Monty

It looks like I should be able to dyno test/tune my TT SBC 427 in a few weeks and I need to order a cam more apporpriate for a turbocharged application.

It seems the popular thinking indicates that minimizing overlap is the way to go. However, I'm not totally convinced that is truly correct.

One thing that bothers me about this theory is that the ratio between back pressure in the primaries and exhaust ports is usually 1.5 to 2 times greater than that of the boost pressure in the intake manifold and intake ports. Since the pressure is higher on the exhaust side, why would you need to minimize overlap in order to prevent the boosted intake charge from escaping out the exhaust port?

I mentioned this to Jeff D'Agostino at Fast Times earlier today, and he agreed with me, and stated he doesn't buy into the minimal overlap theory. Obviously Fast Times is one of the leading engine shops in the country for 'street legal' NMCA/NSCA and NMRA forced induction and nitrous engines, so I definitely respect his opinion, but since this is a street engine I'm not sure which way to go, and he wasn't entirely sure either becuase they've never built an engine like this intended for street use.

He also mentioned that he still feels some amount of overlap is necessary beacuse an engine as large as mine, 427ci, needs more camshaft duration in order to rev. Even with the forced indiction you still need to hold the valve open a little longer as rpm's increase in order to fill the cylinder. I don't plan to run it much over 7000rpm, and when I ran it NA, I used a 256/264 duration cam to make 650hp at 6800rpm on pump gas. We were thinking that soemthing in the mid to high 240's would be appropriate with maybe a 114 LSA, but PTE recommended a cam with no more than high 230's in duration.

I'm curious to know what you guys are using/thinking about using for cam duration?

GMI FAST

My previous 400" Bow Tie turbo project motor used a hydraulic roller camshaft from Crower. The specs were around .560" int. lift, .580" exhaust lift. The duration at .006" lift was split at 296-degree int. and 304-degree exh. The cam had a 114-degree centerline. It was to be a decent cam for daily driving, yet evacuate exhaust easily. Good turbo cam...

87Z-ya

iTrader: (1)

There is a problem with to much overlap on a turbo car, and no its not because the intake charge goes out the exhaust. Its because the back pressure (usually higher than the pressure from the turbo and the draw of the piston) due to the turbo can cause the intake charge to be pushed back up the intake valve! I cant remember his email but Corky Bell from cartech is very knowledgable about turbo engines. We got to meet him on a formula sae event. You should try to find his email if you can. He has a far better background on this than anyone on these boards.

askulte

I've done a bit of digging about this also. There seem to be two schools of thought. The first is minimize overlap and the other is to go for total flow. The older, more established turbo cams will usually have about 6 degrees more intake duration than exhaust (i.e. 224/218, 236/230, etc...). The lobe separation angles all seem to be around 114 LS. The thought driving this minimal overlap is since backpressure is 2x boost, the second the intake valve opens BTDC, the burned, inert gasses in the chamber will go out the intake, since the pressure is lower than exhaust. Since the intake is always lower than the exhaust, you wouldn't want to open the intake valve until the piston has started going down, ATDC, lowering the chamber pressure till it's below the intake manifold pressure. That's mostly from Vizard's book. Most of the old school turbo cams sorta follow this.

The "new school" method treats it much more like an NA setup. Straight pattern cams, with larger durations, and more overlap. I've never seen an LS closer than 112 recommended on a streeted turbo engine. This probably has something to do with the fact that the new turbos have significantly less backpressure than the designs of 10-20 years ago. The more efficient your exhaust is, the closer the turbo will be to an NA cam.

One other thing that's been mentioned w.r.t wide lobe sep's and exhaust durations - Opening the exhaust valve earlier lets a higher pressure pulse of exhaust out of the heads, giving it more time to scavenge the cylinder, as well as spin the turbo better. Apparently this lowers backpressure too (according to those I spoke to), but I haven't quite rationalized that part yet...

Your 427 can use a fairly large cam, and still not act cammy. I'm thinking around a 236/230 on a 114 LS for my twin turbo Dart 400 with restrictive manifolds. A cam is cheap and easy enough to change a year from now when the entire exhaust is upgraded.

