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so has anyone actually made 500bhp from a v12 pre.he

XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

so has anyone actually made 500bhp from a v12 pre.he

 
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Old 05-16-2013, 10:13 AM
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Default so has anyone actually made 500bhp from a v12 pre.he

Im emailing around and people seem to have their doubts, so I wantes to know if anyone had any proven figures
 

Last edited by GGG; 09-06-2013 at 12:39 PM. Reason: edit typo in thread title so it appears in searches
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Old 05-16-2013, 10:35 AM
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I surely don't know...and I sorta doubt that very many people have even tried. What does Chad Bolles say?


Cheers
DD
 
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Old 05-16-2013, 11:17 AM
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Chad siad i could have made a 500hp v12 with the He heads if i ported them the way he did. so i would believe i can. with the 6.5 motor i should only be making about 75 hp a liter.
 
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Old 05-16-2013, 11:19 AM
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Frankly, I'm a sceptic. I think it's unlikely you can hit 500 hp in a drivable engine without using forced induction. We all know how poorly the HE head flows, but the pre-HE was not that much better. Porting the HE head still leaves you with a less-than-desireable chamber shape. Asking a V12 Jag to produce in excess of 1.5hp/ci is going to be a very high bar to reach.

These engines do well in categories of smoothness and panache, but have never been renowned for their power output. Most of the performance claims for Jag v12's are the stuff of internet legend; they just simply cannot do what some folks might think. It sounds harsh, but it is going to be extremely difficult to even reach the same hp/ci levels you can easily reach in a small block Chevy simply because the heads don't work and the cams are limited by design.

What I believe IS doable is a solid 350-360 hp streetable engine with good torque, smoothness, and that gets reasonable fuel economy. With more modern cam profiles, 10.5:1 CR on early flatheads with good porting, good quench, and a newer EFI system that provides better ignition control, I think you can far surpass the HE or pre-HE in power output.

Couple the engine with a five or six speed auto or manual trans and a 3.43 - 373 rear gear, and you would have a Jag that is a great performer.

Also, you really have to ask yourself, suppose you do manage to hit those kinds of power levels? Will the engine be usable in traffic? Can you drive it to work or school? Will it get even worse fuel economy? how does it behave in the lower rpm ranges? Or will this be a track-only kind of engine?

I have a friend with a dyno proven 600+hp big block in his Chevelle. About the same weight as an XJS. It get a whopping 6 mpg. That means on a good day he can drive this car from Orlando to Daytona and back, a total distance of about 90 miles, on a single tank of premium if he's lucky. He doesn't get to drive it much. A big inch, 600hp V12 jag will be the same sort of machine.....
 

Last edited by MustangSix; 05-16-2013 at 11:27 AM.
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Old 05-16-2013, 11:29 AM
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i have the Pre He heads that have been ported with stock valve sizes and a larger profile cam. The isky XM5 and the motor is 400Ci displacement with the broadspeed style piston top.
 
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Old 05-16-2013, 03:23 PM
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Mustang6 pretty much sums it up, 500hp is going to be hard to do with the jag V12, 400hp would be outstanding, 7L or 426 cu.in would be 426hp, or 1hp per cu.in..

XM5 cams were designed in the 1970s, broadspeed was around late 70s-early 80s, with there engineering of that era.

this era we live, 2013, has made encredible engine technology advancements, mostly about air flow and combustion chamber design direct injection, variable cam events, on and on etc. friction reduction, gearing ,transmissions 6-7-8 speeds.

building an old heavy jag to beat new performance cars is just dreaming.
some perspective on hi-performance cars Chad made only 170mph at Maxton mile, and he did have all the mods available at the time.

you can drop by the local dealer,today, and buy 186mph cars all day long(with a warrenty), also some 200mph cars with small mods of ECUs.

i was talking with the guys from LMR houston,(they have some of the fastest modded cars around), i asked, what hp does it take to make an actual 200mph,with say a 3800lb car, quick answer was,800hp or more!

and good drag cars all run solid rear axle, not IRS rears(altho some new stuff is getting better), when was the last time you seen a top fuel car with IRS,hehe.
 
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Old 05-16-2013, 09:25 PM
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reading on the aj6 website. with 6.5 liters 500hp shouldn't be terrible hard. its 75hp a liter. the heads flow incredibly. six intake ports and if you look down them the lack of restriction at .440 lift is a ton. with the oversized liners and broadspeed style pistons. itll have a quench around the right spot.

not to mention, I go to school for engineering, and in an.ideal cylinder has a flat head and the combustion chamber souly in the piston.
 
