retarding the cam from 4 degrees to straight up

[rfoll]

I'm working on a 400 based SBC and the compression is a little higher than I would like for a street engine. With a 76 cc chamber on an iron Dart head static is 10.2:1. This is a street driven 81 Corvette. It will see track time, but it needs to be streetable. The converter is about 2500-3000 and is a lockup. 3.73 rear gear. The cam is a Comp XE274. If I install the cam straight up, will it change the dynamic compression enough to matter? I like to run as much initial timing as I can on everything, but if we have to retard the timing to run pump gas that is what we will do. I have never run anything with this much compression on a regular street car. This is not my car, I'm just building it for a friend, and I'm not wanting to end up with something that rattles itself to death.

[Tom68]

I'm working on a 400 based SBC and the compression is a little higher than I would like for a street engine. With a 76 cc chamber on an iron Dart head static is 10.2:1. This is a street driven 81 Corvette. It will see track time, but it needs to be streetable. The converter is about 2500-3000 and is a lockup. 3.73 rear gear. The cam is a Comp XE274. If I install the cam straight up, will it change the dynamic compression enough to matter? I like to run as much initial timing as I can on everything, but if we have to retard the timing to run pump gas that is what we will do. I have never run anything with this much compression on a regular street car. This is not my car, I'm just building it for a friend, and I'm not wanting to end up with something that rattles itself to death.

Retarding a cam 8 degrees from intended position puts you outside the efficiency window.

XE implies short ramps, you could go for something with the same 050 but more advertised.

Still, good fuel and a cold air intake and I'd have thought you'd be OK.

Running low comp or an excessively late intake closing isn't a good thing to be doing just so you can quote a high initial timing number.

It's when taking ignition timing out doesn't stop detonation (or just as bad preignition) that you have a real problem.

[rfoll]

My race car never had a problem with 10.4:1 on pump gas, but it never had a converter less than 3000 rpm with aluminum heads. The iron head 406 was only 9.7 with the same 3000 converter without a problem. My El Camino at 9.6:1 with Vortec heads never pinged on regular fuel until I got my lockup converter rebuilt. This is outside of my experience and I would hate to have something that requires AV gas just to be safe. The cam is bought ans in the short block. The question remains, is there anything to be gained by running the cam straight up? There is not much room under the hood of a Corvette for cold air stuff, but I am not saying it is impossible.

[Tom68]

My race car never had a problem with 10.4:1 on pump gas, but it never had a converter less than 3000 rpm with aluminum heads. The iron head 406 was only 9.7 with the same 3000 converter without a problem. My El Camino at 9.6:1 with Vortec heads never pinged on regular fuel until I got my lockup converter rebuilt. This is outside of my experience and I would hate to have something that requires AV gas just to be safe. The cam is bought ans in the short block. The question remains, is there anything to be gained by running the cam straight up? There is not much room under the hood of a Corvette for cold air stuff, but I am not saying it is impossible.

Yer you need somebody that's run pretty close to that combo.

Dual Plane or single plane ?

[1980RS]

Me I would advance the cam a little more for better street manners. Never had a cam that I retarded 4° and was worth a darn on the street without nitrous.

[Geoff2]

Every combo is different & the only way to know how the cam timing affects power is to dyno the engine & change the cam timing....

The 400 is going to have good low end tq because of the short R/S ratio. Plus, the car has a 3.73 axle & is not overly heavy. For a shelf grind, cam is a good overall choice as a general performance cam. Comp Cams normally have 4* of advance ground in & the XEs are on 110 LSA. So if all the machining is correct & installed 'dot to dot', the ICL should be 106.I would install it 4* retarded & ICL would be 110*.

[Tom68]

The 400 is going to have good low end tq because of the short R/S ratio.

It'll have good low rpm power because it's 400ci.

[travis]

Dart S/R Torquer heads? The ones that have the conventional open chamber instead of the fast burn style?

[rgalajda]

If the cam was chosen with the proper events initially then you have less room to advance / retard the cam without a drop in noticeable power .
So you probably only have a couple of degrees retard/advance to play with .
As Geoff2 said the dyno would tell you or track testing.

