383 piston, 6" rod with stock sbc stroke?

[wheelman]

I'm planning the rebuild my LT1 this winter and have an idea I want to bounce off you guys.

 

I started looking for new rods as I want to build a bottom end that can handle higher rpm than the stock 95 LT1 and got to thinking it may be possible to build a long rod setup.

Here is what I'm thinking:

Get a set of "stroker" pistons meant to be used to build a 383, 3.75" stroke.

Get a set of 6" connecting rods

Run these parts with the stock crank that has a 3.48" stroke.

 

My calculations are as follows:

Stock stroke and rod length are 3.48" and 5.7" respectively, this gives a total length of 9.18"

383 stroke is 3.75 and rod length is 5.7", this gives a total length of 9.45"

Stock stroke of 3.48" and a rod length of 6" gives 9.48"

 

The combination I want to try is .030" longer than the 383 setup

The stock parts provide a deck clearance of .017 which means this setup is .013 above the deck.

A normal head gasket is .049" thick giving a piston to head clearance of .036"

 

A set of 5.875" rods could be used and give a total length of 9.355"

Using the 383 stroker pistons provides a deck clearance of .095"

 

So, what do you guys think of these numbers and this idea?

[jt1]

It will work fine if your deck height and head gasket choices give you the quench you desire. 0.035" is a bare minimum IMO.

 

However, the deck might not be square with the crank, and when you correct that you will lose some deck height. Also, depending on your head and cam choices you might wind up having to flycut the pistons to give adequate PV clearance. You also need to decide what fuel and compression you want to run, and figure in the deck height, piston cc, and head cc to get to the desired compression.

 

So, yes it can be made to work, but needs to be planned out carefully to avoid any unexpected consequences.

 

To me, a well planned build goes fuel>compression>head>cam>stroke>rod>piston.

 

jt

[Zfan1]

I think standard head gaskets are .039 If I am not mistaken, crushed of course. You can go from as little as .015 to .054 I believe just so long as your quench stays above .036. I personally like to keep it .040 at a minimum. I like to play it safe! On a 383 6" rod set up if you plan on spraying it with a lot of juice, you may want to keep a 5.7 rod and not 6.0 rod, the pistons on a 6" rod are more prone to fail vs a 5.7 piston. The 5.7 piston has a lot more meat around the wrist pins.

[Michael]

Check the arithmetic. To calculate installed-height, take half of the crankshaft's stroke, add the connecting rod length, and distance between the centerline of the piston's wrist pin and the piston's crown. Your approach might work, depending on the piston geometry.

[wheelman]

Michael,

You're correct, my math is wrong, it turns out I have to use pistons with a compression height of 1.25" or 1.26".

They're easily available so I'm thinking I'll go this route.

I'm going to replace the stock powered metal rods anyway and go over size on the bore so might as well get a better rod/stroke ratio while I have it apart.

 

ZFan1,

This is not a 383 build and will never see "juice", strictly NA, so I'm not worried about the piston strength. I'm shooting more for higher rpm reliability than anything.

With my curent gearing setup redline (6300 rpm) is at ~68mph in 2nd gear. I want to be able to run a little over 70mph without requiring a shift to 3rd, so redline needs to be >6500rpm.

I already have high rev valve springs but don't trust the stock rods to handle 6500+ rpm very often.

 

This is an auto-x car, thats why I'm trying to optimize the redline for 2nd gear operations. I normally shift to 2nd shortly after launch and stay in that gear for an entire run. Going to 3rd is an option but costs time and I'm not good at downshifts so reliably increasing the redline a bit is the goal.

[RebekahsZ]

With my curent gearing setup redline (6300 rpm) is at ~68mph in 2nd gear. I want to be able to run a little over 70mph without requiring a shift to 3rd, so redline needs to be >6500rpm.

I already have high rev valve springs but don't trust the stock rods to handle 6500+ rpm very often.

 

This is an auto-x car, thats why I'm trying to optimize the redline for 2nd gear operations. I normally shift to 2nd shortly after launch and stay in that gear for an entire run. Going to 3rd is an option but costs time and I'm not good at downshifts so reliably increasing the redline a bit is the goal.

 

Would you consider going to taller wheel/tire combo to get more speed with same engine RPM?

[wheelman]

No, don't want to go to larger tires, the price for race tires rises pretty quickly as they get larger, besides I like the rims I currently run.

