (cc) 2997 2997 Valves efi/twin cam/24v efi/twin cam/24v Intake valve dia. 33.5 mm 33.5 mm Intake valve lift 7.8 mm 8.3 mm Exhaust valve dia. 29.0 mm 29.0 mm Exhaust valve lift 8.4 mm 8.4 mm Intake opening 3 deg BTDC to 41 deg ABDC 3 deg BTDC to 52 deg ABDC Exhaust opening 52 deg BBDC to 4 deg ATDC 52 deg BBDC to 4 deg ATDC Sequential turbos CT12B None Turbine Size 60/48 mm n/a Compressor Size 62/39 mm n/a Turbine material Ceramic n/a Turbo Scroll area 600 (mm^2) n/a Turbo A/R ratio 0.42 n/a Quote Link to comment Share on other sites More sharing options...
Alex Posted November 21, 2002 Author Share Posted November 21, 2002 Arse....can't sort the formatting....but you can pick out the details Jap then Export spec Inlet Cam Duration 224 deg 233 deg Inlet Cam Lift 7.8mm 8.25mm Quote Link to comment Share on other sites More sharing options...
Rich J Posted November 21, 2002 Share Posted November 21, 2002 Formatted... Model RZ / GZ (Twin Turbo) SZ (NA) Max Power (kw) 209 @ 5600 (280 hp) 164/168 @ 6000 (220/225 hp) Max Torque (nm) 440 @ 3600 (318 ft/lb) 290 @ 4800 (210 ft/lb) Compression ratio 8.5:1 10:1 Induction twin turbo n/a Transmission 6 spd man/4 spd auto 5/6 spd man/4 spd auto Bore * Stroke(mm) 86.0 * 86.0 86.0 * 86.0 Capacity (cc) 2997 2997 Valves efi/twin cam/24v efi/twin cam/24v Intake valve dia. 33.5 mm 33.5 mm Intake valve lift 7.8 mm 8.3 mm Exhaust valve dia. 29.0 mm 29.0 mm Exhaust valve lift 8.4 mm 8.4 mm Intake opening 3deg BTDC to 41deg ABDC (224deg) 3 deg BTDC to 52 deg ABDC (235deg) Exhaust opening 52deg BBDC to 4deg ATDC (236deg) 52 deg BBDC to 4 deg ATDC (236deg) Sequential turbos CT12B None Turbine Size 60/48 mm n/a Compressor Size 62/39 mm n/a Turbine material Ceramic n/a Turbo Scroll area 600 (mm^2) n/a Turbo A/R ratio 0.42 n/a Quote Link to comment Share on other sites More sharing options...
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