1969 Jensen Interceptor drivetrain upgrade

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NigelK
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1969 Jensen Interceptor drivetrain upgrade

Post by NigelK »

Dear All

I’m just about to start a rebuild of the 383 HP engine in my 1969 Jensen Interceptor. My objective is to end up with a “polite” engine for the street consistent with the GT nature of the car, with a reasonable idle and enough vacuum for the brakes, and a 15-25% increase in power and torque over stock. CR should be no greater than 9.75:1 so I can use regular unleaded without pinking.

After long deliberations I have decided on a 438 stroker kit from 440Source, on the basis that the extra lower down torque from increased cu in is more in keeping with my objective than tuning the 383 to add power and torque at higher rpm.

I’m keeping the stock 906 heads for originality, reduced engine bay heat soak, and to remain as close as possible to the factory Interceptor 50/50 fore and aft weight distribution. I’ll get the heads ported, valve seats hardened for unleaded, and larger valves fitted. As far as I can tell the block has never been decked. With 0.019” steel shim head gaskets, static CR should be 9.6:1. Some minor milling of the heads should increase this to my target CR of 9.75:1.

440Source recommend the Comp Cams Xtreme Energy .477/.480” cam which has a 224/230 duration at 0.050” lift. I’ll probably follow Comp Cams recommendation to go up one notch to the Xtreme Energy .488/.491” cam which has a 230/236 duration at 0.050” lift.

For the carb, I’m thinking Edelbrock AVS2 650 cfm. Larger than the factory Carter AVS 4682S (625-630 cfm) but possibly a little small cf the factory 440 Carter TQs rated at 750 cfm. I reckon the improved low down driveability provided by the AVS2 primary annular booster is more in keeping with my overall objective than ensuring the engine can fully breathe above 5750 rpm.

For the exhaust, I’ll have custom 4-2-1 full length headers made with 1.75” primaries feeding 2” secondaries and 2.5” collectors. Then replicate the factory system (one pipe for each bank, with a single muffler and resonator) but maintain the 2.5” diameter and install an H-crosspiece abaft the transmission.

I’m getting the original A727B transmission rebuilt, probably with some upgrades to handle the expected 450-500 lbs-ft of torque. I’ll also retain the stock 10.75” high stall torque converter - the rated 2350-2650 rpm stall speed should give 2500-2800 stall speed behind the updated engine so just about right. The back end is the stock Salisbury 4HA with Pow-r-lok LSD, rebuilt with all new bearings.

Interested in any views on the above!

Best wishes,
Nigel
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Pete
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by Pete »

Welcome, Nigel; I look forward to your build and pictures with interest.

All the best.

Pete
Pete Wiseman; Cambridge.

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DaveB
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by DaveB »

Im doing a similar build for a customers 1960 Polara,383/434ci,although Ive used the 440 source heads,and decked the block to .002",using a .039" gasket,which will give a better quench than the open chamber 906 heads,it will be dynoed in the next couple of weeks,I expect itll make your predicted HP/TQ levels.
Oh yeah,Welcome :thumbright:
HarrySaggs
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by HarrySaggs »

I'm excited to see the pictures as well. Something to look forward to during these slow, pandemic days.
NigelK
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by NigelK »

Slow but steady progress. All the required parts sourced from the States, engine stripped, block dipped and sent off for reboring. Thinking ahead, I’ll want the rebuilt engine tuned and dyno’ed so I have a power and torque curve to give to Burns Stainless so they can design my custom headers (none of the off the shelf header options will fit in an Interceptor). Can anyone recommend an engine tuner with an engine dyno in the West of England (Oxfordshire, Gloucestershire, Wiltshire, Somerset, Dorset) or in South Wales who could do this work? I’d go to John Sleath in Doncaster normally, but lockdown might make this tricky...
60AFF4E6-F16C-4E22-BB7A-3D05E8444F59.jpeg
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mopar_mark
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by mopar_mark »