[rsicard]

Wheelman:

 

With 6" connecting rods, pistons with 1.125" compression height are needed. Divide 3.75" stroke by 1/2, then add the connecting rod length than add the 1.125 compression height and it should come out 9.00". The block deck height is 9.025". This leaves the piston .025" in the hole which is undesirable. The block should be milled to deck height or .005" in the hole or flush with the block deck. I used SRP professional pistons which are forged, forged rods and forged 3.75" stroke crank. With all this there is little expansion of reciprocating components such that fairly tight clearances can be used between the piston top and cylinder head face. The closer the clearance the more resistance to detonation. To further keep detonation to a minimum, ceramic coat the piston crown, cylinder head combustion space, valve faces and exhaust port wall common with intake port.

 

Use high quality AFR-195 cylinder heads as these flow the best on the market. Then top it off with a good cam, lifters and pushrods.

[wheelman]

rsicard,

I guess I shouldn't have said anything about 383 in the subject line of the thread, it implies thats what I'm building.

I plan to use the stock crank (3.48" stroke), with a piston compression height of 1.26" the deck clearance is .025" if I don't mill the deck or I can take .020 off and have a .005" clearance.

Does that sound right to you?

 

I'm planning to stick with the heads I have (Iron LT1) and am currently running an LT4 hot cam with stock stamped rockers.

The hp peaks at ~330 @5800rpm but hardly drops above that all the way to redline. I'm thinking its still above 300hp @6300rpm as it is and I plan to add 1.6:1 roller rockers.

[rsicard]

Wheelman:

 

As long as you are building up the engine, spending a small amount ($200.00) for a stroker crank yields more cubic inches. With 6" rods pistons with 1.25" compression height can be used. I picked SRP Professional pistons with this spec and they are lightest weight on the market. Admittedly they are expensive but with forged rods you will have a bullet proof bottom end. Some grinding must be done to the bottom of the cylinder bores and pan rails to provide clearance for the stroked crank. Scat has good forged rods for $300.00 and a cast crank for $200.00. If you are doing the rebuild, do it RIGHT. Then with lightweight pistons the rotating assembly can be neutrally balanced. The front balancer and flywheel/flexplate both can be neutrally balanced. With a LT-1 that factory compression ratio is 10.0:1. The pistons and cylinder heads can still be coated with ceramic to avoid detonation. The LT-1 is unique in that the cylinder heads get cooling fluid FIRST then the block gets cooling fluid second. This in and of itself allows running 87 octane fuel with 10.0:1 static compression without detonation. Decking the block such that pistons are flush or .005 in the hole boosts the compression ratio providing more power and it still may allow running 87 octane. Ceramic coating adds insurance against detonation. Pay close attention to my previous post as it is TOTALLY ACCURATE based on EXPERIENCE.

[wheelman]

A short update.

I took the car to a local speed shop for a dyno day event, figured I'd get a baseline before the rebuild.

 

The shop has a Dyno Dynamics unit, not sure how this compares to other brands but thought I'd add it so people know.

 

The car put down 295 hp @ ~5000 rpm and 360 ft/lbs @ ~3300 rpm and was still making 250 hp and 200 ft/lbs at the rev limiter (6300 rpm).

 

The engine is an LT1 from a retired 1995 Caprice cop car.

I replaced the cam with an LT4 Hot Cam and the stock MAF with a larger one from a Camaro, it has a mail order tune on the stock PCM but I've "adjusted" it a little to accomodate the MAF.

The fuel was nasty 87 octane pump gas.

 

Based on these results I may go a different direction with the rebuild.

Rather than attempting to increase the rpms I may just do a good quality build keeping the basic power curve as it is and learn to shift to 3rd gear on the higher speed auto-x courses.

[rsicard]

Wheelman:

 

If you are going to do a rebuild, it does not cost that much more to go to a 383 stroker. A $200.00 stroker crank, con rods and pistons and machine work and you have a 383 stroker. More displacement at a reasonable cost. More horsepower and Torque at a reasonable cost. Then the proper cylinder heads and camshaft make more HP + TQ.

[Michael]

If your engine already produces this level of performance, then perhaps a rebuild is not justified? You will be spending lots of money and time to achieve another 100 hp. While that may sound tantalizing, the investment to achieve this increment of performance will be far higher than your initial investment in doing the original swap and engine hop-up. In other words, it's the law of diminishing returns.

 

If you yearn to achieve an altogether higher level of performance, would it not be worth considering an entirely new engine, either with many more cubic inches or from a different generation of engines, while leaving your current engine in operable condition?

[wheelman]

rsicard,

I hear what you're saying but I simply don't want to do a 383, plus to do it RIGHT there are more factors to consider than just the crank, pistons, con rods and machine work. The larger displacement will flow more air requiring larger injectors, a different cam, a new PCM tune, throttle body upgrade, possible port work, yada, yada, yada. Remember the goal is to increase the usable safe rpm range not build more low end torque.

