trying to understand why tb cam specs so different from other non na engines

[gunzandgearz]

ok guys looking for some help here from some of engine gurus on this site. im kind of new to the turbo buick stuff although not new to engine building ive built quite a few engines in my career mostly chevy small and big block stuff and some ls stuff. I haven't gotten a chance fo flow a set of stock cylinder heads yet,but im assuming that they are like any other engine out there where the exhaust side needs help evacuating the combustion chamber. usually any time I build an non na engine I will run the exhaust side with a bit more duration to help evacuate the charge since the problem is not getting the air fuel in its getting it out. so to my question,why are the cams everyone is making for these cars all single pattern cams, ive even seen some patterns with more duration on intake side, scratching my head on this one. any way just wondering if someone could shed some light on this subject for me please,maybe its just because I haven't had a chance to flow heads yet and exhaust flows like gangbusters.

 

[BEATAV8]

Many of the full throttle speed cams are reverse split with extra exhaust duration. I believe the idea there is to put a little extra *pop* into the exhaust to help drive the larger turbine wheels that are available these days.

 

[Nick Micale]

Since you are new to the turbo world, there are many things you will find different about our turbo cars, and the cam is only one small part of the overall performance build.

Yes, with a SMB Chevy, a replacement cam is among the first performance mods, but it is way down the list in our turbo world.

With some rather simple bolt-on mods the stock cam will easily put a GN into the 10's.

The point is we do not need a radical cam to develop power in most cases, but is ALL cases the overall build MUST have the proper combination of engine and bolt-on parts for a desired performance level and budget.

As far as cam selection we look many things but specifically at the heads - alum or iron, valve size, porting level if any, and of course how the car is to be driven and used.

For those of us left after 30 or so years of building GN performance cars, we can cover the performance spectrum from stock to 1000+ HP with about half a dozen cam grinds that will be suitable.

Yes, some owners are ecstatic about their cam because it works for them and that is good, but other similar grinds will usually do just as well.

It took me many years in the "early" turbo days to leave the NA V-8 world thinking behind me, and learn as much as I could about the turbo cars, and I am still learning new stuff about them!

 

[gunzandgearz]

Thanks for the reply guys but I still don't seem to understand because even on a non naturally aspirated chevy motor or one running nitrous,turbo,or supercharger we still run taller duration on ex. side and wider lobe spreads. so why are the tb cars so different. any way just trying to figure out and have some light shed on it because its still just an engine and that's what we've always done on the boosted or nitrous chevy stuff. Again just trying to figure out since im starting to play with my turbo buick car, again thanks for input although I have quite a bit of engine and mechanical knowledge im always up for learning more.

 

[gunzandgearz]

Hey nick I think you were the gentleman that I spoke with a couple of years ago when I was in the process of buying a grand national and you took several phone calls from me and always took time to answer questions that I had and I appreciated that,youre a great guy andhave a lot of knowledge on these cars.As I stated earlier I have quite a bit of mechanical experience building engines,transmissions,rear ends, even spent time working in the engineering dept. at Cessna aircraft as an experimental flightline mechanic,but always have time to learn from you guys that have been doing stuff with these cars and I learned along time ago that if youre not learning anymore youre not doing anything any thanks for the input.

 

[Nick Micale]

Thanks, always glad to help!

Not to get deep into detail, but the head flow conditions in a blown application have different characteristics which become important like port velocity especially on the exhaust side, as well as flow ratio between intake and exhaust.

In normal street/strip usage these numbers are not critical as we usually overbuild for performance and HP, and have additional boost in reserve if we need it.

In competition applications where rules come into play, the build must develop the most HP we can get based upon what we are allowed to do, or the parts we can or cannot use, so the details are critical to the overall results.

 

[turbobitt]

Thanks for the reply guys but I still don't seem to understand because even on a non naturally aspirated chevy motor or one running nitrous,turbo,or supercharger we still run taller duration on ex. side and wider lobe spreads. so why are the tb cars so different. any way just trying to figure out and have some light shed on it because its still just an engine and that's what we've always done on the boosted or nitrous chevy stuff. Again just trying to figure out since im starting to play with my turbo buick car, again thanks for input although I have quite a bit of engine and mechanical knowledge im always up for learning more.

Exhaust valve closing (EVC) is critical in max effort power. Keeping it as close to 0 at .050 and even the biggest cams limits the overall duration. Keeping the intake duration down to minimize overlap and wide LSA. All of this is very general of course. Turbo motors don't rely on exhaust savaging to charge the intake and to late EVC can hurt intake flow at high drive pressures.
AG


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[turbobitt]

For those of us left after 30 or so years of building GN performance cars, we can cover the performance spectrum from stock to 1000+ HP with about half a dozen cam grinds that will be suitable.

The same can be said for turbo selection!!
AG.


