Effect of a 38mm restrictor on an LS7

[C Lee Farquar]

I need to introduce a restrictor for some races that I do, prior to the MAF sensor.

From what I've read it's more at higher revs that it will take effect and wondered if someone more knowledgeable than me might estimate the point this would be. It's a 7 litre engine. I'm running the standard GM ECU. As I have ample power for what I do I considering raising the gearing to negate the effect.

Also any tips on optimising the location would be helpful, thanks.

[C Lee Farquar]

Thank you both, that's given me plenty to mull over.

After much searching I've found a picture (I think) of Robby Gordon's restrictor that he runs in his LS7 Hummer.

[C Lee Farquar]

Sorry Stevie, I did say that because I thought it was preferable to after the MAF, but my reading of the relevant regulations is that either side would be OK.

What would be the pros and cons of the MAF sensor before or after the restrictor?

I've registered on the forum you suggested.

[C Lee Farquar]

The exact wording of the regulations hasn't come out yet. It's a little complicated as it's French FFSA technical delegates trying to accommodate English cars within FIA regulations, some parts of which aren't applicable. But I next race in November.

My reading is that the Regs specify the restrictor must be between the air filter and the inlet manifold, possibly with a maximum 22 litre volume between restrictor and inlet manifold/cylinder head gasket. Any outside air scoop is limited to 10cm.

[C Lee Farquar]

Thank you all.

It is awkward in that I don't need the restrictor for UK events so it is not practical to undertake physical changes to the engine.

David, a quick look at the LS7 torque graph suggests it hits 350hp at just over 4000 rpm. I think I can live with that and short shift, possibly raise my gearing. A bit of a pain as I dropped it last winter.

Couple more things.

Any pointers on relative dimensions for the bellmouth, I'll be coming from 100mm?

Am I completely naive in thinking the standard GM ECU will tune itself to the restrictor? It has a MAP sensor as well as the MAF.

[C Lee Farquar]

Thanks again for your inputs

Are we all agreed that the MAP sensor should go before the restrictor?

On the basis of what Max & David have said the back end of the restrictor would be @ 240mm long if returning to an 96mm inlet pipe (with my school boy maths), which seems to coincide with what Mr Gordon/Shatner/Brewer settled on.

I'm going to have to compromise somewhere to get it in!

[C Lee Farquar]

Ok, that gives me plenty to go on.

I'll measure up the space available this weekend, scrutinise the regulations, and report back.

Many thanks

[C Lee Farquar]

The plan is that my navigator, who owns an aluminium fabrication shop, will do a CAD design to my/forum derived spec and then get a chap over the road to CNC machine it. It is, however, handy to have a back up plan!

I shall spend today working out how I can keep the inlet and outlet of the restrictor straight whilst turning 180 deg from the inlet manifold to air filter box.

[C Lee Farquar]

The engine is in the back and the inlet manifold reversed bringing the air inlet into the car. There is a double skinned roof allowing cold,reasonably clean, air intake.





The centre line of the filter and throttle body are 350mm apart. I could move the MAF into the airbox which would give me @180mm for a restrictor where the MAF is. But, there would be 90 deg bends either side.

[C Lee Farquar]

It would make fitment easier.

Ringram, you may recall a couple of years ago you kindly came up and changed the way my rev limiter works. It's much better now.

Will I need to change some settings in the ECU if I remove it? You picked up I most likely had a couple of wires off the MAF with your computer, which was correct. It seemed to run fine apart from a momentary hesitation at @ mid throttle.

[C Lee Farquar]

An update for you.

I have just done a race with a restrictor. It was done at short notice, the French firstly decided to postpone introduction until next year and then a week before the event changed their minds. They've also reduced it to 37.2mm.

Further to the advice on here, and with off forum help from Ringram, I removed the MAF and fitted a separate intake temp probe. Because of the short notice I ran with a simple bellmouth restriction and no exit cone.

It ran a bit flat and obviously wouldn't rev out. However the standard GM ECU soon adjusted itself and performance wasn't as bad as I was expecting. The engine management light didn't come on.

So, I now have until next April to sort out a more efficient arrangement.

