Has anyone done headers on a 4.2 in an Xj6? I have a 1986 XJ6 and the exhaust is starting to show its age. Was thinking of having headers made and a better exhaust but am afraid that if I open it up too much I could hurt the low end. Any suggestions?
lots of people have done headers on XJ motor, much detail on archives
what you will find to be the almost universal answer is that it will hurt your low to mid range torque, that the factory manifolds are considered better than any header in this respect.
Which means that headers are only beneficial for higher RPM
therefore the consensus is this is good if you intend to race the car, but bad if you intend to only drive on the street
2 Likes
I made a set of headers, and was severely disappointed with the results to the point I tossed them in the Wheelie bin; put the stock exhaust manifold back on with an āupgradeā to the catalytic converter.
The one thing Iāve learned over the years, and many in this forum have pointed out, in an attempt to save me from myself, is that the folks at Jaguar really did know what they were doing; and did what they could to get the most out of the car, while still operating within the confines of the laws and regulations at that time.
Iām an Yankee hot rodder whoās been dealing with American Iron for many years. Unlike Jaguar, American Domestics left a lot on the table to āimprove uponā. I havenāt found this to be the case with Jaguar; all the usual āupgradesā Iāve made on American cars have done nothing but had adverse affects on my Jags. There are a few suspension tweaks, but they are minor and cheap, but do improve cornering without adversely affecting the ride.
wow side tracked again.
1 Like
Save the $'s. Unlike older USA cars, the manifold on the XK engines are quite decent. So, there is little to be gained, if any at all.
And, matching header design to the use is an art. Many available. some might fit, others not so much.
Tis a matter of moving more air. Starts at the intake.
Later USA cars no longer use the restrictive ālogā exhaust manifolds. Even my old Jeep has a decent exhaust manifold.
I date back to the days of ātwin Smittyāsā.
Carl
2 Likes
The fuel injected 4.2 as used in the Series III is oft considered the most powerful production variant of all the famed āXKā engines. At least, that is, the high-compression versions which were rated a 205 net horsepower. The low-comp, emission-controlled version was rated at about 170 net horsepower.
Considering engine displacement, either of those numbers is respectable for the era. Back in the mid-70s thru mid-80s many of the USA V8s were in the 160/net to 230/net range despite much larger engine size.
What really holds these cars back is the 4100 pound curb weight coupled to a 2.88 axle ratio. A modern-ish 4-speed auto with more aggressive final drive ratio is very helpfulā¦and would put the engine in a better position to take advantage of breathing improvements.
Anyhow, Roger Bywater offers (offered?) some good 4.2 enhancement packages that garnered good reviewsā¦but were expensive. I think the full ākitā was about $3000.
A small but satisfying improvement can be easily made by advancing the camshaft timing.
Cheers
DD
2 Likes
Interesting. Iāve always had a āthingā for straight sixes!!!
Not as much as for Ford flatheads, 4,5 or 8, but present.
-
Circa 55, school chum, Bob was an El Paso cop. but, a weekend racer. An āoddā jalopy class. His mount, a skeletonized 34 Ford. Powered by a Ford Flat head 6. Very successful other engines of he era. Including a Pontiac straight 8 flat headā¦
-
I created a mix of Toyota FJ40 6cylinder engines. More compression, better carb and 6 in to one header.
It sounded and ran greatā¦ -
I watched a You tube video. Jeep straight six. Contemporary to mine!!! Bumped from 4.0 to 4.2 by an older crank shaft. (258 CI). Six to one header. Better intake. Modest cam. Ported mildly. Dino. No valve float til 5200. 300 hp at 5200!! It screamed āpowerāā¦
Carl
Next to round engines and straight eights, sixes rawk!
Pre-SBC, the preferred engine for quick hot rods was the 270 Jimmy.
For sure !!
-
Decades ago, local racers used them. Surplus !! Sledge hammer in the side of new or re ordinance rebuilds. Relegated to sale as scrap iron !! Ingenious local racers bought them. Cheap ! Patched them. Raced and wonā¦
-
Watched a You Tube video last night. Brought a 62 GMC ton and a half back from retirement. Ugh, the 270 in bad shape. Swapped for a "mere 215 Babbit pounder from junk. Indeed, no good, In with another
215, Well, a bit better. Still, a afar cry from a heathy 270.
Carl