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Old 05-17-2013, 08:05 AM
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Originally Posted by calvindoesntknow View Post
...not to mention, I go to school for engineering, and in an.ideal cylinder has a flat head and the combustion chamber souly in the piston.
Not really. Chevy tried that with their 348-409 W-motor and while fairly successful it does not even begin to approach the combustion efficiencies of the LSx series of engines.
 
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Old 05-17-2013, 08:16 AM
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The chase for higher horsepower on this engine has been very disappointing for myself as well. Even the Jag/Ferrari techs that I talked with made me feel like a schmuck when I started asking about performance improvements. I seem to have run into the same thing everyone else has...It is possible but limited success with deep pockets. Keep working towards it Calvin, I am sure I can speak for most that we are anxious to see and possibly copy your success. There is a whole bunch of these cars out there that are under-valued/appreciated that sure could use a new lease on life.
 
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Old 05-17-2013, 02:54 PM
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Originally Posted by MustangSix View Post
Not really. Chevy tried that with their 348-409 W-motor and while fairly successful it does not even begin to approach the combustion efficiencies of the LSx series of engines.
the thing people dont adress, is if ine were to rebuild a 409 with all the equipment and tooling tolerances we have today, thar engine would have made remarably higher hp ratings.

by buddy has a 426 that makes 800+ NA and is a.streatable car. everyone said he couldn't until he did.

horse power is a function of air flow. an engine is essentially an air pump. if it can flow the cfms it can make the horse power.
 
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Old 05-17-2013, 03:05 PM
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just took a quick look at the 409 heads. yes they are flat but they have a wedge shaped chamber. so they are really not flat combustion chambers.

the true advantage of the pre he is that there is no cold side.to the combustion chamber. the sparkplug is 7mm from centered, so it makes the flame move outward in all.directions. (a big problem with the lsx heads) my pistons have offset dishes.

the parts are.all there, the magical 500 might not come untill I save up for webber efi units and knock sensitive ignition.
 
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Old 05-17-2013, 03:58 PM
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another thing it my motor is 6.5 liters so its got an extra 1.2 liters over stock
 
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Old 05-17-2013, 05:33 PM
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Originally Posted by calvindoesntknow View Post
just took a quick look at the 409 heads. yes they are flat but they have a wedge shaped chamber. so they are really not flat combustion chambers.

the true advantage of the pre he is that there is no cold side.to the combustion chamber. the sparkplug is 7mm from centered, so it makes the flame move outward in all.directions. (a big problem with the lsx heads) my pistons have offset dishes.

the parts are.all there, the magical 500 might not come untill I save up for webber efi units and knock sensitive ignition.

Why don't you look into ITB's from a motorcycle these would be much cheaper than Webber efi.

Also knock sensing is going to be a trial and error venture. Julian Edgar (Autospeed) did an article on detecting knock and the best way was a mic on the block to headphones on a person might be a good place to start.
 
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Old 05-17-2013, 06:31 PM
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truthfully Warren the best place to start is to get the block back together
 
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Old 05-17-2013, 08:52 PM
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For what it's worth, on a Yahoo XJS group, there are some photos of some really mean looking dragsters with modified Jag V-12s, I would suspect a couple of them have conquered the 500hp.
I didn't read all the posts pertaining to the photos, but it might give clues to their successes too.
 
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Old 05-17-2013, 10:37 PM
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I'll be the first to admit I know zilch about engine tuning. But man, it sounds like doing forced induction would be a hell of a lot easier than what you guys are attempting.
 
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Old 05-17-2013, 11:18 PM
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I thought that, until I started looking at the effort in fabricating stuff in the room provided. plus I'd need pistons rods cams and such for forced induction.

personally I have much preference to driving naturally aspirated engines
 
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Old 05-18-2013, 01:10 AM
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Not trying to derail your thread, but just had a thought... Anyone here know the folks who have done twin turbo XJSs? Wonder if they would be kind enough to join the forum and share some of their experience...
 
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Old 05-18-2013, 02:18 AM
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twin turbos are not worth it. not in a street car atleast.
 
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Old 05-18-2013, 03:38 AM
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Maybe this warrants a new thread: best method of reaching 400RWHP and maintaining the engine characteristics we love.
 

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