Hi,

This is my first post and I hope it's in the right section. I've been looking into camshaft options for my 92 gmc g2500 350 tbi. I've been reading as much as I can here and haven't found what I'm looking for.

The more I read about swirl port heads, tbi flow limitations, and camshaft profiles the more a single profile, tight lsa camshaft with less duration makes sense to me. The downside is that tight lsa's lead to low vacuum and problems with the tbi.

First, most all of the camshaft recommendations I've seen for tbi are 2 profile. Since the stock heads flow so well on the exhaust side in comparison to the intake and the rpm limitations of the tbi in general a single cam profile seems to make more sense. Single profile cams tend to sacrifice peak power and rpm for a broader flatter torque curve. Why wouldn't you choose a cam that would balance the tbis intake restrictions and create lower, broader, torque the same way the heads are designed to flow?

Second is cam duration. Larger duration increases the rpm where power is made.. according to tbichips the tbi system can't support more than 5200rpms anyways so might as well keep the duration moderate.

Finally, it all leads to lsa. You can get away with tighter lsa with less duration. I understand that tighter lsa leads to a small but strong powerband and stronger throttle response. An engine that runs out of steam at 5200rpm doesn't exactly have a broad powerband, so a cam that comes on like a freight train at 1500 rpm and falls flat on its face at 4800 seems like the ticket! I know that tbi setups won't work with a ratty, popcorn idling cam with a 106 lsa, I've learned I have to accept that. Just how low of an lsa can I get away with?

The cam that I've been looking at is a Howard's cam with a 110 lsa and .213 duration at .050 with 420 lift. According to their literature it should idle smooth and make power from 1000-5000rpm with decent fuel mileage. I haven't been able to find anyone who has tried a cam remotely similar. The lsa is lower than what is recommended for tbi, but the duration is also more conservative than a lot of recommended camshafts so it seems it should pull enough vacuum to keep the computer from going nuts. All of this is purely conclusions I have come to based on what I have researched about cam profiles, tbi limitations, and swirl port heads. Am I on the right track? If I am thinking correctly I would think there would be more people running a cam like that. I know most people think only. duration = power, but there is a crowd in the hotrod community that favor lsa to make power. I'm thinking the cam I'm looking at is a compromise that favors a little lsa over duration to make more power where I can use it.

Please, any comments, criticizism, encouragement, or flat out schooling would be appreciated!

 

maybe this will help, I am not the author,this is written by someone I trust and respect.

Here is a list of the best factory roller cams that work with all forms of the OEM EFI systems without having to install chips on OBD I systems or programmers on OBD II.

The durations measured at .050 lift. Total lift at valve measured with 1.5 rockers, add 7% for 1.6 ratio rockers.

- 10241264, 1996 Vortec 190/.414 intake; 195/.412 exhaust; LSA 116. (The Vortec cam will run with 1.6 rockers without problems)

- 14093643, 350 TPI 87-94 roller 202/.403 intake; 206/.415 exhaust; LSA 115

- 12551705, 350 LT1 93-95 roller 201/.447 intake; 208/.459 exhaust; LSA 117 (TBI loves this cam because of the 117 LSA it will also accept 1.6 rockers without issue)

- 12551142 350 LT4 95-97 LT4 203/.446 intake, 210/.450 exhaust; LSA 115 (with more duration and lift with less LSA this cam is a bit edgy with the stock programming, especially if mixed with a 1.6 rocker)

Stay away from the temptation to use the Ramjet cam, although the timing and lift is in the ball park for TBI at 196/.431 and 206/.451, the 109 degree LSA makes it unsuitable for TBI without a new program in the chip.



I happen to be using the GM 12551705 in my build

 

Go with any of the "Compu-cams", they are designed for computer controlled vehicles. Generally you want a LSA of about 114 , maybe 112 at the lowest.

 

Thanks for the replies guys. Unfortunately my block is not roller cam ready so its flat tappet for me. I was hoping for more of a discussion on whether the cam I had in mind would be a good choice despite the fact that it isn't designed specifically for computer controlled vehicles. I have seen camshafts with less duration and lower lsa advertised as computer friendly cams.

Here is the cam that I have in mind:

http://m.summitracing.com/parts/hrs-111021-11

And here is an extremely similar computer friendly cam carb certified and all:

http://m.summitracing.com/parts/cca-12-314-4

My selection has just 1* tighter lsa and 1* more duration at .050. They are almost identical, probably perform identically, and if it comes down to it I really could go with the comp cams, but from what I've read about camshaft profiles according to the specifications of both cams should* perform better, if it will behave with the computer.

A programmer is already in the mail, and I wouldn't even touch the cam without expecting to do some programming. It really comes down to whether the Howard's cam is within the realm of being programmed to run or not. Any ideas?

