Jump to content

3.5 vs 3.9


Bracco
 Share

Recommended Posts

For the longest time it has been my intention to replace my tired 3.5 V8 with a 3.9. My understanding is that they are very similar and that the principal difference between the two , aside from the increase displacement, is that the 3.9 has 4 bolt mains so the bottom end is a bit better. Unfortunately, the 3.9's also have issues with porosity that can only be resolved/prevented by using a seasoned block with top hat liners. I am now in the process of actually looking for a motor and putting together a parts list so that I can begin planning the rebuild that will be done at a leisurely pace and will end in the fall (need to have it completed and installed before winter and low temperatures arrive).

Many of the mechanics and shops I have been in contact with have recommended against the use of the 3.9 as their experiences have been that the 3.5 is more reliable over the long term. Now I'm having second thoughts... should I just rebuild a 3.5? Is there any benefit in using heads from 3.9? I use my 110 3.5 for daily commute to work, taking the dogs hunting, camping, greenlaning and playing in deep mud and snow when the occasion presents itself. I don't intend to build a 300 BHP fire breathing monster, but I would like to give her a bit more than the tired 3.5 has. Any advice, thoughts or personal experiences are greatly appreciated.

Link to comment
Share on other sites

There are some who say the 3.9's are unreliable, some who say they're one of the best versions on the Rover V8 you can have. All rover 8's like to be cared for - regular oil & filter changes and good plugs etc. keeps them happy.

Mechanics will always tell you what goes wrong with anything, sometimes that's handy to know but sometimes it's mis-representative folklore - I was told by a mate that all R380's are terrible because a mate of his rebuilds gearboxes and had hundreds of them in when they first came out, however talking to Ashcrofts it transpired that there was a fault with the early ones that got them the bad rep and hence the mechanic's folklore that "all R380's are carp" :rolleyes:

The extra displacement means extra low-down torque, my 3.9 certainly went well enough and if I had any pennies spare I'd have one in the RR to replace the 3.5.

No point swapping heads, they're the same. The EFi systems are interchangable as a complete system too (one will run the other but many individual bits can't be swapped)

You can get some useful gains from the 3.5 but in a 110 I'd say the extra low-down grunt would be handy, you need torque not big BHP.

Link to comment
Share on other sites

How about a 4.2? This unit was created originally by RV8 God John Eales and Land Rover liked it so much they bought them in to use in the RRC LSE. However, in the end Land Rover stopped using them due to the cost (a result of the high quality internals used by Eales).

Just one other point, the 3.9 doesn't have a crossbolted bottom end - its based on a 3.5 block. The later '4.0' used in the P38 and later discos (which is actually the same dissplacement as the 3.9) is a redesigned block and is cross bolted. The 4.6 is based on the 4.0 block. The cross bolted bottom end is nice to have but not essential; 'Hybrid from Hell' runs a 300 bhp 4.5l John Eales engine based on a 3.5 block.

Link to comment
Share on other sites

and 4.6 more commonly suffer from all sorts of serious block issues.

Then again the are 1,000s of good ones, but the ratio is worse than 3.5 and 3.9s as fridge said.

3.9 Hotwire is definately a good compromise, the Hotwire system even though still neanderthal is a huge improvement over the flapper system, and with a few upgrades (decent timing gears chain, cam tweak, comp head gaskets vs tin etc) makes for a strong unit

Nige

Link to comment
Share on other sites

My nas has a 3.9 V8 and just gone over the 100,000 mile with just normal servicing. As with my previous V8's they get an oil and filter change every 6000 miles and new NGK plugs every 12,000 miles. It's like any mechanical machine, look after it and it will perform!

Link to comment
Share on other sites

Hmmm

I am in the process of building a 3.5 with a 4.2 crank to gain more torque.

I am only going down this route as I have the bit lying round the garage, and want to see how it turns out.

I have the same concerns about the reliability of the 3.9 engine as well.

Will let you know how it tuns out

Link to comment
Share on other sites

  • 2 weeks later...

Gidday gents,

Hope you don't mind an opinion from down under.

