C2- hydraulic or solid cam

**Mark #28455** · June 20, 2005, 05:39 PM

Re: C2- hydraulic or solid cam

There is no one answer to this question - it all depends on how much you intend to drive the car. Remember, your engine has small chamber heads with big ports - the low end torque will be less than with the small port heads no matter what you do. I can honestly say my best driving big block had 9.5:1 compression with small port heads and a short duration hydraulic cam. On the other hand, I've had the most fun with my L89 which has been stroked to a 489 with 10:1 compression and has the large port closed chamber aluminum heads with a long duration solid lifter cam that bleeds off some of the dynamic compression. What are you really looking for? Be honest.

Big blocks don't run well on pump gas once you get over about 10:1 static compression and realistically at 8 MPG you don't want to bother with avgas or octane booster.

I recommend you go to the keith black pistons website and read the tech articles about dynamic compression and selecting comp ratios (www.kb-silvolite.com) and use the calculators to look up whatever combinations you are considering - don't take anyone else's word for it - to check the static and dynamic compression ratios before you pay for the parts.

How does it run now with the stock cam and compression? Remember, a smaller cam won't miraculosly make better low end torque with your big port heads. If you're running stock exhausts, you should go no more radical than the stock 425HP cam for your engine.

Good luck,
Mark

**Duke W.** · June 20, 2005, 05:45 PM

Re: C2- hydraulic or solid cam

It will run a lot better if you convert from ported to full time vacuum advance.

Actual compression ratio is a function of several finished dimensions including head gasket thickness. You or your "guy rebuilding it" should make all these measurements and select a head gasket that yields no more than about 10.5:1 CR, which is probably no more than actual OE build. Chevrolet overstated CRs by about a half point because production engines typically had higher decks than "spec" and some engines were even double gasketed.

The L-72 is really not that radical an engine given that they pull about 14" of idle vacuum which is a lot more than the 10" that 30-30 cam SBs pull. The L-72 cam lobes are the same as the LT-1 inlet lobe - or I should say that the LT-1 cam used the L-72 lobe on the inlet side since the lobe was originally designed for L-72. With effective duration of 231 degrees and 114 degree lobe separation, it is not at all radical for a 3.75" stroke engine and provides excellent torque bandwidth for a high performance street engine.

I recommend you rebuild the engine "stock" using OE or OE equivalent components including the cam. Converting to full time vacuum advance will reduce the tendency to run hot and make it easier to drive in traffic.

If you're interested in about 25-35 more HP at the top end without any negative impact to low end torque, idle and low speed driving characteristics, have the heads pocket ported and port matched and do a multi-angle valve job.

You could do me a big favor if you can measure rocker ratio behavior through the full lift cycle. This is the only piece of information I am lacking to complete my extensive article on Corvette camshafts for The Corvette Restorer.

I've been begging for this information for a good three years now. I don't have a BB handy to run this test.

Duke

**Joe L.** · June 20, 2005, 05:45 PM

Re: C2- hydraulic or solid cam

Bill-----

While it's expensive, you can have the best of all worlds using a retrofit hydraulic roller camshaft and kit. These are offered by most of the major cam manufacturers, including Crane, Comp Cams, Crower, and Federal Mogul/Speed-Pro. Crane offers the best lifters for higher RPM operation (and, also the most expensive) and the Speed-Pro kits are the best value.

Big blocks, being big bore engines, are especially susceptible to pre-ignition and detonation. If you were able to use 91 to 93 octane pump gas with your original '163' solid lifter cam, then I'd suggest that you stick with that grind. The long overlap helps prevent detonation. On the other hand, if you had problems with pre-ignition and/or detonation, I'd reduce the compression and go with an hydraulic roller.

**Jon #40768** · June 22, 2005, 08:11 AM

Re: C2- hydraulic or solid cam

I have big port heads and a 400 horse gm hydro cam and I have way more than enough torque, fair gas economy on the highway, and a decent idle with side exhaust. It is a hard combo to beat for all around driving. My compression is 10.5 and I have the advance at full vacume, have a faster advance spec with one weak spring, reduced the total advance with the inital set up top 12.

Jon

**Joe L.** · June 22, 2005, 04:27 PM

Re: C2- hydraulic or solid cam

John-----

Which cam are you referring to? The L-36/LS-5 cam or some other? If you have the big port heads, does that mean that they've been changed from the original heads used with the L-36/L-68?

**Jon #40768** · June 23, 2005, 10:25 AM

Re: C2- hydraulic or solid cam

Joe I am sorry it took a while to get back. I have an L88 crate engine that a friend of mine bought directly from the chevy dealer. It had a direct gear driven reverse cam from the factory and an open pleneum GM manifold, with big port heads. Back in about 1979 I bought the engine and installed the cam for a 400 horse engine, milled the piston domes to lower the compression to 10:5, and installed a divider in the manifold, which completely sealed the to sides of the pleneum upto and including the carberator. The engine dynoed at 390 horses and 425 ft/lbs of torque on a steady long pull right out of the box with no tuning. It was in the middle of racing season and we did not have time to fool with it.

Thanks for your interest
Jon

C2- hydraulic or solid cam

C2- hydraulic or solid cam

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