 

Michael,

The rebuild is due, the engine is from a retired Montana State Highway patrol car and has over 105k hard miles and I'm sure many long hours at idle on it.

I didn't rebuild it when I did the swap, just replaced the cam and valve springs then dropped it into the car intending to do a rebuild within a year, that was 5 years ago.

There are times when the oil pressure is lower than I'm comfortable with and I get a noticable puff of oil smoke when I close the throttle after a high rpm run.

I'm not lookig for an additional 100hp, just the ability to rev a little higher without risking the bottom end or a sucked valve.

 

Plans at the moment are to bore it .040" over, replace the 1.5:1 stamped rockers with 1.6:1 rollers and maybe move from iron heads to aluminum (mostly for weight savings) and go with 6" rods and appropriate pistons.

If I retain the iron heads I'll probably have 2.02" intake and 1.6" exhaust valves installed.

No boost or nitrous!!!

 

 

Reading back through this thread I noticed I quoted different power output levels.

The first set I gave were at the flywheel calculated based on a dyno session from several years ago and assuming 15% drivetrain loss, the second set are at the wheel levels from a different dyno. If I calculate flywheel hp assuming 15% loss from the current dyno it is 339 which is pretty close to the older level. I'm also suspicious of the rpm numbers from the most current dyno run, I don't remember seeing any cables hooked to the engine to directly read rpm so I think they ran the rollers at a given speed then looked at my tach and entered a conversion. This would produce wrong rpm numbers as my tach is not accurate. When I compare the shape of curve from first run from several years ago to those from yeaterday they are almost identical except for the rpm scale. I know the older session had accurate rpm numbers.

 

Anyway, I have a decent idea of the current output level that can be used as a baseline for comparison after the rebuild.

Edited October 30, 2011 by wheelman

[rsicard]

Wheelman:

 

Usable safe RPM range? Don't quite understand what the goals or budget is. What is the goal with the 6" rods? Weight should not be the only consideration for going to aluminum cylinder heads. What are the thoughts concerning these items? If you want to keep the same tune and injectors then go with 6" rods and appropriate pistons. This along with reworked LT-1 heads will the the economical way to go. Suggest ceramic coating the piston crowns, cylinder head chamber, faces of valves and exhaust ports. This will only yield a modest amount of HP and TQ. If serious about HP and TQ then stroker and HP heads need to be considered.

[wheelman]

Wheelman:

 

Usable safe RPM range? Don't quite understand what the goals or budget is. What is the goal with the 6" rods? Weight should not be the only consideration for going to aluminum cylinder heads. What are the thoughts concerning these items? If you want to keep the same tune and injectors then go with 6" rods and appropriate pistons. This along with reworked LT-1 heads will the the economical way to go. Suggest ceramic coating the piston crowns, cylinder head chamber, faces of valves and exhaust ports. This will only yield a modest amount of HP and TQ. If serious about HP and TQ then stroker and HP heads need to be considered.

Yeah, this thread does sound a bit dis-jointed doesn't it, guess thats what happens when I think out loud and post it online for you guys to see.

The main goal is to increase the reliability of the engine and at the same time moderately increase the power output, on the order of 30 - 50hp. If the increase is larger than that I'll be happy.

I define a usable safe rpm range to be 2500 to 6500 rpm.

The budget I've given myself is $2500.00 or less for all parts and machine work. This isn't a hard number and I haven't done much research yet to determine how realistic it is. Things such as big valves and aluminum heads would be the first items to be cut if they aren't possible within the budget.

The deal with 6" rods is improvement in the rod/stroke ratio. A better ratio makes the engine more resistant to detonation at higher compression, at least thats what I've read. Considering I plan to replace the rods and pistons anyway I figured I could improve this aspect of the engine with very little if any additional cost.

Weight is not the only consideration with aluminum heads but running them would reduce the weight on the front of the car by ~80lbs which theoretically should improve weight distribution and handling. This has not been "proven" but is generally accepted on this and many other car related forums. This is an auto-x car so handling is a very important consideration, if I can stay within budget and meet the other goals using aluminum heads it makes sense to use them.

I'm not looking for a huge increase in power but if I can pick up some with a small additional cost above a standard rebuild I'll make those changes. Your suggestions of re-worked LT1 heads, coated pistons and chambers seem to fit with this goal.

 

Some additional info:

The following items have already been replaced

1. Timing chain and gears. ($100.00)

2. High Rev Valve spings ($300.00)

3. Cam (LT4 Hot Cam, $230.00)

4. Water pump ($100.00)

 

This is approximately $730.00 I don't have to spend during this rebuild, it's already been spent.

