im putting a LT4 cam in my lo3. ive got a set of swirl ports (187's) and a set of tpi heads (081's). ive read that they both have their advantages. which set do yall think would make the most power with that cam?

 

Not the swirl ports... lol. Swirl port heads are GREAT for torque, but if you're going for power in the top end, go with the 081s.

 

Describe top-end for me. With most street-able cams, the stock converter, stock highway gears, stock hydraulic roller valvetrain, and stock bottem end, you are talking about 5,000 RPM or so redline/powerband.

You can get full benifit of the LT4 cam with the 187s. I would keep the 187s, from idle-4,500 rpm they will make more torque than the 081s will. Its not until 4,500 rpm and up that the 081s would pull ahead. By the time the 081s power-gain is realized, you have already lost the race.

 

would a set of ported 187's tear up the computer too bad?

 

Ive been running a LT4 cam on my L98 for awhile. Trust me, its not that wild. Actually, its extremely tame with no lope due to the 116* seperation angle.

The 081's would probably make more power from 3000 and up. Youre acting like they are 227cc heads from AFR and that you need a .600" cam to hit peak airflow.

Answer this for me, when youre racing, what RPM band are you in more? Idle- 3000 or from 3-6k? You want your peak power to be where youre spending the most time during a race. Sure you may lose 0-40mph, but youre going to run him down like he is tied to a rock in 2nd gear.

Also, take into account he has a T-5 and 3.73's. So that throws out your "Converter and Gearing" argument.

 

Sorry, missed the T-5 and 3.73 on the sidebar.

My other comments still remain. The 187s are still not a restriction until past 4,000 rpm or more. Ported they would make more power than the 081s. I am not bench racing, I have used both. The 187s will be faster.

You are under 3K alot more than you think, even with a 5 spd. 3.73 gears still aren't that low and the 5 spd is far from close ratio. 6K would be stretching the hydraulic roller cammed L03s valvetrain to the point where higher strength components would be needed.

 

Oh yes it is a restriction. Like you said, it increases port velocity so you develop more torque sooner. With your heavy van, it makes it feel like its faster. Ive rode in alot of cars that feel faster than mine that I blow away from the line all the way over 100.

IMO, the reason to not spin up a L03 (besides no reason because it runs out of power) is supposedly that the rods are junk. Also the fact that most L03s still around have over 100k miles and the valvetrains are worn out. A fresh L03 shouldnt have too much problem occassionally spinning to 6k.

 

It is not a restriction until well above what you want to turn a L03 on the street. Fore a street car, I typically cam my engines to lay over about 5,000 and have alot more torque down low. The LT4 cam will lay over about 5,000 rpm with the dual plane intake and TBI setup due to its short duration and wide lobe seperation angle.

The van doesn't feel faster with swirl ports, it is. The other LB4/L03/L05 engines that I have built are the same way. My heads aren't stock, but my swirl ports don't lay over untill well over 6,000 rpm when cammed appropriately.

 

hey guys thanks for all the replys. ive got both sets sittin in my garage and im trying to choose which set to take to the machine shop. im going to have them re-decked along with new springs seals and retainers. then im probably going to try to port them myself. i like the idea of lots of lowend torque but then i also like the idea of making power in the upper rpms lol. thats why im having a hard time deciding.

also i will be using 1.5 roller rockers so i wont have to movify my heads for lift. the lt4 with the 1.5's is about .450 lift. as for the rest of my set up ill be using a edelbrock performer (dual plane) with an adapter plate, ultimate tbi mods, 55# injectors and hooker shorties with a 3" exhaust all the way back. im gonna pretty much leave the bottom end stock but since i already have the motor out im gonna put good rod bolts in it.

 

hmmm what about the rods in the LO3's ???

 

Nope, but that cam probably won't even idle on the stock tune. If you are not planning on chip tuning then don't even think about doing a head and cam swap. You will be changing way too much about the engine to avoid chip tuning.


......carefully avoids the old to swirl or not to swirl argument......

 

haha yea i knew i was probably getting into trouble asking about the swirl ports. and yes i will be doing chip tuning a friend of mine has all the stuff ill need and has done it before. so hes gonna help me.

 

Here's my two cents.................I have just moved up to 3.73's two days ago. I have the same setup that you have except for having 1.6 rockers on the LT4 cam and having Vortec heads with a Edelbrock RPM intake. Once through 1st gear [which is very fast] after you shift you will be at the rpm's where the sp's are said to poop out. You be your own judge.

FYI I ran the stock heads, stock intake with headers and LT4 cam with little tuning. I had the chip tuning equipment but with no way to data log back then so I do not know how much I was leaving on the table so to speak.

Oh by the way..........the 3.73's are one of the best mod's I have ever done. I should have done them years ago.

DM

 

Oh I know I have to tune, I am just trying to do things in an order where I can do the least of it lol. I just wanted to know if I threw on a set of ported 187's will the stock tune make it lose power with the stock cam until I figure out which cam I am going for.

 

hey dm91rs how much power did you gain by just adding the lt4 cam when you still had stock heads and cam?

 

How much with numbers would be impossible for me to say.

IIRC it was a noticeable difference. I am sure now with some logging and tuning experience that it would have made an even better upgrade. This was back in 2001. I think it will go good with your other mod's.

DM

 

ok thanks