I ran the standard carb 3.5 for five years in an off road only RR based buggy and through it all, four hours with no radiator (some idiot put the fan through it..... me) and hitting 4500 revs with the oil filter sitting on the ground, it kept going...

But, I have just swapped in a 90,000 mile serpentine 3.9 into the truck and what a huge difference. It runs a long snorkel which may be causing a slight lag when the accelerator is punched, but in every other respect it is head and shoulders above the 3.5. Lots of oomph and power (though in a light ruck) and runs on any angle (don't ask how I know).

Another solution that is more common in New Zealand and Australia is the rover based Leyland P76 4.4l motor. Basically a 3.9 with a longer stroke so it is "square" i.e. revs and torque. I have seen them with the standard twin cabs and with single Holleys or similar. Mine runs a single carb of a Holden 253cu V8 with petrol and LPG.

Go for the 3.9 if you can afford it, it's already been said anything mechanical will go wrong if it's not looked after, not coz it's a 3.9.

Link to comment
Share on other sites

Go for the 3.9 or a 4.6 if you can find one.

I've never heard of any more issues with the 3.9 over the 3.5 but then they are essentially the same engine.

In either case I'd look at a new cam (and lifters!!!!) and maybe some better heads, it will wake up the motor. A nice intake and exhaust system will compliment it also.

A pricey option is to stroke your 3.5 with a new crank and associated parts. The stroker kits take them out to 4.3 litres the 3.9 can be taken out to 4.8 litre however.

Remember displacement = torque

To coin the American phrase " there's no replacement for displacement"

While this maynot be true in race engines for off roaders it's sound advice.

If you want advice on Rover V8's check this forum out: http://www.v-8.org.uk/forum/index.php

For parts these guys seem to offer some good prices (espcially on heads): http://www.v8developments.co.uk/

RPI and Rimmer Bros also do Rover V8 parts but IMO are WAY to expensive.

A company called Reel Steel certainly used to do a lot of Rover V8 bits but I don't think they have a website, they always used to advertise in magazines like Custom Car and Hot Rod Magazine.

Lastly remember most cam makers like Piper will be able to supply a new cam for you.

Link to comment
Share on other sites

Real Steel are good lads - but their catalogue makes for very dangerous reading, lock up your credit card! :ph34r:

RPi and Rimmer are indeed overpriced, Rimmer seem to be basically a V8-specific parts shop (rather than actually interested) and RPi are a piston short of a block :wacko: and still cling to hateful clockwork contraptions like 4-brl carbs and mallory dizzies when the rest of the world has joined the 90's with EFI & dizzyless ignition.

Link to comment
Share on other sites

Real Steel are good lads - but their catalogue makes for very dangerous reading, lock up your credit card! :ph34r:

RPi and Rimmer are indeed overpriced, Rimmer seem to be basically a V8-specific parts shop (rather than actually interested) and RPi are a piston short of a block :wacko: and still cling to hateful clockwork contraptions like 4-brl carbs and mallory dizzies when the rest of the world has joined the 90's with EFI & dizzyless ignition.

I'd disagree about the 4 barrel carb I run a Weber 500 and Offy on my TR7 V8, which proved a marked increase in performance over the twin SU's. It appears to be reliable, easy to start and runs smooth even when cold. Only bugger was getting enough fuel to feed it at WOT. Having said that the car still tunrs out 24MPG as an overall average with mixed driving (much of it wheel spinning and flat out).

Link to comment
Share on other sites

Not saying 4-brls are bad, but on the 109 I went from Holley 390 (unreliable off-road) to Weber 500 (much quicker but still unreliable off-road and hated side-slopes) to Lucas EFi (much more drivable than the carbs and didn't care about angles) to MSEFi (bestest) and just about to finish adding EDIS to it to ditch the clockwork of the distributor.

A carb is a mechanical computer, and you won't find many mechanical Pentiums in PC world :ph34r:

Link to comment
Share on other sites

Not saying 4-brls are bad, but on the 109 I went from Holley 390 (unreliable off-road) to Weber 500 (much quicker but still unreliable off-road and hated side-slopes) to Lucas EFi (much more drivable than the carbs and didn't care about angles) to MSEFi (bestest) and just about to finish adding EDIS to it to ditch the clockwork of the distributor.