Usually 10:1 cr on premium pump fuel will run without detonation unless the engine is running too hot or timing is not correct.
An overly advance curve ( too much advance in mid range rpm ) or too much vacuum advance at cruise speed .Total timing affects top speed more than anything else.
I’m assuming the lockup converter may come into play here . If so you could install a switch for the lockup converter that only gets engaged at highway speeds.

To lower compression.
Can the pistons be massaged/ shaved ? I’m assuming these are flat top pistons. Can you run a thicker head gasket ?

[rebelrouser]

Most of the time in my experience detonation occurs in the mid range or light throttle under a load. WOT most of the time is not a problem if max timing is set correct. And the same goes for light and moderate throttle. Performance trends engine analyzer generates a spark curve with whatever octane fuel you choose. I find if you set up your distributor to match that generated curve, I have not had a problem, it works pretty slick. My street car is a stroked mopar LA with a procharger and FiTech fuel injection. When setting up the timing maps I simply ran the engine on the software normally aspirated and then ran it with 7 lbs boost, took those two generated spark curves and set up the timing maps. It runs perfect on 91 octane made a little over 500HP on the dyno. If you can't find a distributor machine, have someone help you and check the timing at say 250 rpm increments and plot the curve on a piece of paper. Then the only issue is vacuum advance on light throttle. Many vacuum diaphragms have an allen screw you access thought the vacuum nipple to adjust the amount of travel, an if yours doesn't then you can probably buy a diaphragm that does.

[Bigchief632]

What is your elevation in Oregon? If you are worried about it, change the head gasket to .051 thick, it will reduce compression and actually be less sensitive to timing, it'll put it right at 10:1 and will run on premium unleaded. Run steam holes, and a 160 stat, it'll be fine. I wouldn't move the cam.

[rfoll]

Dart S/R Torquer heads? The ones that have the conventional open chamber instead of the fast burn style?

Yes that is what we have. I am totally unfamiliar with these heads and they are still on the ATK crate motor they came on. The large chamber is what gets us down to 10.2 compression. Unfortunately they are the 1.94 intake valve version. This all got started with a nice balanced rotating assembly with flat top pistons and a block with a decent bore to put them in. The deck is .005" and with the head gaskets on hand the quench will be about .045" Not knowing the burn characteristics of the large chambers have me wondering if we will have a problem. We dialed up the cam last night and it comes out almost exactly as printed on the card. It is in fact 4 degrees advanced. Having seen that it made me wonder if installing straight up will tame the compression significantly. I'm not worried about power, but the tires fear for their longevity. Most of St. Helens is 100' of elevation or less. Portland International Raceway is close to 20'. Don Garlits mad his first pass in the 5s there.

[F-BIRD'88]

Use the correct spark plug for the job. (Champion Rv8C) or equal. (I believe those heads use this .460" reach plugs) Correct jetting. Correct timing and curve,
34° @WOT is plenty.. And put decent gas in the tank 92 octane plus and it will run fine.
No need to move that cam and that would not correct a knock problem thats really a result of 1 or more of the tuning elements above being off.
Make sure the vette is cooling the engine under racing conditions.
The next bigger XE284H -10 cam in that series will make a lot more power in this 406 sbc.

Ya this spark plug choice matters.

[Walter R. Malik]

I'm working on a 400 based SBC and the compression is a little higher than I would like for a street engine. With a 76 cc chamber on an iron Dart head static is 10.2:1. This is a street driven 81 Corvette. It will see track time, but it needs to be streetable. The converter is about 2500-3000 and is a lockup. 3.73 rear gear. The cam is a Comp XE274. If I install the cam straight up, will it change the dynamic compression enough to matter? I like to run as much initial timing as I can on everything, but if we have to retard the timing to run pump gas that is what we will do. I have never run anything with this much compression on a regular street car. This is not my car, I'm just building it for a friend, and I'm not wanting to end up with something that rattles itself to death.

"When testing ... if the results differ from your theory; believe the results and invent a new theory".

Do the testing and actually find out; you may be surprised.

[panic]

Give me the stroke length, rod length, static CR, and intake valve closing ABDC in nominal degrees (not .050").
I wrote an Excel app that can (roughly) calculate CCP in psi as well as running CP @ 1,000 RPM, then try alternate IVC closing points.