hymey

Just wondering what most guys are using camshaft wise. In general low overlap is the normal as high amounts of overlap creates flow from the turbocharger to blow straight out the exhaust port hence more turbine speed is required to build a certain boost pressure which can overspeed the turbo and push it out of its efficiency range. This basically is common sense however to turn an engine hard with a turbo you want more duration and in the sense that a larger then normal hot side is selected then it will have to spin harder to get into its efficiency on a small engine. Just look at what the japanese do with GTRs run monster twin turbos and using cams with 270 deg duration plus at 50 thou lift @ 110 LSA's. Is there a hard and fast rule. Cam selection and turbocharging appears to be a black art with some builders prefering stock cams some wide lsa's with reverse split and other very good results are obtained with a typical large NA camshaft with some turbo guru's commenting that overlap does a good job of cooling the exhaust valve between cycles.

I just have a decent turbo here and a good NA combo on nitrous and might just bolt it up and run 8-10 psi through it. The engine is 366ci and the turbo is a TA flange and has a 1.3 hot side.

Any opinions on this?

Joel

turbo'd stang

I've seen cams like you described work very well. There are drawbacks though.

Warhorse

Very interested. What are the drawbacks?

hymey

13 B turbo rotary engines make very good horsepower when they combine bridgeporting with large exhaust housings. They have a large amount of overlap. I know in early times it was against the rules to use turbos on bridge and PP only then builders/tuners discovered it actually works! Hence my original question. A larger exhaust housing is required though as you need more turbine speed for a given amount of boost from the overlap.

engineermike

This is exactly backwards. 99% of turbocharged v-8's run higher exhaust pressure than boost, usually 1.5 - 2.5x. Mine, for instance, was running 17 psi boost and 34 psi exhaust pressure. This means that during overlap, exhaust will tend to travel backwards from the exhaust into the intake port. I never could reason out if this was actually a bad thing, though, because it would help equalize boost and exhaust pressure some, while the total mass flow through the engine could be unaffected. Plus, the exhaust is basically inert so air/fuel contamination may not be as bad as one might think.

I do know that some have commented that they've never lost power by adding overlap on a turbo motor. Though, the dyno testing was done in the <5 deg @ .050 range. I do remember someone commenting that a lot of overlap made spool-up slow, which is believable, but I think he was running way over 5 deg @ .050.

I'm sure it all depends on the combo, though. In one of the few turbo combo's that's built well enough to have a exhaust/intake pressure ratio around 1.0, the best NA cam would make the most power boosted because the relative pressures are the same.

Mike

Louis at GSS

iTrader: (1)

Back pressure plays a large role as well as intended use. Those that have closer to 1:1, or less P/R, can get away with increased overlap and more cam.

hymey

If exhaust pressure is much higher in V8's then most inline 6 applications then possibly the turbo being used may have restricted exhaust housing somewhat. Most supra's and smaller four cylinders in general use large frame turbochargers in comparison to there capacity, in regards to most V8 setups out there. A lot of LS1s use T4 rear flange and you look at some 4 cylinders and 6's that use fairly large rear housings and slow spool time, Which I would presume would have a closer to equal ratio. To have over 30 psi on the exhaust side could have negative effects with overlap. If the turbine housing is big enough it will reduce back pressure through the port?

hymey

Hence larger exhaust housing?