Hot Rod Forum banner

xtreme energy 268h

7K views 37 replies 9 participants last post by  xntrik 
#1 ·
i recently installed a comp cams XE268H into my 305 (86 trans am tpi) with lightly ported stock heads, performer intake, holley 600 vacuum secondays, 4spd muncie, 3.73 posi stock rear end in my 81 malibu. i was driving it today and i have almost no low end power. its a dog till 3500 rpms then its pretty fast up till 5,600ish. does anyone know anything about how this cam is supposed to run? i cant launch very well at all. im planning on checking my ignition timing in the next couple days and see if i can change that a bit. thanks for any help
 
#3 ·
Timing is a good place to start. Did you degree the cam when you installed it? It could be a a bit retarded which will kill off idle power. What is your compresion ratio? If it is 8.5 or lower you may need to advance the cam to wake it up in the lower rpms. I ran that cam in a 1969 10:1 cr 350 and it pulled decent down low and strong in the mid range. I agree with Jake that you may be a bit overcammed and advancing the cam can compensate a little for that as well. It would be worth a try before swapping it. Just some ideas.
Bob
 
#6 ·
Although it is a fairly large cam for a 305, no power until 3500 RPM still seems strange. I had a 355 with that cam and it had great power from ~2200 RPM and above. I had a 1900 stall and that was adequate. I also had a 700R4 with a 3.06 first gear and a 3.90 rear gear though. The fact that it is a 305 is going to bring that number up, but I would not have thought that much.

Adam
 
#7 ·
firestone said:
Although it is a fairly large cam for a 305, no power until 3500 RPM still seems strange. I had a 355 with that cam and it had great power from ~2200 RPM and above. I had a 1900 stall and that was adequate. I also had a 700R4 with a 3.06 first gear and a 3.90 rear gear though. The fact that it is a 305 is going to bring that number up, but I would not have thought that much.

Adam
You are waaay overcammed.

That cam will act 16* larger in a 305 than in a 355.
So it acts like a 284 would in a 355.

About 8* per 25 cubes is a good rule for SBC. (I usually figure 12* from a 350 to a long rod 383, but more cubes requires more lift)

Advancing the intake lobe to increase dynamic compression also advances the exhaust lobe a like amount.

Even if you try to crutch it by advancing it to 6* from straight up (not sure how it is ground, look at a cam card), it will still be a pig in a 305 because the exhaust valve is opening so early in the power stroke, blowing your little bit of power right down the tail pipe.
 
#8 ·
Xntrik, I agree with what you are saying but the comp xe284h (240 246 @.050 .507 .510 lift 110 lsa) is recommended for a powerband of 2300-6500 RPM and a 2800+ stall. According to comp, those numbers are based roughly on a 350. Comp is claiming that in a 350, power should come in good by 2800, that is still quite a bit below the 3500 that he is claiming. Do you think there could be any other factors?

Adam
 
#9 · (Edited)
firestone said:
Xntrik, I agree with what you are saying but the comp xe284h (240 246 @.050 .507 .510 lift 110 lsa) is recommended for a powerband of 2300-6500 RPM and a 2800+ stall. According to comp, those numbers are based roughly on a 350. Comp is claiming that in a 350, power should come in good by 2800, that is still quite a bit below the 3500 that he is claiming. Do you think there could be any other factors?

Adam

In # 1 he says he has a tiny headed 3-0-5 with a low rpm intake, in a 3600 pound runner.

There probably are other factors, we have to conclude the cam is timed correctly, etc.
but perfect ignition timing and carburetion isn't going to make it into a 1970 302 Trans-am race engine.

Take those Comp numbers and add 1000 rpm. HINT, when you read the cam catalog for description based on a 350, for a 305 read the descriptions for one or two cams bigger. If your reading it for a 383, read one step smaller description. One step per 25 cubes.

I saw RickyRedneck try to pull this exact thing off about 7 years ago. :nono: It was a learning experience for him.

I think he needs a single pattern cam about 212*. (224-16=208*)
And he probably needs a heavy flywheel to pull 3600+ pounds.

If it were up to me I'd use the 260H @ 212* or even the 252H @ 206* with that stick. But that is just me. I like snappy mid range. With the stock valves and stick shift he could use a 108* LSA custom grind, but a 110* is not awful. He could use 1.6 rockers for more lift.
If this were a full out drag car it wouldn't be a 305.

Feel free to disagree. :welcome: That is what this board is all about, opinions. ;)

added= Just to give you a rough idea of what I mean..... this is only an illustration.... The 5.0 Mustangs have a 208x212 cam and they do real well with a carburetor (the SFI intake is for below 5300 rpm) The stock SFI car runs pretty well for a 302 cube, huh?
 
#10 ·
8* advance is really pushing it. :nono:

Remember it is advancing the exhaust lobe too. Your power stroke is going out the tailpipe.

There is already 4* more exhaust advance in the grind, and 18* more duration....

When you add that up, the exhaust lobe is opening 30* too soon........ OUT SHE GOES> :thumbup:
 
#14 ·
This is just opinions. :welcome:

In post #1 (copy and paste), he describes his car "almost no low end power. its a dog till 3500 rpms...i cant launch very well at all."

