SpyderD

I would just like a second opinion reality check on this engine combination suggested by my mechanic for this engine rebuild I am having him do.
Currently on the 350 (corvette 1981) engine is a holley 750 double pumper, edelbrock RPM intake, headers, corvette aluminum heads that have been ported and polished, 1.5 rocker arms, 2,000 torque converter, 700R4 transmission.

He is suggesting keeping these parts and adding:
Bore over .030
Replace current comp cam magnum 270 in the engine with a comp cam XE274
flat top pistons to bring compression to between 10.5 to 10.7.

He ran the combination on his dyno program and came up with 376 HP/395 TQ at 5,000 RPM and 392 HP/374 TQ at 5,500 RPM.

My concern is with the high cam duration (230/236 @ .005), I told him I didn't want to sacrifice low end torque with this high duration cam. He got his graph out and told me torque was 351 at 2,000 RPM, 361 at 2,500 RPM, 400 at 4,000 RPM and declines from there. Seems pretty good to me.

My question for you cam and dyno set up people is - does this smell right? Are these dyno programs this accurate? I don't want to be stuck with a cam that is a dog in city driving, but his dyno results do sound good.

One issue is I don't think he reflected the head porting in his dyno program since we don't know how the flow improved.

dman535

I ran that camshaft for a while but I had much less static compression than you did. The low end seemed ok. What gear are you running ?

My greater concern would be the Corvette heads, what are you using for springs ? Those XE cams have a pretty wicked ramp up rate, if your springs are not correctly matched you are going to munch the cam.

SpyderD

The gears are stock (3.08?) but the transmission is a 700R4 with the lower first gear range. I think this has the effect of changing the 3.08 gears to 3.7. Someone correct me if I am wrong.
The corvette l98 heads that they sell in the aftermarket come with stronger springs but I will mention it to the mechanic.

Ryan77

If you get a new cam, get the Comp Cams cam "kit", it comes with everything that you need, cam with matched springs, lifters, and a new timing chain set. They have almost, if not all their cams available in the "kit". I bought mine from Jegs. I bought the 268XE, and I like it very much, a little slow up to 1500 rpm, but from there on out, you're pinned in the seat. Good Luck with whatever you choose :cheers:

The Money Pit

I ran the XE-274 in the 406 for a couple months and it went flat. (Didn't break in right) In the 406 I felt it ran out of rpms too soon,so I went with the XE-284 on the rebuild. In a 355 I'd stick with the XE-268 if you want more low end punch. The XE-274 is getting up there in duration for a 355 unless you run some stall and the 3.70 gears. The compression ratio seems a bit high for you as well. I ran a dyno simulation and found no major gain in power raising the compression from 9.0 to 12.5 : 1 using the same combo in the 406. It's all in the valve timing and heads if you want to make the power.

gdh

:iagree:
That is the cam that CC recommended for my stroker, I am not sure that you would be that happy with it given your existing setup. I would be more apt to go with the 268 or 270.


[Modified by gdh, 8:16 PM 12/3/2002]

mountainmotor

It looks like the dual pattern Crane PowerMax 272 216@.050 on the intake is the cam for this engine/trans/gear combo


[Modified by mountainmotor, 4:16 AM 12/4/2002]

vettfixr

:iagree: I've got this cam and it idles nice at 850 rpm with 15 inches of vacuum on my stock bottom end. you don't give up much bottom end and it really comes alive from 2000 rpm up. Your higher compression should only help.

SpyderD

Can you guys expand on the crane powermax recommendation, why it matches better? The previous owner has the Comp Cam magnum 270 in it (224@.050) which was a fine for daily driving. I don't see a reason for replacing it if it only results in a small power increase.

vettfixr

One of the differences between the two cams is lobe separation. 108 degrees (CC) vs. 114 degrees (Crane) I believe. The wider lobe separation will give you a better idle, more vacuum and a stouter bottom end. If you are going with an engine that will be driven on the street most of your time will be spent well below the peak output of the engine and a cam that's too big or soggy on the bottom end will not be fun to drive. You want to retain as much bottom end as you can while having a top end that's there when you need it. There's an old saying. "Torque wins races, Horsepower sells cars". Others may disagree.