Edited October 31, 2011 by wheelman

[Michael]

Your ideas are persuasive, and certainly you have a reasonable plan. The point is not to disparage the concept of refreshing a tired engine, but rather, that your engine is probably not all that tired. How can one be sure that even a freshly-built "performance" engine won't exude a rainbow of colors from its exhaust system, burn oil, backfire or foul plugs?

 

My point is that either by luck or by skill, you've already hit upon a very elusive animal: a relatively inexpensive engine with a proven track-record, a modest set of modifications, and good all-around performance. A full rebuild would compromise many of these advantages and would return you to an earlier stage, with more uncertainties, more risks, more cost and possibly only incremental gain. So would it not be preferable to limp along with your current engine, performing sound maintenance but eschewing tantalizing new upgrades, until finally the old girl can breathe no more?

 

If you insist on yielding to the siren song of progress, pick up a second engine (junkyard or whatnot) and treat that one as your research project, while retaining your current engine as back-up. A bird in the hand....

[cockerstar]

Ken,

 

Where are you getting the machine work done? Where did you run it on the dyno?

 

Does your budget include the $730 you've already spent, or is the $2500 additional to that?

How well can the stock EMS play with things like a compression bump and aftermarket heads/cams?

Edited November 2, 2011 by cockerstar

[wheelman]

Your ideas are persuasive, and certainly you have a reasonable plan. The point is not to disparage the concept of refreshing a tired engine, but rather, that your engine is probably not all that tired. How can one be sure that even a freshly-built "performance" engine won't exude a rainbow of colors from its exhaust system, burn oil, backfire or foul plugs?

 

My point is that either by luck or by skill, you've already hit upon a very elusive animal: a relatively inexpensive engine with a proven track-record, a modest set of modifications, and good all-around performance. A full rebuild would compromise many of these advantages and would return you to an earlier stage, with more uncertainties, more risks, more cost and possibly only incremental gain. So would it not be preferable to limp along with your current engine, performing sound maintenance but eschewing tantalizing new upgrades, until finally the old girl can breathe no more?

 

If you insist on yielding to the siren song of progress, pick up a second engine (junkyard or whatnot) and treat that one as your research project, while retaining your current engine as back-up. A bird in the hand....

Michael,

You make a very persuasive argument yourself and it is very close to my original plans. I was going to replace the valve stem seals (most likely source of the oil smoke after a high rev run), install 1.6:1 roller rockers and call it good. This last summer I had 2 instances of very low oil pressure while waiting in grid at auto-x events that caused me to change my mind to do a complete rebuild. I haven't pulled the engine yet so I'll consider your view point before moving forward.

 

Ken,

 

Where are you getting the machine work done? Where did you run it on the dyno?

 

Does your budget include the $730 you've already spent, or is the $2500 additional to that?

How well can the stock EMS play with things like a compression bump and aftermarket heads/cams?

CockerStar,

I'll either have S&B Machine or Poynor Machine do the machine work. I prefer Poynor but it's pretty much a one man operation and he takes a long time to get things done but does a good job.

The first dyno run was a place in Yakima, I don't know the name. The latest was at Kaizen Speed during a recent Dyno Day, they were trying to run almost 30 cars in one day so cut a few corners to push the cars through.

The $2500.00 is all new money I have to spend, I just listed the $730.00 as money for parts that won't count against this budget.

The PCM/ECU (EMS as you call it) is fully programmable and I have the software to make changes myself.

The changes I'm planning to make are not very extensive so the tune should be pretty close already and the firmware has the ability to "learn" the new setup over time.

[cockerstar]

Doing a compression and leakdown test will tell us a lot about the condition the motor is in right now, and give you a good idea of what needs freshening. If the bottom end is solid still, $2500 could get you a very respectable set of heads. The low oil pressure probably doesn't indicate a solid bottom end though.

I don't have leakdown gauges, but I do have a compression tester (it's a cheap one, but it gets the job done) if you'd like to borrow it.

 

I don't know if you've ever gone to S&B before, but all of my experiences with them have been negative. Their prices are high, and the guy that is typically working (Ryan) tends to be very condescending. Maybe it's because I'm probably one of their younger customers, but my friends have had similar responses from him.

We've all switched over to Auto Machine in Pasco (on Lewis St.) and have been very happy.

I don't have any experience with Poynor Machine, but my friend speaks ill of them, mainly because of their prices.

 

You're running a set of block-huggers right now, right?

 

You're car is already a beast at 300hp! 50 more to the wheels will be quite a blast

Edited November 6, 2011 by cockerstar