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[ijames]

The stock heads have similar flows to stock small block chevy iron heads, with similar intake/exhaust ratios, so that part of your experience carries over. However, a big difference between a tr and a n/a motor is that to make hp with a sbc you generally start turning the rpms up as far as you you can, which means bigger cam lobes. Also, going to bigger exhaust duration helps keep the hp up as you go up in rpm past the peak hp. The stock tr motors have flexible cast iron crankshafts, heavy pistons (very, very strong but heavy), and weak main webbing, which all adds up to a safe redline of 5200-5500 rpm for a stock short block, and a normal shift point of 5000-5200. You just don't need big duration to get peak hp at 4800-5000 rpm, so why give up spooling and low end torque? Like Nick said, the stock cam has gone 10's (but at that point a bigger cam would probably have gone faster, IMHO ), and is all you really need well into the 11's. Yes, you can spin them higher but you can also play Russian roulette, your choice. Add a 4340 crank, steel center main caps, lighter forged pistons, and a girdle and 6000 rpm starts to get safe, and then a bigger cam makes a lot more sense, as well as optimizing the valve events as turbobitt discussed. To safely go quick in a tr use a turbo big enough to be efficient at peak flow, plenty of octane rating or alcohol injection, enough intercooler to keep the mat close to ambient, enough pump and injector to never be near starving for fuel, heads ported to keep the boost in the mid 20 psi range at peak flow and the shift rpm under 5500 rpm or whatever your shortblock is reliable to, and conservative timing and afr. Yes you can go faster if you push any of those limits, but the tuning skills needed to keep it consistent and together start getting rarer and mistakes get more and more expensive . People can argue about my rpm limits but I think they are within 2-300 rpm of safe for a stock block; the most important lesson is that you just aren't going to spin a stock 109 block to 6500-7000+ like most built sbc's, and you don't need to.

 

[gunzandgearz]

Great, now were starting to get somewhere you seem pretty knowledgeable, I am pretty new to the tb stuff so I am interested in people that have been doing it a while but also people who enjoy this as much as I do and study our hobby. I hope that people on this site are respectful as well as knowledgeable and can have discussions w out getting mad or upset about someones opinion right wrong or indifferent,because I know I will learn a lot from you guys and I hope I have something to offer you guys too. Now back too the discussion,ew I agree with you on the evc but imho I think the intake valve opening point is the most important valve timing event out of the 6 cycles because it determines the effective compression of the cylinder,which is important in any engine. the reason I asked this question was because I just installed a new cam in my engine this last week and when I was researching cams for these engines everything was single pattern no extra lift on exhaust side or duration . Of course the cyinder head port flow on intake or exhaust side has a lot to do with the valve timing events for any engine which I wish I would have time to flow these heads because it determines a lot.

 

[gunzandgearz]

Sweet carl thanks for the input that's what im looking for. my knowledge base on engines is pretty good so im just trying to get some info from some of you guys so my learning curve on these tb cars is shortened. the old saying goes if you know more than your friends its time too find some new friends.

 

[ijames]

At the top of the engine forum page there is a sticky thread "GN1-R's, Ported & Unported Stock Heads", with flow data from several different heads. For some reason the images wouldn't display when I just looked at it, but I just changed browser's and so the problem is probably just mine. Also, do some searches on the engine and general forums and you can find lots more flow data.

 

[bison]

The same can be said for turbo selection!!
AG.


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...and about half that with converters!


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[bison]

People misunderstand bigger. Bigger doesn't mean more duration. Bigger means faster. More duration at higher lifts and less at low lifts. The same cams that work in high hp turbo 6 cylinder engines also work in v8 applications. Adding overlap and a late closing exhaust will limit the mass flow of the engine a crap load on a turbo engine where on an NA engine it will do the opposite with the correct intake/heads and headers. Mass flow means power period. The heads could flow 500cfm on the intake. If the cam has a bunch of overlap and the exhaust is closing late and the CR is low the engine will suck big time. The pressure in the cylinder on the beginning of the intake stroke has to be less than the manifold pressure for cylinder filling to begin. If there's overlap and 70psi ex pressure and you have a late closing exhaust I can assure you that you will have a turd of a performer


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[gunzandgearz]

This is what seperates the men from the boys, who can find the right combo for there parts and pieces.The cylinder head flow numbers do mean something no doubt but something that a lot of people forget about is if we both have heads that flow 500 cfm on the intake side,but mine do it with a smaller port I win because it will fill the cylinder with more air fuel volume that is if everything else is equal this is why I don't know why guys are so sensitive with giving out there cam figures because if everything else is identical which it hardlyever is its no big deal.even though boosted engines do operate under different circumstances it still boils down to air in and air out the most efficient way possible. when I was working at Cessna aircraft there was a guy there that had a blown rat motor in a 69 Camaro and he said to me why do I need to have a good set of ported cyl heads the ports are pressurized, I said to him tell that to the top fuel or pro mod guys and see what they tell you.

 

[gunzandgearz]

Bye the way carl thanks for the info on the head flow this is what im looking for, some flow numbers, some cam specs comp ratios,turbo sizing, and of course some et's from some of your cars as this will all help speed up the learning curve for me on these cars.

 

[deezdad]

Ijames the most underated and underappreciated poster on this board IMHO ... theres also a great article out there somewhere about our cams from a magazine writeup.. has a picture of a glowing motor or something??

 

[deezdad]

Turbo camshaft guide - The truth about camshafts and turbochargers

 

[gunzandgearz]

thanks deezdad al l of this info is of great help. you guys have all been a great help, I have stayed off of the chevy forums because every body on there seems to get there feelings hurt if you try to have a normal conversation about hi tech stuff, a lot of guys think that they know everything and when you try to have a conversation with them on a subject they get feelings hurt because they think they cant learn anything because they know it all already, and that doesn't seem to be the case so far with this forum. im just here to learn from you guys and share some of what ive learned over the years, again thanks to everyone.

 

[gunzandgearz]

bye the way im really starting to get into this tb stuff, its kinda new this v6 stuff,keep it coming