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Effect of a 38mm restrictor on an LS7

Effect of a 38mm restrictor on an LS7

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C Lee Farquar

Original Poster:

1,843 posts

137 months

Saturday 15th September 2012
quotequote all
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.

stevieturbo

12,581 posts

168 months

Saturday 15th September 2012
quotequote all
Interesting question. Worth asking here too

http://efi101.com/forum/index.php

Using it in conjunction with a MAF will raise it's own issues I'm sure. But I'd think the closer to the throttle the better ? ie between the MAF and the engine. Ultimately the restrictor will limit rpm as well as power, 38mm is pretty damn small.

But you wont be able to mount it too close to the MAF as it will affect airflow through it, and fuel/spark tuning.

But will require a lot of dyno time to get set up properly no matter what you do. It wont simply be a matter of fitting the restrictor and leaving it at that.

Max_Torque

11,404 posts

138 months

Saturday 15th September 2012
quotequote all
Right:

1) do the regulation specify the position of the restrictor, and crucially, the geometery of the pipework upstream and downstream? if not, hurray, you can quite get a lot of air through a 38mm hole and with proper geo and get a decent pressure recovery downstream. Normally, the position is mandated to prevent people installing inlet systems that can help negate the restrictors pressure loss! (i.e in WRC, it has to be <50mm from the turbo compressor eye etc)

2) Make sure you feed the restrictor with the highest pressure air availible to you! A 38mm smooth edges restrictor will choke (the point at which the flow through it becomes just critical (supersonic) at 0.270kg/s (assuming 1bar, 15deg C upstream) and at this point the air pressure in the choke will be 52.8kPa (abs). At the critical point the downstream air (and shockwave) can no long propgate up stream (as the flow is now supersonic) so no matter how much you reduce the downstream presure you can only flow 0.270kg/s! But, increase the upstream airpressure, and the density increase will increase the mass flow. So gain say 5kPa (for 105kPa (abs) from dynamic ram in your airbox at high vehicle speed and you get 0.284kg/s ;-)

3) we know the conditions at the choke must be critical (52.8kPa (abs) and Mach1.0) so we need to slow the air back down in as efficient a manner as possible, "recovering" the dynamic pressure back to static pressure with as little loss as possible. This takes a expansion nozzle with an included angle something less that 15deg. Generally by limiting where you can put the restrictor, the regs aim to make it impossible to install a nice pressure recovery nozzle, and it can be difficult to get discharge co-efficients >90% in these cases.

4) a 7.0litre engine, sucking in air at 15degC, with 95% Man Vol Eff, and an inlet manifold pressure of 95kPa will be consuming that same 0.270kg/s we can flow through the restrictor at only 4275rpm. As engine speed rises above this point the plenum pressure will be throttled by the restrictor, and effectively WOT (Wide Open Throttle) will not actually be WOT ;-(
As a double whammy the engines overall pressure ratio will fall as rpm rises resulting in an increase of in-cylinder exhaust gas residuals as the charge scavanging efficiency falls, this will result in a higher end-gas temperature and may lead to detonation phenomina, requiring spark retard (and resulting in even less power output that directly as a result of the reduced airflow)



So what we need:

1) the best intake system & restrictor geometery possible:
- nice long straight, even and slowly tapering pipe downstream of the restrictor to the throttle.
- High static pressure upstream of the restrictor (smooth, large diamter, intake, with a MASSIVE entry diameter bellmouth feading into the smoothest precisely 37.999999999999999999999999mm dia hole you can make
- use the vehicles speed to increase upstream restrictor pressure above ambient

2) An engine that is as efficent at using air as possible
- high static compression ratio
- good detonation resistance
- Valve timing optimised to prevent charge contamination on the overlap
- makes peak power at as low rpm as possible to reduce Fmax (engine mech friction)
- has as low exhaust backpressure as possible (reduce effect of engine pressure ratio falling)
- lowest parastic friction as possible (lowest oil pump, water pump, camtrain & ancilary loads etc

I suggest a good long look at other race series restricted engines to see what has been done to optimise the powertrain for the restrictor!!

C Lee Farquar

Original Poster:

1,843 posts

137 months

Saturday 15th September 2012
quotequote all
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.


Max_Torque

11,404 posts

138 months

Saturday 15th September 2012
quotequote all
yup, thats a restrictor (nice wide bellmouthed entry, shortest allowable length at mandated diameter (including the requirement to machine the OD at that point to within 5mm of the mandated dia to avoid people cheating by building in bypass mechanisms (yes TTE, i'm looking at you here ;-), and a nice long shallow exit nozzle)


note the highly polished finish of this WRC version

http://imageshack.us/photo/my-images/6/s1turbo.jpg...





Edited by Max_Torque on Saturday 15th September 20:52

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stevieturbo

12,581 posts

168 months

Saturday 15th September 2012
quotequote all
In his first post he said he has to mount it before the MAF ?

Surely that isnt the best location ?