A carb is a mechanical computer, and you won't find many mechanical Pentiums in PC world :ph34r:

I agree for off roading Strombergs or SU's or EFI is the answer.

Link to comment
Share on other sites

Go for the 3.9 or a 4.6 if you can find one.

I've never heard of any more issues with the 3.9 over the 3.5 but then they are essentially the same engine.

Not so..............

The 3.9, & 4.6 are essentially the same engine as they share the same bore size and the same liner dimensions, but the 4.6 has 10.9mm extra stoke. In theory the 3.9 is just as likely to suffer cracking behind the liner as the 4.6 ........................ the 3.5 is a totally different beast in this respect as the liners are much thicker due to the smaller bore size.

There are unproven reasons why the 4.6 seems to suffer this problem more than 3.9's ............... however, I can assure that 3.9's /4.0 /4.2's have all been known to exhibit the same issues.

Remember displacement = torque

To coin the American phrase " there's no replacement for displacement"

While this maynot be true in race engines for off roaders it's sound advice.

True to some extent but not the holy grail that some believe it is...............

Carefully compare the max torque together with the torque curves of both the 3.9 & the 4.6 ............. you will note that they are not a million miles removed from each other. Now to flip the coin ..................... the P38 was produced in quite large numbers and are still of an age to make them a reasonable investment for some folk ................ the classic 3.9 in lots of cases is nearing the end of its life (same applies to the 3.9 V8 disco) .............. therefore when a 4.6 sufferers from the dreaded block cracking a P38 owner is more likely to look for a s/hand replacement engine............. this has forced up the prices of known good 4.6 s/hand engines to a level that is now £1000 to £1200 ................ most breakers can sell a 4.6 before they even get the vehicle in.

Apply that sort of money(or less) to a moon mileage 3.9 and with a little know how it can be made to easily exceed the performance of a standard or mediocre tuned 4.6 ......... also you can place the power band exactly where you want it................

The quote you are looking for is from the American V8 racing icon Carrol Shelby .......... "There's no substitute for cubic inches"

In times gone by this may well have been true ............... but modern engineering has now proven that cubic capacity is not always the ideal answer.

If you want advice on Rover V8's check this forum out: http://www.v-8.org.uk/forum/index.php

The info within that forum is no different to what is available here .....................

I'd disagree about the 4 barrel carb I run a Weber 500 and Offy on my TR7 V8, which proved a marked increase in performance over the twin SU's. It appears to be reliable, easy to start and runs smooth even when cold. Only bugger was getting enough fuel to feed it at WOT. Having said that the car still tunrs out 24MPG as an overall average with mixed driving (much of it wheel spinning and flat out).

Well............... if you are happy to have a linear fuel supply to the engine over the entire rev range then those carbs are right up your street.

Most engines, especially those that are tuned, require vastly different fueling dependant upon engine information ................. this is where EFI scores heavily over the mechanical carb ................ EFI on your TR7 would no doubt be worth some extra bhp if set up correctly.............. and the engine would run better into the bargain.

Yes ............. carbs and off road angles are not an ideal mix.

:)

Ian

Link to comment
Share on other sites

A popular (but not proven) theory for the 4.6's cracking was the very lean fuel map they ran to get past emissions laws for the P38, causing high combustion temperatures under load and, well, problems :P

I'm hoping with decent fuelling my 4.6 will be as reliable as any other V8, and it won't be allowed to run lean especially under load.

Link to comment
Share on other sites

Not so..............

The 3.9, & 4.6 are essentially the same engine as they share the same bore size and the same liner dimensions, but the 4.6 has 10.9mm extra stoke. In theory the 3.9 is just as likely to suffer cracking behind the liner as the 4.6 ........................ the 3.5 is a totally different beast in this respect as the liners are much thicker due to the smaller bore size.