Guido

I talked to cam motion when I was deciding on a cam. I called comp cams first and told them my engine specs and since my heads stop flowing at .500 lift (260cfm @ .500, 254cfm @ .550) they ground the cam pretty modest.

It sounds like they went with the old school thought on mine. I told them that the car was built primarily to race but that I would be doing some street driving. Single plane carb intake converted to EFi, 5000rpm stall TH400 and 3.50 rear gears. They wanted more like car weight and stuff I cant even think of right now but what they returned to me was 233/226 .529/.523 113LSA.

Im not real big on what works great for turbos but I know a little. I also knew to leave it up to these people and they came back with something that sounded decent at the time for the research I had done.

Good luck Monty.

rhuarc30

Well you're opening a can of worms here Monty Ok first of all people have brought up some good points. The old school/new school thoughts are right, as well as the thoughts about newer turbos having less backpressure. Also you have a pretty free flowing exhaust set up and that will help. And the flow numbers on your heads are insane . Before I throw in my 2 cents worth I'm gonna recommend that you go over to the turbo buick boards and see what they think and are running, then adjust duration for your increased displacement.

My 2 cents:
1. Remember that the more air you pack into your cylinders the more exhaust your going to have to get out, same as any power adder. With your heads averaging somewhere around 80% ratio, in an NA this would say single pattern, but since you need to evacuate more, I'd go dual pattern.
2. The heads you're running look like some serious pieces, designed for some rev's, so you'll want to run a cam to match. With your displacement that's gonna mean in the 290-s and higher intake duration (advertised). Maybe a little less with a solid roller, as their .05 numbers are so much higher. Also remember you can go as much as 10* higher at .05 with a solid lifter for comparable low end, and still get the high end benefit.
3. The difference between a 112 LSA and 114 isn't gonna be huge, but I'd stick in that range.
4. I'd really recommend a roller cam, you didnt' say what you were thinking, your webpage says solid roller, but since your asking for cam advice I assumed that info is out of date. I'd go solid if you don't mind running the valves occasionally, or hydraulic if you do. If you go hydraulic roller a rev kit will be a must.

Having said that here's what I'd go with, it'll probably be custom, but most of the cam manufacturers don't charge extra for that:
112-114 LSA, I'd go with 114 for a little better vacuum at idle before you see boost.
Don't go less than 110 intake centerline, I'd personally go with 112. Theoretically a little lower will boost low end torque, but in my experience under 110 doesn't really, and may even hurt, though it does depend on the rest of the engine as well.
I'd go dual pattern.
All in all, I'd pick a cam like this:
Solid Roller, 114 LSA, 112 Intake centerline. Advertised duration 285/295, @.05 251/261, lobe lift .380/.400. I'd use the same ratio rocker on both, or maybe the larger on the exhaust (opposite of what you have on your web site), but probably not. If you use 1.7's like on your site that will give you .646/.680 lift.
Having no idea what compression ratio your running, or how much boost, I'd guesstimate that'll give you around 1200 hp and 1000 ft/lbs, with over 500 ft/lbs at 2000 rpm.

Hope this at least gets you thinking, I could be way off base here, and lot's of people know more than I do about turbo's so hopefully they'll keep chiming in.
Good Luck,
John

Heh, just looked at this after it posted, I sure talk a heck of a lot don't I

89ProchargedROC

well see the problem i have with some of what a lot of people are saying here is that this it is more typical information for a 23* head and decent intake

otherwise GREAT FREAKN' INFORMATION....HAVEN'T SEEN ONE THIS GOOD IN A WHILE

But with Monty's free flowing headers, 18* heads, and hogan sheetmetal intake.....its going to require a whole other animal with regards to the appropiate cam

Here is what i would reccomend for you to do Monty:

talk to these people....when searching around and asking people who to talk to these are the ones i got:
-the spetters (turbo people)
-cam motion
-PTE www.precisionte.com
-Duttwheiler www.unlimited-racing.com (duttwheiler link is in there)
-innovative turbo www.innovativeturbo.com (Talk to rick head, it was him and the spetters who really started the turbo craze about 5-6 years ago with Jobs ss/o coupe and silvas pro 5.0 cars)
-John Craig, limit engineering

Those were the people to talk to according to the many who i asked. If i were you, i'd be humble and take all their advise and see what you get and then go get a cam

B4Ctom1

please-please please read this thread https://www.thirdgen.org/techbb2/sho...threadid=83372

JAYDUBB

Wheres that damn DeLorean? Hey Doc!!!