Thanks to those who contributed, no doubt I'll be back with some more questions.

[C Lee Farquar]

Powering down the ECU, which is done each time I turn off the ignition, seemed to make no difference to the mapping. I didn't argue the point because as I don't hack the ECU so it's anecdotal. However I did manage to root my phone and flash the ROM this weekend so maybe I get brave with the ECU

To update the thread, I ordered and have just received a Raetech restrictor. The quality is top notch and looks like it just wants to flow air. I'm moving the air cleaner to optimise the positioning. I'm hoping to race in April but the car is currently in component form.

[C Lee Farquar]

I have a switch for the ignition and starting, everything else is turned off by a Cartek isolator. So unless I particularly want anything running on the car the power is cut to everything every time I stop.

I wired it from scratch so I know the ECU does not bypass the isolator.

[C Lee Farquar]

Regulation change for this year, restrictor size up to 42mm and 15 litres of air allowed between the throttle and restrictor. From 37.2mm this about 30% bigger area, and I believe a bigger percentage increase in flow?

I'd bought one of the Raetech restrictors pictured above at 37mm. There isn't enough meat on this to bore out so I have a local machinist scaling it up. I'm redoing the whole intake to take advantage of the 15 litres and to incorporate a better air cleaner. Separate thread on the air cleaner.

[C Lee Farquar]

I thought I'd update this thread.

The engine has been using a fair amount of oil, about a gallon for 120km of competition, 200km of liaison.

Last event it started running rough on the last day

Earlier in the year, 5.45 into video

Near the end, 36.44 and 41.49 into the video

My car sounds lazy compared to the others as most run five/six speed sequential boxes, I have a three speed auto TH350.

The oil light had come on about four times on a stage the previous night, I had underestimated the consumption.

To me it sounded like it was missing the next day.

It stopped as I crossed the line at the end of the final stage.

Engine is now out



Initial inspection shows one big end has spun and bore wear is up to about 6 thou.

Plan is to take the block in for machining early Jan, I'm hoping a rebore can be done at 10 thou.

There seems to be quite a bit of disparity of opinion as to how much the LS7 can be bored out, 10 thou pistons are readily available, anything bigger is thinner on the ground. I believe this is due to the liner thickness.

A new crank is just over £500, which would be OK. A new block is about £2500 which would be painful.

I've never had the OEM GM ECU tuned for the restrictor, but plan to do so if it will rebuild.

Some oil was being pulled through the air intake from the crank case. I'm sure the bore wear would be from a few years back when I had an ineffective air cleaner.

I'm not convinced the engine sounding rough was related to the big end spinning and would welcome opinions.

[C Lee Farquar]

stevieturbo said:

Clearly you need to establish why it is using so much oil.....and given a restrictor is in place, perhaps you did not re-configure the crankcase breather system and this is why it was ingesting so much oil ?

Has it done this since day one, or is this something that has got worse over time ?

At that amount of oil, there must be some indication of where it is getting used from ?

And a 6 thou out of shape bore...sounds incredibly bad. Unbelievably bad really.

If 10thou pistons are readily available and sitting on the shelf and for standard rod size, then you are probably ok, especially as it's n/a
And if you did need a new block, there are probably better options to consider than an LS7, certainly more affordable ones.
Given the restrictor, there is probably little point in overly worrying about the size of the engine.

Hi Stevie

I think the oil consumption is partly from the crankcase breather (this is connected near the restrictor) and partly the bore/ring wear. There are occasional wisps of blue smoke on some videos.

I believe the bore wear occurred just before I fitted the restrictor.

It used no oil when new.

The bore wear varies between cylinders rather than a 6 thou variation within a cylinder, The engineer who measured it couldn't be sure it will clear at 10 thou and would have preferred to go straight to 20.

I've considered other engines but if I stick with LS I need a dry sump, if I go much less power my car's too heavy to be competitive and I'd need a sequential box. I don't see any cheap solutions (other than an LS7 being far cheaper than the race engines used by other competitors).

I'm also trying to build a house so there's only so much time I have available. If I had the spare cash I'd buy another LS7 and sell my bits. But you are right, for what I'm doing you wouldn't start with an LS7.