There are unproven reasons why the 4.6 seems to suffer this problem more than 3.9's ............... however, I can assure that 3.9's /4.0 /4.2's have all been known to exhibit the same issues.

I think you and many others are WAY over exaggerating any such issues. Really how many Rover V8’s have been produced in the last 40+ years and how many really have issues which aren’t related to poor maintenance?

There are many examples of 4.5, 4.8, 4.9, 5.0, 5.2, 5.3 and even a few 6.0 Rover V8’s out there which have had no problems at all.

True to some extent but not the holy grail that some believe it is...............

Carefully compare the max torque together with the torque curves of both the 3.9 & the 4.6 ............. you will note that they are not a million miles removed from each other.

I don’t have any graphs to hand but PEAK numbers are massively different and I suspect that a 4.6 makes significant amounts more torque and power across the entire rpm range.

Do you have torque figures for both engines say at 1800rpm?

Now to flip the coin ..................... the P38 was produced in quite large numbers and are still of an age to make them a reasonable investment for some folk ................ the classic 3.9 in lots of cases is nearing the end of its life (same applies to the 3.9 V8 disco) .............. therefore when a 4.6 sufferers from the dreaded block cracking a P38 owner is more likely to look for a s/hand replacement engine............. this has forced up the prices of known good 4.6 s/hand engines to a level that is now £1000 to £1200 ................ most breakers can sell a 4.6 before they even get the vehicle in.

Maybe, but you make it sound as if every 1 in 2 P38’s have to have a replacement engine. I suspect the price is strong on them because just as many people want them for engine conversions as opposed to replacing one in a P38.

Remember the Rover is the mainstay performance V8 in the UK as used in countless vehicles, most not even BL/Rover related.

Apply that sort of money(or less) to a moon mileage 3.9 and with a little know how it can be made to easily exceed the performance of a standard or mediocre tuned 4.6

Performance as in PEAK HP yes but unless you are using a power added I seriously don’t believe you can substantially beat the below the graph line torque figures.

HP = torque x rpm /5252

To make HP you either need to make more torque, the same torque at higher rpms or both. Increasing the torque figure is not easy with FI, nitrous or bigger displacement. But making the same or similar torque at higher rpms is quite easy, improved induction & exhaust, heads, high profile cam, etc.

......... also you can place the power band exactly where you want it................

Yes but only to a certain extent.

Link to comment
Share on other sites

The quote you are looking for is from the American V8 racing icon Carrol Shelby .......... "There's no substitute for cubic inches"

While that’s a good quote the quote I was looking for was the one I used which you’ll find used a lot on many US forums.

In times gone by this may well have been true ............... but modern engineering has now proven that cubic capacity is not always the ideal answer.

Again this depends what you are talking about.

A well designed 4.0 DOHC 4 or 5 valve per cylinder engine should be able to produce more power, i.e BHP than a 5.0 OHV 2 valve per cylinder engine.

But this goes to something called curtain area that multivalves engines excel at and DOHC drivetrain allows higher rpms, so back to the HP equation means they can make the most power.

But the 5.0 OHV engine may well actually produce more PEAK torque due to it’s displacement.

Another way is take a modern Formula 1 engine, it makes superb power for the displacement but by comparison the torque figure is quite pitiful.

The info within that forum is no different to what is available here .....................

It is merely another source of info and a site dedicated to V8’s and not Land Rovers, no need to get your knickers in a twist about it.

Well............... if you are happy to have a linear fuel supply to the engine over the entire rev range then those carbs are right up your street.

Well in the real world where’s the problem?

Most engines, especially those that are tuned, require vastly different fueling dependant upon engine information ................. this is where EFI scores heavily over the mechanical carb

Tuning is a good thing with EFI, provided the EFI itself can attain the level of tuning required.

EFI on your TR7 would no doubt be worth some extra bhp if set up correctly

Maybe, but maybe not. Carbs are very good at PEAK numbers it’s fuel efficency and part throttle response where they are not as good.