Toyota cheat ? never lol

C Lee Farquar

Original Poster:

1,843 posts

137 months

Sunday 16th September 2012
quotequote all
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.

stevieturbo

12,581 posts

168 months

Sunday 16th September 2012
quotequote all
C Lee Farquar said:
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.
I honestly dont know the answer to that question.

But I dont think the MAF placed in a vacuum is an ideal location, as would happen if the restrictor was placed before the MAF.

Max is far better placed than me to advise...if he can explain in simple terms tongue out

C Lee Farquar

Original Poster:

1,843 posts

137 months

Tuesday 18th September 2012
quotequote all
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.

Pumaracing

2,089 posts

128 months

Tuesday 18th September 2012
quotequote all
Running my own calcs I find to my surprise I agree with Max's numbers smile

An optimally designed 38mm restrictor should limit airflow to about 274 g/s and power to about 350 bhp and with an engine of that size it should be producing that much somewhere in the low to mid 4000s rpm. Above that point the internal frictional losses are going to be rising and flywheel bhp will probably start to drop. I doubt if it will be worth revving it past 5250.

It will of course, if allowed, still be worth tuning for best thermal efficiency with highest possible CR and cam profiles designed to work best in the limited rpm range indicated.

Well done though Max smile You can pat yourself on the head for me. You're really getting quite good at this sort of thing.

[Ducks and covers]

chuntington101

5,632 posts

157 months

Tuesday 18th September 2012
quotequote all
Would it be worth running a small engine and reving higher? Would this yeild more power / torque even with the restrictor?...

LS platform can go down to 4.8ltr.

stevieturbo

12,581 posts

168 months

Tuesday 18th September 2012
quotequote all
chuntington101 said:
Would it be worth running a small engine and reving higher? Would this yeild more power / torque even with the restrictor?...

LS platform can go down to 4.8ltr.
Bit extreme to build and change a new engine just for a limited number of restricted races.

C Lee Farquar

Original Poster:

1,843 posts

137 months

Tuesday 18th September 2012
quotequote all
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.


stevieturbo

12,581 posts

168 months

Tuesday 18th September 2012
quotequote all
C Lee Farquar said:
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.
Well, if placed between the throttle and the MAF, I guess yes it will. As far as the engine and most sensors are concerned the engine will just be running at part throttle ( even though the blade is fully open )

Will the tuning be optimal ? no.

Is there a chance of it throwing a fault code, as airflow isnt what it would expect for a given throttle opening/rpm/map etc....maybe

The only way you'll find out is to try it.

chuntington101

5,632 posts

157 months

Wednesday 19th September 2012
quotequote all
stevieturbo said:
Bit extreme to build and change a new engine just for a limited number of restricted races.
Sorry thought you had to run one for all races. my bad! smile

Pumaracing

2,089 posts

128 months

Wednesday 19th September 2012
quotequote all
C Lee Farquar said:
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.
The front end of the bellmouth isn't very critical. Just aim for a nice large radius into the 38mm section. The back end is more critical for optimum pressure recovery and usually 7 degrees a side (14 degrees included angle) works well enough. Have a look at a 38mm choke out of a 45mm DCOE carb sometime. The pictures already posted show you what to aim for.

The ecu should cope with the restrictor easily enough.

PS - I never realised William Shatner did a bit of mechanicing for Nascar on the side.

stevieturbo

12,581 posts

168 months

Wednesday 19th September 2012
quotequote all
Pumaracing said:
PS - I never realised William Shatner did a bit of mechanicing for Nascar on the side.
You should hear some of the albums he has made !! lol And this isnt even the worst.


http://www.youtube.com/watch?v=cKo4FMzt_hM

ian_uk1975

1,089 posts

123 months

Wednesday 19th September 2012
quotequote all
Pumaracing said:
PS - I never realised William Shatner did a bit of mechanicing for Nascar on the side.
I thought he was Mike Brewer from Wheeler Dealers at first!

C Lee Farquar

Original Poster:

1,843 posts

137 months

Thursday 20th September 2012
quotequote all
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!

Pumaracing

2,089 posts

128 months

Thursday 20th September 2012
quotequote all
You don't need anything like 100mm pipework though. There's no point whatsoever in the area of the pipe being 7 times that of the restrictor it's feeding. It can't possibly make anything flow more. 60/70 mm would be ample even allowing for a few bends and steps here and there that would be reducing the flow coefficient of a straight pipe.

What's probably more important is plenum volume and generally here the larger the better if there's a restrictor in the system. It gives the engine something to draw on which to an extent can help overcome the abrupt limit on flow that the restrictor sets. However I suspect this might be less important on an 8 cylinder engine which will have an almost continuous and smooth air demand compared to a 4 cylinder's more jagged air demand curve. Max will no doubt have an opinion on this worth listening to.

If you look at the restrictor setup on single seater race formulae engines there's usually something about the size of a small dustbin stuck on the end of the inlet manifold with the restrictor in the front of that.