Here's some numbers from changing cylinder heads only on my ride, a 0.060"-over 283 w/performer, 600 cfm, 6-cyl type T5, 3.42s, cheesy headers & exhaust, cam is a Comp 268H.

Last year I was running 14.3 with my 292 ci combo, running Edelbrock Performer RPM heads. I built a set of 601 casting 305 heads to make more low-end and bring the compression up to almost 9.5 from a lowly 8.0, so I figured I'd take it to the track again.

It's a little more tricky to launch now 'cause it makes some low-end with the increased compression and smaller valves/runners. I still managed a best of 14.49 at 96 Mph.

Goes to show the big heads were hardly being used with so few cubic inches underneath, the reworked smogger castings are making almost as much power, more useable power on the street.

Now I wish that I'd run it on the dyno before and after too.

 

Thats a very minor drop in E.T for a street car. I'd think track conditions played more of a roll in the reduced time then anything. I bet with more test and tune time you will be back where you were a year ago or better.

 

Agreed. A few tenths of a second, in a STREET car, can be attributed to nearly anything. To the temperature of the outside air, to your driving ability that night, shift points, etc. etc. I've gone from 15.1 to 14.2 on the same exact setup (excl. tires) With the same tires, I've averaged a maximum of a 2/10 to a 3/10 difference in the 1/4 from just tires along. Half of it is in the hook, let me tell ya. If you hook it straight, and you get your shift points....then the car will do the rest. But still, the air temperature and several other minor factors all attribute. Look at your TRAP SPEED, not your E/T. Thats what I do.

I gained 1.5 mph in the traps one night JUST by removing my SLIGHTLY dirtied paper air filter and holding the car in 2nd gear longer since I have an automatic. I've since replaced it with a K&N, but my MAF hates it. I'd remove it and run paper again if the damn thing didnt cost so much of my cash.

 

Yeah, I'm really suprised
...you went from a performance, high velocity AL. head to a home ported stock casting.

Why not just mill the Edel's for compression and get on down the road?

 

That's what I'm hoping for. The car feels different, but the trap speeds are within 1 Mph of last year's runs (96, 97 Mph)

Granted I could mill the Edelbrock heads but I'm building a 355 to put under them, I'd sooner leave them unmolested.

 

You should actually be able to beat your times with your aluminum heads, with your low compression in the aluminum set-up you were not keeping any heat at all in the combustion area and actually losing power becasue of this. Do they flow more, heck yes, but was it a optimum combination, heck no. Mill the alumies-angle mill preferably but alot more $$$ than flat milling, you'll have to contact the manufacture to find out how many .000? = a point in compression, they should know exactly what your talking about when you contact them.

Get the compression way back up there, shoot for 9.5-10.0 and run again, I think then the results will be alot different. I've seen guys lose upto .2 switching from Dart iron Eagles to Dart Pro 1's with nothing else changed just from the aluminums heads ability to disappate the heat so fast.

 

You traded flow for cylinder pressure. Probably gave away quite a bit of flow, but almost made up for it with increased cylinder pressure. ALMOST.

Put those aluminum heads on the same motor with 9.5:1 compression and they'll outperform the stockers.

Also, agreed with those above who suggest that low compresion and aluminum heads were sucking a lot of heat out of the combustion chamber. And also agreed that the smaller 305 heads probably give a lot better intake velocity on a small cube engine, partially negating their nominal flow disadvantage.

 

Exactly. I knew that going in to the head swap, but I was curious to see how it would show up in my ET and MPH.

It was the part-throttle cylinder pressure that I was after, and the increased static compression gave me that. Of course it lost some of that top-end hooligan rip.

It's more fun to drive to work now but a wee bit slower at the track.