A few years back Practical Hot Rod took a crate Chevy LS1 engine added a few mods and dyno tuned it with EFI, they then swapped on the GM Performance carb package and found that it produced less than 10bhp difference on a 430bhp motor and had slightly better torque on the low end with the carb.

and the engine would run better into the bargain.

Well as the TR7 was my daily driver for several years in all weathers (including snow) and it’s never missed a beat, stumbled, stalled or been any problem at all I seriously can’t agree. EFI would have been expensive, more complex lack the instant throttle response only available with a carb, lose out on the charm of a carb the visual appearance under the bonnet. And for what, equal running and maybe an mpg better.

Hell 24 AVERAGE mpg from a modified Rover 3.5 Rover which makes ~200rwhp and is still drivable and easy to start can’t be bad no matter the fuelling system.

Yes ............. carbs and off road angles are not an ideal mix.

:)

Ian

Some carbs maybe, such as already detailed, being the Holley 390 and Weber/Edlebrock 4 barrel setups, although many use them to good affect.

But twin SU’s and Stombergs are proven just take a look at how many Land Rover 2.25 petrol and V8 engines use them and never to ill affect unless it’s a maintenance issue. Certainly didn’t have any problems last Sunday competing in a RTV using a factory 3.5 V8 90 running twin Strombergs.

Also a nice advantage with carbs is hardly any electrics to worry about and no need for a diagnostic computer.

Link to comment
Share on other sites

I think you and many others are WAY over exaggerating any such issues. Really how many Rover V8’s have been produced in the last 40+ years and how many really have issues which aren’t related to poor maintenance?

There are many examples of 4.5, 4.8, 4.9, 5.0, 5.2, 5.3 and even a few 6.0 Rover V8’s out there which have had no problems at all.

The 3.9 was only manufactured from cicra 1990 onwards and the 4.6 from 1994 onwards ..........

I don’t have any graphs to hand but PEAK numbers are massively different and I suspect that a 4.6 makes significant amounts more torque and power across the entire rpm range.

Hmmmmmmmmmmm.................................I only need to post the factory figures to destroy your argument..........

3.9 ................. 185bhp @ 4750rpm ............... Max torque 235 ft/lbs @ 2600rpm

4.6 ................. 225bhp @ 4750rpm ............... Max torque 277 ft/lbs @ 3000rpm

4.2 ................. 200bhp @ 4850rpm ............... Max tprque 250 ft/lbs @ 3250rpm

What is the definitive meaning of massive in your dictionary ?

I will try and post up the bhp /torque graphs later ................ the low rpm difference between the RV8 3.9 & 4.6 is minimal ...........

Remember the Rover is the mainstay performance V8 in the UK as used in countless vehicles, most not even BL/Rover related.

Performance as in PEAK HP yes but unless you are using a power added I seriously don’t believe you can substantially beat the below the graph line torque figures.

HP = torque x rpm /5252

To make HP you either need to make more torque, the same torque at higher rpms or both. Increasing the torque figure is not easy with FI, nitrous or bigger displacement. But making the same or similar torque at higher rpms is quite easy, improved induction & exhaust, heads, high profile cam, etc.

Yes but only to a certain extent.

The Rover is used because it is cheap and plentiful ................ also any SBC or SBB has a large weight penalty against the RV8.

Yes, capacity does yield a torque improvement ................ but here we are not talking a huge capacity increase (3.95 to 4.55).......... roughly 15%

In terms of induction and valve timing the standard RV8 is poor ................... and that is where vast gains can be made in terms of torque ......... but yes, it is difficult to push the high numbers into the lower rev range ............. often this is done at the expense of engine efficiency by introducing a large overlap at low rpm.

With regard to the P38's I have just called the local LR independent here (a small west country village) .............. Frank employs 2 grease monkeys plus himself ............. he reckons to get at least one a fortnight ............. his records show that he has bought 28 short 4.6 motors in the last 12 months..........

:)

Ian

Link to comment
Share on other sites

The 3.9 was only manufactured from cicra 1990 onwards and the 4.6 from 1994 onwards ..........

Hmmmmmmmmmmm.................................I only need to post the factory figures to destroy your argument..........