I'm not trying to be argumentative, it just seems like everyone says play it safe and stick the highest lsa cam available in it. I'm curious if there has been any real world data on low lsa, low duration cams.

 

Somebody is flat out guessing here. The Ramjet cam has 1/2* less overlap than the Vortec cam and runs great with TBI. On a 202/202 duration cam of about .450 lift I have run 108* lsa with a little tuning. I ran a Crane 272H10 cam that is 272/272 @.004, 216/216 @.050, .454/.454 lif on a 110* LSA in a 9;1 305 with swirl port heads. I pulled well and had no issue dialing it in with the old 160 baud 1992 only 299 ecm.

I also ran a stock LT4 cam in a 350 in a G-van on the stock chip without issues. Made alot more torque retuned but made within 10 hp of peak untuned. Idle was dead smooth at 500 rpm even with the A/C on.

 

Fast 355 I have a lot of respect for you and I know you have helped a lot of people on this forum, the ramjet cam i know is tune able, just not my first choice for a TBI build

 

I'm glad I heard from someone who has successfully tried a cam with higher duration and lower lsa than what I was looking at. After reading your reply I almost want to look for something with a little less lsa! Did you have any issues with tuning for the 202/202 108lsa cam?

 

Wasn't bad at all to be honest. At that duration there is not as much overlap as the 108 LSA would lead you to believe. General rule of thumb I use and has worked well for me using TBI and swirl ports is......

244/244 @ .006, 194/194 @ .050, .400/.400" on a 106* LSA 4* advanced idle-3,500 rpm
256/256 @ .006, 202/202 @ .050, .425/.425" on A 108* LSA 4* advanced 1,000-4,500
260/260 @ .006, 212/212 @ .050, .440/.440" on a 110* LSA 4* advanced 1,500-5,000
268/268 @ .006 218/218 @ .050, .454/.454" on a 112* LSA 4* advanced 2,000-5,500

I ran a 272/272 @ .004, 216/216 @ .050 on a 110* LSA 4* advanced in a 305 with TBI on just because I could and people told me it couldn't be done. I ran a 232/240 @ .050" cam on a 110* LSA in a 383 TPI just because people said it couldn't be done.

Even the factory 179/194 @ .050, 305 peanut cam is cut on a 109* LSA.

The factory 10241264 vortec cam is not on a 116* LSA as stated above, it is on a 111* LSA and sits 5* advanced on a 106* ICL. The 395 Marine cam is on a 109* LSA but is also 3* advanced. The marine cam has nearly 10* less duration at .006 than the factory vortec cam and actually has 1/2* less overlap than a Vortec cam.

Edit-
I will use an example I found that works well in otherwise stock flat tappet 350 TBI engines used for low-end grunt. I like a single pattern cam ground with Comp Cams 244 @ .006, 196 @ .050, .400" lift lobes ground on a 108* LSA and 104* ICL. If you add 1.6:1 full roller rockers you get a friction reduction as well as consistent rocker ratios and the .050" duration rises to about 198.5* and lift to .427". The 108* LSA may seem bad on paper, but in reality this cam has less overlap (only 28*) than the stock TBI cam and closes the intake valve sooner trapping more cylinder pressure, making more off-idle and low-end torque. Furthermore you could cut this same lobes on a 106* LSA and bring it all the way to 102* ICL and it would still have less overlap than the factory cam and boost the dynamic compression ratio even higher.

The factory 350 truck TBI cam is a "929 aka 274" grind and 256/262 @ .006, 194/203 @ .050, .390/.410" lift on a 112* LSA and 108* ICL and also has 35* of overlap. The cam I mentioned above will make more power in the idle-3,500 rpm range than the factory TBI cam and pull better vacuum.

 

It was GMs choice for the 300-320 HP TBI 350 marine engines when used with Vortec or L98 Aluminum heads. It makes a nice torque curve when used with TBI heads and works even better with 1.6 rockers on the intake.

 

You really have me thinking now... I overlooked that you ran a 216/216 @ .050 110* cam on a 305.... tell me more about that! How'd that work out for you? How hard was it to tune? What were you running it in? That's 50* of overlap if I did the math correctly.

I'm not really looking for a towing cam, I will tow with the truck but that's not its primary purpose. I want it to make most of its power between 1500-3500rpm. I'm looking towards the lower lsa just because of the positive compression it gives at lower rpm, the torque that comes with it, and a snappier throttle As long as what I choose doesn't have a noticeably negative effect on lower end torque I'll be happy. What do you think of something in the 214/.214 @ .050 with a 110 lsa? Something like that should really shine in the midrange... i just assumed it would have too much overlap and too low of a vacuum signal for the computer.

