Guest DaneL24 Posted June 7, 2003 Share Posted June 7, 2003 Most of us know that its not a good idea to use a high overlap cam on a turbo motor. At low RPMs, air/fuel passes into the exhaust where it can ignite from exhaust manifold heat and damage the turbo... in low RPM operation it cools down the exhaust gases and slows down the turbo...turbos improve cylinder scavenging so valve overlap isn't necessary...etc, etc. However, in the appropriate high RPM range, a high overlap cam isn't going to release all that air/fuel into the exhaust and will be very beneficial to power production, which eliminates all the negative affects of high overlap cams on turbo motors. So heres my idea...why not use a wastegate that closes at the ideal RPM where valve overlap actually becomes beneficial to producing power...as opposed to just releasing fuel into the exhaust. At low RPMs when the wastegate is open, the lack of back pressure against the turbine should protect against backfires and such. So what do you guys think of this idea? Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted June 9, 2003 Share Posted June 9, 2003 I don't think the wastegate opening is going to do anything but slow boost response. Also, I have tried a "higher" overlap cam in my turbo engine, and it sucked. Specifically it was the Schneider 460 lift split pattern cam MSA sells, what is duration, 270/280 or something like that. Spool up took forever. It ran ok once it was spooled up, but the turbo cam was way better. Maybe if you opened the lobe centerline angle up, you could get away with more duration. I got a little crower I like. I knew it would work well in a turbo application too. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted June 9, 2003 Share Posted June 9, 2003 I don't think the wastegate opening is going to do anything but slow boost response. Also' date=' I have tried a "higher" overlap cam in my turbo engine, and it sucked. Specifically it was the Schneider 460 lift split pattern cam MSA sells, what is duration, 270/280 or something like that. Spool up took forever. It ran ok once it was spooled up, but the turbo cam was way better. Maybe if you opened the lobe centerline angle up, you could get away with more duration. I got a little crower I like. I knew it would work well in a turbo application too.[/quote'] I didn't know you ran that cam, Lockjaw. I think that's the one I've got installed in my spare head right now, planning on reinstalling it once I get the stocker running at high boost. Is this the biggest turbo cam from MSA, the Schneider 280-70F, with 114* lobe separation, .460 lift and 280 intake/270 exhaust duration? Seems to me it really has minimal overlap, but I've never heard any other opinions on that cam. I'm still planning to put it back in, I guess we'll have another opinion soon enough (mine). Sorry for the post hijack. Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted June 9, 2003 Share Posted June 9, 2003 No it was the NA cam. Quote Link to comment Share on other sites More sharing options...
Guest DaneL24 Posted June 10, 2003 Share Posted June 10, 2003 Were you running an L28ET with stock 7.4:1 CR when you used that cam? Maybe your compression ratio, not valve overlap, was your problem. Even on a turbo motor, which ideally have lower compression ratios than NA motors, need increased compression when used with a bigger cam to compensate for delayed closure of the intake valve. Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted June 10, 2003 Share Posted June 10, 2003 Yeah I was using it in a stock turbo engine. It cammed like a you know what too. It should have still made good power once on boost, but it didn't. I know it had overlap, although how much is anyone's guess. But I think the extra duration on the exhaust lobe was hurting it too. Of course I never got that cam to run right except in one motor. It never performed well in any others. My cam that I am using now is a single pattern one and it works good. You really can't tell its there, except at idle once it warms up. Just a slight chop. But you can feel it when you hit it. I can't tell when it comes up though, it just seems like the stock one only it rev's higher. Feels stouter thru the mid range too. Cam choice is a science, but alot will go in there and work, and there may be 10 hp between 3 or 4 different one's. Anyway, mine is 428 lift, .218 duration at .050 (advertised is 288 yeah right) and a 110 degree lobe centerline. I would have liked more lift, the duration doesn't bother me so much. I need to see about having crower grind me one with more lift, like say 500, and leave everything else the same. Wonder what that would do? Quote Link to comment Share on other sites More sharing options...
Guest DaneL24 Posted June 11, 2003 Share Posted June 11, 2003 I think part of the reason they held back on the cam lift may have been because the L28ET has hydraulic lifters. I actually don't know much about how hydraulic lifters are any different from solid lifters, but it puts limits on the cam lift you can use. With a high duration of 288 degrees and a relatively low lift of .428", the lobes will have less of a "slope" (increase in lift per degree of rotation) than most performance cams, which could help explain why there is so much difference between advertised duration and duration at .050" lift. Its still more lift than a stock cam, so its better than nothing. Another thing to consider is that with a turbo motor, you will need more boost to bring back the cylinder pressure if you lower the effective compression ratio. With that huge cam you had before, you lowered the effective compression ratio by delaying the closure of the intake valve ABDC. Without increasing the boost with this cam, you had lower cylinder pressure, which might explain why it didn't have as much power as you would expect when it was under boost. Make sense? BTW, is that cam of yours a custom grind, or just another cam option to choose from? Crower you said?... Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted June 11, 2003 Share Posted June 11, 2003 It was a custom grind, but if I gave you the part number, they could grind you one. I guess someone ordered it and didn't pay for it or something like that. I like it though, it gives the car a healthy sound without being choppy. Kind of makes people wonder what's up, they don't see any turbo badging, but it does have a big exhaust. I have a little higher compression now then stock anyway, probably around 8 to 1. I am 1mm over, and have flat tops down about 10 thous. My head is a solid lift one to, you could not give me a hydraulic lifter head. I had one and it sucked. I would like to have them grind another cam for me with more lift and see what happens. I don't want to get to radical because then I think I will run into issue's with the engine wanting to turn to 8k, and I am not comfy with that idea. I want it to last, and I don't want it to become a pig on the bottom side either. I see your point about the compression and all that with the other cam. I did not try raising the boost, but it was still lame compared to the stock cam, even when the stock cam ran lower boost. Of course I am going from memory on all that. I still kind of think that there won't be to much difference between one turbo cam versus another. I would like to see someone test them and report the results. I mean James likes his, TimZ likes his, Sleeper liked his, the guy with the ISKY likes his, I like mine, be nice to see what each one does. Same engine, just swap out cams. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted June 11, 2003 Share Posted June 11, 2003 Well, this is somewhat off topic but bear with me. I've been thinking about the turbo cam. And I'm thinking too much duration on the ex. side reversion, bad. So how about tight ex duration cam with larger exhaust valves? Should be an alternatiive way to get more exhaust flow without negative effect. When I was talking to my head machinest, he said he could probably push the ex valve 1-2mm. Ideas, ideas.... Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted June 11, 2003 Share Posted June 11, 2003 I came real close to putting in larger exhaust valves when I had my head built. Only problem was my stock bore would cause excess shrouding of the valve. My head builder advised me against it after flowing the bigger valve versus unshrouding the stock valve; he said the stock valve, with some unshrouding, does better with the stock bore. I've also heard there is a much diminished increase in flow with increased lift. Somewhere after the .450" mark, increased lift is not supposed to flow any more to make it worth the hassle. I'm sure that depends on valve size, some proportion of lift vs. diameter where it won't make any sense to go longer with the lift. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted June 12, 2003 Share Posted June 12, 2003 Good point. I guess I would have to unshroud the valves matching with HKS headgasket plus notching the block for clearance. Quote Link to comment Share on other sites More sharing options...
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