S1 3800 (L27) heads - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 01-25-2010, 11:55 PM   #11
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Unfortunately, I don't have very many options with the Series 1, as I can only port the heads that are available to me. Not that I'd want to put $6,000 into a chevy engine anyway. I have a V12 Jaguar XJS that will easily make 600hp to the wheels for that much money (twin turbo), and will be 10x better built than any American muscle car. You have to own one and work on one to know what its like. A real Jaguar, before Ford screwed it all up. There'* a good reason why a car that small weighs 4050lbs and the engine and trans are only 900lb of that.
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Old 02-12-2010, 11:50 AM   #12
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The S1 has a slightly better rod to Stroke (not piston) ratio stock. The distance from the piston pin to the top of the piston is higher on the S1 than the S2. So, the S2'* rod length isn't 1" shorter in lieu of the 1" shorter deck height. However, you could use a S2 piston in a S1 block with a custom rod to get a killer r/* ratio. That'* the advantage.
A high r/* ratio engine has very low piston speeds at low rpm and it has trouble maintaining velocity in average or larger x-sectional area intake runners. So, low rpm torque is reduced. The advantage is high rpm piston dwell time. This gives a more complete burn and more power at high rpm.

Now, one thing the S1 block does not have is cross bolted mains like the S2.. If you're making a high rpm screamer, which you would be if you were upping the r/* so much, than you would need to make a block girdle or similar to increase rigidity in the bottom end.

Research r/* ratio vs. overall power. You will find that it'* an ongoing argument that can go either way. A higher r/* ratio doesn't constitute more power in all cases. It'* more of a where do you want your power thing.

As far as the 3x00 heads go, aside from the iron heads, they all have splayed valves. Their intake runner roof is also much higher. These two things are why their heads flow so good right from the factory. Point being, ported 3x00 heads vs. ported 3x00 heads, we'll loose every time since we don't have those advantages. Not to say 3800 heads can't be ported to flow decent and make good power, it'* just that our ceiling is a bit lower when it comes to overall flow numbers.

IMO all you can really do with an L27 is make it a torquester. You can't get that high with the head flow and your intake runners are relatively long. So, I'd say get a cam that'* not high on the lift, has a good overlap, mill the heads to get at least a 9.5:1 CR, mill the LIM accordingly, port away (but follow the lead of S2 porting don't hog out the exhaust to a circle, keep a nice short side radius on the intake, etc.), and you'll end up with a car that probably has a great powerband up to 5k.

Also, there is no tuning deficiency with OBD1.5. It'* much faster than OBD1 and you can tune on the fly.
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Old 02-13-2010, 12:31 AM   #13
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Quote:
Originally Posted by 95naSTA View Post
The S1 has a slightly better rod to Stroke (not piston) ratio stock. The distance from the piston pin to the top of the piston is higher on the S1 than the S2. So, the S2'* rod length isn't 1" shorter in lieu of the 1" shorter deck height. However, you could use a S2 piston in a S1 block with a custom rod to get a killer r/* ratio. That'* the advantage.
A high r/* ratio engine has very low piston speeds at low rpm and it has trouble maintaining velocity in average or larger x-sectional area intake runners. So, low rpm torque is reduced. The advantage is high rpm piston dwell time. This gives a more complete burn and more power at high rpm.

Now, one thing the S1 block does not have is cross bolted mains like the S2.. If you're making a high rpm screamer, which you would be if you were upping the r/* so much, than you would need to make a block girdle or similar to increase rigidity in the bottom end.

Research r/* ratio vs. overall power. You will find that it'* an ongoing argument that can go either way. A higher r/* ratio doesn't constitute more power in all cases. It'* more of a where do you want your power thing.

As far as the 3x00 heads go, aside from the iron heads, they all have splayed valves. Their intake runner roof is also much higher. These two things are why their heads flow so good right from the factory. Point being, ported 3x00 heads vs. ported 3x00 heads, we'll loose every time since we don't have those advantages. Not to say 3800 heads can't be ported to flow decent and make good power, it'* just that our ceiling is a bit lower when it comes to overall flow numbers.

IMO all you can really do with an L27 is make it a torquester. You can't get that high with the head flow and your intake runners are relatively long. So, I'd say get a cam that'* not high on the lift, has a good overlap, mill the heads to get at least a 9.5:1 CR, mill the LIM accordingly, port away (but follow the lead of S2 porting don't hog out the exhaust to a circle, keep a nice short side radius on the intake, etc.), and you'll end up with a car that probably has a great powerband up to 5k.

Also, there is no tuning deficiency with OBD1.5. It'* much faster than OBD1 and you can tune on the fly.
Wow, this is some really good information.

Do you think there'* a lot to be gained with intake, exhaust, and 1.8 ratio rockers from porting these heads? I wish I had some series 2 heads to compare this all to, but what I'm mostly going for is better flow with the assumption that the heads are what restrict flow here.

Is there anything that can be done with the plastic insert inside the upper intake plenum on the L27? Or is it best left stock as is?