[C Lee Farquar]

Boosted LS1 said:

Op what are your ring gaps? Your bore wear sounds huge, what are the bore measurements if you don't mind my asking?

I took the block, heads off and pistons out, in before Christmas and the engineer measured the bores in the back of my Land Rover. I didn't write them down, I brought the block back to strip the rest out and am due to take it back in the new year.

I haven't measured the ring gap, I figured I'd be in for new pistons and rings anyway. The pistons are forged, a limited edition run of engines from GM, and the anti slip coating has worn.

stevieturbo said:

If your crankcase breather was connected on the engine side of the restrictor, then it would pull in huge amounts of oil.. not good.

Even on the other side, for a performance application I would never route the breathers back into the inlet are at all, unless there is very good oil/air separators to make it impossible for oil to get into the intake.

The breather fed in between the air filter and restrictor, along with the dry sump breather. The pipe was also small bore which I now realise was more likely to draw oil droplets. I didn't notice any oil in the intake when I changed some pipwork earlier in the year but there was definitely some residue when I pulled the engine.

Originally I had both breathers going to their own small filters. I changed it as I was concerned about contamination of the filters when I steam cleaned the car. I see now I must keep them separate from the intake.

[C Lee Farquar]

227bhp said:

There is a third type which the OEMs use and for some reason aftermarket people rarely do, It's called a fume/oil or oil mist separator and is the most intelligent solution of the lot.

Thank you, I'll look at that.

stevieturbo said:

And presumably when you say dry sump....is this a proper dry sump, or the OEM LS7 semi dry/wet sump ?

I thought I might get that Yes it's the standard sump. This main issue is clearance to a drive shaft running underneath, sub 10mm.

Mignon said:

You're missing a number of important points.

Yes, I have little knowledge of engines but I'm trying not to jump to conclusions.

Mignon said:

So it's more than likely you've always had whatever bore wear is present since you started using it and it ran fine for ages.

It was a new crate engine from GM in 2006, it's never been used in any other application. I ran a particular air intake that was useless (in hindsight) The intake manifold looked like a mud hole.

Mignon said:

So give it a quick hone, stick some new rings in, regrind the crank, check the valves and seats. She'll be right cobber.

I respect your opinion, but can I just check you're not being playful?

GM do plus 20 rings but they are for the standard cast pistons. I assume my Mahle forged ones would need different ones?

The engine works well up to 5k, as was predicated by the sages above.

[C Lee Farquar]

stevieturbo said:

A quick hone, some new rings and bearings and that's mostly it. You can easily get rings in almost any size shape you might need.
If GM dont offer them, then Total Seal will and are easy to get hold of via Dave at Performance Unlimited.

I spoke to the technical chap from Total Seal yesterday at Autosport, he's on the stand with Performance Unlimited.

By coincidence he has an LS7 in a drag car which he has had honed.

He passed on his experiences and I felt I would be better sending my engine up to Dave to be evaluated and honed/bored or relined there.

As I don't know what I'm doing I feel I'm better off dealing with one person who can do the engineering and has the knowledge of the parts available.

[C Lee Farquar]

stevieturbo said:

Dave definitely has the knowledge and abilities....but it could end up expensive.

Which again goes back to what we said earlier about how fussy one should...or should not be....relative to budget.

The chap from Total Seal convinced me that honing them is not easy, you must use a torque plate and the right cutters (or whatever the term is) for honing. He said the liners are so thin that they can be stretched by the machining so it's not difficult to end up needing new liners or a block. He said that he drove to a machine shop 6 hours from his home to get the work done despite knowing of numerous engineering shops in between.

I don't have the facilities or ability to measure or the knowledge of what will work and what wont. I think it makes sense to go to someone who has the experience and knowledge of the parts available. I want to refit the engine, learn the lessons of the failures and get another 10 years use.

Thanks for your thoughts Phillip! I'm sure you're right. My car is also too heavy to be competitive with a smaller race engine, even with a clicky gearbox. A clicky gearbox on the LS would be nice is funds ever allow.