 

... or even iskys 210262 208/208 @ 108 lsa. I know it's a bit out of your rules, but it's relatively mild in its overlap compared to 216/216 @110.. just a thought

 

216/216 on a 110 is about 48-52* overlap depending on which advertised duration you base it from. .004 (52*) or .006 (48*). In a 9:1 305 it is a nice sized cam. Not really able to comment on the low-end torque as it had a 2,800 rpm converter behind it, but it would spin the tires at will in the 1980 C10 that it was in that had 3.08 gears and a TH350. I converted that truck from Q-Jet to TBI using the 93-96 TBI PCM. Eventually put a 4L60E into it as well. I am not going to lie, it only pulled about 15-16 in/hg of vacuum at 650 rpm in neutral and took some heavy rework of the fuel and spark maps because of the 5-6 in/hg of vacuum lost over the stock cam. Once it was dialed in though it had great torque.

I also had a 288/284 @ .006, 224/224 @ .050, 114 LSA cam installed on a 110* ICL in a 10:1 305 with 081 TPI head and TBI. It lagged noticeably under 2,500 rpm with the stock 1600 rpm 700r4 converter and 3.08s in my heavy G20 but really came on strong above 3,500 rpm.

 

Could not find the 210262, found a 201262 listed...Specs look to match what you have described.

Installed 4* advance in a stock bore TBI 350 that is around 9.2-9.3:1 compression ratio, it would have a dynamic compression ratio of about 7.9:1. Overlap calculates to be 46*. The advertised RPM range looks a little high for a cam this mild cut on a 108 LSA. That being said I would probably use the 201258 rather than the 262. It is 258/258 advertised, 202/202 @ .050, .425/.425" lift cut on a 108*. Isky usually grinds their cams straight up, unlike Comp/Crane/Lunati which grind in 4* advance. It is easier to overcam an engine with a cam ground straight up. I prefer 4-6* cam advance on every small block I build, just what they seem to like in a normal everyday road driven car. The smaller 201258 cam also comes in at only 42* overlap and right at 8:1 dynamic compression ratio when installed 4* advance on a 104* ICL. If it were me I might install it all the way back on a 102* ICL for 6* total advance. It would really kick out some grunt from off-idle and rev cleanly through 5,000 rpm although it would peak about 4,500-4,600 rpm. If you used 1.6:1 full roller rockers the .050" duration would come up to about 205/205 without changing advertised duration and the lift would hit ~.454/.454 giving you performance equal to the 201262 up top while retaining the idle vacuum/off-idle characteristics of the 201258.

 

Wow fast355, you really know your stuff! Thank you for all your advice! Yes, I did mean 201262. I've been calculating overlap based on the .050 Using the. Duration + duration ÷ 4 - lsa x 2 formula. I guess it does make a lot more sense to calculate based off of the .004 since both valves would be more likely to be open at that time. I'm liking the isky 256 idea . I'm not sure I want to spend the extra cash on ratio rockers for this truck though, and at the 1.5 lift, and only.202 duration I'm just afraid I wouldn't feel a noticeable improvement in power over the stock cam.

From your rreplies I can tell you have a lot of hands on experience, so I might as well just ask straight up rather than continue to theorize based on numbers... being a daily driver mostly, but needing to tow light loads occasionally, what camshaft do you feel would give me the most "fun" power out of these three. Now by fun power I mean the most power and response on hand that you would feel while accelerating hard from cruise speed. Here's the three

Isky 201256 4-6* adv w/1.6 ratio rockers

Lunati or crane .210 @ .050 110lsa 4*adv

Howard's .213 @ .050 111lsa 4* adv

 

Remember in a ~5,500 lbs vehcle with a torque converter that stalls 1,600 rpm, overdrive and tall axle gearing its the torque that is going to be what you notice. One of the easiest ways to gain torque would be long tube or Tri-Y style headers. I bought a used set of Doug Thorley 315Y headers for my 1983 G20 and setup a nice dual 2.25" exhaust with a H-pipe, high flow cats and flowmaster 40s. The flowmasters don't flow the best but it sounded great.

I am not opposed to the 210 or even 213 cam in a 350 TBI, but I feel its not going to work as well as the smaller cam in your application. I also feel that although your stock cam is 194/203* duration the ramps are quite lazy and the valve timing could stand to be at more optimized points. 202/202* with faster ramps IMO will be noticeable when it is dialed in properly. The goal is to let more air in/more air out and trap more cylinder pressure to make more TQ across a broad rpm range.

I suggested roller rockers because I have had them on several vehicles, even used them on my near stock TBI 350 crate engine in my 1983 G20 once and the difference between stamped steel rockers and the 1.6s with my setup was easily noticeable. I had an edelbrock 3704 intake, 1" open center TBI spacer, 2" bore marine TBI unit, 1 1/2" primary long tube headers and 2.25 to single 3" exhaust at that time. I had a 700r4 with a Corvette 5,000 rpm governor in it, stock 1600 rpm torque converter and 3.08 gears.