Dan W

Precision Turbo did a custom grind for me. Mines more of a street car... 214/218 @.050, 530/495 lift, 112 lsa. This is on a 2200 converter, 3.50 gears, 360 ci small block, Victor Jr converted to multi point and AFR race port 195 heads. Told them I was planning on up to 20# boost and 7000 rpm.

B4Ctom1

cool, that is a pretty interesting grind considering the bigger exh duration at .050" but smaller exh lift. but we use nitrous, which usually turbo blower cams work fine with. we used to use a cam similar to that but when we switched from the NX gemini "quad" to the "titan" http://www.nitrousexpress.com/titan.htm we had to go with over lap because of the cylinder pressures produced on a 13:1 car even o-ringing the block with stainless wire and using copper gaskets would not give us a blow proof seal without bleeding off some cylinder pressure. the biggest we have run so far is a 350 shot. that sytem is obscene. the picture does not do it justice. the selenoids are as big as a high torque mini starter motor, and they have three huge outlets each. You nitrous racers should really check it out. this picture shows it laying behind the others the ones on the ends of the picture are pretty huge too but compare that to the on in the background in the middle, that is a Titan selenoid. the reason I bring this all up is because I intend to run about 15 psi on the street on my '91 and I intend to run a cam with overlap to keep my orings happy too. I am running about 9.5 to one compression and to run that kind of boost and on the street, without a little overlap it would never survive. I would be the "head gasket kid". you know this guy. he is the guy that comes around once in a while with his really fast car but invariably you see him most oft without his ride on race night for problems usually associated with his compression-cam-poweradder combination. I dont want this to be me. I love the GM LT4 HOTT cam, people love to use this with power adders and it makes me so much money changing other peoples head gaskets on blown-nitroused GM's. it works to build cylinder pressure very well, too well on SOME power adder cars.

12 Sec GTA

wow.. 750 hp titan kit..


Anyone want to rent a neon with me ?

Monty

WOW guys, thanks for the responses and feedback. It's quite a bit to digest right now.

I spoke with Dan at PTE yesterday afternoon and he recommended something in the high 230's range for duration at .050". I'm leaning towards something around the low 240's on a 112-114 LSA. It will definitely be another solid roller.

I have a cam we tested on the dyno when I initially built the SBC 427 (NA) that we only put about 10 pulls on. It's a custom Comp solid roller 242/254, 112 LSA. I'm tempted to try that in it and work from there. Cams are relatively cheap and Comp has always been good about getting custom cams to me within 3-4 days, so if we dyno it on a Saturday and find that cam doesn't work well, we can have another cam made and installed ready to test again the following weekend.

B4Ctom1

you know sometimes I get so fixated on lobe sep that I forget to check the duration even with 112 the 230 pretty much says there is going to be a little overlap. I missed that before. maybe one of you could look at thier cam cards and tell me just how much overlap?

rhuarc30

With my cam, 281/289 advertised duration 112 LSA, the overlap is 61* advertised. Even if you go down to 242/242 advertised your still gonna have 18* of overlap, and that's a pretty darn small cam (only 192/192 @.05). Comp cams 260 cam, which is a pretty small cam, still has 40* of overlap, though with 110 LSA, with 112 it'd be a little less.

B4Ctom1

cool

LIL QIK

iTrader: (1)

Hey Monty,
Just wanted to let you know that my twin turbo is running 236/244 @ .050 with a .590 lift on int. and exh. and it's a solid roller. I've talked to a lot of the race turbo v-8 guys here in Houston and they've all said it's going to do real nice with the setup I have. They did comment that it might be a little large for the street but will still do very nicely in the power area. Just wanted to let you know.