And I said= "You are waaay overcammed."


F-Bird, you say the 268 cam is worse than I said, exhaust wise. OK. :)

Even if he crutches his XE268H to make it better,
the cam I recommend, CompCams 260H with 212*@ .050 on 110* cam with 1.6 rockers preferred(or 108* custom LSA), will make more useable power across the board and be much more pleasant to drive in any situation.

I have already seen this scenerio happen with the exact same two cams in a 305. NO matter what you do, the 268 car will always be a very sick puppy to drive. period.

You win F-Bird. Have fun. Time's wasting. :thumbup:
Thanks for the chat. :cool:

Valve clearance depends on valve size, lift, and piston configuration.
Lots of cams will be too close way-way before getting into the 250* @ .050 and 99* ILC range.
Point is, check them.
 
#15 ·
F-Bird


HE SAID IT WAS A DRIVER.

The gist of this thread is that he has a DRIVER he is complaining about being doggy. post # 1 He said it was sick below 3500.


Now YOU are trying to make it a 4.56 gear drag car spinning it 6500 rpm ..... :nono:

Give us a break man-------- :thumbup:

You win ALL the cake awards.. :rolleyes:
 
#16 ·
i wouldnt mind the power band it has right now if only i had enough bottom end torque to launch well. here's something else i could do. i also have a used edelbrock performer plus cam with lobe matching lifters that was in another 305 i had.it has Advertised Duration 278/288, Lift .420/.442. im assuming ya'll are going to tell me to put this cam in. it makes 20 less peak horse on the dyno but has a lot better low end. i just dont feel like ripping stuff off again since i just put it together. i might run this one for a little while then change it this summer. i plan on buying some aluminum heads for this engine with my tax return money, would my current cam be more practical then?
 
#17 ·
Start building a 350, 383, or 400 and quit dumping money in the 305 would be your best move. If you do go with alluminum heads on the 305 you'll want to look for some with around 55cc chambers. The 268 is a lot of cam for a 305 unless you're running 4.10 gears and a good converter to get the rpms up.
 
#19 ·
350's are everywhere!If youre low on money I'd rather put my money in a strong 350 than a 305.Trust it will make a world of difference in real world driving,A 383 would be nice
http://shop.enginekits.com/osb/itemdetails.cfm?ID=81
but if thats still out of your way,rebuild the 350 a set of vortecs are about $500 from summit.Powerhouse sells decent aluminum heads
http://shop.enginekits.com/osb/showitem.cfm?Category=728
• 190cc (Polish)
.100 66/60
.200 134/115
.300 187/154
.400 234/190
.500 268/209
.600 286/227
.700 292/232
305's are'nt really worth the time,350's are easy to find and easy to build,plus the 268H XE cam is perfect for a 350.
 
#20 · (Edited)
take a look at this current thread

http://www.hotrodders.com/forum/329-desktop-dyno-anyone-108201.html

Check out the first post details

and the post # 12 by engineczar with the dyno sheet.... look at the cam he is using.

With a SINGLE PATTERN CAM........Look at the torque curve.......
It will suck the hair off Malibu's 268 cammed 305.

It is putting out

320 lb/ft at 2000 rpm

360 lb/ft from 4000-4500
and 345 hp @ 55-5700 rpm.

That's about 100 hp more.

With Malibu's smaller heads and a smaller cam.... it won't make this much power, but it will make a lot more useable power than the 268 he has now.
 
#22 · (Edited)
firestone said:
In that post, he was using a cam with 224 @.050 duration .470 lift. That is very close to the same as the xe268h (224 230 @.050 .477 .480 lift) other than the fact that it is a single pattern.

Why would it perform so much better than the XE268H down low?

I assume he is using the comp 270h (224 224 @.050 .470 lift) it is rated for 1800 to 5800 RPM which is actually comming on later than the 1600-5800 of the xe268h.

Adam


'Cause it ain't blowing out the exhaust????

(6* difference @ .050 that is 12* @ .006)

Cam specs in catalog are for average 355 cubes.
 
#23 ·
305 or Vortec exhaust port, 6 degrees of duration at .050 additional, over scavenging exhaust...I don't think so. Intake port on Vortec, apples and footballs compared to 305 head.

Wondering what the A/F's are, and what the timing curve is like on Malibus engine?
 
#24 ·
Rick WI said:
Wondering what the A/F's are, and what the timing curve is like on Malibus engine?

Rick, I think that you have a valid point that needs pursuing.

Just because his engine was dyno'd, it is not correct for us to conclude that the dyno process included proper fuel or timing tuning,

even though that is the entire purpose of dynoing, and you and I would do it as due process.
 
#25 ·
i found out my vacuum advance is busted so im gonna go get a new one tomorrow and see how much of a difference that makes in the low end. also, im going to pull the diff cover and check the ratio cuz the guy i bought the car from told me it was 3.73 posi, the numbers are too rusted too read so it could actually be anywheres from a 2.14 to 3.73. that could be another reason my low end sucks
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top