MotorHead

I have the CC 270H in mine with a Holley 750 with mechanical secondaries and it is a great street cam I can drive around in fourth ( with 308's) at 1500 RPM and hit the gas and there is no bog or anything, just pulls and keeps going and with the right setup will keep pulling into the high RPM range.

mountainmotor

Quote:
"Can you guys expand on the crane powermax recommendation, why it matches better? The previous owner has the Comp Cam magnum 270 in it (224@.050) which was a fine for daily driving. I don't see a reason for replacing it if it only results in a small power increase."

A few of reasons one being the heads intake to exhaust flow ratio is pretty poor " ex flows stinks"depicting a desire for a dual pattern cam the next is with the 308's and 700R4 at 60 mph it needs a cam that will smooth up or cruise milaege will stink,,you don't want a car motor trying to lope on the hiway and you can't always cruise at 70.Next a single pattern cam like the Comp 270 we call those one way cams,,the Crane will give a broader power range

When my Son had his stock 88 IROC that car would get better fuel milaege at 60 in third than in O/D" 700R4"

FWIW the Crane has a 112 LCA and the Comp 270 is on a 110

burners

I'm running this cam in the Vette and I like it a lot. I have ported WP Sportsman heads, Comp Magnum 1.52 roller rockers, and Rhoads lifters. With 3.70 rear gears it pulls really good and doesn't seem to have a noticible lack of torque below 2000 rpm (it revs through 2000rpm pretty quickly). On the highway it has great power and gets about 14-15 mpg while cruising at 85-95 mph (4000 rpm @ 90).

What are your goals and uses for the car? If you go that high on the compression then you will want the overlap on the cam but if you want a broader torque curve you may want slightly lower compression and less overlap.

69stingray

That surprises me, LPE uses the Corvette heads on most of his street engine because they have excellent intake/exhaust ratio. Also, the cam the Lingefelter uses for his 350 street engines has 211/219 duration at 0.05 and 0.505/0.525 lift (w/ 1.6 rocker arms). His street engines are set up for torque.

I have started a cam database and a cylinder head database. They aren't finished nor pretty yet.

http://temp.corvetteforum.net/c3/69stingray/head.html
http://temp.corvetteforum.net/c3/69s.../cam_page.html


There is also the TPIS Z10 cam, 208/216 duration @ 0.05 & 0.525/0.525 Lift

I would ask your engine builder to try one of these cams in the dyno program.



[Modified by 69stingray, 7:38 PM 12/4/2002]

SpyderD

Burners - The car goals are not as a daily driver, just a weekend car and some 1/4 mile runs.
The heads have been ported and valves enlarged by the previous owner. The mechanic tried to tell me the heads didn't flow to well either and recommended Vortec heads but I don't want to shell out money for new heads.

mountainmotor

I have the flow numbers before and after in the shop,,will post them tommorrow

burners

If it's just a weekend street/strip car I would say go with the XE274. It is a good cam that makes good hp and torque. Did you mention what kind of gears you are running, I didn't notice? Running at least 3.23's would be good and for your use maybe 3.90 of 4.11's.

SpyderD

What about the compression with these comp cams xtreme energy cams? I was told they actually INCREASE compression. I was hoping the duration would bleed off some of the high compression that the flat top pistons are giving but now I am confused.

The gears are stock on a '81 vette but I have the 7r400 transmission and a 2,000 torque converter. Wouldn't mind changing to lower gears in the future. One thing I figure I have on my side is this particular year model is about 200-300 pounds lighter than the mid 70 vette's.

71roadster

Thats the cam I just installed in my 383. Haven't run it yet and have a question about the builder installing it at 104 degrees rather thatn 'straight up' which is actually 4 degrees advanced from 110 degrees (so it would be 106 degrees). The builder recommended the xe268h but I have heard that in a 350 and it is a fairly mild cam in my opinion so I figured the extra cubic inches would eat up some of the lope of the 274. I called comp cams and they recommended an even fatter cam which kind of suprised me. So I figured the 274 was a compromise between the different recommendations. I look forward to seeing how it runs. I will have about 10.5:1 compression also.

Chris