I would speak to John S, he seems to be working at present. That said, who knows what’s coming, with Covid being on the rise.
"I spent a lot of money on booze, birds and fast cars. The rest I just squandered."
NigelK
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by NigelK »

Just got the dyno results back from Peter Knight. Please see the below. To be honest I'm a bit disappointed with the power, was hoping for mid to high 400s. A quick reminder of the spec:

383 stroked to 432 with 0.030" over pistons
Ported 906 heads with 2.14" intake valves
Head gasket selected and head milled for 9.73:1 static CR
XE274H Comp Cams camshaft, 230/236 duration at 0.050" lift
Edelbrock AVS2 800 cfm carb

Any thoughts?
Dyno plot 080221.png
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Dyno figures 080221.png
Dyno figures 080221.png (435.41 KiB) Viewed 2294 times
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Blue
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by Blue »

Why only 4900 rpm? Looks like the cam is only just getting into its stride, would probably be making over 400hp by 5500RPM.
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NigelK
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by NigelK »

Pete always stops the test at 5000rpm on big block Mopars, apparently...
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MrNorm
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by MrNorm »

That is super low rpm, you can see from the power curve that it has not peaked at all. I'm not a big block guy, but that cam seems too large if you don't want to go above 5000rpm. Or put another way, with that cam you need to keep going up the rpm range - even the comp cams specs for it say 1800-6000rpm. And for a properly built motor you shouldn't have any trouble going quite a bit higher IMHO. My 2c
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NigelK
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by NigelK »

I think my slight disappointment with the power figure stems largely from the factory 335hp rating for the 4-bbl HP 383. But it seems this figure is more hope than reality, and that from factory 260hp peak was more realistic. So I should be happy with a 50% increase to 379hp! Plus 450lbs/ft of torque at 3000rpm will be great for driveability.

I chose the cam based on Comp Cams' own recommendation - I'm happy to go above 5000rpm if road conditions and the A727 permit.

The next question relates to the main oil seal - Pete said the engine was leaking like a b**tard from the main seal. Part of the issue may be Melling high pressure oil pump, but it seems this area is always tricky to seal properly. Has anyone tried one of the kits sold by Hughes or Mancini?
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Pete
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by Pete »

I think a few of us on here are putting 800bhp through an A727 (with the right bits in it!!)
Pete Wiseman; Cambridge.

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Blue
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by Blue »

The trick with the rear main seal is to use the plastic type and to offset it slightly so it does not line up with the mating surface. I wouldn’t use a high pressure pump either, I normally have to chop a couple of coils off the relief spring on a high volume pump to get the pressure down. 20psi hot at idle and 70 psi max is plenty enough for your needs.
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Pete
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by Pete »

I agree with Blue on the Oil pressure. In fact yous is not that high for a high Pressure pump - maybe because the rpm was low on the dyno.

Mine made 98psi at 6800rpm on the Dyno with a High VOLUME pump, not a high pressure one.

Blue and I put an adjustable Milodon pressure valve in it, and it is now as Blue says.

The best Rear seals in my opinion are the Mancini ones. Dave Billadeau will be able to advise on this.

It is unfortunate that the dyno run was stopped before the Two lines bisected. They always cross at 5250rpm.....
Pete Wiseman; Cambridge.

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NigelK
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Re: 1969 Jensen Interceptor drivetrain upgrade

Post by NigelK »

Thanks for the advice chaps. My A727 has been rebuilt so I’m confident it’ll handle the extra torque and power, what I meant was that it might kickdown before I get anywhere near 6000rpm and, with 2.88:1 gears i might struggle to get to 6000rpm in top gear while keeping my licence 😈

I bought the high pressure oil pump on the advice of 440Source, as they reckoned the high volume version might suck the sump dry - mine is a small sump because of poor ground clearance on an Interceptor. The oil pressure regulator is a good idea, I’ll order one of those...
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