GM fastburn heads

[tscott9330]

Easy question, Has anyone used the Gm fastburn heads?

I am giving consuideration to using these because of there 62cc combustion chamber. I have done some calculations using my existing cam numbers and a few different heads. According to my numbers the fastburn's will give a bit more DYNAMIC COMPRESSION.

With
3.75 crank stroke (2.655 Dynamic stroke)
5.7 rods
0 decked
.038 head gasket with 4.15 bore
-6 cc valve relief

With 64 cc heads
Static compression=10.99 Dynamic=8.07
with 62 cc fastburn
Static compression=11.265 Dynamic=8.26

I am giving thought to either the GM's or a set of twisted flow heads with a 64 cc chamber.

I was hoping the smaller cc head would give me a little more low end torque with my current cam. But I am concerned that the 210 intake runner may be a little to much and end up costing me any torque I may have gained from the higher compression ratio.


Any advice would be appreciated.

 

[Twin_Turbo]

what is dynamic stroke?

As for the heads, at a DCR of 8.07 you could use a little more compression but it ain't too bad. The fast burns may be a nice choice, 210cc runners isn't too bad. Only problem is verlocity will be lower so you'll have less off a ramming effect which leads to a bit more pumping loss (because of your long?? cam)

I'm not sure what heads you ar running now but if they are around 190 cc or so the increase in compression w/ the fast burns may be offset by the lower port velocity w/ the 210cc runners at low rpm. There's just no telling what will happen, to give an idea you might want to plot it in DD2K or seomthing. Now, if you are keeping runnersize and things constant and ONLY change the compression then surely you will make more torque down low because the higher compression will make up for some of the pumping loss.

11,2:1 static comprssion, are you able to run that with the gasoline available in your area? (don't know what they have over there) You'd better have one good cooling system tho.

 

[AKRAY4PLAY]

hope you can get 94 octane and have a big overlap on the cam. 11.2 is pretty high on the comp ratio for a street motor, as are the 210cc runners. from my experience, 195cc runners work very good on 383's. for a little more $ i would get some better flowing heads like AFR or Brodix and stay with a smaller port size to retain the low end. the higher flow heads will also make more on the top because of better port designs. just my $.02, Brian

 

[Twin_Turbo]

oh wait, with dynamic stroke you mean the actual effective sweep volume considering intake valve closing point rather than BDC.

The DCR has in fact nothing to do with the port effectivness or even size. The FCR is a constant value. The actual cylinder pressure is dependant on rpm, intake and exhaust efficiencies and other factors.

Ideal DCR for a street engine is around 7,5 to 8,5 so you are already in a pretty good range. Building the engine close to 8,5 will necessitate a very thorugh;y built engine since the cylidner pressure at max efficiency (max torque) will be high. In a full race engine a DCR of 9 or higher is not uncommon but it will require higher octane fules. Depending on cam this translates into static compressions of around 12:1 or even higher. With a 290deg. cam a dcr of 9 is about 12 for SCR

The higher you are in the DCR, the more important cooling and quench height become to prevent detonation. Staying below about 8,25 would be best for trouble free use.
Unless you have cc'ed the heads and checked the crank & rods (and even compression ehgith on pistons) the calculations are a guide, there can be quite a bit of error if you use the figures as provided by the manufacturer.

Obviously long nduration cams reduce the running compression (and cylinder pressure at low rpm) of the engine because of the late intake closing (pumping loss at low rpm)
Because the intake cloing is not only dependant on the cam profile but also on the cam indexing increasing the DCR is possible retarding the cam (and vice versa for increasing)

Another factor to conisder is rod length, the longe the rod, the higher the position of the piston at intake valve closing 9compared to a shorter rod) The effect however is not that big, especially if the rod isn't much longer than stock. (the height diff is small then)

Big overlap won't help you, the exhaust valve has no role in the compression ratio, and since the overlap is merely a measurement of the amount of crankshaft degrees around TDC that both valves are open. It's only dependant on duration and love seperation angle. It IS true that the longer the overlap the more the powerband shifts to higerh rpm, also increasing overlap kills low end torque. Choosing too much overlap is tempting, 1 degree more may seem like a small difference but what matters is the surface area increase under the lobes (the actual lift amount), this means only a slight increase/decrease in degrees here can make a big difference.

What are the cam specs on your engine? Since it has 2.655 effective sweep out of a total of 3,75 it mens that when the intake closes the piston has already travelled 1,095" up the bore.

This means the crank has made 197 of revolution before the intake valve closes. That translates into a camshaft larger than (since the intake valve already was open at TDC) That means the intake had been open for 219 degrees since TDC.

 

[AKRAY4PLAY]

Twinnie,
you are correct about the exhaust valve not effecting the mathmatical numbers of the compression ratio. but it does have a factor to play with total amount of fuel/air charge that stays in the cylinder to be compressed. if the cylinder is not full then that means you have less air to compress into the same volume, resulting in lower cylinder pressure and avoiding detonation at the expense of low rpm performance. rod length has another more important factor to play, piston speed. the longer the rod the more the pistons "hangs around" TDC and BDC. this increases the exhaust's and intake's effecientcy. also the increased speed mid-stroke creates a higher peak vaccum to help increase intake velocity. and finally the longer rod decreases the side loading on the cylinder walls lowering the latteral friction applied to the piston skirts. Brian

 

[Twin_Turbo]

Brian, I know about the exhaust valve, but the overlap can be used for benefit too, you can have the exhuast scavenging pull throught the open intake valve (the exhaust, if done properly pulls many times harder than the piston travelling down)

I agree about the rod length, I am running 6" carrillo's and je pistons, the pistons have a much lower compression height than stock (pin higher up the piston, don't know #'s from top off my head) and yes, TDC and BDC "dwell time" is increased.
I was talking about the DCR only, so it was a purely mathetmatical story

Also, with longer rods (actually longer rod/stroke aration) you have to run a wider LCA )or LDA or LSA whatever you want to call it)




Marck

 

[AKRAY4PLAY]

Marck,
i fully understand your points and agree with you, i was just trying to add a couple of points to show the relationships of parts and how they effect the performance of other parts and the engine on a whole.
as for motors, i am running Eagle 6.0" rods and an Eagle 3.75 crank with JE/srp pistons. are you running a 383?? how is the power band with the turbos?? i am thinking about forced induction on my next all aluminum motor. don't know if it will be a 454 small block or a 572 big block. lots of studying and research to do before buying parts, but i am getting jazzed up the project. Brian

 

[Twin_Turbo]

No,I am running a normal 3,48" stroke. I didn't want a 383 since that would men the chambers would have to be biiger, or the pistons dished more to bring the compression down.

I don't know what the power band is, my car is still in pieces and have not driven it with the turbo's on yet.

An all aluminium block.. COOL

 

[AKRAY4PLAY]

Marck,
keep me posted on the progress of getting her running. i am curious to see the real world characteristcs of turbos on big motors. i like super chargers on smaller motors, but i have never really needed to go with forced induction on V-8's. yet another area of performance to study and master before i die!! Brian