I'm having my LT1 heads ported. Do I need to upgrade the springs too? There are about 75k on the heads.
Also, is there an advantage to replacing the valves with bigger ones? I know there would be on "all out" race motor, but this is a street motor that will see limited drag time and more highway...........

TIA

 

It all depends on which cam you are going to run. When you get your heads P&P they will probably put larger intake and exhaust valves in. I did this all last winter, and I notice you are just up the road from me. PM me and I can give you a call, I have done quite a bit of work to my LT1.

Fred

 

For all practical purposes, increasing the valve size on a stock lt1 is not a great investment.
If you have the stock cam, swapping to new stock springs is a worthwile mod, in my opinion. 75k is enough to fatigue stockers. If you are camming up, match the new springs to the cam.

 

With a good porting job I assume you are getting larger valves (SS, back-cut, necked & swirl polished) and a multi-angle valve job.

It would be a good time to install 1.6:1 RRs with better springs.

 

These are questions you should be asking your head porter.

 

I'm not changing the cam, I'm just doing mods on the heads. I'll be changing the ratio on the rocker arms to 1.6 Harlan Sharps.
Is it worthwhile to go "oversize" on the valves? Or just get a good valve job, P/P the heads and run it from there?
I guess the question is, " Going to bigger valves with a stock cam, will you see a worthwhile increase (money wise, flow = performance) versus the stock valves"
TIA

 

I am surprised that your head porter guy hasn't figured this all out before-hand. I hope you are not polishing and porting blindly. The pros have said repeatedly, you have to look at the entire package (head, cam, gears, converter,etc) to get the most bang for your bucks.
Your port guy better should have told you what kind of gains you'll get from a P& P job before touching them. Otherwise, youmight be throwing your money away. You apparently need heads that are an improvement over the stock ones, with everything else being stock.
Everywhere I go, I see that the really fast cars have a head/intake/cam, etc package that has predictable results. I think larger valves with no others mods...you gonna lose low end torque. anyone?

 

If you aren't going to do anything more then P/P make sure your springs are replaced w/ LT4 springs. Like everyone has told you talk to the guy that is doing the work

 

on what other people have done, what worked what didn't. Trying to "cover all the bases"
I have already built a number of strong running engines. The last one beinga 72 LT1 with 475 HP and 470 torque with the same head guy. I fully realize the importance of the whole "package", but not wanting to spend too much and have too much in the engine to be able to sell the car when I need to is important too.
Been there, done that...............
TIA

 

You should go with the largest performance valves that will fit the stock seats, along with the multi-angle valve job.

Though your cam is stock the 1.6:1 RRs will boost it by about .030" lift and 2deg duration at .050".

In any case, the actual flow of the heads will depend heavily on the valves and the valve job at any valve lift.

When going to larger valves it is very beneficial at low lift to unshroud the intake valve, i.e. part of the head porting operation, as is dressing the spark plug boss.

Definitely have a discussion of these points with your head porter.

 

Here's a good read to get you started
http://www.popularhotrodding.com/tec..._head_porting/

 

Please check you Roller Rocker decision , I'm not 100% sure but I think there are clearance issues with Harland Sharp roller rockers on an LT1 . I am not promoting Crane Gold Roller Rockers for your application. But I think they are clearanced and tappered for an LT1 or LT4 engine. I know about the valve cover mod for the drip tabs , but I think it has something to do with self aligning vs standard roller rockers.
you can do a seach on this forum to help.

Desert

 

At low speed, a slow-moving port can suffer reversion much easier than a faster one. As a result, low-speed output can suffer considerably (Fig 11). With a well-developed combination of cam event timing and cylinder head design, the combination of ramming from a correctly sized port and the returning pressure wave can markedly boost the port pressure just prior to intake valve closure. But the truth is that many engines fail to achieve anything close to the full effect possible because the intake port was simply too big. Part of the reason for this mistake is that air is assumed to be much lighter than it really is. Just to get you grounded in reality, consider that your average school gym contains 50 to 60 tons of air. Knowing this may just be what is needed to have you pay more attention to port velocity than you might have done in the past.