3.9 ................. 185bhp @ 4750rpm ............... Max torque 235 ft/lbs @ 2600rpm

4.6 ................. 225bhp @ 4750rpm ............... Max torque 277 ft/lbs @ 3000rpm

4.2 ................. 200bhp @ 4850rpm ............... Max tprque 250 ft/lbs @ 3250rpm

What is the definitive meaning of massive in your dictionary ?

Ok then how about substantial, the PEAK torque of the 4.6 is nearly a 5th more than the 3.9 and you also claimed a moderatly modified 4.6 as well.

Also I'm not certain on the actualy figures for the 4.6, this website shows the US spec 4.6 Discovery as having 300lb ft @ 2600rpm http://media.ford.com/article_display.cfm?article_id=16731

Another for 300lb ft http://www.edmunds.com/landrover/discovery/review.html

This one 280lb ft http://www.lrfaq.org/RR/FAQ.RR2.US_RR.mk2.html

Link to comment
Share on other sites

EFI would have been expensive, more complex lack the instant throttle response only available with a carb

I'll let someone far cleverer than me (BBC) argue with you over the rest of your post, but I really must take issue with that:

1) When I swapped from a Weber 500 and Edelbrock manifold to MSEFi I made money - a weber setup is £500 whereas you can buy an EFi setup minus the AFM & ECU for about £20 and put a fully tunable MegaSquirt ECU on it for another £225. I make that half price.

2) Complex is in the eye of the beholder - I know exactly what my ECU does, how it works out what to do and how to sort it out if something goes wrong. As far as I'm concerned a carb is a big complicated clockwork thing full of very small and sensitive parts. The only moving part in an EFi setup is the injector needle, and they are either open or closed and don't break very often.

3) Instant throttle response, hmmm. My ECU can react in less than a millisecond to changes in engine load, temperature, speed, exhaust gas mixture etc. and correct the fuelling and ignition just as quickly. I'm no engineer but I'm willing to bet it's faster than the mechanical inertia of most of the components inside a carb or dizzy for that matter.

While we're posting factory figures, here are the carb & EFi numbers for the 3.5 in the Range Rover:

Carb: 134bhp @ 5000rpm 187lb ft @ 2500rpm

EFI: 165bhp @ 4750rpm 206lb ft @ 3200rpm

Link to comment
Share on other sites

I'll let someone far cleverer than me (BBC) argue with you over the rest of your post, but I really must take issue with that:

1) When I swapped from a Weber 500 and Edelbrock manifold to MSEFi I made money - a weber setup is £500 whereas you can buy an EFi setup minus the AFM & ECU for about £20 and put a fully tunable MegaSquirt ECU on it for another £225. I make that half price.

Well I know it can be done, but to go from a twin SU carb setup to EFI is a lot more complex than that and a lot more expensive. How about heads for injectors, the injectors themselves and all the other gumf that’s required not too mention the wiring. The carb was a simple bolt on operation that took half day or less to fit.

2) Complex is in the eye of the beholder - I know exactly what my ECU does, how it works out what to do and how to sort it out if something goes wrong. As far as I'm concerned a carb is a big complicated clockwork thing full of very small and sensitive parts. The only moving part in an EFi setup is the injector needle, and they are either open or closed and don't break very often.

Complex is in the eye of the beholder, but you can tune SU carbs on the driveway with a few hand tools, you need tuning software and PC for proper tuning of any EFI setup, or a hand held tuner if ones available.

But yes I agree as long as EFI is working then it is very easy.

3) Instant throttle response, hmmm. My ECU can react in less than a millisecond to changes in engine load, temperature, speed, exhaust gas mixture etc. and correct the fuelling and ignition just as quickly. I'm no engineer but I'm willing to bet it's faster than the mechanical inertia of most of the components inside a carb or dizzy for that matter.

Well I think this is a long running debate, but this defiantly does not come from me its something I’m voicing. However having driven many EFI and many carb engines there is something unique in the quality of the throttle response of a carb setup, they are just crisper (when working), you have no lag time for the ECU and the injectors it’s just there.