You're the first guy I've talked to who seems to know what he'* talking about, so all and any advice you can give me would be very much appreciated.

How does one go about tuning an OBD1.5 PCM? I'm using TunerPro RT to get ECM data just for monitoring purposes, and I've got a friend who says I can use some information from a 95 Series 2 PCM. I have to send him the memcal I got free from a guy on this board and he can get the bin from that (or so he said). This is going way over my head to be honest, so I'm just taking his steps one at a time. I've never tuned a car, though I can do just about anything mechanical that can possibly be thought of on a car.

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Old 02-13-2010, 01:37 AM   #14
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I've read that for the most part properly sized rockers are about 1/3 of the gain that a cam is capable of. With that in mind and since you're already going to have the heads off, I really think you should go with a cam.

You could do what one member here did a while back.. Have a stock cam reground, then have the heads milled the amount that was taken off the base circle of the cam. This would allow you to use the stock pushrods without the worry of too much lifter pre-load.
IIRC the regrind was around $50 from Delta cams. You should be able to have the heads milled and a normal 3 angle valve job done for under $150 or $200. If you're porting the heads, definitely get a valve job. You'll get more flow compared to lapping.

http://www.fullthrottlev6.com/forums...ad.php?t=56327

TONS of good info in there. Pay attention to talk of raising the roof of the intake runner, not lowering the floor, keeping a nice short side radius, and also how to increase flow in the area where the valve guide comes in.

I'm not a fan of D-shaping the exhaust port in 3800s though. It puts the port at an even more extreme angle to the exhaust primaries which will decease overall flow.

As far as the upper intake, seeing as how the L27'* intake runners are split in half on a diagonal where the LIM meets the UIM, I think you could ovalize the opening to each runner parallel to the runner in the bottom of the UIM effectively making the runner length shorter. I don't have a picture of the underside of a L27 UIM but if you've seen one this might make sense.
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Old 02-13-2010, 01:52 AM   #15
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After all of this is said and done, what kind of numbers at the crank should I be looking at?

I'm trying to stay away from a cam because of the inherent need to tune afterward, which is a whole new ballgame and cost scenario.



I know exactly what you mean in regard to the split on a diagonal. So you're saying I could oval out the opening to each runner on the plastic insert and thereby have a much shorter runner length.

What are the pros and cons of having a shorter runner length? Would I lose much low end power in pursuit of better flow at higher RPMs?

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Old 02-13-2010, 02:49 AM   #16
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I couldn't really tell you the numbers since I don't have any direct experience with camming a S1.

With headwork and 1.8'* you're going to change your engines volumetric efficiency enough to necessitate a tune.

If you're not up to the task of tuning yourself, all you need is scan data and www.gmtuners.com.

And yes, ovaling out the runners to make them shorter. In n/a applications, long narrow runners are for low rpm and short wide runners are for high. I would open it up an inch down the runner at first and then go from there.

It'* all about getting the most out of what you have. With a stock cam, you don't want to do much with the runners. If you go too short, you loose low rpm velocity and it doesn't get made up for in mid/higher rpms.
With more lift and duration, the engine would benefit from even shorter of a runner since it now breathes better mid throttle/up top.
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Old 02-13-2010, 03:09 AM   #17
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I couldn't really tell you the numbers since I don't have any direct experience with camming a S1.

With headwork and 1.8'* you're going to change your engines volumetric efficiency enough to necessitate a tune.

If you're not up to the task of tuning yourself, all you need is scan data and www.gmtuners.com.

And yes, ovaling out the runners to make them shorter. In n/a applications, long narrow runners are for low rpm and short wide runners are for high. I would open it up an inch down the runner at first and then go from there.

It'* all about getting the most out of what you have. With a stock cam, you don't want to do much with the runners. If you go too short, you loose low rpm velocity and it doesn't get made up for in mid/higher rpms.
With more lift and duration, the engine would benefit from even shorter of a runner since it now breathes better mid throttle/up top.
Sounds good. I'll try to mess with the intake tomorrow and see what I can get out of shortening those runners a bit.

Just to make sure we're referring to the same thing. If I remove the insert from the UIM and place it on top of the LIM, I'll have runners that go down into the LIM and take a relatively sharp turn toward the heads.

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The air is at one point going down, and then turns to go toward the heads. What you're saying is that I should remove about an inch of the inner wall of the runner of the insert, thereby shortening my runners by about an inch?

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Old 02-13-2010, 01:17 PM   #18
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Yep. Exactly.
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Old 02-13-2010, 01:24 PM   #19
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Yep. Exactly.
So I'll take it from your experience and your judgment based on my modifications that this will do more good in the mid and top end than harm in the low end, right?

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Old 02-13-2010, 02:42 PM   #20
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I would definitely bet on it and I don't gamble.

It'* essentially the same thing as a less dramatic HV3 on an L36. The HV3 takes about 3 inches or so off of the total intake runner length and really wakes the car up if you have a cam.

I suggested just making your L27 runners 1 inch shorter so that you are not making that drastic of a change.
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