THIS IS A QUOTE FROM THE ARTICLE>....if your intake is too big, you can lose low end power. Again, the article pointed out the important "package" concept that everyone keeps repeating. I think by putting in bigger valves, you will get zero to negative power results(absent other mods of course).

 

I doubt anybody can tell you from experience because everyone does heads/cam together, and most have headers and higher flow cat back when they do the heads/cam.

Think about what GM did with the LT4--very little change to the cam, but better flowing heads with bigger valves. Also, my head porter offered packages and one of the was assembled heads alone, and there was still a substantial gain published with the price.

In addition--there was a text I read where the LT1 was modded bit by bit, with dynos in between--this text inspired me to mod my engine.
They started with a bone stock car/motor, and the first thing they did was the heads. The gain on the LT1 with heads + bigger valves was 27crank hp.

So--I would advise if you are already doing the heads to do the valves and the 1.6 rr's and newer, better valve springs are a must. A 52mm TB also gives a 10-12 crank hp gain after the heads are done, according to the book (TB was next in the sequence). You can always pull the 52mm TB off the car and put the original back when you sell--I did it. If you're good, you can do that with the 1.6rockers too.

I think you are smart to leave the cam alone because it causes more trouble with the timing chain system than it's worth. Keep your LT1 timing set (although at 75k maybe replace the chain with a new stock chain).

 

If you are keeping the stock cam I wouldnt bother with bigger valves. Waste of money IMO.

 

.

I agree with you are far as gaining HP with the bigger valves and the intake. . . HOWEVER, he is going to trade torque for those gains. That horsepower is going to come at a higher rpm...meaning no seat of the pants difference.
Back in the late 80's when I didn't know much, I put bigger heads on my Malibu (chevy 350) and a ridiculous crane cam. My car turned into a dog. There was only a slight difference in power and but the throttle response was very crisp. Only after I put 4:11 gears in it . . .. holy smoolly! did it wake up!! . . and that was a stick shift. Initially, I thought my cam was degree'd in wrong or something...when all it was, the engine needed more rpm to run right, and a lot more ignition timing. (Sure miss the lope of that ridiculous cam tho...). I remember that after the cam was installed, you could hear the valves making sucking sounds when you start it up and shut it down. . .that how you can tell when an engine has a huge overlap.....ummmm........did I digress......attribute to mid-life crisis.....

 

Most of the power does come over 4000 rpm, yes, but with just head and TB, the power peak was still low--5800 rpm I think. Not out of range of the stock LT1.

The torque actually increases by a maybe 10 ft-lb with heads & TB only, according to the book. What was nice about this book was that it showed the full dyno curve, as a table of numbers, for each change to the motor, so you could see if there was any power/torque loss and precisely where any gains were realized. Of course, you can't believe everything you read, but the author was a credible guy with experience in the area.

 

I agree. Valve springs don't have an infinite life....they will eventually break...it may be at 150,000 miles or 90,000 miles or 5,000,000 miles....but they will break.
The more revs you subject them to (cycles), and/or the more stress you put on them (lift), the sooner they will break.
Springs are relatively cheap. I'd replace them.

Bigger valves are not cost-effective on a mild/street engine.

Larry
code5coupe

 

Larry...Thanks for the advice, I'll check the springs on tear down and let you know what they check at, but I'd disagree.
You USUALLY have a valve train problem somewhere else causing the spring to break on a STOCK oem motor. Spring technology in the last 5 years has gone up as fast as the aftermarket is designing new SB heads. The metals are so much better now than 5 years ago. I will venture to say I doubt the springs check out less then 5% off of stock, I'll let you know..................
Regards

 

Hi....I'll have to check my RR's. I bought them for my 72 LT1 and never used them, thought I'd just throw them in. I kinda remember hearing somewhere that you had to use self aligning, but I can't remember the reason...................
THX