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which size cylinder heads ?!

This is a discussion on which size cylinder heads ?! within the External Engine forums, part of the LSx Technical Help Section category; Originally Posted by Camarosseasttn I'm wondering what will be a good gear ratio for the new rear end with 28" ...

  1. #21
    Moderator 98TransAmWs-6's Avatar
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    Quote Originally Posted by Camarosseasttn View Post
    I'm wondering what will be a good gear ratio for the new rear end with 28" slick ? It has a built 4l60e with a 3600 stall .
    I would stick to something in the 3.55 to 3.90 range. There are a lot of variables to consider. I have a max effort NA (for now) 421 ci Lq9 with a fast 102 trickflow 255 Ls3 heads and a 249/262 duration cam and my rear gear is 3.55. I also have a built 8.8 rear with a built 4l60e and a 4000+ stall. A lot of it comes down to the speed and rpm you want to be trapping at as well as the tire.

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  2. #22
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    If you don't mind me asking what kinda HP or ET is ur car gunning since it's a 421 lq9 ? Sounds like a bad ass setup

  3. #23
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    Quote Originally Posted by Camarosseasttn View Post
    If you don't mind me asking what kinda HP or ET is ur car gunning since it's a 421 lq9 ? Sounds like a bad ass setup
    I haven't run it yet or had it dyno'd the engine is currently out of the car right now. However I've been told by a few speed shops while it was getting built that it should be running 10s but I we see for sure soon enough.

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    I don't doubt it a bit if it's a good combination

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    Quote Originally Posted by Camarosseasttn View Post
    I don't doubt it a bit if it's a good combination
    Yep same I've had everything in the build matched with everything. Anyway, what rpm and mph do you want to cross the line at?

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    I honestly don't know what RPM I would like to be at, I'd just like the best setup to net me the best ET in the 1/8 mile? I'm guessing he I'll be around 110-120 mph ?

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    Quote Originally Posted by Camarosseasttn View Post
    I honestly don't know what RPM I would like to be at, I'd just like the best setup to net me the best ET in the 1/8 mile? I'm guessing he I'll be around 110-120 mph ?
    What is your power band typically you'd want it to be towards the end of it.

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    I know it seems to like upper rpms it shifts at 6500 rev limiter is 7200 ?

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    ^With that setup, you should be shifting at 7000+. Move the limiter to about 7600. Go with a 3.89 gear since you have a taller tire.
    Last edited by shortdog; 12-21-2017 at 05:15 PM.
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    You don’t think that’s too many rpms with a 200-250 shot of nitrous? What do u think this setup is capable of running in the 1/8 on a 200 shot with a 3.70-3.90 gear leaving off the line on the 200 shot, permitting traction isn’t a issue?

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    Since your stall is not that big and you're on a 28" tire, you'll be fine.

    It should run low 6s in the 1/8th, maybe high 5s depending on weather, traction, etc.

    What's your race weight w/you in it?

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    A little late here but what heads were you using? Once you get up to a 200 plus shot I personally think a cam change will benefit you more than the heads will. Putting them heads with a better cam even better yet. Any power adder will benefit from a wider split in the cam to help get the exhaust out. What's your exhaust setup? Could be some good gains there also.

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    Quote Originally Posted by tug686spd View Post
    A little late here but what heads were you using? Once you get up to a 200 plus shot I personally think a cam change will benefit you more than the heads will. Putting them heads with a better cam even better yet. Any power adder will benefit from a wider split in the cam to help get the exhaust out. What's your exhaust setup? Could be some good gains there also.
    Sorry for the late response.
    The heads are PRC 247 from Texas speed, the exhaust is 1 3/4" hooker headers into a 3" y going into a 3" borla exhaust , no cats , and I have 1 single 3" electric cut out in front of the rear tire. Cam is 235/240 .629-.615 112 LSA

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    You think it could / should run low 6s with a 200-250 shot with that setup?

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    My setup is 400" (3.9 stroke x 4.04 bore) tfs 245 heads, super Vic intake and 4150 throttle body. My cam much larger than yours at 251/266 for my planned nitrous use. Haven't sprayed it because my converter was showing 12-18 percent slip at 7000 rpm in 3rd on motor. With that said it ran 6.8's in 1/8th and 10.7's in the 1/4 at just over 3800 race weight on motor. So you with a better cam geared to your nitrous use should be able to get high 5's pretty easy on the 250 and possibly on the 200. I think custom cams are a waste if someone is charging you extra for it anymore as there are cams on the shelf everywhere to suite any needs. For yours I'd go 238/254. What little if any you loose on motor (I think you'll gain) you'll gain back huge on the bottle. It's not unheard of with the right combo and cam to gain 300-350 whp on the dyno with a 250 shot being sprayed.
    Last edited by tug686spd; 12-25-2017 at 04:21 PM.

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    Also most y pipes leave a lot to be desired on big power combo's. With the price of a couple of the true daul out the back systems available now that's what I would have went with but they weren't available then. I'm currently running 2" primary, 3-1/2" collector, 3-1/2" true dual with no x or h turned out in front of rear tire. Also running a type of chambered mufflers, they are cherry bomb extremes. Got them because they were cheap but after getting them I think they are restrictive but haven't tested without them. You could test yours with just a 12-18" piece of pipe off each header to see if it improves.

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    So the cam I have now you don’t think I could hit 5s on a 200? Also I’ve thought about taking the exhaust pipe off of the y pipe and let it be sorta like open headers after the y pipe would that work?

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    If you ran a 6.50 on the 175 unless it was a real soft launch you'll need at least another 100hp to get that 5.99. Now we don't know the specifics of the pass so we can't really say for sure. If it was a 1.6 or slower 60' then I can see getting that 5.99 pass with just a little more power, chassis tuning, engine/trans tuning and driver experience. Making power is extremely easy, getting a car to run to its potential is a whole other story. Let's start with what your average 60' times are and see if we can improve there or move on to the next thing.

    My car with factory shocks and springs on all 4 corners has gone 1.4's through the 60' on motor so shocks aren't always needed. A stiff chassis and power delivery help a lot, I don't even have sub frame connectors but I do have a 10pt cage, short torque arm and rod end lower control arms. Here's a 1.4's 60' nice and smooth and uneventful but quicker than it looks.

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