Take a look at the New Defender thread, they comment that the car seems to keep accelerating after you take your foot off the pedal, the TD5 was they same and this is consistent with many other ECU controlled setups.

The Chevy LS1 suffers a similar problem, although in it’s case the poor throttle response is due to running a single large throttle body compared to what would be and equivalent of 4 on a 4 barrel carb setup.

While we're posting factory figures, here are the carb & EFi numbers for the 3.5 in the Range Rover:

Carb: 134bhp @ 5000rpm 187lb ft @ 2500rpm

EFI: 165bhp @ 4750rpm 206lb ft @ 3200rpm

Sadly stock numbers reveal little of the potential, first all other engine components need to be the same (SCR, DCR and so on).

Then is the breathing ability of the stock carb setup equal to the breathing ability of the EFI? If not you need to align them to make a direct comparison. Having a compromised carb setup compared to more substantial EFI is not proving the carb is the main cause of the lower HP, it simply means that particular setup produces less.

Now don’t get me wrong, I like EFI. Hell my Camaro makes more power stock than pretty much any Rover V8 is ever likely to make. And I wouldn’t ditch the EFI setup on it as it allows control of so much, plus the additions of anti-knock and all the other attributes associated with PCM/ECU controlled setups.

But for some reason I think (well read and hear often) in the UK carbs seem to be viewed as old school and inferior which for want of another word is total bollox, lol :lol::)

You might find this article interesting, here’s an extract:

http://www.popularhotrodding.com/tech/0409...ted_intake_ls1/

Old Schoolin'

With our numbers checked, tested, and rechecked it was time to peel off all that modern age high-tech EFI stuff and give the age old carburetor a try. We disconnected connectors, pulled wires, and unbolted the intake from the aluminum block. We used a set of intake manifold gaskets from Weiand and bolted the single-plane intake in place. Since the intake has no coolant pumping through it and there's no distributor to deal with, the swap only took a short time.

The fuel mixing duties were given to a Barry Grant 750 Mighty Demon. These carbs work very well and are priced right as you basically get the center section of a Race Demon plus have changeable air bleeds. Without the ECU on board, the next hurdle in converting to a carburetor is how to fire the individual coil packs. This is where MSD and Edelbrock come into play.

Edelbrock worked with MSD Ignition to develop an ignition control that would drive the eight coils. The ignition is an inductive spark design, unlike the common capacitive discharge ignitions that power most race cars. This features factory connectors that simply plug into the main coil connector for each bank of cylinders, the crank sensor, the cam sensor, and into a Map sensor. There is also a power and ground wire to connect. One other feature is that you can choose between six pre-programmed timing curves by simply plugging in a different module (just like MSD's rpm modules). MSD is said to be working on their own system that will incorporate their Pro-Data+ software for easy programming through a PC.

With everything connected, we gave the throttle a couple pumps and hit the key. The LS1--an engine that was never intended to be crowned with a carburetor--fired right up. The throttle response seemed crisp and we were on our way. After a few light pulls, the engine was shut off so we could take a look at the plugs. It seemed to be a little lean so we set about swapping the jets. The Demon was supplied with 75 primary jets and 83 in the secondary. After a couple different changes we stuck with 79s up front and a pair of 90s in the back. With everything set, we revved it up and performed three more pulls to gain an average to compare to the EFI.

The results were surprising. We kind of expected to see a drop in power and torque at lower rpm due to the single-plane design, but the carb stayed right in there with the EFI system across the entire rpm range. The EFI peaked torque at 4,900 rpm with 425 lb.-ft., while the carb tipped 420 lb.-ft. at 5,000 rpm. Power wise, the two induction systems were equal at about 430hp at 5,600 rpm.

Trying to compare the two systems is apples-to-oranges as the runners of the EFI system are longer, yet they are only carrying air and the squirt of fuel has been optimally positioned. The open plenum of the GMPP carb intake seemed to be able to easily fill each of the shorter runners with plenty of air/fuel mix. We felt that the intake would really begin to shine at higher rpm, but we were limited due to the valve springs.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy