In my experience with the hyperpak intake in a drag racing situation, I am losing top end, If I retard the cam will this help?

And if so, How many degrees should the cam be retarded?

thanks,

Greg Ondayko

69 Dart

I just talked this morning about this with Doc.

The Hyperpak has a 'limited' window, and if you're revving it past 5500 you want to use a 'shorty' intake, the long runners are great for mid range torque but limit top end....

Mine pulls good, but I limit my use to 5500 and that's it (only for 'power launches'), most of the time it runs in the 800-2300 rpm range and has great 'punch'.

You'll have to wait 'til he gets home tomorrow for a follow up...

-D.Idiot

Yes I kind of figured it would be limited, but I wanted to try some other modifactions before I went to the short intake.


Greg Ondayko

I had breakfast this morning with Doc and my brain hurts....


We talked about this and the best analogy for short runners vs. long runner intakes is watching people lining up to get onto a subway....


At low speeds the 'pulses' of air fuel line up and are farther apart...similar to a handful of people at the off hour catching the metro....

At higher RPM the pulses start quickly catching up to the last pulse and can 'jumble up' and stack up while waiting to get through the open valve.
Similar to 'rush hour' where suddenly 'everyone' wants into the only door to the subway car... This can limit your top end and RPM excessively.

Doc had covered a topic when I started working on my installation about limiting cam lift to prevent reversion, where high lift cams can cause some pulse 'back pedaling' in the intake causing some problems, and limiting your overlap with the cam grind.

Other problems that 'crop up' are fuel distribution in the intake.

-D.Idiot

Greg,

What performance are you trying to reach? More than 5500 RPM is quite a bit on a 225. If you are high 14s or slower, you may not need much more RPM than that anyway.

The Clifford standard 4V intake is the best bolt-on 4V manifold for high RPM racing. This is what the fastest guys running 4bbls use, and I can vouch for it too. It is slightly worse than the Offy for lower RPMs though.

Lou

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Home of Slant6-powered fun machines since 1988

Well I am not trying to get a specific number, I just would like to shave some Time off of the quater mile.

I have a dutra Hyperpak, Small port, Mopar purple cam, 500 CFM Carter, Head Milled .090", Ported/ Polished Head,

Stock Valves/ Pistons
A-8333 4 speed - no OD, 4.10 Rear,

Best time hs been 17.2 in the quater,

What are your cam specs?

Have you played with the timing advance curve much? You should not be running more than 30 deg total advance (not incl vac adv), and that'll kill power if you are. I've picked up more than 1/2 sec at times from having my "set by ear" timing too far advanced.

With that drivetrain, you oughtta be in the 16s.

To give you an idea, my '68 Dart runs in the low-mid 17s @76-78MPH, and it currently has a stock rebuilt head (no porting, stock valves), 8.0:1 comp, 500 Holley 2bbl on Offy 4V, 3.23 gears, 2200 rpm converter, MP 244 cam (degreed to 1 deg advanced). It is also probably heavier than your car since I have the OD automatic (A500) in it. 2.5 sec 60 fts.

I assume you've played with carb jetting too?

Just some thoughts...

Lou

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Home of Slant6-powered fun machines since 1988

What exhaust are you working with, and definately what are the cam specs... I'm running mine with a Comp Cam 252 which is slightly better than the MP 244/244 in duration @.050... the Hpak really wants more overlap that the MP cam can deliver...

If you're running a short cam the 500 will need to be a bit leaner than what's in the box, something tells me the secondaries might not be tipping in correctly...

hope you have headers...

-D.Idiot

Oh yeah, I have good exhaust - Dutra duals w/2.25" headpipes into a single 2.5" out the back w/turbo muff.

Lou

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Home of Slant6-powered fun machines since 1988

I have clifford long headers running into duals 2.25"

and I have also rejetted the carb Sorry I do not have the complete cam specs right now, I believe it's .480 lift, 52* overlap, 276 Dur, and 108* centerline, but I am not completley sure of those numbers, I have to dig them up.

Greg Ondayko

Since you have a big cam, good exhaust, decent carb...

I guess next is, what size valves do you have in the head, and what springs, and what's your compression ratio?

Somewhere in here you have a bottle neck that's holding things up...

-D.Idiot

Valve springs could be an issue. I just was using that cam (MP 276 deg/0.490") and a big valve/milled head, and it had been running high 15s to low 16s in my '68 Dart with 3.23 gear. I would shift at 5200-5500 for best perf.

The performance dropped slowly over a couple of years to where it would rev much past 4000 rpm. It was not due to carb or ignition, and I am quite certain it was fatigued valve springs. It ended up running in the mid 17s, right where I am now w/ a much milder setup.

So yeah, checking springs might be a good thing, and if they're stock then you pretty much know that's the problem. You need a couple of steps up from stock springs to support that cam to 5500 rpm.

Lou

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Home of Slant6-powered fun machines since 1988