89ProchargedROC

have fun checking lash with those manifolds.......you have to remove the turbos to get the valve covers off unless you modified the valve covers for just part of them to come off

Monty

Well, we ordered a camshaft today. After discussing it with several people we came up with a camshaft that is probably larger than what most people would run, but you ahve to remember I'm running 427 ci's.

We're going with a custom Comp single pattern mech roller using their Xtreme Energy Street Roller Lobes (lobe # 4876).

It's 254/254 @ .050, advertised 292/292, on a 115 LSA, they recommended running it at a 111 ICA. With my 1.7 intake/1.6 exhaust ratio shaft rockers it'll end up being about .660"/.621" at the valve, minus lash.

This cam should give it a pretty healthy lope at idle, but compared to some of the cams I ran on the SBC 427 when it was NA, it should still pull around 10-11" of vacuum at idle.

Jester

That one sounds more right to me. Esecially on a twin turbo setup.

Traditional wisdom for NA motors says that for a street car running a full exaust, ie an exit path longer and more restrictive than the intake, to run a dual pattern cam with extra exaust duration to help even things out. It seems to me that the same logic applies to turbo motors. Even moreso than a normal dual pattern NA cam, as turbos are even more restrictive than most full NA exausts. A higher LSA than normal also seems to be logical to me, I wouldn't use less than a 114lsa. The cam I chose, which is as yet unproven, is a 234/244 510/544 114 off the shelf cam. That seemed like a good compromise between what I should really run and cost. I think if I were to do it over with more funds, I would get a custom solid grind, in the 220ish/240ish range on a 114 or even 116.

Now on single turbo cars, where the turbo is large enough that lag is a significant problem, then I can definatly see going with a reverse split pattern with a smaller exaust lobe.

askulte

How would a reverse split lobe combination help fight turbo lag?

A.

Jester

because it helps keep exaust pressure and velocity up...at least by my understanding.

askulte

Hmm... I'm trying to figure that one out in my head. (um... All circuits busy! Please try again.) A shorter exhaust duration is either closing the exhaust earlier or opening it later. If it opens later, the exhaust doesn't have as much energy in it as opening it earlier in the cycle. That's one of the arguments I've heard for more "normal" exhaust durations, since the earlier opening would make a stronger blowdown. I guess it's the closing earlier part that would do it. <shrug>. I've got a copy of Gordon Blair's book "Simulation of 4 Stroke Engine" (or similar), and have been trying to read it every few nights along with "Going Faster" Blair's book is very math heavy, but if you skip over the derivations, he explains the "why" of it all. Pretty good coupled with Prof. Heywood's book on engine design.

rx7speed

saw this in the sticky and thought I would bring it back up in the boards since it has some really really really good info

B4Ctom1

I like this one too https://www.thirdgen.org/techbb2/sho...threadid=83372

cosmick

Responding to the original post only, you're not trying to keep the intake charge from escaping out the exhaust valves, you're trying to prevent the exact opposite.
Non-overlap, which is best for turbocharging, is also the biggest factor in whether a conventional v8 can be made to pass an emissions test.
With boost, you don't need duration for high rpm cylinder filling, even in a stroker. Not do you need overlap. What you need is a late intake closing. It should close when compression pressure exceeds boost pressure. Adding overlap can't help, and more overlap is all you get from more duration.
What strokers need, after the most port flow possible, is the most valve lift possible. But high ratio rockers are murder on pushrods and any lifters other than solid rollers.
With boost you want the smallest intake valves possible, to keep valvespring seat pressure requirements realistic. Otherwise you can't run as much boost.
Part of the benefit of turbocharging is that you don't need to add stroke. In fact, more stroke is counterproductive for turbocharging.
These days a 2.0L 4 can make 1200 HP, so can a pump gas 4.8L LSx, with only a 2" intake valve. I doubt your short block assembly can survive 1200 horses, but if your crank breaks first, replace it with a shorter stroke.

86CamaroDan

Original post is 14 years old