jimlab

I just saw an ad in a magazine the other day for Lunati "4/7" cams, which apparently switch the lobes of cylinders #4 and #7. All that's required, it said, was switching the plug wires for those two cylinders. The results (in the ad) said ~20 horsepower on a higher performance engine, and 5-10 horsepower on a lesser engine, with no other changes. Here's an article on Holley's web site which lists roughly the same information, but with different numbers...

http://www.holley.com/HiOctn/NewsInfo/NewsRels/l-36.htm

My question is, how is it possible to do this? Are the journals for the #4 and #7 rods at the same position so that it's irrelevant which cylinder is fired first?

Second, is it coincidence that this is "half" the difference in the change of the firing order made by GM for the LS1? I've occasionally wondered what the results of cutting a new crank and camshaft for an SBC/LT1 with the revised firing order of the LS1 would be, and it looks like there's some merit in going at least "half way".

SBC/LT1 firing order - 1 8 4 3 6 5 7 2
Lunati 4/7 firing order - 1 8 7 3 6 5 4 2
LS1 firing order - 1 8 7 2 6 5 4 3

Thoughts?

Eric Bryant

Yep - they both hit TDC at the same time.

I've got friends who have done this on their carbed big-blocks as it is indeed worth some extra power. I'd be real interested in seeing how it works on the LT1, as the lack of plenum volume should indeed favor moving the #7 event away from the #5 event. It might even go so far as to make the engine less-susceptable to split BLMs at idle.

OldSStroker

The "4/7" swap works with single plane (carb-type) manifolds, but not dual plane. I find it easier to visualize by drawing diagrams with 8 circles and arrows going from one cylider to the next in the firing order with firing degrees noted next to each cylinder. Connect the last back to the first for the complete picture.

You'll notice a few things:

you still get 2 adjacent cylinders on one bank firing 90 * apart.
(5-7 on SBC, 4-2 on "4/7 swap", and 3-1 on LS1. That should only affect 4-2-1 headers, not 4>1s)

you still have the center 4 cylinders firing consecutively and criss-crossing.

there are two extreme corner consecutive firings in the SBC and LS1 (1>8 & 7>2) although they occur at different times. 4/7 swap has only 1>8.

In the 4/7 swap, 4 goes from firing @ 180* to 540*, one revolution different. Same thing for 7 (540* to 180*) of course.

LS-1 doesn't have consecutive firing (and therefore induction) of 1 & 2, those nearest the plenum inlet, so perhaps they don't cause as much turbulance of incoming air which might disrupt the cylinders farther away. In fact, the LS1 has 7-8 at the end of the plenum consecutive. Maybe that might work on an LT-1 also.

I don't think you need a different crank, just different cam lobe orientation and timing and injector wiring swapping. AFAIK, cores for the LS1 firing order aren't readily available for SBC/LT1.

Part of the LS1 firing order was for less vibration for a smoother engine and therefore maybe a little more long term durability. My guess is the other reason was power. The 8100 big block has the LS1 order also.

SStrokerAce

I saw that add too, funny thing is Comp has had the 4/7 Swap cam core available for over a year now.

In fact I have one LT1 cam out there with it now, it's a carbed setup so it's basically a tradtional setup that 4/7's are used in. Never mentioned it on the boards, because nobody has brought it up, and it's not something I'm going to just throw out there unless you talk to me directly about it. You have to change a few things on the LT1 motor to get the 4/7 to work (other than the plug wires) the reason I've never done it on a LT1 is the ECU. Give me a aftermarket ECU and we are golden, the problem is that you are trading the 4 and 7 injector plugs across the motor and now the O2 sensors are reading and controlling 3 cylinders on one bank and 1 on the other, and that's bound to cause some problems if the **** hits the fan.

I think there are 8 different firing orders you can put on a V8, with the same crankshaft we use today. The LS1 firing order would be nice but the cam blank is going to run about 3 times the price of a 4/7 swap cam blank, my 4/7 swap cams are only billet and I would imagine that from Lunati also, and they cost slightly more also.

From what I have seen the 4/7 swap is worth at least 5-10hp and more depening on the application. This stuff is not that new to the world, Reher-Morrison pioneered this a LONG time ago, but it's finally a commerically available item.

Bret

jimlab

Well, let's put it this way... I have LS1 coil packs and instead of using the "generic" FAST eDist that I have, I could sell it and buy the LS1-specific version and fire the coils in LS1 firing order if I had a cam to match.

If I'm going to replace my cam with a smaller grind (which seems likely at this point), I might as well have a billet cam made with the LS1 firing order if there's a power benefit associated with it.

My only concern was the orientation of the rod journals on the crankshafts. I did not know that the LS1 crankshaft was the same as any other small block crank and that it was only the configuration of the cam and firing order of the coils that was changed, if that's what I understand you to be saying.

SStrokerAce

Well Jim,

Between these two threads it sounds like a possiblilty. A LS1 firing order on a LT1 is going to cost BIG money, even on your scale. The reason being is that the cam would have to be cut on a core with round lobes not precut and heat treated lobes. So the cam would have to be roughed in, then heat treated and then final ground with the lobe. Not sure on the cost but expect it to be around $900 for the finished camshaft. The 4/7 Cam is about $360 and it still needs special provisons to get it to work in a LT1 block, and it's going to cost you enough to make those changes.

Have you driven the 260/266 112 cam in your motor now? I would see how well you like or dislike that camshaft and then see where you want to go from there in terms of power band and driveablity.

Just giving you options and what kind of price scale you are looking at.

Bret

OldSStroker

They are the same. If a cylinder fires 360* in one firing order from what it does in another, the crank pin is in the same position in the cylinder just a rev earlier or later.

Here is the firing order comparison:

SBC is 18436572 vs LS1 is 18726543

Cyl #1 0*(SBC) vs. 0*(LS1)

Cyl# 2 630* vs. 270* (LS1) [630-270=360 diff.]

Cyl# 3 270* vs. 630* [360 diff]

Cyl# 4 180* vs. 540* [360 diff]

Cyl# 5 450* vs. 450*

Cyl # 6 360* vs. 360*

Cyl # 7 540* vs. 180* [360 diff]

Cyl # 8 90* vs. 90*

jimlab

What other changes are required? There's almost nothing LT1 about my engine but the block and the front cover now.

SStrokerAce

Well there are a few things and they gotta go too! Sent you a pic in your e-mail.

I figured this out a while ago, wait till someone else says what it is then I'll tell the tail in public. Maybe

Bret

IllusionalTA

Someone was reading my mind i seen it in Hotrod i believe.. And asked the same question.. Good info now spill!!!

engineermike

What's even funnier is that there is a local dirt-track engine builder that's been doing this for 3 - 4 years now.

Moreso than that is Ford did this on the 302's sometime in the late '80's. But the Ford cylinder numbering is screwed up, so you have to convert it over.

Mike

mastrdrver

I think it was Super Chevy or CHP, but one of them gave like 8 different firings that will work for the stock crank. One, which I don't remember, was said to give a large hp boost and all was needed was a cam swap.

I don't think those cams will work on the LSx or LTx engines bcuz of how the engine figures which cylinder is coming to TDC. Atleast that is my theory.

jimlab

All of this brings up the question of whether an LT1 with an LS1 firing order will still sound like an LT1...

SStrokerAce

Probably sound more like a LS1, isin't this more akin to a bear in the woods?

Bret

dnz28

I brought this up a while ago, Im going to attempt it on the lt1, I' ll give results but its going to be a while.